How does the coolant reservoir work? - losing coolant after head replacement...

[Dan_89]

Hi everyone,

I'm hoping that someone can describe how exactly the coolant reservoir on this car (2003 1.8T Quattro in my case) is supposed to work as I am experiencing a mysterious rapid loss of coolant after doing some major work on my vehicle: timing belt/water pump, head replacement (swapped my AMB with an AEB) and check valve simplification + removal of SAI pump system (AEB head can't use this anyway.) This was all done after my timing belt failed prematurely (after just 35,000 miles due to a seized water pump) and resulted in 12 bent valves

Anyway after 2 months of work my car was finally ready to return to the roads! Being anxious to see how it ran I quickly filled the coolant, power steering, checked the oil and went to start it: somewhat to my surprise it actually came to life and ran smoothly nearly right away (idled rough at first but quickly smoothed out.) I took it for a drive and was very happy with how everything was performing: engine was smooth and quiet, and besides a noticeable turbo lag (which I understand is to be expected when going to a large port head while still using the stock turbo) it seemed to be performing 100%. Not long into the test drive my temp started to rise above the 1/2 way mark and I immediately pulled over and let the car idle as the gauge rose to about 3/4, coolant light came on briefly and quickly went off once the temp dropped back down to 1/2. Being just a few miles from home I carefully drove it back and the temp gauge stayed at the 1/2 way mark for the rest of the drive.

After some research it occurred to me that I more than likely had enough air trapped in the cooling system to prevent the coolant from properly flowing and I would need to bleed all the trapped air out...of course! I proceeded to re fill the coolant reservoir (which was almost completely empty by now) open the bleed valve and run the car with the heater turned all the way on to see if I couldn't get all that trapped air out of the system. Sure enough I first got a lot of air bubbles coming out of the valve and eventually mostly coolant but once the engine had heated up I was getting hot coolant sputtering all over out of the bleed valve with intermittent pockets of air but still no heat in my car. Being that this was creating such a mess I decided to shut it off and continue to bleed the system by attaching a hose to the bleed valve opening and collecting the hot coolant in a bucket. Continuing to bleed the system I still wasn't getting any heat so I decided to rev the engine a bit while carefully watching the temp gauge. After doing this my heater finally kicked on and I continued to get heat after letting it idle for some time. Satisfied that I had removed all of the air from the system I first pulled the the hose and closed the bleed valve before shutting off the engine (I was told to do this by a mechanic friend of mine to prevent any air from being drawn back through the open bleed valve after shutting off the engine. I then re filled the coolant reservoir and took it for another drive. This time I was able to drive it hard and keep it in the power band for a sustained period of time without the temp gauge ever rising above the 1/2 way mark. I drove it for maybe 10 miles all while doing a number of hard pulls with no signs of cooling problems until I was nearly home. Yet again the temp gauge began to rise above the 1/2 way mark and I immediately let the vehicle coast while the temp continued to rise just slightly above 3/4 before quickly returning back to 1/2 again (the coolant light also briefly came on and went off once temps were back down.) I very carefully drove it the remaining 2 miles or so to my house with the gauge staying at 1/2 and to my surprise my coolant level was suddenly back down to nearly empty! I carefully checked everything over for leaks and after not finding anything gave it some time to cool down before filling the coolant again and taking it on another test drive with nearly identical results: the vehicle performed flawlessly for about 15 miles of spirited driving then temps started to rise and yet again I was nearly out of coolant

After shutting it off I carefully inspected the engine bay as everything cooled off and noticed the sound of air leaking from somewhere. I found that the air was coming from an opening underneath my coolant reservoir that appears to simply be an open channel leading to the top where the cap threads on the reservoir and my guess is that it's purpose is to prevent overfilling the reservoir? I'm wondering if this channel should be open to the reservoir when the cap is closed (which was the case on my vehicle as air was rushing through it as the vehicle cooled off with the reservoir cap securely screwed on. Is there supposed to be some sort of O ring or gasket that is meant to seal this channel off when the cap is in place that was perhaps misplaced during all the work I did on the vehicle over the last 2 months? If anyone could post some high res pics of a properly functioning reservoir cap that I compare mine to that would be extremely helpful! I am continuing to check all coolant lines and connections for leaks and can't find anything so my guess is that when the coolant reaches a certain temperature it is vaporizing out of my reservoir and after looking closely around the engine bay surrounding the reservoir this does seem likely as I can see a fair amount of condensation. Any help is greatly appreciated, I have gone through a ridiculous amount of coolant already and would prefer not to lose another gallon of it before understanding how exactly this system is supposed to work

Thanks!

Dan

[Seerlah]

You were supposed to fill the coolant system properly before taking off.

When doing a coolant flush, you fill the reservoir to the max line. When you turn the engine over, you need to let it run for roughly 10 min with the cap off the reservoir and top off as necessary. During this time you are bleeding the system as well as filling the system, once the thermostat opens up.

[MmmBoost]

I hate to say this......but you may have fucked your new head up in your haste to get out driving.

[Charles.waite]

I hate to say this......but you may have fucked your new head up in your haste to get out driving.

The car takes around two gallons of coolant. If you notice, the reservoir DOESN'T hold that much because most of the coolant is always hanging out in the block or the heater core. I hope for your sake you didn't fry your headgasket.

I just replaced my thermostat and drove my car around at VERY low speed/load in my neighborhood (never above 20mph or 2k rpms and never farther away than a block or two.) in order to cycle the coolant through my engine. It sucked in about half of the tank of coolant. Then later that day after topping it off and driving about 30 miles, again NOT pushing the car, I topped off the tank again.

I don't think the Jeremy Clarkson School of Mechanics method that you followed was a good idea at all....

[Dan_89]

Chris, definitely not what I wanted to hear! :( Are you thinking that I could have blown the head gasket? This possibility has crossed my mind, but the vehicle runs flawlessly until the coolant disappears: smooth, quiet, no vibration, no white smoke, and it seems to be making full power I would think if coolant was mixing with any other channels in the head the engine would run like shit but maybe not? This open rectangular channel on the bottom of the coolant reservoir is what has me puzzled, it seems like something should plug into it or at the very least it seems it shouldn't open the reservoir to the atmosphere when the cap is secured. Should the reservoir not be completely sealed when the cap is in place?

[road_tripper]

try removing your spark plugs and see if any are sparkly clean. obviously the coolant has to be going somewhere... if it's not leaking and not turning your oil to chocolate, it is probably going out the tailpipe. unless you've not added enough coolant to fill the system?

Also... if you fill the reservoir with coolant, leave the cap off, and let car idle up to operating temp, does the level change? Does it bubble in the expansion tank? If it bubbles do the bubbles smell like exhaust fumes?

[Seerlah]

That little void on the reservoir is normal. I would assume it is to alleviate pressure from having to undo the cap if it wasn't there.

[catamount90]

Tell us this: are you getting heat?

[Seerlah]

The guy has no coolant. Easy solution. Refill it and bleed/fill properly. I personally would drain all the current coolant out first, and start from scratch. But that's just me.

[NyteEscape]

when i did my timing belt i filled coolant and thought i was done. i then proceeded to drive 20 miles back home with my normal driving habits. got home and realized coolant was really low. topped it off and kept an eye on it. it's been 40k miles and my head gasket seems to be fine (knock on wood).

[vinny.dtw]

Personally I think he has a small puncture in a hose somewhere thats leaking when the car heats up while driving. No way in hell if you drive for 10 miles, would you burn enough coolant in the resevoir through a headgasket...It's obviously leaking, and leaking fast going somewhere.

Check all the hoses like radiator hoses, and the lines going to the coolant. Often times, it may appear fine, until something heats up, and you'll see a rip in them due to expansion. let me see if i can go take a pic of my tank and cap.

[Charles.waite]

Both times I was "low" after doing my T-Stat, I had to add around 1/2 -1 quart. Also my heat didn't come on until after it drank almost the whole tank the first time. It just takes a while to get the air bubbles out.

OP: How many times have you had to fill up the reservoir. Twice is normal after a full drain and fill. More than that and you've got a leak.

[Dan_89]

Ok sounds like thinking my problem was the coolant reservoir was wishful thinking, I'm afraid you guys are correct I probably have blown the head gasket in my haste to get it on the road I do get heat now but it takes forever to come on and I haven't been able to get heat out of the vents after just letting it idle, always have to drive it for some time first. I'm just shocked that the engine could seemingly run so well if it is in fact burning up all the coolant that I am losing but I've decided to stop running it to prevent doing any damage to the engine (hopefully I haven't already!) I guess I'll be ordering a set of head bolts and a new head gasket yet again and re doing much of what I already have done This time I'll be sure to do a thorough coolant flush after getting it all back together this time, I had no idea this was required (I'm really new to working on vehicles and decided to try and tackle this project myself after seeing all of the very detailed DIYs related to this vehicle online and not having the funds to pay a shop to do the work.) Should I be checking any other components on the head/engine or anything else in the vehicle for damage as a result of driving with a likely blown head gasket?

Thanks,

Dan

[vinny.dtw]

Ok sounds like thinking my problem was the coolant reservoir was wishful thinking, I'm afraid you guys are correct I probably have blown the head gasket in my haste to get it on the road I do get heat now but it takes forever to come on and I haven't been able to get heat out of the vents after just letting it idle, always have to drive it for some time first. I'm just shocked that the engine could seemingly run so well if it is in fact burning up all the coolant that I am losing but I've decided to stop running it to prevent doing any damage to the engine (hopefully I haven't already!) I guess I'll be ordering a set of head bolts and a new head gasket yet again and re doing much of what I already have done This time I'll be sure to do a thorough coolant flush after getting it all back together this time, I had no idea this was required (I'm really new to working on vehicles and decided to try and tackle this project myself after seeing all of the very detailed DIYs related to this vehicle online and not having the funds to pay a shop to do the work.) Should I be checking any other components on the head/engine or anything else in the vehicle for damage as a result of driving with a likely blown head gasket?

Thanks,

Dan

Hold on a second here...how did you come to the conclusion that your headgasket is blown? Did you drain your oil to see that it was milky? Or can you smell coolant burning through your exhaust?

[Dan_89]

Ok as I was typing that last response it appears that some of you think all hope is not lost and I may still just be dealing with a coolant leak instead of a blown head gasket (I also find it very hard to believe that I could have a full coolant reservoir, drive for 10 - 15 miles and manage to burn all of it without the engine running terribly.) So is there a definitive way of determining whether I am dealing with a blown head gasket before I throw a bunch of time and money at replacing it possibly for nothing? I have had to fill the reservoir much more than twice now...closer to 5, maybe more Could I really be burning up that much coolant and still have a seemingly perfectly fine engine? Sounds like I should be searching for a massive coolant leak once everything is heated up, but at the same time I don't want to run the vehicle anymore if I also have a blown head gasket and could be damaging the engine at the same time...

[Sales@RAI]

If it didn't get pinned in the red you didn't toast anything

[Charles.waite]

Ok as I was typing that last response it appears that some of you think all hope is not lost and I may still just be dealing with a coolant leak instead of a blown head gasket (I also find it very hard to believe that I could have a full coolant reservoir, drive for 10 - 15 miles and manage to burn all of it without the engine running terribly.) So is there a definitive way of determining whether I am dealing with a blown head gasket before I throw a bunch of time and money at replacing it possibly for nothing? I have had to fill the reservoir much more than twice now...closer to 5, maybe more Could I really be burning up that much coolant and still have a seemingly perfectly fine engine? Sounds like I should be searching for a massive coolant leak once everything is heated up, but at the same time I don't want to run the vehicle anymore if I also have a blown head gasket and could be damaging the engine at the same time...

Common leak points are the Thermostat (located under the intake manifold) and the Rear Coolant Flange (located underneath the Combi-Valve on the back of the block. The Rear flange leaks down onto the transmission bellhousing so thats an easy leak to spot usually.

I would aslo check your Coolant temp sensors (one located on the rear flange and on located in the lower Radiator coupler) to make sure the O-rings aren't leaking.

[Sales@RAI]

It honestly sounds like you just started filling the engine with coolant you might not even have a leak

[Dan_89]

If it didn't get pinned in the red you didn't toast anything

Ok these last posts are reassuring, especially this one! The highest the temp gauge ever got was ever so slightly above 3/4, and this was only very briefly after which the needle quickly came back to 1/2 and idled at that temp smoothly for some time before shutting it off. I've been closely watching the areas around the coolant flange, temp sensor on coolant flange, and thermostat for any signs of coolant leaks and haven't seen anything in these areas yet. I'll have to keep an eye on the temp sensor located on the lower radiator coupling as I haven't been checking this location. Considering the amount of coolant I've already put into the system I'm sure I'm losing it somewhere but nothing obvious has jumped out at me yet. Maybe I'll take it for another drive now as I did a more thorough coolant bleed last night and re filled the reservoir this morning while the engine was running and see whether I'm still losing coolant since I haven't driven it after becoming concerned that I blew the head gasket. What do you guys think, is this a good idea?

[catamount90]

First of all you need to get down on your hands and knees and look at the ground under your car. Is it wet?

Secondly, pull the dipstick. Is it higher than it was before you replaced the head? Like... way too high?

Pull a plug, get a flashlight and look into your spark plugs holes. Is the top of one of your pistons shiny?

Then report back. All of this talk about replacing the head before you've even doing a pressure test or compression test is just silly, when you haven't even look for leaks as far as I can tell.

[Dan_89]

I took it for another test drive today with the goal being locating where my coolant leak is originating. Before driving it I took some white rags and strategically wrapped/secured them in locations where a leak seemed likely: around the coolant flange, temp sensor and coolant lines connecting to it, the lower radiator coupling and sensor, thermostat, and major rubber coolant lines. Next I started out by first driving the vehicle lightly around town while of course watching the temp gauge carefully and running the heat (which was working.) During this process everything seemed perfectly fine: engine ran smooth and quiet and I stopped a number of times to pop the hood and check for any signs of leaks (none) as well as the coolant level which remained un changed. Next I drove the vehicle a bit harder: did a few rapid accelerations keeping it in the power band as well as some hard pulls from 3rd gear on the highway and initially the vehicle performed just as it should: power was good and everything was smooth and quiet. Then predictably the temp gauge began to rise above the 1/2 way mark and I immediately let it coast while the needle steadily climbed to 3/4, the coolant light warning came on and the temp gauge quickly dropped back to the 1/2 way mark and the warning light went off (identical to what has happened in my previous tests.)

After this I immediately jacked the front end up, threw some plastic under the vehicle and began looking for coolant leaks. The coolant level was yet again nearly empty and it was immediately apparent that I had lost a significant amount of coolant from somewhere on the driver's side of the vehicle. Looking underneath it puddles of coolant had already formed on the plastic and there were coolant droplets a long the entire length of the driver's side plastic molding underneath the vehicle. I removed all of the rags that I had secured a long various points on the cooling system and they were all dry. However everything directly underneath and surrounding that rectangular channel exiting the bottom of the coolant reservoir was drenched in coolant! I could actually still see coolant dripping from this channel. I had wrapped a rag around most of the reservoir which remained dry but I didn't have it around this channel. It sure seems to me that for whatever reason when driven hard I'm losing all of my coolant out of this channel...does this make any sense at all? From what I an tell I could drive the vehicle at 3000 rpms all day without a problem and never lose any coolant, it's just when I push the vehicle a little that I suddenly lose nearly all of the coolant in my reservoir.

Oil level is normal and I will pull the spark plugs tomorrow.

Dan

[Charles.waite]

Sounds like the coolant reservoir is dumping coolant out of the pressure relief valve at the bottom. Is there a chance it's a faulty tank and the pressure relief is cracking under too little pressure?

Have a shop pressure test the tank. If it checks out then it sounds like your cooling system is getting overpressurized somehow. Maybe exhaus gasses? Given your car's inability to hold coolant, that theory is going to be tough to prove.

[road_tripper]

don't mean to beat a dead horse, but try smelling your coolant reservoir, then your bottle of unused coolant, after you've overheated your engine and lost the coolant during your many test drives.

if they smell different (especially if your overflow tank smells like coolant+exhaust) you'll be a bit closer to ruling out or confirming a head gasket problem.

[Dan_89]

Thanks for the tip on smelling the coolant reservoir and bottle of unused coolant, would this need to be done right after I have lost coolant or would I be able to notice a difference now being that I haven't re filled the coolant reservoir since I lost nearly all coolant in the tank again yesterday? Also what would you consider "overheating" the engine? After replacing the head the temp gauge has never reached the red line, I always let it coast to a stop and cool off as soon as I see the needle begin to rise above the 1/2 way mark and the highest it has ever gotten was ever so slightly past the 3/4 mark before quickly returning to 1/2. I'm not losing any coolant until the temp gauge begins to rise after pushing the vehicle a bit and this is when the tank seems to suddenly purge nearly all of the coolant so I'm thinking I might want to just start by replacing the expansion tank and cap. I've been checking the exhaust for white smoke during all my testing and am not seeing anything other than what appears to be normal exhaust fumes. I'll check the spark plugs next.

[road_tripper]

Thanks for the tip on smelling the coolant reservoir and bottle of unused coolant, would this need to be done right after I have lost coolant or would I be able to notice a difference now being that I haven't re filled the coolant reservoir since I lost nearly all coolant in the tank again yesterday?

You'll be able to tell by sniffing the empty expansion tank. pop the cap, sniff, open your bottle of unused coolant, sniff.

I'm thinking I might want to just start by replacing the expansion tank and cap.

there's a lot of good advice in this link; you should try reading it and follow the suggestions (not necessarily my suggestions).

[Charles.waite]

there's a lot of good advice in this link; you should try reading it and follow the suggestions (not necessarily my suggestions).

ISWYDT...

[Dan_89]

there's a lot of good advice in this link; you should try reading it and follow the suggestions (not necessarily my suggestions).

haha I see your link points to my own thread, I'm thinkin this is intentional? I just figured that ordering a replacement reservoir isn't a bad idea seeing as it's only $20 and in the meantime I'll find a shop to pressure test the existing reservoir. Thanks so much for all the help guys, I really appreciate it!

Dan

[old guy]

Here's how the reservoir and cap work: Clicky click. Unfortunately your signs seem to point toward a blown head gasket.

[Dan_89]

Awesome thanks for that old guy, I'll pressure test the reservoir so I can at least eliminate that. I'm in the process of going through my cooling system at the moment and have already removed and tested/confirmed that my thermostat is working correctly. Today I plan on checking the heater core for any blockage, pressure testing the reservoir and probably doing a proper coolant flush as well to see whether that doesn't solve my problem seeing as I failed to properly re fill the system with coolant initially and it seems likely that I still had some air trapped somewhere in the system possibly causing blockage and over pressurization.

[Dan_89]

Alright so I have performed a thorough coolant flush, replaced the coolant reservoir tank and bled the system of air. I am now able to drive the vehicle aggressively for quite a bit longer than before but after pushing the car hard for awhile I'm still suddenly losing 1-1.5 liters of coolant through the coolant tank's pressure release valve. It's the same routine as before: the temp gauge is locked on the 1/2 way mark the entire time until it suddenly begins to rise to 3/4, coolant light comes on and after coasting to a stop the temp needle quickly returns to the 1/2 way mark. After pulling over and popping the hood sure enough my coolant tank is yet again nearly empty and it's very obvious that all the coolant has been lost through the pressure release valve on the bottom of the tank.

I have ruled out any head gasket problems and besides suddenly losing coolant when driven hard the car is driving better than ever. I have put at least 100 miles on it since putting the new head in and it consistently runs smooth, quite, and efficiently (in fact I've noticed that my fuel economy has improved rather significantly.) I'm beginning to wonder whether my coolant loss problem could be related to the thermostat. I remember reading something about the thermostat for the AEB engine being cooler than the AMB engine (80°C as opposed to 100°C if I'm not mistaken.) I'm currently using my original thermostat which is of course for the AMB engine however keep in mind that the cylinder head on the vehicle now is an AEB. I have also noticed that my engine does seem to consistently run warmer after the head swap: if I remember correctly before installing the new head my temp gauge usually read just over the 1/4 mark while now the temp gauge is consistently at the 1/2 way mark once the engine is warmed up. Does my thermostat theory make any sense?

Thanks!

Dan

[Dan_89]

Anyone have any thoughts on this? Can a lower temp thermostat even be installed in an AMB block? Or perhaps I should just replace the thermostat with an OEM B6 one and see if that doesn't solve the problem...the vehicle continues to drive flawlessly besides running a bit warmer than before the head swap and purging about a liter of coolant everytime it is pushed hard for any length of time...

Thanks!

[Charles.waite]

Anyone have any thoughts on this? Can a lower temp thermostat even be installed in an AMB block? Or perhaps I should just replace the thermostat with an OEM B6 one and see if that doesn't solve the problem...the vehicle continues to drive flawlessly besides running a bit warmer than before the head swap and purging about a liter of coolant everytime it is pushed hard for any length of time...

Thanks!

If the thermostat currently in there isn't the correct B6 version, its possible its the wrong temp and could be causing pressure buildup. It could also be faulty. I would take the chance on the Thermostat. Why not? it seems you've explored every other avenue...

[CO AVANT]

Sounds like the same issue I had after I installed my rebuilt motor. Anything more than ~27psi out of my turbo would cause coolant to blow out the reservoir pressure relief. Mine had no signs of coolant/oil mixing. I was using brand new stock stretch head bolts at the time torqued down in the correct pattern. Without even taking the head off I one by one took each stock bolt out and replaced them with ARP head bolts (non stretch) and torqued down to spec in the correct pattern. Have driven 8000 miles since then without a single issue boosting over 30psi.

[Dan_89]

If the thermostat currently in there isn't the correct B6 version, its possible its the wrong temp and could be causing pressure buildup. It could also be faulty. I would take the chance on the Thermostat. Why not? it seems you've explored every other avenue...

Hi Charles,

The thermostat that is currently in my vehicle is the correct B6 version with an opening temperature of 100 °C (I pulled the thermosat out some time ago and actually put it in heated water and confirmed that the thermostat was consistently opening right at water's boiling point. After doing this I figured that this ruled out my thermostat as being faulty but perhaps there is more to it. The cylinder head that I replaced my destroyed AMB head with is a re built AEB head and after doing some reading it occurred to me that the AEB engines use a thermostat with an opening temperature of 87 °C which is why I am wondering whether there is any way I can adapt a cooler temp thermostat to fit the housing for my AMB block. I picked up one of the 87°C thermostats for a 1999 Passat (same vehicle my rebuilt head came from) from a local auto parts store a few days ago hoping that it would fit right in my thermostat housing but after pulling the thermostat out again today I realized that it doesn't fit in the housing correctly at all.

Sounds like the same issue I had after I installed my rebuilt motor. Anything more than ~27psi out of my turbo would cause coolant to blow out the reservoir pressure relief. Mine had no signs of coolant/oil mixing. I was using brand new stock stretch head bolts at the time torqued down in the correct pattern. Without even taking the head off I one by one took each stock bolt out and replaced them with ARP head bolts (non stretch) and torqued down to spec in the correct pattern. Have driven 8000 miles since then without a single issue boosting over 30psi.

Interesting, I wonder what the head bolts would have to do with this problem, any idea? I am running stock everything (besides having an AEB head and having the PCV system and SAI pump removed) and you said that this was happening only when running very high boost. Still it does seem like switching to the ARP head bolts might be worth a shot, I was shocked at how much torque was called for when tightening the stretch head bolts (44 ft/lbs. of torque + an additional 1/2 turn seemed crazy, it was tough to make the final 1/2 turn even using a large breaker bar!)

Thanks,

Dan

[Charles.waite]

THere could be something with how the AEB mates to the AMB that is causing excessive coolant pressure. Not sure if thats actually possible, as I've heard they're all interchangeable, but its a thought...

[Dan_89]

That does seem like a very good possibility, maybe I'll have to start a separate thread asking about head bolt choice and torque specs when putting an AEB head on an AMB block and hopefully I can get some input from others who have done this swap.

Thanks!

[CO AVANT]

What head gasket manufacturer are you using? Victor, Reinz, Elring? Another forum member in CA is having this same issue right now and we both have Victor head gaskets.

[Dan_89]

I'm actually using the Fel-Pro PermaTorqueMLS head gasket.

[Dan_89]

Before I get into potentially replacing the stretch head bolts with ARP head bolts I would like to explore the possibility of trying a lower temp thermostat in my vehicle. Can a lower temp thermostat even be used on the B6 A4 with an AMB block? I'm looking at a photo of the thermostat housing for the B5 AEB engines (87°C) and although it appears to have the exact same fitting to the engine block it doesn't have the temperature sensor or any of the same coolant line connections as the B6 thermostat housing. It is a relatively cheap part compared to the B6 housing ($16 or so) so I wonder if it would be worth picking one up and seeing if I can somehow modify it to work on my vehicle. Does anyone know what the temp sensor on the B6 thermostat housing does? Is it just for the temp gauge or does the ECU use it to control other systems on the vehicle? I wish I could just find a lower temp thermostat that is compatible with the B6 thermostat housing...

[Charles.waite]

Its not a Temp sensor, its used to actuate the Thermostat itself I believe. It was a B6 only experiment as the B7 has a purely mechanical Thermostat like the B5.