Coolant system pressurizing

[CELison]

Having an issue where the coolant system is pressurizing under boost and blowing coolant out of the expansion tank. Driving normally it’s fine. It can sit and idle fine, cruise fine, mixed driving it's fine, doesn’t burn any coolant and level remains the same when driving normally. The coolant is nice and pink too. But if you get on it it’s spews coolant from the tank. I did notice however, the cap doesn’t “click” into place like it used to. That seems odd so I’m buying a new oem tank and cap and seeing if that helps. After it dumps coolant, I can pull over, pop the cap off and the coolant level rises, but it doesn't come blowing out or overflow. It's not boiling either.

The car doesn’t misfire under full boost and runs great. Cold starts just fine, drives around fine, no smoke from the exhaust. I don’t get what’s pressurizing the system. I would think combustion gases getting into the system would pressurize it all the time, not just in boost. I would also think combustion gases leaking into the cooling system would cause misfires. I was under the impression the cooling system was always under the same pressure once up to operating temp. Thinking back now though, anytime I blew a rad hose off, it's always been at full tilt, never under light throttle. Idk. Head lifting under boost? Seems unlikely though. I used OEM head bolts, and the motor was assembled professionally by Zillarob.

Fuck this car, any input is appreciated.

[killerkali]

A HG not leaking at idle and only at full boost is pretty typical. I've blown my heater core hoses off 2-3 times at WOT or when nolift shifting. Only thing I could imagine is the head lifting like you said. OEM head bolts also

[CELison]

I would think I would get misfires though if that was the case. The cap is definitely not locking in place though. There's not "click" on that last 1/8 of a turn like there should be. Maybe it failed, and because of that the pressure relief in the cap failed too? What causes the system to pressurize at WOT? Sounds like a newb questions but I'm stumped here.

Here's the cap. Obviously going to start here.

[james 408]

can get the tester kit to see if combustion gases are in your coolant, which would point to head gasket, block, or head lift... or something along those lines. Might be as simple as retorquing bolts...

but a leakdown test on the cooling system might be worth... I wonder if it's just some airbubbles in the coolant system? the air could build heat as the block heats at a different rate than the coolant that's circulating and expand...

[ReggieNoble]

First thing I would do is get a tester to test for hydrocarbons in the coolant. If that checks out ok. Then maybe you have a blockage in your cooling system. I dont know what the ratio is on the water pump, but at 5-6k rpms it should be moving some water

[james 408]

what about just a bad radiator cap valve sticking/not regulating pressure right?

[CELison]

Where even is it? I have an a6 rad btw. I’m assuming they regulate pressure the same way though

[james 408]

god i don't even think there is one on the audi... maybe that overflow one does the function... and the check valve would be in the little... where's a diagram

[CELison]

I think the relief in the cap regulates pressure for the whole system. So if that’s fucked.....

[james 408]

Yeah, go with your first guess.

[LJS]

FYI-Regarding the CAP
If you have a replacement tank ( OE or OEM) it will need the "newer" cap for it to ,as you said, CLICK---there's a dimensional difference in the tank requiring a bigger "ramp" on the cap.....
ALSO
Since it's a radial seal o-ring there's no need for the cap to click to have a proper seal--you only run the risk of the thing unscrewing....

[james 408]

makes sense, but that the ramp is gone is a pretty good sign that cap is as old as some b5 owners, so... I'd still grab a new cap.

apparently this is a thing (not worth effort):
https://www.youtube.com/watch?v=0EuVM5JkEC8

[ElementR]

The only way your're going to blow coolant out like that is combustion gases getting into the coolant system. A bad cap will vent, but it will do it regardless of load. If you only see this under heavy load, it's combustion gas... usually a head gasket, but can also be a small crack. I'd lean toward a head gasket as your symptoms are classic for head gasket leak due to head lifting under boost. You can do the combustion gas test, but you might not see a positive reading if this is under load. You can also pressurize the coolant circuit with about 10 psi and see if you hold or lose pressure. This will help pinpoint external leaks but if you lose pressure and see no obvious external leaks, you can stick an inspection camera into your spark plug holes and see if there's evidence of coolant (a clean paper towel can also work as it will suck up any coolant in there)... again no guarantee you'll see any since the head is not lifting in that case.

I had a car on the dyno once with a head lifting under boost. It would fill half of a 2 liter bottle through the radiator vent line during a pull. The car made great power but just blew the coolant out as the gases were being forced into the coolant system.

I wouldn't necessarily expect misfires if gases are escaping into the coolant system. If you had coolant in the combustion chamber however, I would expect a misfire.

[ElementR]

And if the cap was bad, you should see a lot of action in the bottle when you shut down after a normal easy drive around town even with normal coolant temps... that's because the coolant system pressure is essentially atmospheric and therefore the coolant is boiling. The cap raises the boiling point by maintaining pressure around 1.5 bar in the bottle.

[james 408]

makes sense. wouldn't it still be possible though if the cap weren't providing enough pressure for the fluid to be forced out and through the cap? Temperature related I get, but going WOT produces a ton more heat for a few seconds doesn't it?


but the no boiling factor reduces likelyhood a lot, i get that, just thinking that maybe it's only at the very high levels that the cap fails rather than a consistant fail. Edit: like say it's holding 1.3 bar instead of 1.5 cause the spring is worn out a bit...

[CELison]

Head gasket makes sense. Idk if it’s going to worth keeping if that’s the case

[CELison]

Maybe just replace the head bolts if that’s the case? Hope the gasket is still ok? I ordered a combustion gas test kit. Figured that’s a definitive test.

[james 408]

yeah I mean i've had powersteering boil and spit even though it's fine most of the time... just thinking the reduced pressure might lower boiling point enough to piss out selectively, but appears fine by the time you stop, and works fine most of the time cause it's still holding most of the pressure.

and I think he's saying that the gas test might not work, cause it might not have enough gasses present at idle when you test the air... only WOT. I thought i remembered there being 2 methods though, one that's a chemical that changes colors by testing the coolant itself, and one that's a gas test on the tank. Edit: no the fluid stuff tests the air too, but the other method to testing the open bottle at idle, is to have you go WOT with the cap on, then come back and test the gas that's left in the tank. Not sure that'll work though if it's just pissing all the air and coolant out though... hrm.

[CELison]

I got the color changing one

[travis_w]

The color changing one is nice. Kinda like a little science experiment.

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[james 408]

what about just sticking a camera on the tank... might be able to get some info by the amount it rises relative to temperature readings vs WOTness.

like pin it when it's barely up to temp, and if it doesn't spit, see if it's only under later attempts where you finally see it boil. Swapping caps with another car might be easier though just to rule that out.

[CELison]

I’m going to put a bottle on the end of the expansion tank overflow line and do some wot pulls at varying levels of coolant temp and see what happens. That way I can see how much I’m losing.

I’ve never heard of these cars lifting heads thoigh. It’s only 30psi max. Seems odd that it would be happening at lower (relative to some 2.7s) boost levels.

[vavJETTAw36]

I would think the pressure relief valve in the tank should be superseding anything else if your coolant system was tight. I too am skeptical about your cap. I feel like that’s your issue right there. Otherwise pressure would just escape out the bottom via the dump tube.


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[CELison]

Lets say a head is lifting. I'm assuming the gasket itself is still ok since I'm not getting oil mixing with coolant or burning coolant, no misfires or anything performance related. I would assume I need to replace the head gaskets as well though, unless I do 1 bolt at a time maybe and keep pressure on the gaskets the whole time?

Also, it did blow a cam plug out the other week prior to all of this. It was very cold that morning so I figured pcv related. Never happened again after that but maybe a possible relation?

Update: Decided to take it to pick of some corned beef and potatoes (happy St Pattys day boys). Let the coolant temp get to 1 tick before center, and gave it the berries in 2nd gear. Before this I had pointed the overflow line directly at the Y pipe so that if it pisses out I would be able to see the coolant on the y pipe and shit around it. Pulled over immediately, popped the hood and it was dry. Let it get a little warmer (been at center for about a minute) and repeated. Same result, no coolant blowing out. It's about 40 degrees out, the car was not sitting in traffic at all, moving at about 50mph, so plenty of air flow. When I got there I popped the cap off, fluid level rose, but didn't come pouring out like when the coolant is on the warmer side. Not the greatest test, but it seems like coolant temp may have an effect which points me back to a bunk cap/tank not relieving pressure properly. What are the odds the cap stops clicking in place the same time this is going on....

[ElementR]

That doesn't line up well with your original post. Perhaps you do have a bad cap. I would replace it and see if that resolves the issue.... if you're head was lifting, I would expect it to have happened on those pulls too.

[MacDaddy]

Part out

[james 408]

.... What are the odds the cap stops clicking in place the same time this is going on....

Probably pretty good if it's 17 years old, heated up with boiling coolant, and you heman'd it off of there in a 'wtf is this piece doing' moment.

[Trb1]

Heads lifting seems unlikely. Did you have your heads decked when you put your car back together? A small warp in one of your heads sounds more like the issue. Takes a bit for it to become apparent as the gasket has to wear down. Had that issue in a Wrx I had. As soon as I had the heads decked my head gaskets stopped exploding every 5k Miles..

[CELison]

They were the heads that were always on the car. Never overheated in 9 years. Idk why they would be warped all of a sudden.

[Zba]

Too much speculation in here. Is there anyone local who can test the man's cap to get a definitive answer?

[CELison]

Ill wait for the new one to come. Need it either way. I could borrow one but I’m just too lazy about the car and busy with work to deal with it the next couple days.

[Trb1]

They were the heads that were always on the car. Never overheated in 9 years. Idk why they would be warped all of a sudden.

Yeah. But spec for warped heads is in the thousands of an inch. 9 years of heat cycles on aluminum. Doesn’t have to be because you over heated it. You broke the original gasket surface when you built your engine, right? 99/100 times it’s not headgasket issues with these cars. But blowing coolant out of your overflow in boost is a classic sign of combustion gas getting by gaskets.

Another way to check: Turn your heat on, warm car up, get into boost, and check the heat. I guarantee it gets cold in boost.

[RHawk]

My 2 cents here: if the cap is questionable, replace it anyway. If your symptoms continue, then take a deeper look at the head gasket. You may have a warped head from improper assembly....over or under torque head bolts, improper torque sequence, a bunch of things. Maybe a bum gasket from the factory, maybe a small piece of debris got under the gasket, who knows? To me it seems like a bad gasket. But start with the easy stuff first, like using the sniffer in the coolant reservoir. Maybe it's a simple fix? Good luck keep us posted!

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[CELison]

I would think a bad gasket would cause issues all the time. Not just at high boost. That sounds like a lifting head, which is pretty uncommon on these cars. For what it worth, the motor was built professionally by Rob (Zillarob) at 82 Autowerks. He knows his shit, so improper assembly I would pretty much rule out.

[ElementR]

You did originally post classic symptoms of gases getting into the coolant whether from a lifting head or warped head or some other cause of an improperly sealing head gasket.... but your later post threw a wrench in that conclusion because you did not see coolant spewing out after a couple hard pulls. When head gaskets leak under boost, it's consistent. I would find it very odd that it suddenly fixed itself which is why I would suggest you replace the cap first (it's cheap and no time involved to do it) and then see if the issue remains or not before jumping to conclusions.

[CELison]

First step is new cap and tank (ordered already, OEM for both, no aftermarket shit) and a combustion gas test (also ordered already). If that doesn't show anything then the heads are coming off. Not looking for a definitive answer, just curious if anyone else has dealt with this. I know HG issues are rare on these cars. I know of 1 that had a head lifting with stock bolts, but it also made 800whp...

Also, I popped the hood a week or so ago and noticed the cap had actually unthreaded itself a 1/8 turn or so. I really thought I mentioned that originally. My fault, I've been basically been in a drunken stuper all weekend for St Pattys. That piece of information seems very relevant to my issues, again pointing to the cap not regulating pressure properly.

[Zba]

If the test doesn't show anything the heads ARE coming off, or AREN'T coming off...

If it were me, and there were no signs of combustion I would leave that shit alone.

[CELison]

If there are combustion gases in the coolant the heads are coming off. Never did it in the car, but it can't be bad. Timing belt off, Intake mani off, unbolt turbos from exhaust manifolds and remove coolant crossover pipe off. Then just unbolt the pieces of shit and see if I can figure out what went to hell.

[Timtheguru]

Part out

Lmao

[Trb1]

If there are combustion gases in the coolant the heads are coming off. Never did it in the car, but it can't be bad. Timing belt off, Intake mani off, unbolt turbos from exhaust manifolds and remove coolant crossover pipe off. Then just unbolt the pieces of shit and see if I can figure out what went to hell.

Don’t forget about coolant feed lines which are behind motor mount brackets. Enough to make you want to start the thing on fire.