Crank no start, coolant temp/level codes and more(please help me)

[Crevecouer]

Hello!

Recently i've been having some issues with my 2002 A4 Quattro 1.8T Sedan AMB Block that started happening during an abnormal cold snap. A few things to mention before i dive in. I do not have VCDS. I am ordering one as soon as I am finished writing this. A cheap amazon VAG reader has done me well the past few years and 2 cars. It reads codes in most modules and that's all i've really needed. I have a Bosch OTC at work that's given me access to certain values i've been after. Kicking myself for not ordering one sooner. Prior to this, I have had no issues during the winter months and -35C is pretty normal here in the winter. Things started getting weird around -35C on the morning of the 15th of December.

I went to start my car as normal and it was cranking great but would not catch and fire up. A few times it would catch for a split second then back to cranking. I tried for a bit before catching a ride with someone to work. Battery is brand new and was installed at the end of November. Starter is kicking in and everything sounds absolutely perfect but it just wouldn't catch. When i was home i checked the basics that i could in the cold and with the garage not usable at the time. Put a booster/tender on the battery to be safe and checked for codes. Nothing really worth mentioning at this point besides a terminal 30 voltage too low which i later ruled out as an issue. Fuel pump runs when i flick the key on and there was pressure at the bleed valve on the backside of the rail. Ended up getting it towed to the shop at work and after letting it sit inside for a few hours to warm up it started. It idled rough and when you would give it any gas it really wanted to die. I had fueled up the day before with 91 and figured maybe bad gas? I put a bit of water eliminator in the fuel tank and eventually the roughness went away and it drove fine. Drove home that night and took the long way but the next morning the exact same issue. Got it towed again Saturday, drained all the fuel out (it 100% smelled bad) and noticed when it started up there was a good bit of water coming from the exhaust when i was revving it up. I assumed something had frozen in the lowest point of the exhaust. I have a CTS Turbo high flow cat installed but nothing besides that was different from last year besides removing the SAI pump relay. I have been driving less for fun this year versus last and idling more as these engines really don't make a ton of heat till you start driving (heater core was replaced 1 month ago) I drilled a tiny hole in the lowest point on the exhaust and drove it home for the weekend to see what happened. Put half a tank of 94 from Chevron in it and drove it spiritedly home. I should mention that as soon as the roughness went way the car drove great. No issues, no hesitation at all. Turbo spools up great still for almost 300,000km but now it gets a bit weird.

Sunday morning, 6AM again around -35C i go to start it and same thing. Cranks great but won't catch. Left it for a few hours and after checking out a couple more possibilities i decide to start it around 2pm and it fires right up first crank at -25C. No rough start, when you give it gas it doesn't stumble or hesitate one bit. I let it run for about 20 minutes before taking it out for a good hour and a half drive. I figured at this point there may have been some of the old bad fuel that had gotten frozen up in the lines and decided to drive it pretty hard to try and burn through it as well as clear up any remaining water that may have been frozen and not made it through the small hole in the exhaust. Parked it for the day and next morning cranks... But doesn't catch. I had it towed to work yet again and really got into things. Solid fuel pressure at the line when cranking it. 55-60PSI cranking measured from the pressure relief valve at the back of the rail. Oil is good, coolant is good (and brand new) battery tender on it and battery reading good. Alternator checked good when it was running. No check engine light besides for my high flow cat and the SAI relay being out. I FINALLY got a code for an open at the G62 coolant temp sensor. Did a bit of digging and calling around to a German shop and after reading many reports of these causing a crank no start at times i decided to get it replaced with an aftermarket sensor. Right after the new sensor was installed and everything put back together i fire it up and the low coolant light is on the dash. Checked connections at the expansion tank and looks good. Level was good, between min and max. Decided to get an OEM sensor and install it hoping it was just a bad sensor and the light is still on. Checked to make sure the sensor is plugged in and seated correctly. New o-ring. Car has been inside for a few days now and fires up every time so i currently don't know if the cold-no-start issue is resolved but i'm really not sure where to go with this coolant level error on the dash. The code for the coolant temp sensor is now gone and the car behaves as i would expect it to when starting it, but it keeps yelling "LOW COOLANT" at me. Temp gauge is working and reads accurate to the reading from the scanner, although i can only see a reading from one coolant temp sensor on the tools i currently have. While idling up to normal operating temp the top rad hose is nice and hot but the lower is noticeably colder. Am i looking at a possible thermostat issue here? I would expect to see a code if there was anything abnormal as far as a large variation in ECT sensors go.

I'm at a bit of a loss here. The expansion tank is less than a year old too and this didn't start happening until the 4 pin ECT sensor was installed. I assumed the aftermarket sensor was the culprit but same thing with an OEM one. Maybe i had a bad coolant sensor for a while and now that the 4 pin is replaced the 2 pin is acting out? I plan on replacing it either way to see what happens. Any suggestions or advice would be appreciated as i would really like to get this figured out before it warms up again which will be during the weekend/Christmas extra long weekend. VCDS won't arrive for a week or two. I will include a list of maintenance that could even possibly come close to being related as well as modifications i've done.

To be perfectly clear, i don't even know at this point if my cold crank-no-start issue is resolved and i won't know till Friday morning. The next step i'll take in the morning is replacing the 2 pin ECT sensor that's on the lower rad hose and try using a wire to connect the pins on the coolant level sensor on the tank and see if the warning goes away, maybe that's as simple as my newer expansion tank being bad.

Mods/Maintenance: (All done in the past 2 years, less then 10,000KMs since all this work has been performed)
Timing belt/Water pump
Coolant expansion tank
Heater core
Coolant flange
Spark plugs/coils
Fuel filter
ALL fluids recently changed, oil changed every 6 months or 3,000KM. Only approved 0W-40
Rear 4 pin (G62?) ECT Sensor
Oil level sensor
Brand new battery

CTS Turbo high flow cat
Wolfsburg tuning diverter valve
Mufflers deleted
Lots more for both lists but nothing else i can think of that could have any relation

Sorry for the massive wall of text but i like to give as much info as absolutely possible. Any suggestions are appreciated and TIA!

[TobiqueSlang]

Hate to say it but your expansion tank might have kicked it despite being newer. I'd also say the 2 pin couldn't hurt to replace regardless and you might want to replace the thermostat and j plug.

another issue with the cooling system might be that it needs to be properly bled for air. replacing the 4 pin doesn't make much of a mess but it still introduces air into the system.

you also could be experiencing freezing in the system if your coolant mix is just 50/50 and the weather is dipping below the threshold. might be "frozen" but it could be gelling up.

you don't appear to have a fueling issue but you could also have an ignition or air problem. you said you deleted the SAI, I THINK that could be a cold start issue in some cases but I could be wrong. either way, I'd check for ALL the vacuum leaks and inspect your ignition coil harness if the coils are known good

[rollerton]

Do you have a CEL?

You intermittent no start sounds like possible G40 / cam position sensor. When it can't reference the cam position vs the crank position the ECU just guesses where TDC and times it from there. It can be very dependent on coolant temp also because the ECU adds timing advance on cold start so it may start way easier when cold with bad g40 from my experience. Luckily cam sensor is super cheap and widely available.
Without a good OBD that can read measuring blocks you're just guessing though.

[Crevecouer]

Hate to say it but your expansion tank might have kicked it despite being newer. I'd also say the 2 pin couldn't hurt to replace regardless and you might want to replace the thermostat and j plug.

another issue with the cooling system might be that it needs to be properly bled for air. replacing the 4 pin doesn't make much of a mess but it still introduces air into the system.

you also could be experiencing freezing in the system if your coolant mix is just 50/50 and the weather is dipping below the threshold. might be "frozen" but it could be gelling up.

you don't appear to have a fueling issue but you could also have an ignition or air problem. you said you deleted the SAI, I THINK that could be a cold start issue in some cases but I could be wrong. either way, I'd check for ALL the vacuum leaks and inspect your ignition coil harness if the coils are known good

Luckily when i put the 2 pin temp sensor in at the lower rad hose the coolant level warning went away! But that sensor is leaking coolant. New one should arrive tomorrow. Had it bled with the amount of coolant that comes out when you swap out the two pin but will of course do that again tomorrow. After i replaced the sensor this morning the car sat outside for 6 or so hours in -40C and fired right up so i'm hopeful. Will know for sure tomorrow morning and will update with my findings. This was also the first really cold start with the SAI relay put back in but i am for sure confident that the coolant temp sensor was faulty even if it wasn't the sole issue as i was getting some high resistance readings from the old one. Will triple check coolant in the morning too just to be safe but i have done 4 or 5 full flushes/changes with the flange at the back, heater core flush then eventually replacement as well as the quick connect hose that blew on the rad using premix from VW dealer every time.

Thanks for your help!!

[Crevecouer]

Do you have a CEL?

You intermittent no start sounds like possible G40 / cam position sensor. When it can't reference the cam position vs the crank position the ECU just guesses where TDC and times it from there. It can be very dependent on coolant temp also because the ECU adds timing advance on cold start so it may start way easier when cold with bad g40 from my experience. Luckily cam sensor is super cheap and widely available.
Without a good OBD that can read measuring blocks you're just guessing though.

I have the regular CEL from my high flow cat and from the SAI relay being taken out (which i put back in this morning) but nothing else. The cam sensor was in my mind for sure though. The code for the failing G62 was stored in the gauge control module even before i started having the "low coolant" error but the code for the G62 sensor went away and has not came back since i replaced the sensor. When i cleared it before it came back right away. Now that i think about it i replaced the cam sensor about 2 years ago. The issue seems resolved after replacing both coolant temp sensors as it sat outside for about 6 hours in -40C today but will know for certain in the morning.

Still kicking myself for not getting VCDS as a proper full scan probably would have revealed the issue long ago but soon!

Thanks for your help