04 1.8t AWM headgasket or just air?

[crazyquik22023]

Can't figure this one out. I have a 04 1.8t AMB, 160k. Have been having some problems with the coolant system. Sometimes when driving the temp gauge will go slightly past 1/2 way to 3/4 past, but will always go back down to 1/2 and hold there the rest of the trip. Sometimes I can drive and the temp gauge never goes past 1/2 and when I log temps they are right between 90-102. Today when I was driving my temp gauge went a little past but then went back to 1/2, and when I got home I left engine running and opened hood and coolant level was just above "min". I shut engine off and opened the expansion tank cap. There was pressure in the system and level rose slightly after opening cap. I left the cap off and then started the engine, the coolant started bubbling and overflowing out of the expansion tank, but eventually stopped and level was right where it should be. So there is definitely air in the system somewhere. Last week I replaced the timing belt and water pump and had a hard time bleeding the system but did get it bled properly. (I think). In the last 15k miles I replaced the following. Water pump (Hepu metal impeller), thermostat (HT brand), j-plug (OEM), rear coolant flange (OEM), coolant temp sensor(OEM), heater core(OEM). My question is do you guys have any other suggestions as to what could be sucking in air besides a bad head gasket? There are 0 signs of oil mixing with coolant. Also I pulled the plugs the top of the pistons are dry.

[old guy]

Perform a block gas test. It will confirm if you are getting exhaust gases in the cooling system

[crazyquik22023]

I've actually never done that before. What is needed to do that?

[old guy]

I used this method to confirm my cracked AMB head.

[crazyquik22023]

I used this method to confirm my cracked AMB head.

Thanks old guy. I'll give it a try

[old guy]

Check with your local Autozone for their free tool rental program. Leave a deposit and get it back when you return the tester. Make sure you get fresh fluid with the tester even if you have to pay for the fluid. It has a shelf life.

[crazyquik22023]

EDIT: For some reason in my first post and title I stated I had an AWM 1.8t. It is in fact an AMB..I must have been thinking about my other audi when I was typing..not that it makes much a difference anyways.. I order a block tester and got it today. Did the test and fluid never changed from blue. Which is good since I dont have a cracked head or gasket, but now I need to figure out where I am letting air get into the system.

[ozMatt]

All the parts you have replaced were OEM, except for the water pump and the thermostat.

Reading on here over the years suggests the thermostat is typically a part you would always go OEM with. (Behr i think is the OEM manufacturer).

Having said that, you said the temp logs were within spec. Have you been able to log temps when the gauge has gone to 3/4?

[mobilesuit818]

Sounds like your fans aren’t working properly...went thru a similar thing.


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[Let it snow]

The coolant will boil over with the cap removed. It needs the cap for pressurization. The rear coolant sensor does two things. One is the
signal for fuel. The other for your dash gage. If your temperature does not rise when you log it but the gauge spikes it may be a faulty sensor.

[crazyquik22023]

All the parts you have replaced were OEM, except for the water pump and the thermostat.

Reading on here over the years suggests the thermostat is typically a part you would always go OEM with. (Behr i think is the OEM manufacturer).

Having said that, you said the temp logs were within spec. Have you been able to log temps when the gauge has gone to 3/4?

I agree. I always try to go OEM, I was being cheap when I bought the thermostats. To me it doesnt seem like a thermostat issue though since it is pretty consistent when it does go over 1/2 way mark. It will go over the 1/2 way mark slightly sometimes only a small amount past, sometimes almost hitting the 3/4 mark, but then always returns to half way mark and hold temps there. It never overheats more than once per trip. And there is definitely air in the system, since when I open the expansion tank cap and run the engine with the cap off, i see air bubbles escape and the coolant overflow out the top of the tank until the thermostat opens and sucks the coolant down. I logged it once when it was overheating and yes temps were high..115-120 I believe.

The coolant will boil over with the cap removed. It needs the cap for pressurization. The rear coolant sensor does two things. One is the
signal for fuel. The other for your dash gage. If your temperature does not rise when you log it but the gauge spikes it may be a faulty sensor.

I will log again tonight or tomorrow, but I am almost positive gauge is reading accurate.

Sounds like your fans aren’t working properly...went thru a similar thing.


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I will double check this but I do believe they are working properly. I know for a fact they both come on full speed with the a/c on and last night on the way home the temp gauge went over half slightly and I had a/c on from when I started the car, so fans were running non-stop

[mobilesuit818]

Fans without the AC on should be turning on randomly.


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[crazyquik22023]

Fans without the AC on should be turning on randomly.


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Correct. I will verify that the fans are working as they should.

[crazyquik22023]

Logged temps today. Coolant heated up normally kept rising steadily at about 1 deg every 2 or so seconds. Temps kept rising steadily until it got up to 120. Gauge was almost at 3/4 mark. After around 120 temps dropped very quickly (I'm assuming thermostat is opening and air is out of the system at this point) back down to 100 deg and gauge dropped to center line again. Temps remained between 95-102 rest of trip home. So temp gauge is now verified to be good. Also let car sit in driveway for a while running, when temps get up past around 100 fans come on and go off as needed. So seems like fan module is working as it should.

[old guy]

Sounds like a sticking thermostat.

[crazyquik22023]

Sounds like a sticking thermostat.

I was thinking that too.. but it happens so consistently that it seemed unlikely to me. But I guess that is my next thing to replace with an OEM one. Although I did just replace it less than 5 months ago.

[crazyquik22023]

Put in new OEM thermostat today. Car never went past 102 deg C in 80 deg weather with a/c on full. So in my opinion Mahle, and HT thermostats are poor quality and definitely not worth it since neither lasted more than 1 year or 15k miles for me. I would definitely recommend not trying to save the extra $75 (like I did) and just get an OEM thermostat. I know most of us are aware that the thermostat on b6 1.8t's is definitely an item to get OEM. Even knowing this I unfortunately tried going cheap with aftermarket brands and it ended up costing me much more in the long run. Thanks for the help all