Ok before i get the typical response to NOOB postings I want to say I did have been researching this for a couple of hours.


I have a 1999 A4 2.8 with 145xxxx.Timing Belt and Water Pump Replaced at 90k. Along with scheduled service at 120k.

Problem: Over heats with AC on in stop n go traffic.

MY DIAGNOSIS: HEAD GASKET, PROBLEM #4 BELOW. HOW MUCH WILL THIS COST?

Items I have Trouble shooted:
1. Thermostat: Cold start check hoses for temp increase and they do get hot, and does not take long for it warm up 12 min park at a Idle.

2.Oil/Anitfreeze color: Both coolant and oil appear clean, no milk in oil and no oil in coolant tank.

3. Electric radiator fan: Turns on only when AC is turned on. Let it run with out AC on got over normal temp and never turned on. Is this a two speed fan or switch system.

4. Remove Tank cap and let warm up: ran fine for about 10 min then started bubling out the top of the coolant tank.( I believe this is the problem, head gasket).

5.Exhaust: Looks ok no white smoke

 

I feel like if it was a head gasket you'd be seeing mixing oil and coolant. Have you been losing coolant at all? What mix of G12 and water are you using? Just to clarify, you said it also overheats when the AC is NOT on, is that correct?

 

When the AC is on it gets up the second line from the 12 o clock or middle of the gauge in stop n go driving. The coolant was only down about a quart still visible in the tank. I checked the spark plugs and the look normal, light brown in color.I am going to try to check the pressure and for bubbles after is is completely cool in the morning and double check the exhaust for white smoke.

The fan has me curious too, if it is a two speed fan maybe one of the relays is out because it only came on when i bumped the ac on. I dont believe it is a head gasket after inspecting it today. maybe its a relay, thermo switch, thermo stat or water pump IDK....

 

I wouldn't jump directly to a head gasket. You've checked the main culprits with the exception of the engine cooling fan. What I'm wondering is whether your fan clutch is failing. It's hard to tell because it always spins. I say this because with some fan clutch slippage, you may still have enough engagement and drive to cool the engine adequately without the AC, but with the AC on, it may be that there's enough clutch slippage (or serpentine belt slippage - have you looked at the serp tensioner for wobble while running?), that it can't compensate for the additional strain on the engine caused by the AC compressor. Coolant bubbling up in the overflow simply confirms what the gauge says - that you have excess engine heat.

As for oil/coolant mixing, it happens when a HG goes bad and gets a tear between a coolant jacket and an oil galley, but there'd be no fluids mixing if the HG opened up between a coolant channel and a cylinder, and that'd cause an overheat for sure (happened on my wife's Civic a few years ago). Do a compression check first to see if any cylinders are low, which would indicate an opening to a cylinder, and follow that with a chemical test of the coolant - if you're getting combustion bypass heating the coolant through a HG rupture, it'll detect combustion gases in the G12 and confirm a HG leak.

 

I had a big fight with a Subarau once. Overheated, tried everything. Till I finally just broke down and put a new radiator in. Cut open the old one and it was partially blocked with stop-leak crap. So I took off the oil cooler and blew through it. Almost completely blocked....and that means one bank was getting almost no cooling.

Any chance you've got some stop-leak blockage?

 

easy way to check if theres oil in the coolant - warm up the car splash some coolant from the tank of the manifold. if it boils and bubbles theres oil. It doesn't sound like a HG tho. I know oil coolers cause head gasket related problems on the Audi. But as Mtroxel said, check the rad touch it and see if the temp is different in places it should be constant across the whole thing.

 

IMTHEDEVIL, well i took a look at this morning double checked the radiator pressure and exhaust and no bubbling or smoke. I tried to peek at the belt and tensioner and the tennsioner seems smooth at a idle. How would be a sure way to check the fan clutch, I am assuming when cold its moves freely and when hot it gets hard to spin, or is designed to use centrifical force as well.


MTROXEL where are you located im in ST Cloud MN

 

Close, Eden Prairie. Are you near me?

 

I believe the problem here is with the fan clutch. I looked at it this morning and it freely moves with the engine off at cold. I started it and ran it till it was hot and shut it off and it still was free spinning hot. I started it again and took a small stick and was able to stop the fan with little resistance.

I am located in St Cloud MN.


I am going to try to replace the fan clutch on it, is this hard to do on this vehicle I have most hand tools and sockets no special tools to move the raditor support in the service position.

 

not hard to do but it is reverse thread.