hey everyone havent been on here in a koo min. so here the deal with my car. for some strange reason my coolant level in the expansion tank slowly drops say every 1000-1500 miles of driving so im always topping it off. havent had any overheating at all BUT having to see that level slowly drop is annoying as hell. my oil is clean its not milky, no white smoke coming out from exhaust, dont see any coolant drips under the engine, and the water pump and thermostat are both new replaced them last April 2012 along with timing belt. id like an insight before we start taking things apart since the car doesnt overheat its just annoying and frustrating seeing that level drop slowly.

car info:

2001 Audi A6 Avant quattro with 2.8L V6

171,xxx miles

thanks!

 

is the system holding pressure, grab one of the coolant hoses when the engine is hot and see if it is hard. It might be as simple as a bad cap not holding pressure. Any coolant smell in the car? or do your windows fog up? Wet rugs? if you don't find something it might be worth taking it some where and have them pressure test it

 

I have seen on the V6 engine where coolant is leaking into the combustion chambers and there isn't any white smoke or any other concerns besides a rough cold start and sometimes misfires. I agree a cap could be the cause and its boiling off because the cap isnt holding pressure. I would deffinetely check the rear coolant pipes and ECT sensor bung for leaks.. You can remove the spark plugs and then pressurize the system for 20minutes when the vehicle is hot and check the combustion chambers to see if they are wet, then I would do the same cold...

 

It could be that the o-ring on the CTS is bad.

 

any coolant hose? and no there is no coolant smell inside the car my windows dont fog up and my rugs arent wet.

i highly doubt the coolant is in the combustion chambers cuz car starts rite up and runs smooth at cold starts and doesnt have misfires. the rear coolant pipe is the one under the engine cover? and sorry for the stupid question but what is the ECT sensor and what does it look like and where is it located?

CTS? what is that? again sorry for the stupid question...

 

Coolant temperature sensor. Same as the ECT. There is a coolant cross over pipe at the back of the engine. Removed the flexible plastic intake section from the airbox to the throttle body, and you should be able to see it behind the passenger side head.

For your viewing pleasure...

 

ok so my dad and i did some superficial inspection over the weekend and we were stumped to find that the expansion tank is missing the o-ring gasket on the cap. also i drove the car to operating temp and then touched the hoses and they didnt feel all too hard. could it be the o-ring on the cap thats missing causing the problem?

 

yes, if your system is not sealed you will have evaporative losses.

 

You said the timing belt was just done within the last year, was this a job you did yourself?

It is possible, but not likely, that you may have an air pocket in the coolant system allowing it to not flow properly and sit in the block for a long period of time and boil over, thus causing the mysterious loss of coolant you are speaking of. You need to bleed the system from the top coolant bleed valve (It's a 6mm [?] allen bold found in the general location of the oil dip stick) as well as bleed it from the heater core. Doing a quick search on here will land you the how-tos. They really aren't that difficult to do. I say this is not likely because the amount of time that has gone between the timing belt change and this problem. You would have seen this act up a long time ago if that was the problem.

I would also check the secondary coolant pump as they are common things to go on our cars that will cause a mysterious coolant leak.

Lastly, you could have a water pump that is freewheeling, which is a condition that occurs when the pulley on the pump spins, but the impellers on the back do not. When this happens, the coolant will not circulate and a light overheating issue will occur. I'm guessing you haven't seen very warm weather yet, and as the spring progresses this issue will becoming dangerous to the car's health and you will see a very rapid loss of coolant as a result. A lack of flow from the top radiator hose will indicate that the water pump isn't doing it's job. Or the thermostat could be stuck in the closed position.

To sum things up, I would first have a respectable shop pressure test the system and see if they notice anything coming out. They will probably charge you about $50 to do it. Depending on what the outcome is, you may then be able to do the repairs yourself depending on your experience.

Keep us posted.

 

ima go ahead and order the expansion tank cap with o-ring gasket first and see how it holds up. im crossing my fingers that that solves the issue if not on to step 2 and do the pressure test at the shop...