Tip & How-To about Audi A4

How to check your Volkswagen & Audi oil cooler for leaks.

If you own a Volkswagen or Audi and have coolant in your engine oil or oil in your coolant you may have a failing oil cooler. This may have been misdiagnosed as a bad head gasket.

Before spending hundreds or thousands repairing a head gasket that may not be the problem you can perform this relatively easy check of your oil cooler which is a common problem on many Volks/Audi models that use oil filter type of oil cooler. This type of cooler is located between the oil filter and the engine block. The oil filter screws onto the oil cooler. It has two rubber coolant hoses attached to it and is made from aluminum.

The oil cooler works by passing engine oil through the body of the cooler. The engine oil passes over what resembles a radiator inside the cooler. This 'mini radiator' is passing engine coolant in through one of the hoses and out through the other. When working properly the oil and coolant is kept separated. Over time the passages inside the cooler can corrode and the oil and engine coolant inside can mix. This can lead to coolant in your engine oil (which may produce a milky substance on the inside of the oil filler cap) or oil in the coolant (which you may be able to see in the coolant reservoir as a dark ring or oily film).

To test your cooler for leaks you will need a few basic tools and about 3 hour of your time. (To allow for the engine to cool)

-Basic ratchet set and set of screwdrivers.

-One straight 5/8 inch plastic heater hose connector. You can find this at most auto parts or plumbing supply stores. Cost about $1.

-A pan to catch some coolant that will spill.

This procedure is best done on a lift but can be done using jack stands as well. This MUST be done with a cool engine to avoid burns from coolant.

-Lift or jack the front end of the car. (If you don't know how to safely jack a car onto jack stands, DO NOT proceed any further. Take the car to your repair shop and have them follow these instructions)

-Remove the plastic cover under the front of the car (if your model has this) to expose the oil filter. The oil filter is a cylindrical part that sticks out from the side of the engine.

-The oil filter is screwed into the oil cooler. You will see two rubber hoses connected to the oil cooler.

-Using a flat screwdriver or socket loosen the clamps connecting the hoses to the cooler. Now have the plastic connector handy. Remove the hoses from the cooler, you may need to pry on them with a screw driver. I find that twisting them to break their seal works well. Catch any coolant that spills in the pan.

-Take each rubber hose and push them onto the plastic connector being sure to keep the clamps on the hoses. Tighten the clamps. (Not too tight, just snug) Now you have just eliminated the cooler from the system temporarily. The rubber hoses are now connected to each other.

-Wipe the oil cooler clean with a rag so it is easier to see any leaks that might develop.

-Now start the engine and let it run to normal operating temperature. This means at least 15 minutes but to be sure you find any leaks I suggest half an hour.

-Keep a close eye on the 2 metal tubes on the oil cooler. (The ones you removed the rubber hoses from) You are looking for engine oil to begin seeping out. It will appear golden brown to black in color.

If you see oil coming from those tubes you have found your problem. You will need to buy and install a new oil cooler.

If you don't see any oil leaking you will need to look elsewhere for the trouble. Possibly a head gasket, unfortunately.

-Let the engine cool for a few hours before reattaching the oil cooler. It is the reverse of removal.

-Replace any lost coolant with the same amount you caught in the pan.

And there you have it. A pretty simple procedure that may save you a bunch of cash in repairs.




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1 Answer

oil mixing in resevoiur


the most common fault for this is the oil cooler, where the oil filter screws on ,these fail inside and allow oil and water to mix, usually in the coolant system, not a big job or cost to fix, unusual for head gaket to blow on these engines.I worked with Audi for 15 years and cannot remember any A4's that had a head gasket issue.

Jun 20, 2012 | 2000 Audi A4

2 Answers

hi, i have automatic passat 1999 model and recently water happens to get into the gear box and it control board, i experiences a boom sound anytime the gear is changed, the oil has been changed by the problem still exist. pls give me an advice ?


If no external leak and the transmission fluid colour looks white, the problem belongs to the automatic gearbox oil cooler. The oil cooler has broke mixing the gearbox oil and coolant causing this milkshake. It's a small radiator core type oil / water heat exchanger. It is cracked inside and transmission oil is mixed with engine coolant. Because the transmission oil pressure is greater than engine coolant pressure, the transmission fluid level drops. See also the coolant colour inside recovery coolant reservoir inder bonnet! It could be like this:

tdisline_495.jpg
The automatic gearbox oil cooler type oil / water heat exchanger (oil cooler):

tdisline_496.jpg
In the picture above the transmission oil pan was removed for oil filter replacement.

May 05, 2011 | 1999 Volkswagen Passat

1 Answer

the oli is getting into the water but not water into the oil?


Is coolant oil color now pink or brown?

If pink, transmission oil cooler failed.

If brown engine oil cooler failed, oil pressure is higher than coolant pressure.

Check the oil cooler for corrosion internally, problem is common.

Possible head gasket failed or worse, head cracked..

Jan 08, 2011 | 1997 Audi Cabriolet

1 Answer

transmission fluid level need regular top up and there is no external leak.Fluid color looks white.


If no external leak and the transmission fluid colour looks white, the problem belongs to the automatic gearbox oil cooler. The oil cooler has broke mixing the gearbox oil and coolant causing this milkshake. It's a small radiator core type oil / water heat exchanger. It is cracked inside and transmission oil is mixed with engine coolant. Because the transmission oil pressure is greater than engine coolant pressure, the transmission fluid level drops. See also the coolant colour inside recovery coolant reservoir inder bonnet! It could be like this:

9d86426.jpg

The automatic gearbox oil cooler type oil / water heat exchanger (oil cooler):

138b4e6.jpg
(In the picture above the transmission oil pan was removed for oil filter replacement).

Oct 29, 2010 | 2005 Audi A4

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