My 4.2 QS Engine Repair + MOT!!

S4Phil · #1 25th March 2012, 09:19 PM

Thought I had better start a thread on the work I'm doing on the A8 currently. I have a tradition of starting project threads even if the car doesn't get finished

Long story short, the engine is mixing oil into the coolant system (potential heat exchanger common problem) As well as it having been off the road for around a year so obviously no MOT currently.

Here's the A8 in question

A 4.2 QS, black leather and S8 wheels.

Here's the events of today after a few hours work.

Can you tell which one is coolant and which one is gear oil

After a compression test the engine has come up healthy enough. I now need to do a coolant pressure test and get to the bottom of the mixing coolant.

I've also got a bad leak from the p/s cam cover breather, the two exhaust sleeve clamps are knackered and blowing adn the inside is filthy!!

More to come through the week when I steal the coolant pressure tester from work.

Nollywood · #2 25th March 2012, 09:40 PM

Looks like you've got all angles covered. The plastic coolant pipe is a well-documented weak point. I would get an aluminium replacement whilst you're in there, even if it looks okay.

Probably a good time to do a timing belt kit, water pump and thermostat replacement, having come this far.

S4Phil · #3 25th March 2012, 09:42 PM

Which coolant pipe is that sorry?

Belt and gubbins are pretty new as I know the previous garage who worked on the car. I will probably do the thermostat though and new rocker gaskets etc.

Nollywood · #4 27th March 2012, 07:31 PM

Quote:

Originally Posted by S4Phil

Which coolant pipe is that sorry?

Belt and gubbins are pretty new as I know the previous garage who worked on the car. I will probably do the thermostat though and new rocker gaskets etc.

There's a plastic pipe that carries coolant from the cylinder block to the oil cooler (just behind the alternator, incorporating the oil filter). The pipe is sandwiched between the engine and the oil cooler itself, only visible once the cooler / oil filter assembly is removed.

This pipe cracks, as it becomes brittle with age, resulting in coolant leaks. It's a bitch to replace with the front of the car in place, with your front end stripped as it is now, access and replacement is cake.

Conan_the_Librarian · #5 27th March 2012, 08:29 PM

S4phil,

I have been there. With water in the oil thare are many options for gasket failure. Remenber that your oild system is running at approx 6 to 8 bar. Your coolant is only running at 1.7 to 1.9 bar. So for oil to get into your water, the oild must be close to your water system wher the oil is under pressure.
For water to get into your oil, the water must be close to an oil gallery that isnt under pressure. As the entire water system is under pressure but only the oil supply system is under pressure there are vcery few places where oil under pressure is adjacent to your coolant. Those places are the supply to the cam bearings in the heads and the oil cooler. If its thye heads then it's new head gaskets and maybe a skim of the heads. But only microns, no more, as the valves are interferance fit.

However, in the oil cooler, the casseste is bathed in water with the oil under pressure inside. if there is a leak in the cassette, then when its warm oil will mix with the water; this happened to me and it wasn't until I pressure tested the cassette at operating tempreture that it was discovered. When it was cold it didn't leak at all

S4Phil · #6 28th March 2012, 05:58 PM

Much appreciated guys

I did see that small pipe on Vagcat and then read up about it failing often so it will get done too, although if this plastic one has lasted 10+ years I'm quite happy for another plastic one on there!

I still haven't pressure tested it, got a bit side tracked playing with the S4 again