TomF

I was able to change out my Failing AOS on my 2000 C4 Tip Cab and I just drove the car 50 miles or so, with no overheating or other issues. Also, no more smoke on acceleration. I then went to check my fluids and discovered a serious amount of oil in the coolant reservoir. It looks like a milkshake... It seems suspect that it happened at the same time as the AOS replacement.

Is there any chance that in my tired state that I reversed the two small hoses on the AOS? Would that even make a difference? What if I received a defective AOS from Porsche?

There was no sign of any oil in the reservoir previously. I have been trying to find a diagram of the internals of an AOS and the hose connections, but haven't had any luck.

Any thoughts would be appreciated. I would thank you in advance for assuming that it is something other than intermix and not jumping to conclusions.

Thanks!

Best,
Tom

Macster

Oil in the coolant suggests a high pressure source of oil. My guess is the AOS does not have any high pressure oil routed to it.

I do not know of the two hoses you speak of but if there is any chance they're reversed and they are not interchangeable/reversible that possibility should be eliminated pronto.

'course after the source of the intermix is found and fixed, the coolant will have to be drained and the system cleaned -- flushed -- to remove traces of oil so this does not contaminate fresh new anti-freeze.

To ensure a good job you want to be sure you fill the cooling system completely with whatever fluid is used to flush the system then run the engine long enough to make sure the t-stat is fully open. Be sure the heater is on to make sure coolant flows through that to flush it as well.

Before the engine is run again, the oil should be checked for any presence of coolant/water.

You can drain the oil filter housing oil into a clean drain pan and look in this oil for signs of coolant/water or have the oil analyzed which will report if any anti-freeze compounds are present in the oil.

Oh, I have to ask: Are you sure you didn't by accident add oil to the coolant tank?

Sincerely,

Macster.

TomF

Thanks for the suggestions. No, I didn't put oil in my coolant tank. After 25 years of 911 wrenching, that is something that even tired, I would somehow manage to avoid.

I just checked to see if the hoses are routed correctly- they are. I still cannot believe that this is a coincidence that I change the AOS and then I have an intermix problem. I know that the coolant side of the AOS had cracked and that the bellows and the main body of the unit were leaking oil. I wonder if the old AOS was sucking up oil somehow? It is interesting that the car hasn't been running abnormally hot at all. As a matter of fact, it has been running cooler in the 50 miles since the AOS replacement.

I pulled a couple of coolant hoses and thank goodness there is no milkshake in the ones I pulled. I guess I will have to really dig in this weekend.
I will check my oil and drain some coolant from the bottom for a cursory look.

Does anyone know of a detailed diagram of the cooling and oiling systems? I have looked at the workshop manuals and PET, but they aren't much use.

I am still holding out hope that it is the oil cooler and not a cracked head!

TomF

BTW, does anyone have a cutaway of how the AOS works?

b8_rdc

Hose with 90* comes from the pulley area of the engine where the coolant line attaches to tiny plastic fitting that is bolted near the TB. The other hose fitting on AOS I believe goes to the funky T on coolant hose that comes out of water pump.

You AOS could have failed in coolant passage and drip down into the oil but I dont think it can work in reverse. Sounds like a leaky oil cooler?

philooo

is there any oil cooler in our cars ?

Macster

Oh yes. There's a water (coolant) and oil heat exchanger that works to speed up the heating of the oil when the engine's cold and to help remove heat from the oil when the engine's hot.

There are o-rings/seals that keep the coolant connections from leaking and ditto for the oil connections.

If the cooler developed a leak at one of these connections it might be possible -- I've never seen one up close so I'm guessing -- oil could be forced into the coolant side.

Or of course another way is if the thing developed an internal leak through a hole in tube/passage wall that allowed oil to be forced into the coolant side of the thing.

I seem to recall this oil cooler accounting for at least one and maybe more than one ugly intermix problem.

Sincerely,

Macster.

philooo

where is this part located ? is it external to the engine ? I don't remember seeing this as I was following other people engine DIY

TomF

Phillooo, the oil cooler (or a better term is heat exchanger) is located on the right top of the engine underneath the plenum and intake manifold. It cant' really be seen without removing the throttle body and plenum.

Macster, you are correct- there was a case here on Rennlist of a documented failure of the oil cooler/exchanger. The intermix was very similar to what I am seeing.

I removed the two AOS coolant hoses from the front of the engine and blew compressed air though them. There was no evidence of oil in the lines.

Is there a diagram somewhere of the coolant system flow? That might help me a bit.

Since it is such a beautiful day here in Seattle, I am trying to look on the bright side. If it is a cracked head, 120K on the motor isn't too bad, and I get to attend Jake's engine rebuild class! I attended Jerry Wood's class on 911 engine rebuilding in 1996...what a blast!

Thanks for the comments.