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I am trying to locate a coolant leak on my '99 996. It is leaking pretty significantly near the rear of the engine (by the timing chain cover, closest to the rear seats), somewhere up here:


I have ruled out the expansion tank; I disconnected a few pipes and was able to run my hand all along the back of it and it came back bone dry. Meanwhile I can see rivulets of coolant running down the rear of the engine.

I put the car up on jackstands, but visibility up there is extremely limited. I unfortunately do not have access to a lift. I was wondering if there was any way to diagnose/resolve the issue without having to drop the engine.

I appreciate any insight into this. Also, I noticed several coolant hoses that disappear into oblivion back there. Where do these hoses connect to on the other side, and is there a way to check these connections?




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You need to pressurise the system when it's cold. The leak will then show up. The workshop manual says it should hold 1.5bar (22psi) but I think that's a bit high. I pressurised mine to 10psi to find a leak. I did a temporary fix and it held at 10 so I have left it for now until I change the header tank relief valve.

I bought a pressure tester that will do VW's as the cap on my 996 is the same as some of the older VW models.


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Broken fitting on the AOS? Coolant runs through the AOS. It is way back up on the engine, drivers side. Is the coolant on the left side (drivers side)? Helps us if you tell us where the coolant is leaking...left side of engine, front, rear, right side of engine, etc.

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That location (on the driver's side) has the AOS sitting above it so there are coolant lines.

The skinnier hoses on the reservoir are usually vent lines (to vent air). Can't think of an easy way to check those hidden hoses other than removing the intake manifold (as if you were replacing the AOS) or using an inspection camera (boroscope).

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Sorry, the coolant leak is running down the rear of the engine (i.e., the side closest to the front of the car) close to the drivers side. I can see the coolant running down when I'm under the car, I just can't figure out how to access the source.

So remove the intake manifold, you say? Other than the AOS, how many connectors are way back there? Can I access them all if I remove the intake manifold, or are there some hose connectors that are completely out of reach? I should probably take a look at the AOS, at least. Oofah, this looks like fun: http://www.renntech.org/forums/index.php/tutorials/article/238-air-oil-separator-replacement-aos/

That should fill up a weekend pretty well.

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There are two coolant hoses going to/from the AOS. One from tank (via a T connector) to AOS, then the second from the AOS to block just near alternator.

I've been able to "repair" those leaks before by reaching up from underneath and tightening clamps on those hoses. However, do be careful not to over tighten because the AOS is plastic. Or perhaps you don't have screw clamps and/or the hoses or AOS have just failed/cracked/split at which point replacement is the best advice.

You can remove the intake manifold to get to the AOS. Don't drop anything into the intake, and put tape over the ports once the intake is removed.

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Thanks. I have a lot of miles on the car and I don't know if the AOS has ever been replaced, so it's probably worth my while if I'm going through all this trouble just to replace it, and some hoses while I'm at it as well. Are there any other obvious failure points in the coolant plumbing system back there, or is the AOS most likely the culprit? Based on where it sounds like it is placed (rear of the engine, on the driver's side), that's where my leak is coming from.

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I would also replace the orings in the 2 hard plastic pipes that connect to the AOS.

The rubber coolant hoses are also cheap (3rd and 4th pics), I would replace those as well and also use high quality worm drive hose clamps.

If you remove the intake always replace the bolts and intake orings, and torque properly.

It sounds like an AOS to me too.

The only other possibility for your leak as I mentioned was the coolant hose going to the oil cooler (2nd pic in your post) which goes over that area as well, however it would have to have a split due to being chaffed or something like that since it's connection is on the passenger side (US) of the engine on top of the oil cooler.

I guess perhaps also a coolant tank leak making it's way toward the rear of the engine, but that is doubtful. Worth looking at, and consider replacing the tank too, if you have the intake apart it is very easy to replace the tank.

DO read up on how to properly bleed the coolant system.

Edited by logray
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Will do, many thanks. I already have a new coolant tank, because I originally thought that was the culprit. While there was still a lot of dampness on the back of the engine, the back and bottom of the coolant tank was completely dry, as was the area directly below it. Still, since I have the part and it's an older car, it won't hurt to replace it.

I really appreciate your detailed response.

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Have you checked the water pump? I know it's not located closest to the rear seats but leaks can run in different directions. If you haven't replaced that yet (you mentioned having a lot of miles on the car) then this could be starting to go.

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