rmccomiskie

I just bought a 98 A6 Avant for cheap. PO says it overheated a couple of times and then started running poorly. Local mechanic (Audi savvy) says 2 cylinders have low to no compression. While R&R of the heads might fix the compression, the mechanic recommended to replace the engine because of unknown condition of the bottom end.

I'd rather diagnose further to see if it's gaskets, valves, rings, whatever. How should I go about it?

DaveInSaltLake

compression tests:
dry
wet
leak down

search Austionado16's posts for numerous bits of info the the hydrocarbon tests.

If you're lucky it will just be a head gasket. Worst case scenario you'll have some scarred piston walls to deal with.

I can't see the bottom end being impacted unless there's water in he oil and the motor was run that way.

The_Hamster

Leakdown test, find where the pressure is escaping.,

austinado16

Sometimes aluminum heads (in the VW 4 cylinders) can warp so bad that the cam will rock in the cam bearings. When the head warps like that, you can't just machine the surface of the head because the cam will still be on a teeter-totter. You have to either line bore the cam bearing towers, or you put the head in a jig and bake it at about 450*F to straighten it back out.

If this were my new purchase, I'd do a compression test just for kicks, probably a leakdown test, and then throw a set of headgaskets at it, and do whatever work was needed to the heads.

But if I saw scoring in the cylinder walls, I'd pull the plug on the whole thing and drop a used engine in it.

4Driver4

My guess is that the low compression is a bad head gasket.

IME, overheated engines come in two varieties: those that overheated quickly, and those that overheated slowly. On a quick overheat (blown rad hose on the highway), the loss of compression that stopped the car is usually fixed with a couple of head gaskets and maybe a plane of the heads. On a slow overheat (slow coolant leak), the main bearings may be damaged. In this case, the car is fixed the same way that the quick overheat car was, but when all seems fine and the car is taken out on the highway, oil pressure just drops as the car comes up to speed.

Bottom line: if in doubt, replace the engine to avoid throwing good money after bad.

rmccomiskie

Here's my plan:

Find out more from the seller about the symptoms of the overheating.

Do some testing:
-dry compression
-wet compression
-leak down
-hydrocarbon tests
I didn't see any frothing in the oil so I think the bottom end is okay.

Best case: blown head gaskets, maybe burned valve(s)
Solution: R&R the heads (gaskets, valves, seals, etc.)

Middle case: warped heads
Solution: Replace the heads.

Worst case: scarred piston walls, loose rings
Solution: Replace with a used engine.

kday

If you end up needing new heads, let me know...

DaveInSaltLake

but I don't doubt it for a second

kinderutz

head gasket or cracked head/block between those 2 cyl

austinado16

Very common in the Rabbit diesel engine. I've got 2 here. Our 12v heads are the same almost.