MattS2Turbo

Hi, I'm new here, but I'm hoping to shed some light on a funny noise my car has been making.

Basically, it's a 91' S2 with the 16V engine. It's been slightly modded, mostly exhaust stuff, but I doubt thats the problem. 7k miles under it's belt since it's last modification so I'd say that rules it out.

There is a knocking noise in the bottom half of the engine relative to it's RPMs - sort of just a knock, knock, knock. Oil pressure is a little less than normal but nothing strange.

I'm thinking... maybe a Rod Bearing or Piston Wrist Pin? Any idea how much $$$ I'm looking at for a repair?

Also, I have a friend who put a lot of work into his 944's engine but then totalled the rear end. Could I just do an engine swap? It's an 83.

Any help would be appreciated.

mark944turbo

I just had a noise in my S2, a slapping knocking noise, worst at idle, cant hear it as much as engine speeds up.

It turned out to be piston slap, 1 piston was totally worn out and the bore scored.

MattS2Turbo

Ok, I got a stethoscope (Medical supply surplus stores - $.99) and payed really close attention to the noise.

It's the worst at cold idle. As it warms up the sound becomes less and less loud, and when you're driving, you can't hear it at all. Oil pressure stays normal for the most part, and it seems to be loudest in cylinder #2.

Any ideas?

mark944turbo

I think you probably have the same problem I did. To confirm, take the head or oil pan off and try to shake the piston back and forth in the bore, you will see it moving slightly if it is piston slap. Taking the head off would be the easiest to see, but if you take the oil pan off you are at the rod bearings anyway..

The cause of this condition is overheating causing the piston to expand too much, which wears through the iron coating, and wrecks the piston quickly.

mark944turbo

http://users.wpi.edu/~mark944/pistonwear/untitled.JPG

There is the worst piston. Noise was the same way, worst on cold start, fading after that.

MattS2Turbo

SO, the question is, if it is a piston slap, how much $$$ should it cost to fix and is it something I need to address absolutely immediatly?

Luis de Prat

Address the problem when you can, but park the car for the time being. S2 engines are not cheap to replace.

mark944turbo

No need to address it immediately, the previous owner of my car drove it for miles like this. You already need a new block and pistons (if it turns out to be piston slap), so the damage isnt going to get any worse than it already is. Power on my car was not even down that much, and it still somehow had good compression. It was just the embarassing and annoying noise all the time.

Cost to fix is however much you buy a good 3.0 block and used pistons for, plus the cost of new rings, having pistons coated by swain, plus cost of having the silicon in the new block re exposed so that the new rings will seat well. I would estimate around 3000 total, unfortunately, to do it right, and that is doing your own labor on the rebuild. If you pay someone for the labor, add another 20-30 hours * hourly rate.

You can take shortcuts and just use the pistons and block that you get with the used rings, but not really worth it in my opinion since you are doing the complete rebuild anyway.

Another option would be to sleeve your current block, either all cylinders, or just the bad one, and use a custom piston. I am not sure what the pricing on that would be, depends on what shop you use. I would replace all pistons if one has started slapping, when I pulled the other 3 they were not as bad as the worst one, but well on their way.

Luis de Prat

I wonder how these things happen. My S2 has 150K miles on it and no problems (knock on wood). Could this be related to infrequent oil changes early in the car's life?

Fishey

Rod Knock on the #2 piston... Sounds like a good bet thats the problem...

mark944turbo

My theory is that it is overheating causing the piston to expand faster than the cylinder.

I hope it is a rod bearing, but unless you were low on oil or on track tires and doing some serious braking and cornering, I think the slap is more likely. Especially if oil pressure isnt down, it always is when a bearing is thrown.

mark944turbo

You also have to wonder why porsche went with piston oil squirters in the 968, probably from experience with the S2. My car had a lot of track time on it as well.

MattS2Turbo

Well, according to the maintainance records, the radiator was replaced about 7k miles ago due to failure and overheating. Could this problem have existed but not made any noise until now? That would explain the overheating theory.

So, you say the problem won't get any worse? Since I'm looking at a complete rebuild anyway, can I just run this block into the ground while I'm saving the 3 to 4k needed for a used short block and pistons, etc? I don't have access to a second car and I have a 30 mile commute to work, round trip.

Also, any of you guys in Northern California? I'd bribe you with beer to help me with the rebuild

MattS2Turbo

Also, Mark, how many miles did the previous owner drive your car with this problem?

mark944turbo

Wow, now it looks even more similar to my problem. It is likely that the noise would develop slowly. What happens is it overheats and wears through the iron coating on the pistons, then the aluminum or whatever it is underneath wears quicker. Evenutally it wears enough to cause the slapping noise.

You can run it to the ground no problem, but I would change the oil filter every 2000 miles or so, just because it is likely to have some aluminum in it each time, and it is better to do this than risk it clogging and costing you and expensive crankshaft.