Sharky

This problem happened on my 90 S4.

I took the car out of a spin on Saturday after spending the last few days casing limp home mode problems. The drivers side bank was shutting down and the green light on the monitoring relay in the passenger foot well was on. I had the MAF rebuilt, change the rotors and caps, and the spark plugs. Started up the car and it ran fine.

I decided the take for a spin to check it out. I was driving very aggressive and all of a sudden the car went dead. No crank and it seemed like the battery was dead. Call the wife to bring the battery pack. Car cranked slow but started. It ran for a few seconds and died. Called AAA, flatbed home.

Got the car to start but it wouldn't idle. Cranking was very slow as if the starter didn't have enough power to turn the engine. Checked rennlist post and found a post that talked about the flex plate TBF where there was too much pressure on the back of the motor. Got up this morning and checked my flex plate and it was forward about a half of an inch. I couldn't believe it because I checked it before SITM. I also swapped the starter relay.

The car started up fine with a strong fast crank. Idle was normal and it's back to its old self. I still can't believe that the flex plate was the heart of my problem. Is it possible for the flex plate to put so much pressure on the motor that it will stop the car from idling and give a hard slow crank on start up?

Jim bailey - 928 International

Yes Wayne you are in the initial stages of thrust bearing failure. You MUST check the crankshaft end play ! If you continue to drive the car you will probably destroy the engine. It is that serious ! At best you will probably need to do a complete rebuild of bottom of the engine.

AO

Wayne... I wouldn't drive it until you've checked your oil filter (cut open) for metal particles. You may have ruined your thrust bearing, but saved the engine. Basically, when there's that much pre-load on the crank, the bearing is getting worn down. When that bearing goes, you're literally pushing the crank forward along with the pistons, etc.

Eventually, the pistons wear away at the cylinder walls and you can lose compression. At that point the engine is toast! Hopefully you still have descent compression and the motor can be refreshed to run another 100k miles, but for now, I'd park it until you know what if, any damage has occurred.

I think you might be a very lucky, unlucky fellow.

Bill Ball

Well, you've gotten the nasty news from Jim and Andrew. Endplay check. It should be less than 0.4mm.

Anybody want to speculate how a half inch could accumulate in a month or so? Why was this not found on the previous flexplate checks?

Mrmerlin

for a quick check use the dipstick to put some engine oil onto a black piece of paper or plastic go into the sun and look for metal particles if there are any you will see them.
When you check the endplay/flex plate connection you should absolutly check the rear pinch bolt make sure to torque both bolts to 66ft /lbs, also use new bolts if possible, you will need to remove the exhaust system to get to the rear driveshaft pinch bolt. If the endplay is beyond the specs dont run the engine you might be able to save it,it will have to be removed and the crank removed before further diagnosis is possible

Bill Ball

Stan: Isn't the rear TT bolt position limited by a groove in the TT?

Mrmerlin

Yes, but dont count on it it can move enought to cause problems if the front bolt is tight

Bill Ball

Half inch (12.7mm) is a lot to move.

Sharky

I just got home from work and after reading the replies, I think I'm F---ed!!! That's not what I wanted to hear.. I'll check the end play tomorrow, but what other simptoms are there other than metal fragments in the filter?

OHOOO man I'm not feeling this at all!

FlyingDog

Wayne, sorry about the bad news. Let me know if you need help with the motorectomy.

Bill, for the half inch of movement... how much did the splintered driveshaft you helped (edco?) remove lengthen, or did it?

Bill Ball

Your problems with it dying when warm and slow turnover (assuming good battery and starter) are rather classic. However, you had quite a load on the flexplate at that time.

You need to check the endplay. If it is more then 0.4mm, it is out of spec. If it is between that and 1mm, you MAY be able to rebuild, as there should still be a good amount of TB left. If more than that, the odds are very high you have lost the block (spun bearing heats the TB journal and cracks it).

Bill Ball

We never measured it. AFAIK, no one has ever documented a shaft stretching significantly.

FlyingDog

I hadn't heard of it either. I was trying to think up possibilities for that much movement.

928autobahndreamer

This may seem like a really strange question, but is there any chance that this may have been affected by the prev "limp home" situation? I remember reading that TT were more likely to fail after driven in limp home mode. It would make sense that this may put extra load on the TT and in some way transfer pressure to the flex plate?

Sorry, I have no experience in this area, just thinking out loud.

Charley B

An arguement can be made that a limp home mode may have contributed to TT bearing problems, but I haven't heard of a connection between Limp home mode and Thrust Bearing Failure. Then again, what the hell do I know?