Nicholbry

Hello fellow 928 Rennlisters!

Question: My '90 S4 Automatic starts up fine w/o hesitation cold, but drive it 15 min.+ and it idles low at stops, seemingly critically low, fearing that it will stall . If I shut the engine down and try to restart it, she cranks with some difficulty requiring 6-7 sec. to finish turning over. Oil pressure gets low as well; although this may not be related (but just thought I would mention it). The source of that problem may be the plastic journal pins under the cam covers that I am going to replace w/ the metal pins.

I've read up on my concerns, but keep getting pulled in different directions to source the problem . One idea out there is related to the drive shaft movement. Perhaps it is binding and requiring the starter to work harder to turn it over once it's warmed up??? The flex plate was replaced recently (it was cracked as the previous owner didn't service the pinch collar on the drive shaft) and the collar was reset by my mechanic when the flex plate was replaced.

Recently completed service: new starter and vent shroud, new battery, timing belt, all accessory belts, rebuilt tensioner, new tension idle bearing, water pump, thermostat, coolant hoses, search for vacuum leaks under the manifold (none found), new power steering pump, pwr. steering reservoir, pwr steering hoses, Bosch spark plugs, Beru wires, distributors, new intake tubes, air filter, Differential flush/refill, new positive battery cable w/ uprated heavier wire for better conduction, and a bunch of other stuff that would be unrelated to this topic.

Any help would be appreciated. You guys are the greatest!

Mrmerlin

even tho you have already looked it sounds to me like a vacuum leak, if tyhe intake is original then you might want to do the intake refinish / refurbish, you could also check the fuel dampers and fuel pressure regulator to see if the hold vacuum and if they are leaking, also to consider is replacing the fuel filter this would be the easiest thing to do first

IcemanG17

It does sound like you have a vacuum leak or a faulty idle stabilizer.....one easy "fix" you can try is to spray WD40 down the vac tube to the stabilizer valve to help clean it out a bit.......if this improves things it will help you narrow things down

The hot start problem is also very common.....generally it falls to one of three things: 1 fuel pressure check valve 2: leaky fuel injectors 3: bad pressure regulator

If your fuel pressure check valve has never been replaced...it needs to be changed anyway....my car had a slight warm start problem that got better with the check valve...

You mention low oil pressure....is this when driving or only on a hot restart?? IF it is on a hot restart then it sounds like the springs on the oil pressure "regulator" (or whatever its called) above the oil pressure sender are bad...mine were and it did cause low oil pressure warnings on hot restarts.... Bonus is the springs are cheap like a couple bucks each.....just make sure you have a shorty 27mm to remove the sender

ROG100

I do not want to be alarmist, however Nicholas has shared a lot of the cars detail with me and I have a concern that there could be an element of TBF here.
Car has 65k miles and the last mechanic reported "extreme pressure" on the flex plate and replaced the flex plate due to a crack???
I have recommended that the "end play" on the crank be checked before we go any further.
At the very least this would alleviate concerns that Nicholas has.
We need some help in doing this from someone in the Mississippi area.
Once we know Nicholas's home town we can pin point someone hopefully.
Shoot me down in flames if you disagree.

Kevin in Atlanta

I would think 65k is a really too young for TBF. But, there are other indications that would suggest that TBF is a possibility. I know of a 928S that had cracked the flex plate. But, I know the owner and he abused the heck of it.

Let's drain and inspect the oil and cut the oil filter open before we jump.

ROG100

Only done about 1000miles so may not show in the oil.
Just as easy to check the float on the crank.

Jim bailey - 928 International

I would agree that the crank endplay needs to be checked because it does sound like it may be a Thrust Bearing issue. The new starter was to try to fix a cranking issue ? There are not very many things which cause the engine to labor/ crank slowly.

Nicholbry

Thanks guys for showing up here to help. Indeed, as Roger mentioned the car has only done 1000 mi. since the flex plate replacement and collar adjustment which seemed to have been neglected by the previous owner.

Regarding the oil pressure, I don't think it is related, but thought I would throw it out there just in case it had something to do with it. The oil pressure isn't affected by the warm start; rather, the oil pressure just gets very low after the car has warmed to normal operating temp. The pressure rides in the middle during 55 mph driving and drops to only 1/4 at stops.

I live in Southern Mississippi. I am 1 hr from Natchez, McComb, and Baton Rouge,LA for anyone who might want to take a peek. I am new to this DIY stuff and enjoy it, but I am limited to things like fuel filters, fluid flushes, interior repair, Aero mirror replacement, etc at this stage. and even require a tutorial for those things. I'm afraid that the Torque Tube may be a bit out of my league; although there are some great tutorials out there. I'm afraid of disconnecting the transmission or getting in there and can't get something undone. Additionally, I am not aware of a mechanic locally that understand these cars fully. One guy down here is knowledgeable of Merc. & BMW, but even he was left scratching his head regarding the idle and he seemed put off working on the car; I'm guessing, due to his lack of knowledge and tighter confines.

Just to recap, the car has only 65K miles. I am going to try and tackle this Drive Shaft End Play if I can and release/re-anchor the collar again just in case the torque the mechanic used wasn't 66#. I will paint for monitoring, but it will likely take me a week as I am a little slow at this stuff. It's the Mississippi thing you know!

Keep chiming in, anything you guys have to say will help. Ya'll are terrific.

Nicholbry

Oh Jim! Yes, the starter was replaced to try and remedy the problem. Although the mechanic would not say, I'm guessing they were "shooting in the dark" trying to find the source of the problem costing me thousands along the way, only to deliver a car with labored starts when warm. These same guys recommended a different wheel/tire arrangement saying that they were too big for the car and causing inner fender rub. Turns out, the arrangement was OK; rather the previous owner lowered the ride height too much causing occasional rub. Too late, I had already shelled out the $$ before I had the car to inspect.

Thanks,

SharkSkin

Oil pressure 2-3 bar on the highway? That doesn't sound good. Whatever else you find, look into that as well. Proper filter, check the oil thermostat, etc.

Jim bailey - 928 International

What needs to be measured is how far the crankshaft inside the engine can be pushed forward and backward. Driving an engine with a thrust bearing problem will destroy the engine ! Even in the early stages it would require a full engine rebuild but then it grinds away too much of the engine block overheats the main bearing and cracks the engine block. The axial play started out .06 -.192 when it gets over .40 mm it is too much. If your local mechanic needs help have him call me but I highly recommend that you NOT DRIVE the car until you ruleout TBF.

Kevin in Atlanta

Agreed. I guess it depends on if anybody worked on the TT or transmission and did not follow the WSM's. I was just thinking that 65k was early for TT or tranny replacement so that the car might still be set up as it was from the factory.

Nicholbry

GULP! I'm really nervous now. I am 37 yrs old, never smoked, never been drunk (really), never done drugs; but I think I'm starting them all today.

RngTrtl

I wish I was closer to you, I would come on down and we could give it a look over. and yes, now is the time for a beer or 3

IcemanG17

A thrust bearing check is a very good starting point.......your oil pressure is WAY too low....beyond the WSM spec of 1 bar per 1K rpms...or 5bar at 4K rpms....... but I don't think low oil pressure is a TBF issue?