FeedNfrenZ

My engine starts fine, idles at 1k (cold) then as it warms the idle starts lowering to about 600 rpms. The idle will then hunts up and down and may stop. If I gun the engine it'll rev., undershoot rpm (almost stop) then settle out. When the electric fan kicks on at 170 degrees (16" fan added with adjustable thermostat) the engine idle suffers then recovers. If I get on the road it seems to run fine, I let off accelerator and the engine rpms dip way down (500-400 rpms) and may die. It'll start right back up. I've turned the crank with a socket hot and cold with no real differance. How can this be related to TBF?
I had my vent line from the rubber boot between the MAF and throttle body plugged for supercharging ALA Tim murphy but as of now the plu is out and that is vented wide open. Other than that and the #30 injectors I am back to stock. I'm running 30lbs fuel pressure on and adjustable reg., superchager off, no air box (straight MAF) I've confirmed Temp. 2 ok, MAF ok, fuel pressure ok, voltage to coils ok, new spark plugs (old ones black), alt. ok (13.8 volts), new o2 sensor, cats bypassed. I've put the engine in gear and loaded it down by holding the brake while I've rev'd it (fine).
Next thought is to check engine timing at idle other than that I'm ready to punt!

michaelathome

Check your O2 sensor, I had similar problems w/it disconnected. Did you try putting your old one back on?

Michael

Imo000

Wait a minute………., this is exactly how my SC’ed ‘85 928 acts. I had a slight idle recovery problem before the SC and it got a little worse after the SC. During driving, if I don’t push the clutch in before the rpms fall bellow 1500, the engine will end up staling. In neutral, if I rev up the engine, 50% of the time it will almost stall before it recover to an idle. This is a small issue for me so I’m not fixing it this winter. I’m pretty sure your idle stabilizer is a bit slow to react. Don’t get too depressed cause this doesn’t sound like a TBF.

heinrich

No airbox is part of the problem

FeedNfrenZ

Heinrich, why would that be?

Bill Ball

Sounds a lot like the idle stabilizer is stuck. Have you tried the WD40 (Kroil, PB BLaster) fix?

Also, you say you have TBF, and I know we went around and around on that. You now say the crank turns as freely warm as cold and shows no signs of very slow turnover when hot that you had before. The only change is that you released 5MM of preload on the flexplate, right? So, other than the idle irregularity, things seem fine? I recall some ambiguity about how the crank endplay was determined. Are you still sure it exceeds 0.4MM?

Anyway, idle stabilizer or a vacuum leak.

Jim M.

What Bill Ball said, Idle stabilizer. BTDT

Jim Mayzurk
93 GTS 5-spd

FeedNfrenZ

Bill, crank endplay measured without prying and holding for measurement was .010 end to end however prying and holding during measurement was .050. I believe the belts that were still connected account for the fact that the crank was springing back to a given spot. SO the accurate measurement was .050.
I cleaned and tested and oiled the idle stablizer when it was out. .................I'm lost?

Bill Ball

0.05 inch. OK, that's 1.3MM. If that is correct, then you have TBF and all of this may be TBF symptoms. I'm not 100% sure the measurement is being done correctly. I mean, I think there is some uncertainty about how it should be done. But if it is 1.3MM, you should not being fiddling with the idle or anything else.

I would repeat the crank measument, measuring at the crank and not the periphery of the flywheel. That way, prying should make no difference. If that is what you did, then by definition you have TBF and you are further damaging the block by running the motor. There is a possibility (maybe slim) the motor could be rebuilt with a new thrust bearing. Every time you rev the motor you reduce that possibility.

SharkSkin

If your crank is walking forward and rubbing on something occasionally, it could account for this behavior -- maybe. It's your call, but I fail to see the point in trying to get it to run "just right" when you have a bigger project looming on the horizon....

Warren928

Brent,
I have had a similar problem.
When its hotter outside and the engine is warm it tends to do it at intermittent stops. I just assume its the idle stabilizer since I never have checked it.

Imo000

PEEPS.... after numerous mix of Gibson's Rye/Pepsi...... and me readind the replies........it has to be, l.............ike I said before, the stable idleizer valve........sometimes the WD40 trick will not fix the S4 idle problem!!!!

PS. This is the first and last time I'm trying to post after a "few drinks"

heinrich

Brent run a healthy 928 with the box off and run your hand past over the MAF about 1-inch away and tell me what happens

Imo000

During the SC build, I ran my '85 with the filter box off and it ran fine. I can't imagine the S4 being that much different. The removal of the filter box should smooth out the air flow even more.

You are right, even at idle the 928s engine (or any V8 for that matter) has some serious vacuum power. It will pull you palm right into the MAF.