924Board.org

justin's 931 · Joined: 18 Mar 2007 Posts: 11 Location: Medina, Ohio

My son's 1980 931 runs extremely rich. To the point of getting about 10-15mpg. Even at idle the car pumps out black smoke, if driving it spiritedly and you rev it out, whomever is behind you is left in cloud of black smoke. This car has the CIS, and I was told that there is an idle mixture screw in between where the aircleaner bellow attaches and where the injection lines are located. I attempted to adjust this screw and it made no difference at all. I was told it adjusts the big round throttle plate underneath where the air cleaner bellow attaches.

The car had gotten to the point that once it warmed up, it was barely idling (100-200 rpm) and would stall from time to time. I adjusted the idle a little bit so that it now idles at 750-800 rpm when warm.

Also, when changing the oil in this car, we were in for quite a shock, as soon as the drain plug was removed the oil shot out of the oil pan about three feet making at the time a very frustrating mess (now we look back at this an laugh) I was told that the oil shot out because there was gas mixed in with the oil from it running too rich.

How do I adjust the fuel mixture on this car.

You helped me out in the past with this project car, cannot thank you enough. Other than a few quirks, the car is an absolute blast to drive.

Thanks

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Check out the "K-Jet Overview" thread, and do a

Search on "control pressure." That should point you in the right direction.
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Paul · Posted: Thu Aug 23, 2007 9:26 am Post subject:

Stop driving the car until you have this corrected or I see new rod bearings in your future.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

justin's 931 · Joined: 18 Mar 2007 Posts: 11 Location: Medina, Ohio

Thanks for the input Rasta and Paul, you guys helped me out in the past. I looked at the K-jet page and will see if I can find the 3mm allen that adjusts the mixture

Paul · Posted: Thu Aug 23, 2007 11:14 am Post subject:

That's the last thing you want to do.....

Check the control and system pressures first.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

morghen · Joined: 21 Jan 2005 Posts: 9102 Location: Romania

check all the intake rubber too. My car had crappy milage and was idling at 300-500 RPM and i reseted the idle but that was not the problem....the problem was the big o-ring before the throtle body was shot and the metered air got the injectors runing while some of this air escaped trough the o-ring.
_________________
Supercharger and EFI kits
https://www.the924.com

924RACR · Posted: Thu Aug 23, 2007 9:52 pm Post subject:

Indeed, that was my first thought, that there could well be a substantial vacuum leak. But this would usually cause a lean condition, right? Except maybe on boost...

The gas in the oil, in that quantity, is usually caused by a long period where the car won't start, but is cranked a lot in an attempt to start it. All the fuel is sprayed in, but just dribbles down the cylinder walls into the sump.

Definitely check the pressures, like Paul said... hell, go through EVERYTHING.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

justin's 931 · Joined: 18 Mar 2007 Posts: 11 Location: Medina, Ohio

Thanks to all of you for your responses. The more I thought about this, the more I think it is related to a vacuum leak. More air coming in would make more sense that the injectors would overcompensate and cause a rich running scenario. Therefore as you had suggested, the last thing I really want to do is start messing with the adjustments if it is something as simple as a vacuum leak. Thanks again, I will keep you posted.

Smoothie · Posted: Fri Aug 24, 2007 1:50 am Post subject:

I don't want to discourage you from checking for vacuum leaks - that is a major problem with these old cars and yours may well have a few, but it really wouldn't account for your always-rich running.
Air is metered and the fuel quantity determined by the air metering plate at the fuel distributor - any vacuum leak downstream of that point would cause lean running at idle and part-throttle (anytime there's vacuum (aka- negative pressure) present in the intake system) because additional air that was not metered is being pulled into the intake and added to the A:F mix, leaving you with more air than was metered, and a too-lean mix. You'd have rich running only under boost (mainly full throttle when there's positive pressure in the intake), because already-metered air would be getting pushed out (and removed from the A:F equation, leaving you with less air than had been metered, and a too-rich mix).
Long story short - if you're overly rich always (idle, part-throttle, full-throttle), your main problem is either an incorrect mixture setting or incorrect system and/or control pressure, not vacuum leaks.
The mix adjuster is where you were at, but remove that screw - it's just plugging the hole - the 3mm hex key adjustment screw is hidden down that hole. (Do this only after fixing any vacuum leaks.) Feel around for it (straight down), don't drop the tool in there, note the position of the adjustment before doing anything, then (don't apply any downward pressure that would affect the mix) turn counterclockwise (that's toward lean) in very small increments and see if you can eliminate the black smoke while keeping the car running (if not, turn the adjustment back to where it was previously and get access to a CIS fuel pressure tester to check the system and control pressures).

Normally, you'd leave the adjustment for last, but since you're finding a screw covering the adjuster access hole (+ the car's 25+ years old), it's easy to conclude that someone's already messed with it in the past (that hole was plugged with a ball bearing from the factory). My own 931 was way rich when I got it in '92, with black smoke at idle, and all it needed at that time was an adjustment toward lean.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

justin's 931 · Joined: 18 Mar 2007 Posts: 11 Location: Medina, Ohio

Thanks Smoothie, the more I thought about any intake leaks I was thinking what someone else had posted, that indeed a leak would make the car run leaner. The funny thing is, the boots look like they are in pretty good shape, the boot that connects to the turbo charger under CIS is tight, but I know that when I had it off last time, it looked like it should be replaced, not that it had major cracks, but it looked kind of beat up where it attached to the turbo. Also, there is a hose coming out of the airbox and is attached to the lid of a black plastic canister on the drivers side. The lid does not secure to the canister and though I doubt it would cause any of the problems, thought it might be worth looking into sealing it better.
I really appreciate your thoughts, and I will look into the adjustment tonight.

Smoothie · Posted: Fri Aug 24, 2007 5:10 am Post subject:

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Also, the numerous elbows and short sections of hose all over the intake can look fine until they are in your hand. If you are serious about keeping and enjoying your car, I would refresh every piece of rubber in the intake tract, then forget about vaccuum leaks for the next 20 years or so.
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3