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CorsePerVita · Joined: 25 Jul 2008 Posts: 1992 Location: Redmond, Oregon

So I did a full tune up on the 931 today and went through the vacuum lines as well.

Now, when I bought the car it started doing this weird thing where once it was warmed up the idle would go down to basically nothing and the engine would die. Faster so or worse if you hit the brakes.

I figured "huh, that's weird, must be a vacuum issue." so I replaced the vacuum lines. Now after doing that, the issue is 10x worse and instead of it doing this once ever so often, it's doing it a lot. I can save it from dying if I continue to blip the throttle when it goes down this low.

Anyone have any ideas what might be causing this? My thought was maybe that the aux air valve wasn't working right and maybe the idle adjust screw is turned so far that he aux air valve sticking causes "high idle" to emulate "true idle" - then when it warms up it just wants to die. Now, that's only an idea, not necessarily true. I think tomorrow I'm going to adjust the idle up a bit and see if that's what happens.

What should high idle be on these cars when it's cold?
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- 1977 Porsche 924 2.0 N/A (Trackday Project)
- 1979 Porsche 924 2.0 N/A (The other daily)
- 1980 Porsche 931 (Daily)
- 1987 Lamborghini Jalpa
- 1999 Ducati 900SS

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

Ask again after you have tested fuel volume, and system/control pressures, hot and cold.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

CorsePerVita · Joined: 25 Jul 2008 Posts: 1992 Location: Redmond, Oregon

I don't have a CIS pressure tester, so unfortunately testing the pressure isn't something I can do at the present moment. :-/
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- 1977 Porsche 924 2.0 N/A (Trackday Project)
- 1979 Porsche 924 2.0 N/A (The other daily)
- 1980 Porsche 931 (Daily)
- 1987 Lamborghini Jalpa
- 1999 Ducati 900SS

9xx · Joined: 18 Jun 2006 Posts: 627 Location: Jarvenpaa, Finland

But you could drive it to your local garage and have the mixture measured with exhaust gas analyzer. And have it adjusted of course. Doesn't cost much at all.
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Mikko

All gone: 931 '82 Alpine White, original option "220" G31 with LSD + 3 x 944

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

I have a proper tester, Derek. If you pop over this weekend, I can give you a hand.

As for the tailpipe sniffer, utility is diminished on a US car with a cat.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Joes924Racer · Posted: Sat Feb 23, 2013 1:53 am Post subject:

faulty wur. I skirted that issue for ever until i replaced it.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

CorsePerVita · Joined: 25 Jul 2008 Posts: 1992 Location: Redmond, Oregon

Nein37 · Joined: 29 Dec 2009 Posts: 513 Location: New London, CT

If the car is dead cold the idle should be around 1500 rpm, 950 + or - 50 when fully warm. You need a timing light to properly set idle speed on a DITC equipped car. Pull the air temp sensor on the front side of the intake, plug the hole and pull the oxygen sensor connector under the car. Warm the car up and turn the idle adjust screw until the 0 is just inside the timing window. If the idle is still too low or too high you need to adjust A/F mixture. Once you get the A/F somewhat correct to keep the car running good, readjust the idle speed again. Keep going back and forth until you get it running right. Then re-install the air temp sensor and reattach the oxygen sensor connector.
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1981 931 CGT replica, OEM CGT intercooler, .8 BAR WG spring, GTS Headlights, Innovative Wide band A/F, A/C delete, 16" Fuchs, Weltmeister 200lb lowering springs, Bilstein HDs front, Koni Sport rears.

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

His car is an 80.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Nein37 · Joined: 29 Dec 2009 Posts: 513 Location: New London, CT

I'm battin' 0 for 2.
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1981 931 CGT replica, OEM CGT intercooler, .8 BAR WG spring, GTS Headlights, Innovative Wide band A/F, A/C delete, 16" Fuchs, Weltmeister 200lb lowering springs, Bilstein HDs front, Koni Sport rears.

CorsePerVita · Joined: 25 Jul 2008 Posts: 1992 Location: Redmond, Oregon

Ok so I tried adjusting the idle. That's not the issue.

- Issue is not related to brakes after all, appears that was a coincidence. The dying engine issue will happen regardless if I am on the brakes or not. Actually, if I sit and pump the brakes rapidly when at a stop, the idle goes up just a hair.

- The engine appears to die after and only once the car is warmed up. Sometimes it dies entirely, other times it bobbles at nearly no rpms running very rough until I give it gas. If I blip the throttle, it'll work just fine.

I do suspect a mixture issue at this point as a quick glance at the fuel dizzy shows the missing cap on the mixture screw, leading me to believe it's been messed with. Unfortunately, hard for me to tell if it's lean or rich. But yeah, I've got to side with Rasta at this point, seems something is off a bit on the pressure/mixture.
_________________
- 1977 Porsche 924 2.0 N/A (Trackday Project)
- 1979 Porsche 924 2.0 N/A (The other daily)
- 1980 Porsche 931 (Daily)
- 1987 Lamborghini Jalpa
- 1999 Ducati 900SS

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

Trying to diagnose a poor running 931 without a CIS tester is folly.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

CorsePerVita · Joined: 25 Jul 2008 Posts: 1992 Location: Redmond, Oregon