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Car runs great, but some odd issues
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CorsePerVita  



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

PostPosted: Thu Feb 21, 2013 12:29 pm    Post subject: Car runs great, but some odd issues Reply with quote

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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Rasta Monsta  



Joined: 12 Jul 2006
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PostPosted: Thu Feb 21, 2013 2:49 pm    Post subject: Reply with quote

Ask again after you have tested fuel volume, and system/control pressures, hot and cold.
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jazz guy  



Joined: 26 Nov 2002
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Location: Colorado

PostPosted: Fri Feb 22, 2013 3:59 am    Post subject: Reply with quote

Rasta wrote:
Ask again after you have tested fuel volume, and system/control pressures, hot and cold.
^+1
If you had vacuum leaks and you fixed them while doing the tune up, your mixture could be out of whack. The PO probably adjusted it and the idle screw to get it to run with all the leaks. Plugging the leaks might make it run worse until you get it all sorted.
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CorsePerVita  



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

PostPosted: Fri Feb 22, 2013 7:41 pm    Post subject: Reply with quote

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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9xx  



Joined: 18 Jun 2006
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PostPosted: Fri Feb 22, 2013 10:31 pm    Post subject: Reply with quote

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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Rasta Monsta  



Joined: 12 Jul 2006
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PostPosted: Sat Feb 23, 2013 1:00 am    Post subject: Reply with quote

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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Joes924Racer  



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PostPosted: Sat Feb 23, 2013 1:53 am    Post subject: Reply with quote

faulty wur. I skirted that issue for ever until i replaced it.
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RC  



Joined: 25 Mar 2007
Posts: 2634
Location: Australia

PostPosted: Sat Feb 23, 2013 2:48 am    Post subject: Re: Car runs great, but some odd issues Reply with quote

CorsePerVita wrote:
Faster so or worse if you hit the brakes...

I figured "huh, that's weird, must be a vacuum issue." ...

Anyone have any ideas what might be causing this?

Yep, leaking brake booster is my first guess Derek. Particularly so if RPM varies when pumping pedal or there`s a whistle noise. Easy to isolate with a plug/ bolt in the vac hose to the booster. May be more issues but try that first.

Nice score BTW.

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CorsePerVita  



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

PostPosted: Sun Feb 24, 2013 4:44 pm    Post subject: Reply with quote

Rasta Monsta wrote:
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.


So does anyone know what the cold idle should be when the aux air valve is giving extra air? I don't have a workshop manual. Isn't it 1500rpm or 2000rpm?
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Nein37  



Joined: 29 Dec 2009
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PostPosted: Mon Feb 25, 2013 1:19 am    Post subject: Reply with quote

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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Rasta Monsta  



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PostPosted: Mon Feb 25, 2013 3:14 am    Post subject: Reply with quote

His car is an 80.
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Nein37  



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PostPosted: Mon Feb 25, 2013 6:02 am    Post subject: Reply with quote

I'm battin' 0 for 2.
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CorsePerVita  



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

PostPosted: Mon Feb 25, 2013 11:35 am    Post subject: Reply with quote

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.
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- 1977 Porsche 924 2.0 N/A (Trackday Project)
- 1979 Porsche 924 2.0 N/A (The other daily)
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- 1999 Ducati 900SS
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Rasta Monsta  



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PostPosted: Mon Feb 25, 2013 12:17 pm    Post subject: Reply with quote

Trying to diagnose a poor running 931 without a CIS tester is folly.
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CorsePerVita  



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

PostPosted: Mon Feb 25, 2013 12:22 pm    Post subject: Reply with quote

Rasta Monsta wrote:
Trying to diagnose a poor running 931 without a CIS tester is folly.


Yeah, I suppose I need to get one. Any good references or recommendations on where to get one?
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