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Idle Fluxuation and Popping

1987 930 with Fabspeed exhaust.,

What is causing the idle to fluctuate when you downshift and stop at at a light. RPM is up and down and the engine sometimes stalls. It usually will correct itself after a 10 seconds. Also, backfires (Pops) at a constant rpm or deceleration. I had the mixture adjusted a while back and this was the outcome. Car runs with a hunting feel at a constant RPM. Kind of a small lurching feel, Not a smooth feeling...

Help fellas what do you suggest?

THANKS

Bob
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Old 07-22-2007, 05:43 PM
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Can't tell you much except I had the same symptoms when I was running lean.
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Old 07-22-2007, 05:51 PM
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I have always been under the impression that popping from the exhaust was a lean issue. Reading some other Pelican threads, a few people said it was a rich symptom for CIS. ???? I know carborated engines will miss and pop when lean, it seems the same would be true for fuel injection.

Sorry about the Title Fluxuation S/B Fluctuation..Must have had a magnetic moment..
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Old 07-22-2007, 06:15 PM
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My guess is lean. I once tried to cut the smog factor at idle by leaning out the idle mix and it just made my car run like crap until the boost kicks in. 12.6 AFR is the best AFR for maximum power, you could try that pre-boost and then go richer on boost, with an idle of around 13.0. At least as a starting point before fine tuning...

Just a thought...
Old 07-22-2007, 09:26 PM
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DDDD,

What causes the Idle to fluctuate? I'll richen it tonight and see, but last time when the car would die at stop lights then it would idle after a while...No vacuum leaks... ???
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Old 07-23-2007, 06:23 AM
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Malfunctioning Decel valve (AAV), maybe?:
http://forums.rennlist.com/rennforums/showthread.php?t=228944
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Last edited by sand_man; 07-23-2007 at 06:58 AM..
Old 07-23-2007, 06:37 AM
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Might also be a AAR problem.
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back in the saddle: '95 993 - just another black C2
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Last edited by sand_man; 07-23-2007 at 06:48 AM..
Old 07-23-2007, 06:45 AM
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The popping is caused by unburnt fuel igniting in the headers/muffler. Fluctuation and popping can be caused by a cylinder that is not firing causing an uneven idle and power fluctuations at cruise. If you have a timing light hook it to each plug lead and make sure you have spark at every cylinder. If you don't have a timing light just pull each plug wire (one at a time) and hook up an old spark plug to it (and ground the body of the plug) and crank the engine over with the fuel pump relays pulled out. Should see a nice spark, if you don't then your distributer cap and rotor could be failing or plug wires need replacing.
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Old 07-23-2007, 08:21 PM
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Magic,

I just replaced Cap, Rotor and wires. It is a mixture issue. Most CIS systems I have seen in time have this problem Idle RPM up and down. I have replaced intake boots and have no vacuum leaks. I was wondering if it has anything to do with the air bypass for idle. also where do you unplug the O2 sensor? I want to lean it out and richen the mixture per the CIS manual. Then adjust it at a smog shop if needed. My O2 sensor travels throught the sheet metal and up under the intake manifold somewhere is there a plug that can be unplugged easily.

Thanks

Bob
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Old 07-24-2007, 08:03 AM
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