RPM flux on Startup

Cebu · 06-01-2001, 06:34 AM

86 944na (stock), On first engine start after sitting overnight,the engine stumbles for approx. 3-sec and then smoothes out with a steady 900 RPM. There is no RPM hesitation on acceleration or starts the remainder of the day.Vehicle accelerates smoothe and strong to red line. Only after car sits for 12 plus hours does the RPM surge from 9-1100 on start. Items replaced,Fuel filter,fuel pressure regulator,fuel pressure damper. All imputs are appreciated.

krankenwagen · 07-12-2001, 07:50 PM

My '86NA does it too. It started about a week or two after I installed a Europroducts DME chip, but I don't think that's what's wrong. Anyone else have issues after installing one of these chips? I haven't replaced anything fuel system related yet, but I am going to replace the O2 sensor next week to see if my full throttle performance get's better. I'll let you know if it helps.

Magnus_944 · 07-12-2001, 09:48 PM

It could be a leaking injector.

/Magnus

fty · 07-14-2001, 08:03 PM

I have the same problem..does it all the time bobs from 600-110 then flattens out after a couple mins...

Lawrence Coppari · 07-15-2001, 07:06 AM

My '86 with MAF has a low speed rough idle for the first couple of minutes. It has not gotten any worse or better in 3 years. I just live with it. It does not fluctuate; it merely idles too low for the first couple of minutes.

Ehab · 07-15-2001, 10:50 AM

When it fluctuates, that is an indicator that the mixture is either too rich or too lean for that period of time, or a leaky injector flooding a cylinder. I know this because I can cause the idle rpm to fluctuate by setting the A/F mixture using the AFM bolt.

Coppari ... sounds like your Aux Air valve needs cleaning/tuning, or replacement if faulty. My idle starts at 1100 when cold (steady), then slowly drops to 950 as the engine warms up.

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MAS · MAS's Garage

"... My idle starts at 1100 when cold (steady), then slowly drops to 950 as the engine warms up."

Same with my 87 924S.

-Zog

dmac · dmac's Garage

The variation in initial idle speed is due to the action of the Idle Control Valve (the ICV is in 85.5+ cars, while a Auxillary Air Valve (AAV) is in previous cars). As the speed drops the computer signals the ICV to open slightly, causing the RPM's to rise. If it overshoots the valve is closed slightly reducing RPM. Some initial hunting is normal for this engine.