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echrisconnor's Avatar
 
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cold running problems

I'm in need of some help. My 1987 924s runs like crap in the cold. At startup on days when the temperature is below 40deg F. it runs rough, stumbles, has poor acceleration and mileage and pops from the tailpipe. It looks to me like it's running rich, because the tailpipe has a lot of black around it, but I also know that most cold-running problems are attributed to a lean mixture.

It gets better when it warms up. After 15-20 minutes of running, the idle goes up a couple hundred rpms, it smooths out and becomes more driveable. In warm weather it runs like a champ. One other thing to note is that the problem is erratic. It almost always has problems in the cold, but every now and then, I start it up and it runs like a champ.

I've checked for vacuum leaks and found none. I ohmed out the head temperatur sensor and found it in or close to spec. Not sure what to check next.

Help!
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'76 911 Carrera 3.0
Old 04-13-2007, 10:55 AM
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Idle stabilization valve. Located under the intake. 2 hose lines and one electrical plug. Pull it and clean it or buy a new one$$$. Also open the lid on the AFM and check the black wiper strips for deep grooves/breakage/chips. Look at the air intake boot on the underside for a crack.

Dal
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Old 04-13-2007, 11:59 PM
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So you think it's unmetered air bypassing the idle air stabilization valve? Does that effect cold running or just idle? As for the AFM, I can look at that, but why would cold weather change it's performance?

I'll check these things out and look for some more vacuum leaks.

chris
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Old 04-15-2007, 06:13 AM
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The ISV has a piston action-open/closed when cold it can hang up if dirty which is usually the case rather than needing a new ISV, when it warms up the valve may be releasing thus good idle. You have an idle issue so rule it out by dealing with what controls idle.

The AFM when bad can cause irratic idle too when cold but the issues with it are usually a miss at idle or a 3500 to 5000 rpm burp and/or misfire while on the throttle. Or not start at all if the wiper material is broken. Weather is what the meter is all about really, determining what air temp is passing so it can signal the DME/ECU to make the correct air fuel mix and timing. You say it is rich so the AFM is a part of that and in your case is not allowing enough air in to lean it out causing a poor start up. Simply speaking it is not getting enough airflow at cold start. The ISV is somewhat dependant upon the AFM by means of airflow, if the air is misread(AFM) it will put out a wrong signal thus air entering the ISV may be insufficient for a controlled idle. My bet is it is sticking.

My idled just as yours and replaced my ISV, problem solved.

The areas I suggested are what is most probable to cause poor idle.

You can clean the ISV w/brake kleen using a couple jump wires and the battery. One lead to neg the other 2 wires to the 2 posts (3) reverse the pos wires back and forth to free the valve...to that extent.

Dal
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Last edited by 924Sman; 04-15-2007 at 07:03 AM..
Old 04-15-2007, 06:58 AM
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Thanks for the explanation. I find that really helps me get more of an understanding of the big picture. I'll try these things and report back.

Thanks again,

Chris
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Old 04-15-2007, 12:45 PM
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