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Smart quod bastardus
 
fredmeister's Avatar
 
Join Date: Oct 2002
Location: Baltimore, MD, USA
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strange miss when engine cold

I have recently started having a miss in the engine when I first start the 79 930 turbo when it is cold (sitting for awhile).
I have changed the distributor cap, wires are in great shape.
When the car is first started it goes thru the normal high idle and eventually drops to around 1200rpm......but it has a distinct miss. Really noticeable when you blip the throttle.
Now if I let the car idle for 10 minutes, the idle speed drops to 950rpm and the miss seems to disappear.
Have not driven the car under load since this started.
What could cause this?

Fred
Old 09-17-2007, 07:51 AM
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Fouled plug? The stock plugs are really cold...

Cold control pressure (or any other part of the warmup system, like thermotime valve, switch, AAR etc) out of spec?

I never idle mine to warm it up, the owner's handbook specifically says not to. Mine doesn't fast idle on a cold start anyway (although it does do the obnoxious high idle on a warm - but not a hot - start). So I always run it @ 1800-2000 RPM (for good oil flow) until the oil pressure bounces a few (say 4-5) times from the engine thermostat opening and then drive it.

If the miss disappears when hot, I'd be inclined to either check/clean the plugs or treat it to an Italian tune-up. YMMV...
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'77 S with '78 930 power and a few other things.
Old 09-17-2007, 08:19 AM
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The plugs are replaced with NGK W4(?) not sure the number but they are the next heat range higher plug than the expensive stock Bosch plug.
I was wondering how the AAR would effect the system if it malfunctioned because of 2 reasons.
You mentioned that your car never high idles even during cold start-up, mine seems to idle at 1200rpm for extended amounts of time, sometimes even after driving it for a few minutes it will still hold a high idle speed. Could this indicate an AAR valve hangin up?
Second, if this valve got contaminated with alot of oil from the intercooler would this cause it to malfunction like this.....hanging open slightly or taking long time to finally close? Reason I am asking is that the turbo oil seals was going and it was blowing alot of oil into the intake and intercooler under high boost loads. This oil eventually would make its way into the AAR thru the hose that connected to it from the intercooler.
So, does this indicate more an AAR valve issue or should I still be leaning towards fouled plugs?

Fred
Old 09-17-2007, 11:14 AM
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Originally Posted by fredmeister View Post
The plugs are replaced with NGK W4(?) not sure the number but they are the next heat range higher plug than the expensive stock Bosch plug.
Uhmm. The expensive stock plug is platinum electrodes, IIRC. Not plated or coated, but real platinum. Which resists fouling much better, apparently - and probably why they were used, given that a lot of them only get changed with a valve adjust and they're real cold, as spec'd from the factory.

Quote:
I was wondering how the AAR would effect the system if it malfunctioned because of 2 reasons.
You mentioned that your car never high idles even during cold start-up, mine seems to idle at 1200rpm for extended amounts of time, sometimes even after driving it for a few minutes it will still hold a high idle speed. Could this indicate an AAR valve hangin up?
Mine's always behaved like that. I get a fast idle for about a minute on a warm start (like when I buy groceries), but not on a cold one, no idea why, other than it's probably the AAR. Original setup would idle perfectly cold, but really didn't want to run between 1000-2000. Now, it'll cold idle much too low for comfort if you let it - but is drivable almost immediately. I figured this was the WUR or setttings thereof (fitted one from Brian Leaske), and could care less about the cold idle, as I can easily keep revs up manually, now that it'll respond to the throttle when cold...

If you've got a fast idle on cold start and that is controlled by the AAR letting in extra air until it's bi-metallic strip heats up, then it's obvious that either the AAR - or something else - is letting in extra air, no?

Maybe your bi-metallic strip isn't getting power, or is broken? Then the AAR would stay open and let in the extra air until heat soak warmed it enough to close.

Given that the high idle eventually goes away, then whatever is letting in the extra air is closing, just not as quickly as you'd like. Could even be a manifold leak or something else that closes up when it gets hot?

There's a recent thread on the 911 forum (last couple of days) about adjusting the resistance of the AAR to control the length of time the high idle lasts for. You could do worse than check the resistance of yours (38 ohms gives about 5.5 minutes, apparently).

Mysterytrain also posted this thread about how he rebuilt his SC AAR, modifying it to give him the resistance/delay he preferred.

AAR Revisited

Quote:
Second, if this valve got contaminated with alot of oil from the intercooler would this cause it to malfunction like this.....hanging open slightly or taking long time to finally close? Reason I am asking is that the turbo oil seals was going and it was blowing alot of oil into the intake and intercooler under high boost loads. This oil eventually would make its way into the AAR thru the hose that connected to it from the intercooler.
So, does this indicate more an AAR valve issue or should I still be leaning towards fouled plugs?

Fred
Uhm, I'll guess that the AAR by itself shouldn't cause a miss in a correctly-tuned CIS system, even if it's not closing fast enough. It's just bleeding in more air for a faster cold idle, as though you had your foot on the throttle, or a hand throttle.

But if you've still got oil in the intake, I vote for cleaning it out. It's not going to help anything work better, and it could conceivably foul a plug or two.

I still vote for looking at the plugs - if for no other reason than because you could eliminate fouling easily with a visual inspection before you start looking at other things.
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Old 09-17-2007, 06:17 PM
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