Advice - 3000 rpm idle

[Dorsh]

Hi Guys,

First, thank you all for sharing your knowledge on this forum. I'm a regular reader on here and only post on here when I've absolutely had it and and need help. Having said that, here is my issue.

Background
I have a 78SC that was sitting for 1.5 years due to a charging issue and a broken rear window (Targa). After getting a window in and replacing the alternator and the battery the charging system is now operating correctly although the car idles at 3000 rpm (cold or hot) and runs good and bad. The car would kick and jerk while cruising but run great under acceleration.

After cleaning the injectors the driving issue was nearly cleared up except the high idle. While looking for more issues - ie vacuum leak, I found the air box split apart where the two halves meet. Naturally, I'm thinking this has to be my issue. Well after spending an afternoon replacing the air box, I'm disappointed to report my car still idles at 3000 rpm! Driving down the road at 4500 rpm - push in the clutch the idle will drop quickly down to around 3700~4000 rpm's then float down to near 3000.

I have seen alot of posts about the AAR and comments saying this can get stuck. I have lubricated this component but have not done the freezer test. Also I've seen comments on the decel valve being faulty - at this time I have not tried anything with this either.

Any suggestions are greatly appreciated.

Thanks guys,
Kurt

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78SC

[pete3799]

Have you checked the return spring?
Tried lubing the bell crank on the side of the trans.
Idle screw not having any effect?
If the air box was split your probably running rich now that the leak is fixed.

[James Brown]

1+ after a airbox change, you should re set all the components to run now that the airbox is sealed up. Try the idle screw first. disconnect the deceleration valve and see if that has any effect. Spray some carb. cleaner around the intake runners and the airbox to check for leaks. After that, time to get the gauges out and the LM1.

[Nazgul280]

Hi Kurt,

I'm not an expert but I can tell you are on the right path with air leaks. If I can relate to a story of mine, it may help you think of other things to check systematically.

I restored an old Z car of mine and had idle problems with the new engine that had so many mods it was unrecognizable from stock. My first startup it raced directly to 4500 rpm and stayed there until I killed the ignition about 4 seconds later. I had numerous leaks all through the custom intake and injector bosses that all added up to an uncontrolled idle.

I've had friends (in non-Pcars) that had vacuum leaks in the climate control heater cocks that caused similar problems. Now who would look for a vacuum leak under the dash?

My point is this: you have already identified at least two potential leak sources. Check those first until you can rest assured that you have ruled them out. Next, go to the more improbable. I would say ANY air line coming from the intake manifold, a Head gasket leak, exhaust gasket are a few more ideas. What does your vintage 911 use vacuum for? Find out and start checking.

A couple of tricks to track down leaks: Simple propane like a Burns-O-Matic torch can be used around your gaskets to check. Here's how. Start your engine and grab the unlit torch with the propane flowing and run it close to all your valve cover, exhaust, and intake manifolds that you can get to, all the while listening to the engine. If the engine stumbles, you found a place that is sucking the propane into the engine and it doesn't like it as much as your Fuel/air mixture.

Another trick that I never had too much luck with but others have is to take a few feet of vacuum or small diameter fuel hose and use it like a stethoscope. Place one end near your ear and with the other you can probe around and listen for a hissing sound, that is, if you can hear it over the engine!

I'm sure others here can enlighten you with some specific paths for your 911, but the problem is basic: the engine has to be a sealed pump and the only air entering is on purpose.

PS: I know this sounds dumb, but are you sure your Throttlebody is returning to fully closed when you release the pedal?

[Dorsh]

Thanks guys for the comments.

One thing to point - this issue existed before the replacing the air box and was noted when the car had a charging issue. Having said this one of my other cars (Dodge truck) had a bad battery and the truck idled very fast similar to this to compensate for the bad battery. Does my Porsche have a similar system?

The return spring and linkage are all good. I've replaced all bushings and springs associated with the throttle while apart and also confirmed this by running the car without the linkage connected. Is the throttle body returning to idle position? When apart the linkage would allow the valve to close so I'm assuming this isn't an issue.

I will look into the decel valve and idle screw. I agree the car was running very rich with the split air box. After one quick drive since back together it seemed to lean out so I'm not sure at this point. I will look into these items and report my findings.

Thanks again for taking the time to comment. I'm an ex AF jet mechanic so I know my way around a wrench and have two boys under two so taking this to a shop is not in my nature or an option.

Thanks again,
Kurt

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78SC

[James Brown]

Well there's your problem Kurt, I'm a Navy jet mechanic! Anyway, keep plugging away. Try to disconnect the throttle linkage just to make sure and check that CP, do you have a set of gauges? I have a set you can borrow (went to carbs, no use for them), let me know.

[Nazgul280]

Kurt,

Have you found your problem on this yet?

[T77911S]

the decel valve is a good place to start.

i was a little confused by you last post. not sure if you just miss spoke, but the air box leak will make it run lean.

the aux air valve could be an issue too. although i have not heard of any problems with them. dont get this confused with the AAR. i dont think the AAR can make the idle go that high by itself.