Very Fast Idling problems

[Rocc]

I'm having problems setting idle on my 2.7 CIS system. The problem is that it works the exact opposite of a choke, when I first start the car it idles fine at about 1000 rpm but as the car warms up the idle goes up as well, sometimes as high as 3000 rpm.
Now if I adjust the idle screw down to about 1000 rpm idle, when I start the car cool it won�t run it just cuts off , so I adjust the Idle screw back up it runs fine until it warms up again. �See where I�m going with this�. Oh yea, when I adjust the idle screw down I lose power and the get up and go it normally have when the idle screw it turned up before the engine warns up and the idle goes out of control.
Could it be :
A. Bad cold start valve, and I�m running the idle screw to high so It runs good on cold starts.
B. Or the auxiliary air valve going bad letting to much air in as it warms up.
C. Or None of the above.

[Paulporsche]

Rocc,
Welcome and be sure to post some pictures of your car when you can.

Usually, if your engine starts, the cold start valve is working. It sounds like your WUR is out of spec or not working and someone has raised your idle to compensate. It could also be your AAR or possibly your decel valve. It could also be a vacuum leak anywhere along the intake system. If you search these items you will see a lot of info/help on this. It would also be a good idea to get a Bentley manual for the SC, which will still have a huge amount of pertinent info for you. And lastly, you should probably get a CIS fuel pressure gauge which will help you check your WUR (warmup regulator) and a long 3mm allen wrench for adjusting the fuel mixture.

If you are not doing this yourself, your mechanic should check your control pressures and CO, and look for air leaks. These are the most critical areas of concern for setting up CIS cars to run correctly.

[Rocc]

Hey, thanks for the reply,
I don't think it's a vacuum leak because a vacuum leak would be constant fast idle.
The problem I have is that the idle may float or creep up from the time I start the car. It seems sometimes the faster I drive the car the faster the idle may get, like right off the freeway the idle would get as hight as 3000 RPMs, like the throttle is stuck. Or just cruising the hood it may idle as high as 1700 to 2000 RPMs. Ether way when I leave home the car idles perfect at about 900 to 1000 RPMs ?????
We've checked Timing...OK
vacuum lines...OK
Fuel mixture...OK
Adjusted idle screw so tight I've lost power

[Kirk911SC]

Was this problem ever resolved? My '82SC has started showing similar problems.

Kirk

[Crowbob]

I richend the mixture about 1/8 turn (clockwise), then reduced the idle until I got around 900 RPMs.

[Kirk911SC]

Mixture is 3.5% CO - set with analog Gunson.

Kirk

[HawgRyder]

I have the same prob with my MFI setup....I'm starting to think the distributor is not returning to rest position (advance sticking).
Bob

[Kirk911SC]

My idle starts out around 1000 rpm, will climb to 1800-2000 rpm after a highway run after idling for about 5 minutes (like when I get in the coffee drive-thru lineup in the morning after commuting) and will stay there. Always nice to pull up to the window to pay with the engine racing and the SSIs drowning out any conversation! After the car is shutdown for 20 minutes or more and restarted the idle is very low (have to feather the gas to keep it from stalling). Eventually with some driving the idle will return to around 1000 rpm, and...............eventually will race again!

Kirk

[azasadny]

My '74 does the same thing. It idles at 1000rpm when cold, them promptly goes to 2000rpm after it's warmed up. I've replaced almost every component (WUR, cold start valve, vacuum hoses, etc...) and the problem is still with me. I'm getting used to it as I've had the car over 3 years, but I'd like to find out what the cause of this anomaly is. I have a throttle microswitch that was bad and replaced, the idle was good (1000rpm) for ahwile, then the replacement throttle microswitch "died" and the problem returned.

[Kirk911SC]

Could a leaky oil breather hose cause this problem. My car has an oil leak coming from the rear top of the engine (I'm guessing the oil breather hose). Heats up, expands, vacuum leak, idle increases?

I haven't had a chance to investigate the hose yet.

Kirk

[Mysterytrain]

If you ride the clutch will it bring the idle down? Could be throttle linkage hanging up. On 78 and later cars you might have an air leak. The AAR should be fully closed when the car is hot. Easy test...pull of the hose that is connected to it and use a mirror and flashlight to see if its closed. If it is not then you need to try to clean and lube it. Another problem can be the Air sensor plate hanging up. Ck to see that it floats freely and is not binding in the housing. You might want to clean the housing, the arm, the plate and the plunger with some carb cleaner. The plunger can also hang.

[Kirk911SC]

Linkage is not hanging. The arm, plunger, plate, etc. are all clean, and move freely. RPM does not hang after high rpm operation, it creeps up from ~1000 rpm and will stay there.

I have not checked the AAR yet, but it would cause a constant high idle, and not one that creeps up, correct??

Kirk

[Kirk911SC]

Just an update.

I recently checked my AAR for proper opening and closing. After removing it and placing it in the freezer, the valve did appear to open more than it was at ambient temp. Electrical specd at exactly 33 ohms, and with voltage applied the valve will close entirely.

Checked voltage to AAR and found 0! After tracing the wiring, and some head scratching, a broken wire was found IN the WUR harness connector. Spliced the wire, voila, 12 volts to AAR.

Idle now starts off at ~1400 rpm and settles down to 1000 rpm in short order. One problem solved!

I still have the increasing idle problem though after driving for some time. Idle always seems to come up after idling for some time. Heat soak of some part causing vacuum leak?

WUR pressures arew within spec, and CO set to 3.0% with Gunson analog gas tester (still not sure about it's accuracy...........recent Ebay purchase).

Ideas?

Kirk

[smestas]

Rocc,

What was the outcome of your problem? Was it the AAR? My car is suffering from the same exact symptoms and Im trying to get a good idea of where the source of the problem might be.

[Lorenfb]

As many, e.g. Paul, have indicated, the mostly likely problem is the AAR.
This unit provides an air bypass to increase the idle when the engine is
cold. It functions by a shutter which varies the amount of bypass air.
The shutter is controlled by a spring which is heated by a resistive element.
This heater element is powered by the same voltage as the WUR and
receives ~ 12 volts when the fuel pump is activated the result of the
fuel distributor sensor plate movement.

[sballard]

I am embarassed to admit that I had a problem very similar to this on my '72T. I started the car and revved it a few times then noticed that one of the spark plug wires was interfering with the throttle linkage. When I tied it down with a tie, no more problem. See, those wire holders on the MFI cars realy do have an important function. I'd definitely check this first before re-tuning.

Steve B.

1972 911T

[coldstart]

I also had a similar problem with 72 911T. It turned out that the accelerator pedal bushing was shot and the pedal was not returning to fully upright. A very simple (and cheap!) check would be to attempt to pull upwards lightly on the accelator to confirm that the pedal is not sticking. Sounds silly, but that was the simple problem with my high idle....

[911S Targa]

Good post. (even though I replied to it this morning, but my post is gone ??????)

Anyway, mine starts up, sputters for around 2 seconds, then goes to normal idle. Once I hit engine temps of 120/140, my revs go up, and my tach gets real nutty, going as far as 4500, but usually jumps up and down around 2500/3500, but I know the eng isnt reving that high, just by listening to it. It did this problem for about 3 weeks, then went completly away. Last week, it returned.

Thanks for the thread, now I have a direction to take, since we have the same motor.

[Brett San diego]

My car is a 1978 911SC, all stock. I have a fast idle, of which I've slowly been ruling out components as the root cause. The whole story... Did valve adjustment and had mixture and timing set several months before fast idle developed. Idling at 950-1000 rpm and running beautifully. One day, the idle simply stayed fast after a drive. Now, cold start up is normal (~2000 rpm). As it warms, the idle will drop to about 1200-1400. As it reaches full operating temp, the idle will slowly increase again to 2000 rpm.

So far, AAR is good. Pulled and plugged the hose to the AAR while engine was running, and there was no change in the fast idle. Turned engine off and used mirror to look through the AAR tube. It was visibly closed. Allowed the engine to cool, and the valve had fully opened. Checked fuel pressures. Primary pressure, 70 psi. Cold control pressure, 22 psi. Warm running engine control pressure, 47 psi. Heating element in WUR is also fine. I wiped out the intake venturi and sensor plate as best as I could. I don't think the sensor plate is sticking. It seems to move fine. Most recently, I pulled and plugged the control vacuum line on my deceleration valve. No change with the fast idle.

I now suspect the problem is the auxiliary air valve (AAV). I believe, it's not closing completely, and upon warming up with the engine, it is slowly opening more accounting for the slowly increasing idle.

This seems to be the same fast idle problem that the original poster, Rocc, described, so I figured I would resurrect this thread with my thoughts. I'll post my results with a new AAV when I get around to swapping it out. It will probably be a while, though. I'm about to drop the engine to do Carrera tensioners, some maintenance and clean-up, and I will do the AAV at the same time.

Brett

[Mysterytrain]

You might want to check out your distributor. If the advance is hanging up the rev's will hang too. The plate needs to move freely. As me how I know..78SC also!