'86 911 Erratic/Low Idle when cold.

[AJA]

I have a '86 911 with a 120,000 miles that experiences trouble when starting cold. Turns over, but I need to give it some gas to keep the idle up. If I take my foot off the gas, the idle will be low (or very low and stall) and a bit erratic until it warms up.

The car runs and idles fine when warmed up (5-10 minutes).

I don't have much history on the car since purchasing it last summer.

Any thoughts on what to explore first? Idle Control Valve?

[BGCarrera32]

In all honesty it sounds like classic Motronic (the Bosch EFI system your car uses)...bear in mind your car is 24 years old.

First thing I would do is have a competent 911 shop check the mixture, which is adjusted via a relatively hidden 3mm hex screw located underneath the airflow meter (gray box thing that your air filter housing is mounted to). Do not mess with this without a wideband O2 sensor.

As you are in IGH the best shop I can recommend is Auto Edge in Mahtomedi, sort of North of White Bear Lake.

Auto Edge - Home

[BGCarrera32]

BTW, what have you done so far? Is this car new to you? I see this is your first post here...

[rusnak]

How long does the car need to run before it idles ok? I'm wondering if there is a vacuum leak or something that threw off the open loop mixture? It sounds like when it enters closed loop, the O2 sensor takes over and adjusts the mixture.

[AJA]

Thanks for the feedback. I really appreciate the forum, feedback, and your time.

The car is new (to me). Not much history was passed on with the car. To date, fluids have been changed and new plugs, wires, cap and rotor were added (note that issue occurred prior to plugs, wires, etc being swapped out).

The car seems to run OK after about 5 to 10 minutes from cold. If it is a warm day or the car has been running, it will fire up and idle without incident. It appears to me (admitted rookie that likely knows little to nothing) related to temperature.

Quite frankly, it is likely in need of a professional review/tune-up.

As a side note, I love the car. Ever since I was a little kid, I always wanted a 911 and sometimes these little opportunities come up between life, bills, and kids!

[techman1]

Check the idle control valve. Maybe even give it a cleaning. Easy to find remove, and clean.

[Targa Me]

You can check the idle control valve by turning on the ignition, but not starting the car. the ICV should be vibrating.
Next i would check the Cylinder Heat Temperatrue (CHT) sensor. Depending upon the temperature of the cyl head; weather stone cold or warm or hot, this sensor should have a specific resistance valve in direct relationship to the temp.
My 84 had similar symptoms and the CHT was at fault.
I would do all the simple Checks before I start messing with things like the idle mixture.
If you don't have a Bentley manual, buy one. It is a necessity if you work on your own car and interest reading even if you don't work on your own car. Oh, and you'll also need a good multi-meter.
Welcome to this site, there are a lot of peeps here that really know their stuff. This is a great place to hang out.

[BGCarrera32]

Checking idle mixture is easy-peasy with a wideband o2 placed upstream of the cat, something most enthusiasts don't have. Wear in the AFM can throw the mixture off slightly, especially after that kind of mileage and it should be checked.

The o2 sensor itself is ignored at startup for (I think) like the first 45 seconds to a minute.

A faulty CHT sensor usually makes an engine go pig rich- and it doesn't sound like that's the case here after it's warmed up, but possible.

The ICV check is easy and a good recommendation. However, it is good to blow it out with brake clean anyway. The ICV is the round cylinder thingie hanging right smack in front of the air intake boot at the top of the engine. 2 short hoses and an electrical connection. It is basically an air bypass valve.

If you don't know when the last valve adjustment was performed, then you're due for that. Not knowing what your skill level is, either purchase the 101 Projects book from this website or take it to the shop I recommend for a good lookover. (btw I am in no way affiliated with them.)

[rusnak]

Originally Posted by BGCarrera32

Checking idle mixture is easy-peasy with a wideband o2 placed upstream of the cat, something most enthusiasts don't have. Wear in the AFM can throw the mixture off slightly, especially after that kind of mileage and it should be checked.

The o2 sensor itself is ignored at startup for (I think) like the first 45 seconds to a minute.

A faulty CHT sensor usually makes an engine go pig rich- and it doesn't sound like that's the case here after it's warmed up, but possible.

The ICV check is easy and a good recommendation. However, it is good to blow it out with brake clean anyway. The ICV is the round cylinder thingie hanging right smack in front of the air intake boot at the top of the engine. 2 short hoses and an electrical connection. It is basically an air bypass valve.

If you don't know when the last valve adjustment was performed, then you're due for that. Not knowing what your skill level is, either purchase the 101 Projects book from this website or take it to the shop I recommend for a good lookover. (btw I am in no way affiliated with them.)

I agree. An air-fuel meter showing %lambda would be my logical starting point. The dme operates from a pre-programmed map until the O2 sensor heats up.

Since the car runs well after it is warmed up, the issue is not with ambient air temp, it seems to be with engine start up temp. It could be one of the temp sensors, which could be checked with an ohmeter. But I doubt it, because it sounds like the car is running ok in closed loop. The problem sounds like a open loop running problem.

Try this: remove the oil filler cap, and try to start the engine from cold. If it won't start unless you put the cap back, then the start up mixture is lean. You might find an air leak in the intake system from a cracked or missing vacuum hose, crankcase vent tube, ambient air valve hose, etc. Look at the back of the throttle for missing or deteriorated hoses. Check the cruise control hoses, and check tightness of your hose clamps. You can have many little vacuum leaks.

[CCM911]

I have had my 1984 Targa for about ten years, and it has done the same thing since the day I brought it home. One of these days, I may even replace the Cylinder Head Temp Sensor and the Idle Control Valve. I have had it out and have cleaned it, but still no vibration.

Enjoy the new car.

[AJA]

Thanks to all for the feedback...The car is currently being serviced and based upon feedback from the shop (Flat Six - Minneapolis), issue was resolved through the replacement of the idle control valve.