3.2 idling intermittently

[junjun11]

Hello, this is a great community and appreciate all the input. It helped a lot on many issues.

I have an '85 cabriolet which runs very well until two days ago. After the engine warmed up, the idle went up to about 1800 RPM and then came back down to 1000 RPM then back up to 1800 RPM and down to 1000 RPM again and so on. It is idling like this since. So the car sounds like someone is stepping on the accelerator and rev it between 1800 RPM and 1000 RPM. It would only do this after the engine has warmed up.

I tried to suppress it by putting the car in gear and, without stepping on the accelerator, released the clutch and the car would accelerate until RPM reaches 1800, then it would decelerate to 1000 RPM, and would accelerate again til it reaches 1800 RPM. It would do this no matter what gear i was on.

But the car runs fine above 1800 RPM and on the highways, no issues. unless engine speed drops down to 1800 RPM. So you can imagine what a pain to drive it in traffic.

My mechanic says it is a fuel pressure or fuel damper issue??? But i am wondering why it is more relative to the engine speed (goes down at 1800 RPM and goes up at 1000 RPM) rather than inconsistent idling.

Any input would be highly appreciated. Many thanks.

Joe

[ischmitz]

There can be several reasons for what you see. First, check that the idle micro-switch properly engages when the throttle is closed. With the engine off actuate the throttle in the engine bay and listen carefully. There should be an audible click when the throttle returns to idle. If you have to pull the throttle against its stop to make the switch close either the linkage or the switch needs adjustment.

Your mechanic thinks it is mixture-related and the fuel pressure in the rails is one thing that affects mixture. You could have a bad fuel pressure regulator.

I just fixed a 3.6 (very similar Motronic) with the same symptons yesterday. It turned out to be the O2 sensor. It was old and too slow in its response. As a result the idle walked between 600 and 1500 RPM with a 2 second interval. The symptom showed up a couple of weeks ago from one day to the next according to the owner.

Ingo

[86 911 Targa]

Along with the previous post, check these vaccum lines.

[junjun11]

Wow, thanks guys, that narrowed down the possible causes. Takes out a lot of guessing work on my side. I'll make sure to have an update about this when all goes well.
Joe

[junjun11]

Hi, checked all the lines and the regulator. All seems fine. Turns out to be the idle valve control. Cleaned it with Brake cleaner... psst, psst, now idling normally. Thank you very much.

[DRACO A5OG]

Congrats,

But I am concerned that it is reading 1000 RPMs. Is your baby chipped or Stock?

85 should be set at 780-800 RPMs if chipped 880-900.

I would check the OHMs on that ICV to make sure it won't fail later down the road.

[junjun11]

Draco,

now it is idling normally at 800-850. I am running stock chip.

Now, i have another problem that has arised: When the engine is cold, the car would start and idle fine. When i step on the accelerator and release it suddenly, the engine would die, unless i release it really, really slowly. Once the engine warms up, then everything is fine.

Before, i noticed that the rpm would go up to about 1,100 until it warms up, then comes back down to 800-850 rpm. Oh by the way, if i turn on the A/C, same problem: engine dies after sudden release of accelerator.

So basically have this problem only when (a)the engine is cold and; (b) A/C is on, compressor running.

Does the 3.2 have an "idle up" control for the A/C? I'm guessing it does because i didn't have problem with it before. I also cleaned the MAF sensor with brake clean, but no effect. A/F ratio maybe??? Vacuum leak?? checked the lines, all seems fine.

Thanks.

[techman1]

The symptoms you relate sounds like the Idle Speed Control valve you cleaned is the culprit. It is responsible for regulating the idle speed, just as it sounds.

The valve should open and close, relating to engine load ( air con., cold engine, warm engine, lights, etc.) to keep your engine at a preset idle speed. It gets its signal from the DME (engine motronic brain) to open or close.

Check the connection to the idle speed valve. Maybe not fully plugged in, or a loose wire.
It is possible your cleaning did loosen it internally, allowing it to close some, which dropped your engine speed down where it should be, when warm. The only thing is, when cold, it needs more air, so the valve should open to let more in. The valve may be shot, not allowing it to adjust.

In a nutshell:
1. Bad idle speed valve.
2. Bad wiring from DME to valve.
3. Bad idle circuit on DME

Shade tree mechanic advice, comes with no warranty.

[ischmitz]

Is the ICV vibrating with the ignition ON but the engine not running. You should be able to feel a 80 Hz vibration with little jerks every second when you touch it. That is a quick test and tells you whether the wiring harness and the DME are O.K.

Ingo

[techman1]

Ingo is the "Man" when it comes to this!
Love your latest diagnostic tool, the DME tester!

Neal

[redranger]

Originally Posted by junjun11

Now, i have another problem that has arised: When the engine is cold, the car would start and idle fine. When i step on the accelerator and release it suddenly, the engine would die, unless i release it really, really slowly. Once the engine warms up, then everything is fine.

My 86 911 does the exact same thing. When I start it cold and back it out of the garage, the rpms will drop sharply and the car will sometimes stall (w/o the A/C on). If I let the car warm up for about 2 minutes until rpms at idle drop from 1100 to it's normal level of 700-800, it acts a lot better.

I've had my ICV cleaned and it is in working order.

Anyone have any thoughts on this?

[rcaradimos]

It could be just a touch to rich, check CO setting

[junjun11]

Originally Posted by ischmitz

Is the ICV vibrating with the ignition ON but the engine not running. You should be able to feel a 80 Hz vibration with little jerks every second when you touch it. That is a quick test and tells you whether the wiring harness and the DME are O.K.

Ingo

Hi i turned the key to the "on" position, without starting the engine. I tried to feel for a vibration but didn't feel any. So i listened little closer and did hear a faint whizzing sound.

Should i really be able to "feel" the vibration? Or is the faint whizzing sound telling me that i should check on the lines to the DME? Loose connection maybe?

By the way, my engine is a euro version so i don't have an O2 sensor, just in case this small info helps.

Thank you guys for all your help.

Joe

[techman1]

Joe,

If you take the valve off, I believe you can look inside it and see the orientation of the valve. If you reach in with a screwdriver, you can actually rotate the valve.
Try this- go ahead and move the valve. Look closely at the location, and then carefully re-install it. Start your car, the idle should be off, either high or low, depending which way you moved the valve. The system should quickly adjust, so fast you should not notice. If this doesn't happen, take the valve off and look inside. Did it move? If not, for some reason it is not adjusting like it should.

This caution- be prepared to rotate the valve back to the original setting, so your car will at least run like before. (if it is not working) until you resolve the problem.

[carreraken]

Mine used to do this years ago - intermittently. I believe my shop adjusted the mixture and it went away...

[toysRus]

Hi Guys! i was just reading this post and i have the same problem with my car dying with the AC on. The car dies out on cold as well without the AC running. Sorry if i sound like a moron but does anyone have a picture that show where this idle valve is located? and what is a DME?

I just recently acquired a 1979 911 SC with the 930 slantnose kit. my friend used to own it and before selling it to me he had a 3.2L engine installed. I ordered the Bentley shop manual and i was going to wait and find these parts you all are referencing but i can't wait!

Hi Joe, have you figured out the problem with your car?

thanks everyone in advance!
bobby

[junjun11]

Hello again guys, a friend of mine dropped by today and brought his 1986 coupe.

I lent the MAF, idle switch and ICV from his car and installed into mine. the idling problem didn't go away. So is it safe to say that none of the 3 components are the problem?

However, when I used his DME to my car, it idled smoothly, no more dead engine, hot or cold, a/c on. What gives? Do i have a defective DME? If so, wouldn't my engine not start at all?

By the way, his car is a US version car (with O2 sensor), my car is a euro spec. So, US version DME into a EURO spec engine. Is it mixture problem? because i assume, with the US DME, it is not reading any O2 sensor signal from the EURO engine, hence the US DME is making rich mixture which eliminated the problem.

Thank you guys.

[twistoffat]

The DME is the electronic manage system that controls all the engine electronics in your car. Its a little box located underneath the seat which contains the ECU or "Chip". The Idle sensor if you look at 86 911 Tagrsīs post above is part number 29

[twistoffat]

engine not start at all?

By the way, his car is a US version car (with O2 sensor), my car is a euro spec. So, US version DME into a EURO spec engine. Is it mixture problem? because i assume, with the US DME, it is not reading any O2 sensor signal from the EURO engine, hence the US DME is making rich mixture which eliminated the problem.

Thank you guys.[/QUOTE]

It sounds like it could be the DME but I am surprised that it wasnt there before. Who long did you wait before trying your friends DME. Could it be that by the time you tried it your engine had warmed up sufficiently to negate the original MAF problem which wasnīt evident anyway when the motor was sufficiently warm. The Motronic systems are pretty reliable and fail safe. When something goes wrong it is usually one of the sensors. It could be a relay from the DME to the fuel pump.

[JJ 911SC]

junjun11


Attach to the DME harness, do you have a "floating" brown wire that may be connected together or not?

If so, if connected, try it disconnected or vice versa.

It's something to do with California Emission but mine was plug in (and it should not) indication that my 3.2. transplant probably came from a CA car.

Maybe sending you on a tangent... but it's worth the try.