Surging Idle '88 911
 

Hello - I have a surging cold idle from 1000 to 1200 RPM when cold starting. I checked for vacuum leaks doing a smoke test and couldn't find any. I then swapped the ICV from another 911 3.2 that runs fine and also cleaned and tested the original one to make sure it was working with no problems, but the surging persists.

If I jumper B / C the idle doesn't surge. I have checked the CHT and it ohms within spec. I have also done a ohm test on the 2 pin to 5 pin on the DME connector.

Any thoughts on what I should trouble shoot next? I have another 3.2 that I could swap parts from that is running fine. Should I try switching the DME or the DME Relay?

I've also posted this on rennlist, but not much help so far.

Thanks

This puts the DME into the idle screw adjustment mode, i.e. no idle valve control.

You mean here; https://rennlist.com/forums/911-forum/1384321-help-fixing-surging-cold-idle-on-88-911-3-2-a.html

Testing with a known good DME sounds like the next step.

Also, read here; Idle Valve Testing, post #20, http://forums.pelicanparts.com/porsche-911-technical-forum/360766-bench-testing-carrera-idle-control-valve.html

My 89 did that also. I think they all do a little searching for correct idle. I would swap the DME for a Focus 9 Technology unit. They are solid state and helped mine greatly. Cleaning the ICV with a little brake clean can’t hurt either.

Will have NO effect! Let's avoid guesting & wasting money by "throwing parts" at the problem.

Please explain. Mine ran much smoother with the Focus 9 DME. It was even recommended by my Indy.

It should NOT effect the engine's running. It's just a power switch (another relay)! Did you re-install your original DME relay?

Listen to Dave. If it ran differently after swapping the DME, something else changed. Try swapping back.

It surged with my old DME! I have seen many threads about keeping an extra DME in your car because they are prone to go bad. Every thread I have read about surging mentions changing the DME……

If you do any swapping, try swapping the idle control valve to see if it makes any difference. Sounds like the idle needs to be set (I would do this first). Take the ICV out of the loop using jumper wires, then adjust the idle to correct settings. Remove the jumper wire & see how it does.

Where?

When testing for a potentially bad DME ECM with another good running ECM, always consider using the potentially bad ECM with the good running engine.

Sorry. Maybe I confused intermittent idle surging with intermittent cut out of the engine.

Yes, that's the typical reason for a new DME relay. Although, some have just properly re-soldered their DME relay, and it has been reliable for years.

So far I have swapped the ICV - no change, base adjusted the idle - no change, swapped DME ECM - no change. Took off ICV, closed valve and kept it disconnected, idled low and the once connected, surge came back.

ICV and DME seem ok, and it doesn’t appear to be a vacuum leak. What should I try next knowing I have a spare everything on another working 911 3.2?

How low? Should be 400-500 at most. If not, you have an air leak.

Did you swap out the AFM and test? Possibly the "barn door" tracks are worn and or the Air temp sensor is faulty.

Curious, what happens when you unscrew, remove the Oil filler cap with it running? Does it pull a vacuum? Any changes with RPMs? Does the engine stall or does it generate larger swing in RPM?

FWIW, check your flywheel speed and reference sensors. These tend to get gummed up with dirt, oil and fine metal shavings. You'd be surprised. Remove, wipe and blast them with some brake cleaner.

Good luck.

I am a broken record, but recommend checking your FPR and Damper as well.

I didn't notice any changes in RPMs with the oil filler cap off, which was a surprise. I bypassed the ICV with the engine cold and set the idle then. Should i set the idle when the engine is warm?

I'll check the flywheel speed and reference sensors. I haven't done that yet.

I have not swapped out the AFM. I'll try that as well.

My car had a horrible idle for a year. Went all over the place. As soon as the bad intake gasket was replaced, idle went right back to normal. No kidding.