Just a quick bump to see if any new thoughts with additional info

thanks

I don't have anything helpful to add, but I will say that my new-to-me 86 targa is doing the idle surge too, when at operating temp. One thing I noticed is that if I turn the engine off and then turn it on again, it's fine until I give it some throttle. As soon as I release the accelerator pedal, the surging begins and won't stop until I turn the engine off again.

This is a great thread, full of useful info for troubleshooting. Thanks to all who contributed.

One thing I found helpful was the big improvement that happened when I disconnected the O2 sensor - the surge goes away, or is reduced to a level that isn't embarrassing at stop lights.

I didn't get to drive it today, but as of last night I'm feeling like the idle adjustment may have corrected the problem. I will drive it to work tomorrow I think, to see how it starts and drives through the warm up period.

Good luck to you!

Disconnecting it gets rid of the bad readings being transmitted to your DME....the system needs a working 02 sensor for optimal performance, air/fuel ratio and ultimately better mpg. No 02, the system defaults to less than optimal load. You could be running rich or lean. Rich, you foul plugs.....lean you burn pistons.

It WILL run with the 02 unplugged....just not as well as it should....for awhile and won't pass a smog check if you have one in your area.

I think Ingo mentioned that in the L-Jetronic/ Motronic DME, there is no "default" map. He seemed to be saying that the DME will continue to operate on a single map even without the O2 sensor. That means that it's even more critical to have a good O2 sensor, although with a lack of voltage it would be operating at the most rich part of the range possible.

Craig, there is one other thing that you might have a qualified mechanic check, and that is the base CO% mixture. I think it would be doing you a dis-service to even mention it, as I do not know if you are up to speed on the basic idle mechanism. Basically, there is a way to bleed air past the air flow meter with a screw. But you must have a wideband air fuel ratio meter hooked up and running. Also you must be aware of the risks of operating too far outside of the safe stoiciometric range of the ideal air fuel ratio, or "Lambda".

Craig - so to clarify (for me), did the idle speed adjustment solve your problem? My '86 idles at a constant rpm when cold at just under 1000 rpm. After I drive for a while the idle surges between 1000ish to and about 1150. The weird thing is that if I am coming to a red light and disengage the clutch at about 1150 rpm fairly often the idle seems settle at about 1000 rpm (still high) and doesn't surge. If I disengage the clutch at a slightly higher rpm (say 1250 or 1300) the surging at idle is much more common. Undoubtedly I need the idle speed reduced but I hope it also cures the surging. My car is new to me (my first Porsche) and I love driving it but I know I have a lot to learn.

Cheers

My '84 exhibited some of these idle qualities as well. New O2 sensor, new intake gaskets and spacers (BIG improvement) and unplugged "brown wire" to the DME. (Brown wire for California emissions--unplug to slightly richen the mixture.) The biggest improvement so far has been changing the intake gaskets and spacers and the car idles very smoothly at about 950-1000 cold start-up and settles down to 800-ish as it warms up.

My surging idle in my 85 was rectified with a new 3 wire O2 sensor...This was after cleaning the ICV, operationally checking the WUR, and assuring no manifold leaks.

I recently acquired an 87 3.2 and it was running rich. It would stumble when taking off in 1st gear and when warm, it would hunt at idle. When backing out of the garage it would easily stall while engaging the clutch.

Replaced the air flow meter with a spare one and all is well, or rather, she goes like he** now.


Cheers,

Joe

Hunting Idle
 

I just purchased an 84, and have been searching the threads for information. The previous owner kept excellent maintenance notes on the car, and the engine is showing 5% leak down at 131,000 miles and runs strong. While driving in slow, hot, city traffic, I experienced the hunting idle that folks are talking about. As there is no mention of the O2sensor being replaced, I figure it's probably time to do it while also checking for air leaks, clean the ice, etc.

Are people happy with the universal Bosch O2 sensor, or the 86 Ford Taurus sensor, or should I spring for the overpriced Porsche one? My other Porsches all run Weber carbs, so all this DME stuff is new to me. However, in the land of ethanol contaminated fuel (which my manual says I should try to avoid), I feel that the DME system should be a better running system when working as intended.

Thanks for your advice and motor in good health!

The O2 sensor is a 60,000-mile replacement item so if the records don't show it being done then now's a good time. Be careful pulling the connector apart up top, you don't want to break the male end on the harness. (At least one thread on that exists here.) I think I went with the OEM Bosch unit from Pelican when I did mine at 60K.