WolfensteinBC

I have a 97 C2S with an erratic idle problem when the car is rolling with clutch no engaged. Revs go up and down but then returns back to normal once the car stops moving.

I can't check if it's the O2 sensor as my car was in storage for some time with battery disconnected so it will be some time to cycle through the test parameters before it will show any results.

Just wondering if anyone had this experience before as it's a fairly specific idle problem.

Thanks!

pp000830

I believe the engine has a function so when one takes their foot off the gas the throttle doesn't cut off too abruptly and stall the engine. In your case it sounds like you have developed an awareness of this process and the perception that the recovery of RPM seems a little slow. I would run the OBD Codes and see if an idle control issue is listed. The responsiveness of the process and the speed at which it recovers idle may also be influenced by how cold the weather is and how warmed up the engine is.
I am not an expert so the above is just some food for thought.
Andy

nine9six

Wolfenstein,
To me, the odd key is the fact that you're idle hunts, only when the car is moving; so I'm assuming the idle is rock solid when the vehicle is stationary, and right at Porsche stated idle parameters, +/- 50 rpm, yes? Normally, I would think ignition issues, but not as you state the issue...

The issue could simply be that the ECU needs to relearn parameters, since having the battery disconnected. Give it some time and drive the car and see if issues don't work themselves out...

If, things don't work themselves out, and without knowing the mileage of the car, I would tend to think vacuum leak or lazy O2 sensors. Also don't know what kind OBD2 reader you have but mine (Actron 9850) will read O2 sensor voltages in real time, regardless of drive cycles being set or not.

earossi

+1 for recommendations made by Nine9six. It was unclear from your description if the car exhibits hunting idle when rolling to a stop AND while sitting. The ISV is relatively easy to remove and clean. Look up threads on this forum that explain it all. But, if you pulll the ISV and don''t hear a rattle when you shake the valve, then you may have a gummed up ISV which will hunt whenever the throttle is closed.

Another cause of unstable idle is a vacuum leak somewhere. There are a large number of vacuum hoses on the 993 that are now reaching an age where they are brittle and prone to leaking.. You can soap the connections to track down a vacuum leak source.

WolfensteinBC

Thank you all for the suggestions. To give a little more detail regarding my problem. Yes, the idle is rock solid at stop. No fluctuations at all.
I have cleaned the ISV just last year so it's good. One other note is that this idle fluctuation aside from only happening when in motion also only
occurs when the engine is hot. Doesn't happen after cold start. My mileage is about 160K and the top end of the engine was rebuilt about 7K miles
back. I am unable to check the OBD2 as the parameters have not been reset as of yet. Takes time for the car to cycle but I travel extensively for
work so unfortunately my car needs the battery to be disconnected frequently. The garage where I park the car does not have a power source nearby
for me to use a battery tender unfortunately.

One other unusual characteristic is the idle also tends to hunt when going in reverse. I have stalled the car a few times before and wasn't sure if
that was normal. My car also has a RS Flywheel and clutch modifications. I had thought that might be the reason for the idle fluctuations at first
when going in reverse. This had been happening for some time.

I'll try to look for the vacuum leak and look into the O2 sensors but I suppose it's best to try to run the system parameters and check the OBD codes first.

Thanks again everyone.

W

Dplus

"an erratic idle problem when the car is rolling with clutch no engaged"

Why are you rolling along out of gear?.

WolfensteinBC

When coming to a stop light. Car is slowing down and as soon as you step on the clutch the idle goes off. Especially when parallel parking.

GC96

Since you mentioned that you disconnect your battery frequently, is this problem worst after you re-connect the battery and then it slowly gets better or is it always the same? As nine9six mentioned above, the ECU does take some time to re-learn the parameters for your specific engine that will allow it to transition to idle properly. If this is the case there is a procedure that I have found helpful, see post #5 by Steve Wiener in this thread: https://rennlist.com/forums/993-foru...s-sitting.html
If you're having this problem long after you've re-connected the battery and had many cycles of re-starting the car, you might have a vacuum leak.

GC

Steve Weiner-Rennsport Systems

This happens when the battery is discontinued for some time and the MAF/throttle body loses calibration. Its one of the main reasons why keeping these cars powered up using a battery tender of some kind is so important.

I do a quick recal using my Hammer or PST-2, but it can be done with other similar scan tools (not a code reader). If you have no access to use a battery tender, this is going to be an on-going issue for you.

Just drive it and it will eventually do a self-adaptation and fix itself.

Railmaster.

You can connect another battery through the cigarette lighter outtake while the battery is avay for charging and keep the codes that way!

SwayBar

You do not need the codes to be set to read other problems.

After cleaning my SAI passages, OBD2 reported a failing O2 on the driver side, which was then fixed.

I also had a hunting idle, and OBD2 reported an issue with my throttle position switch was out of bounds, or something like that. Turns out the TPS was good, but there was an issue with my throttle cable not allowing my throttle to close all the way, thus never activating the 'throttle closed' switch. A faulty reinstallation/connecting of the cable was the root cause, and was fixed.

After doing the 15-minute drive-reset, my codes were reset, and I was able to successfully smog my car.

Get an OBD2 on it and see what it tells you.

IainM

95_993

If there is any sunlight where you store your car, you could use a solar powered battery tender.

pp000830

There are some OBD codes associated with idle do you see any?

WolfensteinBC

Thanks everyone again for all the feedback. I'm currently away again for work but when I get back I'll try to do the 15min recalibration first and do the scan. Thing is I find it very hard
to do the recal drive sequence in Los Angeles without interruption. Anyone around the area that could recommend a good place to do this? Otherwise it takes a really long time. For
me as I don't drive regularly, could be months.

She is parked in the underground parking of an apartment compound. No light and no electrical plug close by. I supposed I could buy a really long extension cord and run the cable but
you never know what other people thing might think and complain. The problems with apartment living and extended work trips.