Randall G.

Has anyone seen this? A few weeks ago, I noticed my engine idling about 150-200
RPM high. The engine normally idles at 880 RPM.

The idle is not hunting. It's just high.

I've tried disconnecting the idle stabilizer--no change whatsoever. I also
considered that perhaps the CCU was sending a "permanent" AC-is-on signal to
the engine. But, I believe the signal goes to the stabilizer, and disconnecting the
stabilizer has no effect on the idle. Also, an AC-on signal would not raise the idle
so much.

Oh, and FWIW, turning on the A/C has no effect on the idle speed.

I had the car at the shop today, and they were unable to find anything amiss. To
get a full diagnosis, it looks like I'll need to take the car to Protomotive (a bit of a
drive), where Todd Knighton has fashioned an adapter that allows his OBDII
computer to communicate through my pre-OBDII interface.

Thanks in advance ...

Bill Wagner

I've seen problems like that occur on cars with something wrong in the engine temp sensor circuitry. The cars think the temp of the engine is always cool and keep the car idling high when malfunctioning.

I don't know if that info is of any value to you since you have a 993 engine with what I assume is a completely different engine management system.

In any case, Good Luck,

Bill Wagner

Randall G.

Hi Bill,

You may be on to something. I believe the 993 uses the same temp. sensor arrangement as the 964.

Now, I need to figure out which sensor tells the DME how hot the engine is, where it is located, and how to test it. This is going to be a lot easier in a few days, when my shop manuals arrive ...

Thanks ....

MikeF

Randall,

Please post your results. I have the same situation with my 94 964. It's always idled around 1000-1050. Car runs great otherwise and my mechanic couldn't find any problems.

Thanks,

Bill Wagner

FWIW, the temperature sensor is located in cylinder number 3.

Here's a cut-n-paste from the AllData instructions on how to replace it:

Service and Repair

1.Remove heated air elbow.
2.Remove speed/reference/temp II sensor connector retainer.
3.Disconnect the temp II electrical connector.
4.Remove wire and cover grommets near engine speed and reference sensors.
5.Pull speed/reference sensor wires out of grommet.
6.Remove temperature sensor from engine.
7.Reverse procedure to install, applying a light coat of high temperature anti-seize compound (i.e. MolyKote HTP).

I don't know if it's the same on a 993 and a 964 engine. I'd check the wiring and test the sensor before replacing it.

Cheers

Bill Wagner

Adrian

Dear Mike,
The most common problem of a high idle (is not adjustable) is the idle microswitch on the throttle body. I would check this to make sure at idle
a). It is being activated to the closed position and
b). Make sure the microswitch is actually making a short circuit.
The other potential problem is the idle control valve is not in the idle position. It may be stuck and this will cause a high rpm. A calibrated tap sometimes solves this problem.
Please remember that the DME controls the idle 100%. It get a heap of inputs but the most important is the idle microswitch. It then says to itself, yes I am at idle and kicks in the idle mapping which sets the idle control valve servo motor.
Ciao,
Adrian
911C4

PS: Th 993 and 964 systems are different. Plus hybrid installations are different again.

Randall G.

Some more information to add to the discussion ....

I started my car up cold twice over the weekend, and each time I paid close attention to the idle's behavior:

-Initially jumped to about 1200 RPM, stayed there for less than 10s.
-Dropped to about 1050 RRM.
-Once warmed up, the idle does not change from the initial cold start value.

This seems to suggest the idle is staying in the "cold-start" mode?? Perhaps someone with a "normal" idle can verify what the cold start sequence is? I'm guessing is should be:

-Initially jumps to about 1200 RPM for a few seconds.
-Drops to around 1k RPM.
-Drops to normal idle speed after the car is warmed up sufficiently.

I still plan on looking into the microswitch tonight, assuming the 993 engine has one. It certainly has an idle stabilizer valve (I think you're calling this an idle control valve, right Andrian?). Disconnecting the idle stabilizer had no effect on my idle speed. I also banged on it for good measure--no help.

Adrian, good to hear from you. Any projection on the publishing date for your book? It's going to make very interesting reading .

MikeF

Adrian,

Thanks! I will check the microswitch at lunchtime today (assuming I can find it). Boy, it would be nice if it was that easy. Thanks also for the e-mail regarding 18" vs 17" wheels on a 964. I will take your advice and stick with the 17's.

Kind regards,

Adrian

Dear Mike,
You cannot miss the microswitch. Pop open the engine lid. Get somebody to put their foot on the gas pedal (as you guys call it) and you will see the throttle mechanism move. As the throttle moves away from the idle psotion you should hear a click as the microswitch releases. You can see the microswitches on the throttle body.
Dear Randall,
I have cut back some involvement in list and board involvements to spend more time on the book. The book is finished in its rough draft form. I am now filling in details and editing. I hope to be finished this process by the end of this month.
To your idle issue. I can simulate this happening two way. One is to have foot on the gas pedal before I start or if the outside air temp is VERY cold. I have no data on this but I am sure there is a cold weather map programmed into the DME. The reason is because in winter people have everything burning and turning and this loads up the engine, like the aircon in summer. The engine temp sensor plays a major role in this. During winter my idle is around 1000rpm for sometime. I will check over winter,
Ciao,
Adrian
911C4