911 3.2 Motronic Question(s)

[johnman001]

One of my cars has a stock 1986 3.2 engine. I believe the DME is stock, but I haven't had it open to see if there is an aftermarket chip or not.

The engine was completely rebuilt stock and runs well. My only remaining item that I haven't been able to figure out is that this 3.2 does not quite act like my other 3.2's.

Idle is rock solid, and the idle microswitch seems to be adjusted and functioning.

The issue that I am having is at very light throttle (during the motion where the microswitch is opened and before the throttle butterfly opens), the RPM's climb and hold at approximately 2,000 RPM.

It seems as if the timing becomes advanced slightly and/or the mixture MAY be weak (lean) until the throttle plate actually begins to open. I mention that it MAY be slightly lean, only because when idling around a parking lot or driveway, the car tends to surge until the throttle plate actually begins to open. (I can feel the difference between the slack being taken out of the linkage and when the primary throttle return spring takes over.)

This seems to me more of a nuisance than a real issue, but it is annoying, and like I mention, this only occurs on this 3.2 and not my others.

I can find no vacuum leaks or other oddities, and otherwise, the car runs very strong.

Any suggestions would be greatly appreciated.

[gomezoneill]

ICV sticking?

[johnman001]

Originally Posted by gomezoneill

ICV sticking?

I don't believe this to be the case since it doesn't "hunt" when the throttle is closed. It drops almost immediately to 880 or so RPM and remains there until the linkage lifts off of the microswitch at which point the RPM rises to approximately 2,000 RPM.

When I have had a sticking ICV on the other 3.2's, the idle RPM undershoots-overshoots for some time before stabilizing (sometimes never stabilizing).

[Trakrat]

This is probably a dumb question... but where is your throttle cable?

Reason I ask is because mine attaches to the heater plastic part that goes down the hole on the left side... You don't have the heater on here...

[johnman001]

Originally Posted by Trakrat

This is probably a dumb question... but where is your throttle cable?

Reason I ask is because mine attaches to the heater plastic part that goes down the hole on the left side... You don't have the heater on here...

911's don't seem to call them throttle cables. I believe there may be a "cable" that is used with cruise control that this car does not have.

The throttle linkage starts at the butterfly throttle plate and is a split design with a helper spring (this linkage activates the idle microswitch). The secondary linkage at the butterfly throttle plate is spring loaded and allows the throttle pedal to actuate the microswitch without opening the butterfly throttle plate.

The throttle pedal is connected to a rod that extends back through the center tunnel and ends in a rubber connector at the firewall and then extends outward to the bel crank located on the transmission.

The bel crank at the transmission has a rod that extends to a second bel crank located at the top of the motor. The second bel-crank located on top of the motor has another bent rod that actually actuates the split linkage at the butterfly throttle plate.

So, to make a long story short, to the best of my knowledge, 911's prior to 1989 had no throttle cables unless one would count a cruise control cable as a throttle cable.

[Tippy]

2000 is high, mine did 1300-1400 once you cracked the idle switch

[Tippy]

Try a timing light to see the jump.

[steely]

Check your base idle setting with the ICV test connector pins B & C jumpered out. The test connector is on the left of the eng bay by the coil.

[johnman001]

I'll try setting the base idle with pins B&C jumpered together. The idle is so nice, that I hadn't even considered checking that lately, but will tonight.

I will put a timing light on it to see if it does indeed jump when the throttle just lifts off of the microswitch.

At least these are a couple of items that I can check and if one of them resolves the nuisance, it's one more item to check off the list of small things to be done on the car.

Please keep ideas coming. Perhaps it's something simple that I keep overlooking.

Thanks!

[Tippy]

What's the idle timing and post idle switch timing?

[911 sc]

What about a worn afm track? Try reposition the arm for a new set of tracks.

[johnman001]

Originally Posted by 911 sc

What about a worn afm track? Try reposition the arm for a new set of tracks.

The car shows no signs of stumble anywhere through the RPM range. The only symptom is that the car revs to 2,000 RPM when pressure is removed from the idle microswitch (I believe this opens the circuit).


Testing the AFM with a battery, showed no signs of jumpyness or any voltages out of the ordinary. The voltage smoothly rises as the flapper is opened.

[gomezoneill]

With an engine compartment that nice you gotta fix it.

[mysocal911]

Originally Posted by johnman001

The car shows no signs of stumble anywhere through the RPM range. The only symptom is that the car revs to 2,000 RPM when pressure is removed from the idle microswitch (I believe this opens the circuit).

Disconnect the idle valve to determine whether it's related to the idle circuit in the DME ECM.
It may idle a little lower, but for test proposes it may help isolate the problem.

[steely]

Originally Posted by johnman001

The car shows no signs of stumble anywhere through the RPM range. The only symptom is that the car revs to 2,000 RPM when pressure is removed from the idle microswitch (I believe this opens the circuit).

Yes, opening the micro switch tells the DME your no longer in idle position so there is no need for it to control the ICV. (Bentley 2xx Eng Mgmnt section somewhere).

Doing the jumper test is another way to tell the DME chip to stop controlling the ICV, without messing with your linkage.

Disconnecting at the ICV will certainly stop it from working, but may leave the valve in an unknown position, possibly affecting the base idle. Who knows maybe it defaults to 50%.

Since you mentioned the idle is fine until the idle switch is off, I am wondering if the resulting increase in idle you are experiencing is the base idle (which is set hi).

[johnman001]

Originally Posted by steely

Check your base idle setting with the ICV test connector pins B & C jumpered out. The test connector is on the left of the eng bay by the coil.

DING, DING, DING!

We have a winner folks. Got home from work and perused the WSM and set base idle with pins B-C jumpered together. Had to turn bypass screw in a bit to bring the idle down to the 850-880 range (with pins B&C jumpered out).

Problem resolved. Now when the microswitch is opened (butterfly just about ready to open), the RPMs rise just about 500 RPM

[mysocal911]

Originally Posted by johnman001

Had to turn bypass screw in a bit to bring the idle down to the 850-880 range (with pins B&C jumpered out).

So who was messing with the idle bypass screw to cause the problem, i.e. it takes more than "a bit"
of turning to cause the engine to rev to 2000 once the idle valve has no more control? Disconnecting the
idle valve would have immediately indicated your problem. Turning the ignition switch "on" and not
starting the engine would have placed the idle valve in its center position (no mod to engine idle).
So when the idle valve was disconnected and then the engine started, the very high idle (2000 RPM)
would have been immediately noticed indicating excessive air bypassing the throttle plate.

[johnman001]

Originally Posted by mysocal911

So who was messing with the idle bypass screw to cause the problem, i.e. it takes more than "a bit"
of turning to cause the engine to rev to 2000 once the idle valve has no more control? Disconnecting the
idle valve would have immediately indicated your problem. Turning the ignition switch "on" and not
starting the engine would have placed the idle valve in its center position (no mod to engine idle).
So when the idle valve was disconnected and then the engine started, the very high idle (2000 RPM)
would have been immediately noticed indicating excessive air bypassing the throttle plate.

It really didn't take much to turn the idle bypass screw in to drop the idle down into the upper 800 RPM range.

Setting the base idle procedure in effect does disconnect the ICV, so in a way that was part of it. By inserting the jumper, the ICV becomes inoperable and one can adjust idle with the air bypass screw. (and yes, it became immediately obvious when using the WSM procedure)

The only one I have to blame is myself. Being the only one to really work on the car. I don't recall having to mess with anything to get it started after I completely rebuilt and transplanted the engine into this 1974 chassis. The other 3.2's that I swapped into 914's were a bit more effort (possibly because they weren't completely rebuilt fresh engines).


Thanks to all for your help.

[jamesrg]

thanks for this thread, I had a similar issue and was able to tighten up my return spring and spray some electronic cleaner on the micro switch and my surging to 2K RPMS after shifting into neutral seems to have been fixed. Fingers crossed need to test it more but appears to be working better, I think I am on the right path at least. If it continues I will adjust the idle bypass screw and/or replace the idle position switch.