idle variation on C4

[mdoherty]

My 92 C4 has an idle that is very sensitive when first started. If the gas pedal is pushed and then let off, the car almost stalls but recovers just in time. In addition, the car has poor throttle response for the first 4-5 minutes of operation. Basically it acts like the mixture is to lean. I'm suspecting a the mixture flapper valve is dirty or sticking slightly. Anyone have this problem?

[Nickshu]

Could be that. Check the Temp sensor on the left cam cover as well, I think it should read 2000-3000 Ohms when stone cold. May not be sending the correct information to the computer on cold starts. Also check the inline fuse for the preheater on the 02 sensor. A blown fuse here can also cause wierd cold start behavior.

Good Luck.
Nick.

------------------
_ _ __ _ _
Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region

[dsentel]

I'm pretty sure that when my floormat gets pushed up toward the gas pedal too far, it causes a similar condition. I forget the technical explanation (a mechanic told me this), but it is easy to check and try it.

[john walker's workshop]

find a shop with a bosch "hammer" and do a system adaptation. this often straightens out odd running problems.

[964Cab]

My '90 C4 Cab does the same thing, I have only owned it a week. I just sit for 2 minutes and wait for the engine to settle down. Contrastingly, my '95 993 starts instantly and is ready to drive in 2 seconds.

Thanks

[Mark Lintott]

I've had my 964 93' C4 for about 5 months now and it behaved in the same way for the first month or so until warmed up. Improved a lot after a recent service - minor only - oil etc. Must be just using it and a bit of TLC. Mine had virtually sat idle for the previous 3-4 years.
Mark
C4 Turbo body

[beetos]

My 90 C2 did the same until today, just finshed a full servce including valve adjustment, new caps rotors and wires, new o2 sensor, primary muffler bypass and weltmeister chip. I think the new o2 sensor and wires fixed the unstable idle. The new wires are a royal pain in the ass to get on. Idles rock solid now and runs like a gem. cost about 1200 bucks and a full weekend.

[sms1305]

What exactly does the hammer's "system adaptation" routine do?

[Bill Wagner]

A "system adaptation" clears and resets the ECU so it can re-learn it's sensor inputs. It also clears any sporadic or odd signals it received from one of the sensors and may have stored as errors when in fact they were just oddities. Nothing is perfect.

Bill Wagner
'91 C4

[beetos]

Would disconnecting the battery overnight do the same?

[Bill Wagner]

beetos:

Yes, but it probably doesn't need to be overnight (but what the heck...it's easy). A quote from the service manual regarding the checking of the battery supply to the DME:

"If there is no B+, the fault memory is cleared".

B+ is the 12V supply line to the DME from the battery. The procedure is to check the voltage at pins 24 and 18 of the control unit plug.. if this is zero (or near zero volts) the DME has been reset. If the battery is disconnected, the only thing that could keep a voltage level active for hours is an LCR circuit feeding a VERY high resisance circuit (probably a CR circuit with a diode representing the "R") in a closed loop, which may exist within the DME control unit itself or elsewhere in the car. How long this really takes to drain down is unknown to me. This may be possible...some Japanese cars are this way.

The DME uses some type of (what I assume) is static RAM to hold it's settings. If the supply to this RAM is set at zero (or near zero) volts all information to the DME is lost and it must go into a "re-learn" mode.

I should probably state that a "system adaptation" may be different from what I'm suggesting. I'm talking about a FULL RESET of the DME where the ECU needs to re-learn everything, whereas a "system adaptation" using the ultra-expensive Bosch tool may only reset "problem" items....I don't know, but the Bosch units don't seem all that sophisticated to me.

Hope this helps,

Bill Wagner