CF2 2.0TS High Idle - Not the usual Fix
I am having some trouble with the CF2 2.0TS in my 916 GTV. It keeps idling up around the 1500 to 2000rpm mark.
A bit of background:
It is a March 2000 build car, which has the Bosch Motronic 1.5.5 ECU. It has a Throttle Cable on one side of the throttle, and the Throttle Potentiometer and Idle Stepper Motor on the other side.
I had a leaking radiator so while replacing that, I also replaced the Thermostat and as the Temp sensor broke upon removing it from the old thermostat, i replaced it with a new FACET unit.
As well as this I replaced the intake manifold with a good 2nd hand unit due to another issue. These repairs were done in the last 2 months and a few hundred miles ago, while this idle issue has only occurred in the last week or so.
So during my investigation so far I have found:
When starting cold it idles around 1200rpm, slowly drops down as the temp increases, then at a certain temperature, the idle starts to creep up from around 1050rpm to the figure of 2000rpm.
I have removed and cleaned the Throttle. No difference
I have re-adjusted the throttle cable preload/adjustment screw. No difference
I removed the throttle cable whilst the engine was running to ensure it was not binding, no difference.
I have tried the ICM "tap" trick, where you tap the Idle Control Motor with a screw driver handle, no difference.
I have removed and disassembled the ICM and cleaned it up. No difference
Using MES I have re-adapted/reset the Idle Control Motor, you can hear (and see) the throttle moving when it does this. Readings After adjustment:
@ Throttle closed, i have:
Absolute Min Throttle Pos: 31.2 Degrees
Current Throttle Pos: 33.0 Degrees
Delta Throttle Pos: 2.8 Degrees
@ WOT, i have:
Absolute Min Throttle Pos: 31.2 Degrees
Current Throttle Pos: 109 Degrees
Delta Throttle Pos: 77.8 Degrees
So that says to me that the Throttle Potentiometer is working. The stepper motor moves, so I believe the ICM is working also. I also have a spare (used) Idle Stepper/Throttle Potentiometer, I have fitted this and the same fault persisted even after running a new throttle adaption. So i swapped back to my original one. None the less I feel that both units are good.
I start the engine, left running until the cooling fans have kicked in atleast once. Plugged in MES and monitored these figures
Coolant temp: approx 98 Deg C with bonnet open, sat stationary @ 2000rpm. As i have a new CTS I trust this reading.
Target Idle RPM: Approx 2000rpm
Current RPM: Approx 2000rpm +/- 100rpm variation.
Target IDLE MAF reading: 14kg
Current MAF: 25kg
Lambda Reading (Volts): Fluctuates between 760mV and 130mV with every parameter update.
Measured Battery Voltage: 13.47VDC (measured via MES, i have not verified this reading is correct yet)
If I unplug the MAF, it then idles like a dog and is very rough to rev, also takes longer for revs to settle. so I think the MAF is okay as it is better when its plugged in. Plus it does measure and that measurement varies with engine RPM.
This Lambda reading indicates that it is measuring a Lean state @130mV, so it was suggested that it is an air leak.
Armed with a can of WD40 i have sprayed around the intake, and none of these places seemed to effect the engine speed (i would expect it to bog down if there was a leak):
Intake Pipe work between the MAF and Throttle Body. No splits/tears/cracks either.
Around the plastic Intake manifold
Around the Throttle body.
Around the Brake Booster Vacuum line/feed. either ends.
Cam Cover to Intake Pipe hose
Cam Cover to intake Manifold Hose
Spraying under the intake manfiold (near the Evap Solenoid) makes no difference either.
Unplugging the Evap Solenoid makes no difference either, so it is not stuck in an "open" state and if there was a break in the hose inbetween the Evap Solenoid and the Intake, i think I would have found it with the "leak test".
So I am a bit stuck as to the cause.
EVAP Solenoid isnt stuck open
I trust the Coolant Temp sensor
I cannot find any intake side air leaks.
But something is tricking the ECU into thinking it needs to rev to 2000rpm.
The only other thing I can think of is that if I unplug the Lambda sensor, the engine runs, and will Idle around 1500rpm, but will drift slowly down to 980rpm which I believe is the correct idle RPM. So maybe a duff Lambda sensor is causing the ECU to over fuel, which is then making a rich AFR, allowing it to rev up to 2000rpm at Idle. This would also make sense as it idles fine when cold, but as it goes from Open Loop to Closed Loop EFI, it would start reading the Lambda Sensor.
So would anyone agree that it is safe to assume that I have either:
A Cracked/Leaking exhaust (I cannot hear any Chuff or noises i would associate with an exhaust leak, nor see any signs of an exhaust leak)
A dead Lambda O2 sensor?
any comments/suggestions are welcomed!
Last edited by Taylor93; 21-06-17 at 14:44.