engine refuse to REV - intermittent

[prj]

Here is an update. My mechanic came and check the fuel line (in the engine bay), attempt to start and there's a lot of fuel coming out from the line. So definitely not any issues with the fuel supply. Then finally start the engine but very unstable, almost stall. Remove the throttle position sensor, no difference the engine remain runnig in a unstable condition; Repeat the process by removing the maf, still the same. Then we go through the resistance check on the throttle position sensor and found the reading does not match with the testing procedure online (when the throttle good to WOT, the resistance didn't go high enough).

I didn't check the maf test because I did attempt to run a known good condition 968 maf on my car but remain can't start the engine.

Still I can't explain why the engine didn't show a fault code from this sensor but I think I'm going to try a new part with this sensor and hope the problem can go away.

TPS has no effect on how the car runs/idles.

[hunker7]

TPS has no effect on how the car runs/idles.

My TPS failed couple of weeks ago and idle was perfect. Car is unresponsive when driving that's all.

[UrS4boy]

TPS has no effect on how the car runs/idles.

Sorry. That makes no sense.

[prj]

Sorry. That makes no sense.

The sensor is only used for boost control. And in case the MAF fails for substitute load calculation. It has no effect on the car not starting/stalling.

[UrS4boy]

The sensor is only used for boost control. And in case the MAF fails for substitute load calculation. It has no effect on the car not starting/stalling.

So the throttle potentiometer does not provide the ECU with any information that the ECU uses to increase/decrease the injector duty cycle?

[pcfnet]

My TPS failed couple of weeks ago and idle was perfect. Car is unresponsive when driving that's all.

This was the sympton I've experienced initially. Not only the car is not responsive but also the engine is getting unstable to run, ie. stall.

I was aslo suspecting my own fault when doing the MAF connector cleaning, I took out all the pin and clean individually and put it back. I was wondering maybe I accidentally put the pins back to the connector incorrectly.... However after this I did manage to start the car finally with smooth idle but the engine is still unresponsive intermittently when the throttle is pressed/opened.

[pcfnet]

One more thing I've tried is the ICM, I tried to start the engine by replacing 2x ICMs.. but it remains the same.

[prj]

So the throttle potentiometer does not provide the ECU with any information that the ECU uses to increase/decrease the injector duty cycle?

If the MAF is operational, yes that statement is correct.

[pcfnet]

Throttle Position Sensor would be my bet. I've had this when I've forgotten to reconnect it after doing work on the car and had a brief problem with my new build, as I was using a TPS that had sat for a while but that's cure it. It can be replaced with a spare easily enough. HTH

From your experience can the TPS itself cause the engine refuse to start at all? I am ordering the TPS now but I am trying to dig out any other potential cause which cause the engine refuse to start at all.

I am not sure if this makes sense, but if the TPS is not working correctly the ECU may think I am putting some load to the throttle while starting the car, which could inject too much fuel, flood the engine and wont start...

[pcfnet]

BTDT. Scared me.

Here is some info that might help:

G69 / F60 Throttle valve position potentiometer /idle switch information:

http://forums.quattroworld.com/s4s6/msgs/20405.phtml

Based on this schematic, it makes the test procedure a bit easier to understand:

Testing the F60 Closed Throttle (Idle) Switch is as follows:

Across Pins 4 and 6 on the switch:
Closed = 0 ohms (continuity)
Open throttle (slightly) = infinite ohms (open)
If either fails, adjust switch as per procedure above.

Testing the G69 Throttle Position Potentiometer:

Across Pins 1 and 2 on the switch:
Must be 1500 to 2600 ohms

Across Pins 2 and 3 on the switch:
Throttle closed = 750 to 1300 ohms
As throttle is opened from closed to WOT = increase to 3600 ohms
If NO, replace sensor.

The above test info is now available as a single sheet that you can print-off and take out to your UrS when the need arises.

http://12v.org/urs/G69F60ThrottlePositionSwitchTestSheet.pdf

I've gone through the TPS test, and the only thing that is not agreed with the test above is when the throttle open from closed to WOT, it only increase to ~2600ohms... I thought it means my TPS is faulty BUT I found some people have same experience as well...

[prj]

From your experience can the TPS itself cause the engine refuse to start at all? I am ordering the TPS now but I am trying to dig out any other potential cause which cause the engine refuse to start at all.

I am not sure if this makes sense, but if the TPS is not working correctly the ECU may think I am putting some load to the throttle while starting the car, which could inject too much fuel, flood the engine and wont start...

I already said, the TPS does not affect anything during start.

[mike74]

It might sound obvious but, have you checked to make sure the engine earths are ok? I had similar symptoms in a previous car (not quite as bad though) and it turned out the earths were FUBAR. Quick to rule out if you've got jump leads handy

[pcfnet]

Just to update, I swap the tps last night (ordered it from audi tradition) . I install in a way that when the throttle is just open I can hear the click at the same time. The vcds reveal the "TA angle" to be about 8 degree. Then the engine start! I run it for 10mins and partial open the throttle to around 2-3k rpm slowly, everything seems working fine. I'll try to get the car running on the road later tonight.

[UrS4boy]

Just to update, I swap the tps last night (ordered it from audi tradition) . I install in a way that when the throttle is just open I can hear the click at the same time. The vcds reveal the "TA angle" to be about 8 degree. Then the engine start! I run it for 10mins and partial open the throttle to around 2-3k rpm slowly, everything seems working fine. I'll try to get the car running on the road later tonight.

http://12v.org/urs/DeephousesThrottleBodyDIY.pdf

[prj]

That means your MAF is bad. And your TPS was bad too.
It does not use TPS info unless the MAF signal is wrong.