2000 C4 - "limp mode"? during heel-toe braking

[DWS964]

This happened twice to me today on the track, and once over a year ago on the track.  During braking and downshift, the car loses power and will not rev past idle.  The engine doesn't die.  No CEL. Seems like a "limp mode".  I pull over, turn it off, and restart, and no problems.  I have heard/read somewhere about this happening if brake and gas are pushed at the same time.  But, I have also done hard braking and blipping the throttle for downshifts on multiple laps in several sessions.  Interesting that this happened at same point on track today - end of straight, braking and downshifting to go into the bus stop at NCM.

[DWS964]

"Porsche fault code 408 - Function monitoring Torque comparison Signal implausible" shows up in Engine error codes.  

Does not throw a CEL.  Cleared the error code, and went back out.  Happened about 4 more times.  Always on hard braking at end of straight, with a blip and downshift.  (still not really heavy braking, not going real fast as still learning track). But it seems that engine went to limp before I could blip to downshift.  Perhaps went to limp before clutch went in.  I was heel-toe'ing on two other corners without incident. (and with less heavy brake)

I am wondering if my fat foot is touching the gas pedal during heavy(er) braking prior to clutch-in and blip, and this is when the limp mode starts.

Quick turn off and restart and all is fine.

[Loren]

P1508 (408) Torque Comparison Function Monitor – Signal Implausible

- Fault detected during function monitoring. – DME control module

 

Try disconnecting the battery for 5 minutes - re-connect then perform an accelerator adaption.

1. Switch on the ignition.

2. Wait one minute. Do not press the accelerator.

3. Switch off the ignition for at least 10 seconds.

4. Read out the fault memory

[DWS964]

 

P1508 (408) Torque Comparison Function Monitor – Signal Implausible

- Fault detected during function monitoring. – DME control module

 

Try disconnecting the battery for 5 minutes - re-connect then perform an accelerator adaption.

1. Switch on the ignition.

2. Wait one minute. Do not press the accelerator.

3. Switch off the ignition for at least 10 seconds.

4. Read out the fault memory

 

Thanks for the suggestion - but tell me what this is doing and how it relates to the P1508(408).  And why would this manifest as a Limp under braking scenario?

[thewheelman]

How long are you overlapping the throttle and brake?

 

I've noticed when left foot braking, that after a second or so, the gas pedal becomes inoperative. After letting off the brake, everything returns to normal. This is a standard behavior that many automakers have implemented to prevent runaway automobiles.

 

It sounds like your issues may be that the car is operating outside of that normal behavior. Maybe sticky TPS/throttle body, or failing rheostat in gas pedal. 

[Loren]

The DME and transmission talk over the CAN network.

In this case the DME senses that that the torque curve in the transmission is "out of range" compared to the torque (applied) in the engine. So it thinks the DME must be malfunctioning.

By doing the accelerator adaption you are resetting the baseline in the DME. If the fault continues then either there is a communication error (usually another fault for that) or the DME is not responding as it should.

[DWS964]

The DME and transmission talk over the CAN network.

In this case the DME senses that that the torque curve in the transmission is "out of range" compared to the torque (applied) in the engine. So it thinks the DME must be malfunctioning.

By doing the accelerator adaption you are resetting the baseline in the DME. If the fault continues then either there is a communication error (usually another fault for that) or the DME is not responding as it should.  

The questions continue (I'm an engineer...)

What is there in my manual transmission that would be communicating with the DME?

I would guess that disconnecting the battery first in your procedure would erase an existing code?  Or, would I expect it to remain? I have been clearing the offending code with my Durametric.

I am unsure whether I will be able to duplicate the conditions that caused the failure, on the street.

Coincidentally(??), during this same time, I was having trouble getting full power out of the motor.  It would not rev past about 5K when floored and would just run out of steam and not go any further.  I have noticed some inconsistency with this engine in winding out and getting full power.  It has the Vortech supercharger installed and I know that the chip has been modified by the kit mfg, injector rate increased, etc.  I suspect a MAF contribution, as a KN filter is installed, so I will clean and/or swap out.  Maybe the knock sensor is holding things back.  As I said, the performance is inconsistent.

 

Loren, THANKS for your help - always first class service.

 

Darrell

[Loren]

Sorry thought you had a Tiptronic.

Do you have Traction Control or PSM?

If so then the mismatch torque signals would come from PSM (via the wheel speed sensors).

 

Ok - now that we know there is a supercharger and huge increases in torque. I would say that is pretty normal.

996TT has similar issues when folks the the hard there is so much torque combined with wheel slip the the car's electronics think there is a fault.

So based on what you have said it does not surprise me and it will likely be an incurable fact of life for those mods on this car (same for the 996TT).

[DWS964]

Sorry thought you had a Tiptronic.

Do you have Traction Control or PSM?

If so then the mismatch torque signals would come from PSM (via the wheel speed sensors).

 

Ok - now that we know there is a supercharger and huge increases in torque. I would say that is pretty normal.

996TT has similar issues when folks the the hard there is so much torque combined with wheel slip the the car's electronics think there is a fault.

So based on what you have said it does not surprise me and it will likely be an incurable fact of life for those mods on this car (same for the 996TT).

 

Yes, the car has PSM (all C4's have PSM, I assume?)

 

Sadly, although the car has a supercharger installed, it is not achieving major torque increases - so I am inclined to rule out that failure mode.

I was tempted to turn off PSM and see if the failure continued to surface, but didn't want to take chances on a new track.

I am trying to make the connection to hard braking.  But it may be something in the PSM, which I assume is collecting signals from brakes, wheel speed sensors, throttle setting, ...etc.

And there is the connection between a bad MAF causing PSM and ABS lights to come on, that I have experienced.

So, I will go after the MAF (cleaning or swapping) and the Throttle adaptation and go from there.

 

THANKS for the input.

[Loren]

What tool are you using to read faults? Durametric? PST2? PIWIS?

[DWS964]

What tool are you using to read faults? Durametric? PST2? PIWIS?

Durametric

[Loren]

Latest software version?

No PSM or ABS faults?

[DWS964]

Latest software version?

No PSM or ABS faults?

yes, latest software - v6.x

No ABS faults.  I assume any PSM faults would come under the ABS codes section.

I do not think that PSM nor ABS lights flash during the event, but I am not looking at the dash at the time.

 

THANKS

[LostSouth]

Have you had any luck figuring this out?  I am experiencing a similar problem on my C2 race/track car.  Happens at fairly low speed and not under heavy braking.  I just installed a new clutch with light weight flywheel and am using slicks for the first time.