Throttle Faults (e-gas)

[Costas]

What could be causing the attached faults simultaneously? I parked the car last night - this morning it would not start.

I checked and cleaned (contact spray) the DME connectors to no avail. In the past, I had a few occasions when the throttle would "stick" causing the car to idle at around 2500 rpm. Moving the DME connectors around would solve it (I assumed a bad contact on one of the signal cables).

I have also checked all the fuses and the plug to the throttle body.

The live values on Durametric do not change, no matter how I press the gas pedal. Normally, they will vary - e.g., "Pedal Value" going from around 0%, to 100%. Now, Pedal Value is stuck at 30%.

Of note, but I don't believe it is relevant: The engine is an M97.21, the car has been running fine with it since 2010 (I reprogrammed the original DME myself).

Any suggestions on how to approach this would be greatly appreciated. I am at a loss since the faults suggest a problem on both ends (pedal and throttle plate). I tried my best to ensure good contact for the wires plugging into the DME - I even removed each of the two "sections" that slide into the safety clip and checked that each wire is pushed all the way forward and locked.

I am assuming it is extremely unlikely that the DME itself can go bad only in the circuitry involving the e-gas?

EDIT (per the suggestion of JFP below):

The car is a 2003 986 s. No modifications to the wiring harness, with the exception of an extension to control the M97.21 engine's two-stage valve lift.

Edited May 1, 2014 by Costas

[JFP in PA]

What could be causing the attached faults simultaneously? I parked the car last night - this morning it would not start.

I checked and cleaned (contact spray) the DME connectors to no avail. In the past, I had a few occasions when the throttle would "stick" causing the car to idle at around 2500 rpm. Moving the DME connectors around would solve it (I assumed a bad contact on one of the signal cables).

I have also checked all the fuses and the plug to the throttle body.

The live values on Durametric do not change, no matter how I press the gas pedal. Normally, they will vary - e.g., "Pedal Value" going from around 0%, to 100%. Now, Pedal Value is stuck at 30%.

Of note, but I don't believe it is relevant: The engine is an M97.21, the car has been running fine with it since 2010 (I reprogrammed the original DME myself).

Any suggestions on how to approach this would be greatly appreciated. I am at a loss since the faults suggest a problem on both ends (pedal and throttle plate). I tried my best to ensure good contact for the wires plugging into the DME - I even removed each of the two "sections" that slide into the safety clip and checked that each wire is pushed all the way forward and locked.

I am assuming it is extremely unlikely that the DME itself can go bad only in the circuitry involving the e-gas?

You should always include the car year and model, as well and any pertinent modifications (later engine, altered wiring harness, custom DME flash, etc..) when opening up one of these dialogs.

It would appear that your DME has lost contact with your TPS, which is why the live values do not move. I am also concerned that you can correct a "sticking throttle" by wiggling wires about, that also looks like a bad connection issue.

Unfortunately, the factory method for checking TPS circuits involves using a rare and expensive factory "pin out box" to check various circuits for correct voltage ranges, etc. in order to trace down the fault(s).

[Costas]

Thanks JFP, I have updated the original post.

So it is likely a connection issue, not much chance of, e.g., the throttle potentiometer(s?) going bad?

I have the attached pinout for the 7.8 Motronic. In theory, should I be able to check with a voltage meter if the gas pedal is sending any values?

Then again, if the connection is lost, why are the live values 4.54 and 5.00 Volts? If I unplug that section of the DME, durametric shows no values at all.

Also, why would there be a lost connection on the throttle plate values as well (and at the same time)? Those are on a separate connector I think (connector III in the pdf).

Thanks again!

Porsche_DME_7.8_Pinout.pdf

[JFP in PA]

Thanks JFP, I have updated the original post.

So it is likely a connection issue, not much chance of, e.g., the throttle potentiometer(s?) going bad?

I have the attached pinout for the 7.8 Motronic. In theory, should I be able to check with a voltage meter if the gas pedal is sending any values?

Then again, if the connection is lost, why are the live values 4.54 and 5.00 Volts? If I unplug that section of the DME, durametric shows no values at all.

Also, why would there be a lost connection on the throttle plate values as well (and at the same time)? Those are on a separate connector I think (connector III in the pdf).

Thanks again!

You could test the potentiometers separately, but I doubt it.

Problem with trying to read the voltages at the DME, that is not going to be easy with everything connected, which is why the factory put the pin out box into the circuit so you can use a volt meter with everything still connected.

You could still have a failing DME, but I keep going back to being able to alter what is going on by wiggling wires, that just smacks of a connection or wire issue.

[Costas]

Agreed on the wiggling being suggestive (very). That is why I looked there first. However, I think I probably have a good connection near the DME now.

Interestingly, the values in an old thread match my own: (pedal value 30.07% exactly and similarly for the voltages)!

Update: the guy had also posted the same question elsewhere and there he updated saying the issue was resolved by re-establishing the ground to the MAF! I will have a look - it is pin 9 on connector III for the 7.8 DME.

Edited May 1, 2014 by Costas

[JFP in PA]

Agreed on the wiggling being suggestive (very). That is why I looked there first. However, I think I probably have a good connection near the DME now.

Interestingly, the values in an old thread match my own: (pedal value 30.07% exactly and similarly for the voltages)!

Update: the guy had also posted the same question elsewhere and there he updated saying the issue was resolved by re-establishing the ground to the MAF! I will have a look - it is pin 9 on connector III for the 7.8 DME.

Bad grounds are worse than a loose wire or connector, and harder to isolate.

[Ahsai]

Second the wiggle test reco. The 30.07% pedal position looks like a substitued value for limp mode (due to lack of connection like John said).

You can also remove the intake so you can see if the throttle plate is fully open like Durametric indicated. You can do the following tests with a multimeter.

1) Unplug the pedal position sensor (PPS - two individual potentiometers inside). With key on engine off, test the socket side of the PPS for +5V and GROUND for BOTH potentiometers (page 2 of attached pdf). To test for GROUND, connect the red probe of the multimeter to 12v and the black probe to the pin you're measuring. 12v indicates ground. 0v indicates an OPEN.

2) Repeat 1) for TPS (page 1 of the pdf below for DME 7.2 but 7.8 should be similar but please verify)

3) Unplug the battery, DME, PPS, and TPS and test all the wires for continuity between the DME and both sensor sockets.

TPS-and-PPS.pdf

[Costas]

Thanks guys. I just tried wiggling that particular MAF ground. Also disconnected the MAF completely and the plug to the throttle body. No change.

I will try Ahsai's suggestions tomorrow (it's nighttime here). Might need some help locating the pedal position sensor - as far as I can tell, there is a metal wire leaving my throttle pedal, which probably ends up to the potentiometers, somewhere behind the dashboard(?).

Many thanks again!

Edited May 1, 2014 by Costas

[Ahsai]

On a 996, it's under and behind the dash right where your right knee points to when you're in the driver's seat. Just trace that cable from the pedal and you should find it.

Figure 1 here http://www.pelicanparts.com/techarticles/Boxster_Tech/94-ELEC-Sprint_Booster/94-ELEC-Sprint_Booster.htm

of course without the sprint booster in place.

Edited May 1, 2014 by Ahsai

[Costas]

Thanks Ahsai, I managed to unplug the TPS, as a test, and it made no difference. Can we draw any conclusions from this? i.e., if the wiring to the DME were OK and I unplugged a bad TPS would I see anything different? Are these values (30.7% etc) what you get when there is no connection whatsoever to the TPS? I am thinking that, normally, with the TPS unplugged Durametric shouldn't show 5V for it. This suggests a short somewhere in the wiring from the TPS to the DME?

[Ahsai]

Did you mean the pedal position sensor or the TPS on the throttle body? It almost sounds like you lose the ground connection to these sensors (hence the maxed out voltages you saw) but yet these sensors have separate ground pins at the DME. That's why I suggest unplugging each sensor and test the socket side and see if you have +5v signal and a good ground from the DME. If you do, at least you know the connection between the sensor and the DME is fine.

[Ahsai]

If possible, the most accurate way is to back probe the sensor pins when the sensors are still connected. Then you can push the gas pedal and see if you measure any voltage changes at the sensors. Short of that, disconnecting the sensor and measuring the socket side is the next best test.

[Costas]

I was referring to the pedal potentiometer as TPS! Thanks a lot for the guidance - I will seek some assistance with testing.

[ECU Doctors]

In my experience, whenever we see those codes, there has either been an issue with the memory in the DME OR a shorted DME (broken ground tracks).

I would say you could send the DME here to test it but I'm not sure how you feel sending it all the way to Florida.

My suggestion, after you finish testing your wiring, to save some time, is to open the DME and check over the board and make sure it does not have any broken tracks or melted components. Make sure to smell it for burnt electronics, as things can look good on the outside but be melted on the inside.

[Costas]

Hello, thank you very much for the suggestion. It was one of things I tried: I plugged in the DME of another car and, although the engine would not start with the "foreign" DME of course, this DME also came up in Durametric with the exact same errors and live values.

Currently the car is at my mechanic's who has found a severed wired somewhere between the gas pedal and the ECU - I will ask for the details when I get the car back and will update here.

Thanks again.

[Ahsai]

Good call trying another DME. I'm curious to know why the throttle position sensor also flagged codes. Maybe the TPS is fine and those values shown by Durametric are only substituted values, after the Pedal position sensor has been detected offline. Please keep us posted.

[Costas]

It's been a while - I thought I would post an update although the problem remains:

 

My mechanic is a friend so he's done many things so far without charging much: replaced the complete wiring harness with another used one, checked and fixed vacuum leaks, tested throttle body, pedal potentiometer, cleaned fuel tank, lines, & filter, etc.

 

We believe it has to be the ECU! Incredibly, the alternative we had tried may also be faulty so we were misled. My DME is being checked by a local specialist - cannot say if they have the expertise which I see on ECU Doctor's website but hopefully they will come up with something.

 

The only other thing that would be left at this point is the wiring in the front part of the car - though my mechanic says he's had that checked for continuity (including checking behind the fuse box, etc).