My car has not been run for since last November and I just got it in the shop to resolve some prior issues before this track season. One thing that I uncovered is the fact that it's not quite boosting where it should only getting about 0.9 to 1.0 with the occasional 1.1 instead of the normal 1.2 to 1.3. Boost also begins to taper approaching red line while before it used to hold a solid 1.2 to redline. I can clearly see it in the logs. Car runs great but is definitely down on power. I've done some logs today which did not uncover anything unusual. On the way back to the shop while at part throttle (about 40% load) at 0.5bar and 96mph, car suddenly lost power and went into what I guess is limp mode. I coasted to a stop on the side of the road and the throttle was not responsive as the car continued to respond only to very mild throttle, not enough to drive the car. A solid CEL was on with a msg to drive to the workshop. I shut the car off and restarted it with everything back to normal. I scanned the DME with PST2 and it had the following fault code:

P1508
408 Torque comparison function monitor - signal
implausible

The maintenance manual basically says this:
P1508
408 Torque comparison function monitor - signal
implausible
Diagnosis conditions
• Engine speed greater than 1,120 rpm
Possible fault cause
♦ DME control module faulty
Affected terminals
Diagnosis/troubleshooting
Work instruction Display OK If not OK
1 Replace DME control module ⇒ Step 2
2 Perform adaptation ♦ Switch on the ignition
♦ Wait one minute
♦ Do not press the accelerator
♦ Switch off the ignition for at
least 10 seconds
♦ Read out the fault memory

So basically they have you replace the DME / ECU. I'm a little skeptical here at this diagnosis. I have not touched the DME or done any flashing or programming to it. Anyone have any ideas here? I'm wondering if it may be related to the lack of full boost. I have pressure checked the system and it's tight as a drum holding 20+ psi without leaks. I also installed a new OEM Bosch MAF and fresh plugs 200 miles ago at the end of 2015. Next item I'm looking to swap will be the N75 valve but the current one is only 4 years old. I've always driven the car regularly at least once a week and never had any issues. Now after sitting for 5 months I'm chasing all kinds of gremlins. Basically the car boosted fine when parked in November and now has issues.

One thing I will mention is that earlier this week while testing the car I broke a gearbox output flange while in boost. This caused the engine rpms to bounce off the rev limiter, actually about 300rpm past it as the flange broke. I wonder if this could have fried the MAF? Any way to check the MAF? I have a spare that only has about 5K miles on it and can swap it in. Could this be the reason boost is off also? Hmm....

I was not logging with duremetric while the car went into limp mode but have data from my Motec which logs continuously when the key is on. I'm looked at it and all looked normal except one thing that jumped out at me and that is the throttle pedal potentiometer number (1 and 2) on the throttle pedal. Both were quite a bit different from each other. When I hooked up durametric to it after I got home, they did not match up either.

Can someone with a duremetric please do me a favor and do a scan of the following three items and post their result? You wanna obviously do this with the engine off:

Please log the following: Pedal value, Pedal encoder potentiometer 1, pedal potentiometer 2. I am interested in the pot 1 and 2 values you get at the pedal value of 0,25,50,75 and 100%. You can simply record the values as you press the throttle pedal (engine off) to the % values above. Thanks guys!

 

I'm at work for another 5 hrs. If you don't have a reply by the time I get home, I can check mine.

 

First clean the throttle body because they dont look dirty but the oil causes issues if not cleaned once a year. It happened to me.


Could be your maf or you could have a big boost leak.

 

If you suspect the MAF, a common diagnostic is to unplug the MAF and see if it'll drive w/o going into limp. You'll get some CEL's related to the unplugged MAF, but it should drive fine otherwise.

 

Throttle body is clean and both throttle body potentiometers show accurate values.

I've done a pressure test. It's tight as a drum. No boost leaks..

 

I wish it was so clear cut. I've cleared the cel and car drives fine without going into limp mode. Drove it for an hour after it went into limp mode with no issues. I'm gonna swap in another MAF tomorrow and see what my boost is. Got my fingers crossed its the MAF. I also have a freshly rebuilt LSD that was installed two days ago but that should not have anything to do with it.

 

Do those look about the same as yours? That's not your problem, is it?

%...V1... V2
00 0.75 0.38
25 1.46 0.83
50 2.08 1.05
75 2.52 1.35
100 3.38 1.70

 

Jean,
Thank you very much. Nope, those numbers match up with what I have when I scan it. Of course, this may have been an intermittent problem or a glitch as the car is running fine now. Like I said, there was a pretty large divergence in throttle % just prior to the car going into limp mode. Maf change is next. I'll know more in a couple of hours. Thanks again

 

OK. I get the bonehead of the year award. The Y-pipe clamp was completely loose and there was a massive leak around the coupler / Y-pipe joint, hence the loss of boost up top. I'm sure this also tripped the throttle body shut down when there was a sudden loss of boost thus tripping limp mode. I had done a full boost leak check just prior to getting the car to the shop two weeks ago. While trouble shooting the fuel system we decided to replace the FPR. I had removed to Y-pipe for easier access and just forgot to tighten the work clamp upon reassembly. It's irritating to make stupid mistakes... Thanks to all those that offered to help!

 

Glad you found the problem! I for one enjoy seeing these case studies if you will, so that we're all the more educated in troubleshooting when it's our turn.

 

if only all the problems we see on here were that easy to fix!!

 

Told ya right where the throttle body is lol. P1508 is right at the throttle body or its the throttle body thats dirty which = boost leak or clean throttle flap.

(HAPPY)

 

Here ya go

But seriously. Glad it was something simple and you can get back to the track to beat up on some GT3's

 

Good to hear! I guess we start thinking deeper than necessary half the time.

I had the same instance at the track and should have noticed it by your description.