powdrhound

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. When I got home and scanned the DME with PST2 and 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. 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!

Lucid Moment

So I have had the same code come up before, just by itself. On the track in the rain, car went into limp mode, engine would run, could not drive car any further. Got towed, scanned for codes. I cleared codes, restarted and ran fine. I actually ran again since same day, no further issues.

I did not have an issue targeting boost though. Also I have an MAFless tune.

Hope you figure it out.

Atrox

Simple change your air filter, car is suffocating. Potentiometer is all within throttle body. New air filter and problem solved. Do an egas reset for good measure after new filter install.

Thank me later.

powdrhound

Wish I could. New OEM air filter was installed last week. Egas reset was also performed several times.

Carlo_Carrera

I would try swapping out the MAF.

"02996ttx50

me too. esp since you have a spare maf that you know hasn't been fried.. but then you've done a whole lot of stuff to that old dme since youve had the car and then the rebuild. it's kinda difficult to rule out the chance that it might possibly just "be time".

"dme control unit faulty" is fairly telling, and as a code quite rare. just a thought. gl. wierd it ran fine when you parked it, then this.

powdrhound

Swapped out the MAF but still low on boost up top. I do however get the bonehead of the year award. Went to do another pressure test and found 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...

Carlo_Carrera

^ Good trouble shooting.

"02996ttx50

lol. it is kinda funny since a tug on the y pipe hoses etc is like the very first thing even i do..and i cant fix sh*t! lol.

glad you sorted it out. it couldn't have been much. it was parked! congrats!

elite1

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.

Glad it wasn't your air filter. Those tend to get pricey real quick!

mdurbahn

That sucks, did you miss out on the DE too? Sorry I didn't see this earlier I have a Durametric and could have logged those values for you.
Matt

powdrhound

I have durametric and Motec logs. None showed anything out of the ordinary besides loss off boost up top. It's all sorted now. Just a clamp. Thanks for offering to help Matt.