odd reduced rpm limit after 30 min run time

[Quadcammer]

So i have a 99 boxster with the 5.2.2 dme.  the car has a 3.2l box s engine with a 987 airbox and maf sampling  tube.  fairly standard swap.  has headers, 996 throttle body, and underdrives.

I had softronic do the engine tuning.

So for approximately 30 minutes after key on, the engine runs fine, revs to 7300rpm redline with no issues.  after 30 minutes though the car will run fine but it hits what feels like a rev limiter at 6200rpm.  the car will not rev through it, wot or not.  if you pull over and restart, you get another 30 minutes before this reoccurs.

I asked softronic about this and they said the dme programming is not the issue.  i have checked the maf wiring and obviously the maf is plugged in.

Any things to check or input would be great.

Thanks very much

[Ahsai]

Odd problem indeed. Having an aftermarket tune makes it even more challenging. I assume no codes? A bit silly but have you checked the throttle cable and make sure the butterfly can rotate in its full range?

 

If I were you, I would focus on the throttle body and MAF and make sure the MAF returns the correct voltage/flow rate at the high rpm (and not under reporting it). Can't explain why the symptom goes away after restarting the engine though. 

 

How about logging the MAF voltage and measured flow rate at that rpm both before the 30min (but with engine fully warmed up) and when the symptom occurs? Maybe hard to tell effect from cause but at least it's a start. Should also log the air intake temp (measured by the MAF), throttle plate angle, position and coolant temp the same time.

 

It's hard to imagine what prevents the engine from spinning past 6.2k though. Knock sensors acting up?

 

Sorry my reply is not very coherent....just throwing out ideas.

[Ahsai]

BTW, stick shift or automatic?

Also maybe worthwhile to check the fuel pressure and flow rate.

Edited October 18, 2017 by Ahsai

[Quadcammer]

Thanks ahsai. Manual transmission.  no codes are thrown nor are any pending.  car runs completely normally otherwise and doesnt miss or break up.  literally feels like it hits a hard limiter. 

 

The throttle body goes through its full range and the maf is nearly new.

 

For some reason i get the feeling that in the tune they are running it almost like thr maf is unplugged and that causes a sort of limp mode.  im disappointed that softronic was not more interested in trying to solve the issue.  i will try logging the maf voltage.  i cant imagine fuel flow would cause such a problem but can check that also.

 

Any other input would be great

Edited October 19, 2017 by Quadcammer

[Ahsai]

I agree the tune is the easiest explanation. I would expect the limp mode should limit the rev lower than 6.2k but only softronic would know.

 

All the other things I mentioned are not fully consistent with the symptom. They're more like the closest things to check that may contribute to the problem.

[Loren]

Which MAF are you using? The one from the 987, 996, or 986?

They are all different and the software needs to know which one is in use.

[Quadcammer]

Thanks guys for the assistance.  Loren, the MAF is the standard pre-drive by wire 986 MAF from a 99. 

 

Maf readings were checked and appear to be fine, as is the throttle opening percentages. 

 

I have a feeling that it may be the result of an intermittent Crank position Sensor.