Intermittent dramatic loss of power at 2.5k rpm

yacht broker · 22 Jun 2017

My car is a 2005 997 3.6.

Recently it has developed an intermittent and dramatic loss of power at 2.5k rpm. This can happen in any gear and whether the car is not or cold. It literally feels like your foot has slipped off the accelerator. Ease the throttle for a moment and it goes.

This is really frustrating as it usually happens when exiting roundabouts. Less than ideal.

The car has been into a Porsche indy who struggled to replicate the issue and couldn't find any evidences from interrogating the ECU. They could only find some very minor missfiring which was not serious enough to throw a warning light but pointed at the original coil packs being past their prime. These have been replaced along with plugs etc.

The power loss definitely doesn't feel like a minor missfiring, so what else could it be? Throttle position sensor etc.

DucatiRob · 22 Jun 2017

Hi YB

I was having a very similar issue until recently, a sudden and almost violent loss of power, but at around 3000rpm. I had fitted a Y pipe dual cone intake to my C2S last summer and had been experiencing running issues, occasional misfire, lumpy idle etc, the issue got worse since I had a Hartech engine rebuild a few months ago. The only thing I had changed was sports headers fitted when the engine went back together, the sudden power loss issue was on,y when accelerating and most noticeable in top gear strangely, it recovered almost immediately but made driving quite unpleasant.

I have now reverted back to the standard airbox and the issue has gone away completely. My issue sounds worse than yours but the root cause may be similar. I had been getting CELs and error codes for over rich fuelling and the occasional misfire code. Essentially the ECU could not cope with the extra air intake and and adjust fuel trims to suit, I think it has something like a 30% tolerance before throwing codes. Have you made any modifications to the intake or exhaust? If not, a vacuum leak somewhere or a leak in the exhaust could cause similar symptoms. The other common area to look at is the MAF sensor, a dirty or failing MAF could also be the culprit. If you are sure there are no vacuum leaks the it is worth giving the MAF a good clean with MAF cleaner!

Hope this helps!

yacht broker · 22 Jun 2017

Cheers.

The only mod is a gundo hack which is downwind of everything so surely can't be a culprit. I ask the indy to check the MAF as this sounded the most obvious to me but maybe they didn't.

The engine has recently had a full rebuild so whether there is a leaking vacuum hose somewhere could be a factor.

deMort · 22 Jun 2017

Its nothing to do with the exhaust bypass .

I could just about argue for a cat fault but not going to as it doesnt strike me as that .

I assume there are no fault codes , indy should have checked adaptions to check for rich / lean running so im left with 2 things ..

Fuel pressure wont give a fault code but will give lack of power problems .. trouble is its the wrong sort of rpm for this and it should be noticable to the garage .

If its more intermitant then im with Rob here .. MAF which is reading incorrect at certain rpm ( air flow ) .. it wont give a fault code as it will be reading inside its limits but will certainly affect performance .. in theory it should show on the adaptions for FRAU .. thats mid range rpm adaptions .

Unplug it , start the car .. give it a chance to log the code and drop to defaults then drive the car and see if the fault is still present ..

Code will need to be cleared afterwards so might be best if the garage and you do this .. you can drive the car as you know how to produce the fault and can see if it makes a difference .

Or take a gamble on it and replace .. not cheap though .

Throttle sensor .. it doesnt have one as such .. it has throttle pedal potentiometers , x2 of them and a similar system in the throttle itself .

Any fault here will log a fault code pretty dam quick as its safety related .

Actually i just thought of another although rare as the proverbial rocking horse ...

An ancillary thats over loading .. something like a partially seized scavenging pump , not this but think of a drive belt idler pully that was siezed .. it will turn under huge force but put a strain on the engine to turn it .

rare .. but when you eliminate every thing else then no matter what remains ...

ragpicker · 23 Jun 2017

Has it been or have you had it mapped?

Bleeding off the fuelling map early at partial throttle loads to create the pops and crackles in the exhaust on the overrun can cause this issue. This would be reflected in the fuel trims, and also in the idle AFR. Similar effect to low fuel pressure and is unlikely to throw any codes initially. Long term you may get 'o2 sensor levels too low at idle on bank 1 and 2' (or words to that effect).

jonno_ · 23 Jun 2017

I'm going with failing MAF here.
ETA: esp. with no fault codes.

Intermittent dramatic loss of power at 2.5k rpm

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