yardpro

I drove the 928 today on a few hour trip to pic up the wife's christmas present.

When i got on it it was "surging" as it accellerated.

here is a little history..
this summer i noticed a sudden power loss of a few hundred rpms as the car got close to redline... it would fall off then come back on the power.

The next symptom i had was that when it dropped down and the revs jumped up, there was a hesitation in the power, then it would surge back up and be fine.

today was much more pronounced... it had the recently consistent near red line hesitation... but something new started happening....

as the rpms increased in third climbing from 80-120 the car would rythmically loose power and rpm, then pick back up.. there was a consistant rythem to this "surging".

It was a quick loss of power but not abrupt like an ignition cutout... almost seemed like a fuel delivery issue.... could this be a bad fuel dampener, etc?

Jadz928

Hall sensor or knock sensors retarding timing 6 degrees?

Mike LaBranche

Yeah, my first thought was knock sensors.

yardpro

so the knock sensors would cause a temporary power loss?
mine are fairly new, done within the last 6-7K miles...
still think they could be the culprit?

Jadz928

More than likely not is they're new. Your also referring the having serviced the hall sensor too?

Fuel delivery rate okay? Fuel pressure regulator holds air under vacuum?

yardpro

the hall and knock were done just before i bought the car by smith motorworks... a reputable shop....

not sure about the fuel pressure.. have a gauge and vac pump.... how do i test it?

Maleficio

Vacuum leak being revealed under intense load?

yardpro

no vac leaks that i can find...
i have checked the vac system with my mightyvac, an cannot find any leaks..

John Speake

Have you carried the fuel volume test at the return to the tank as per the WSM ?

S4-on-your-back-door

fuel filter

WallyP

Probably a fuel flow problem. You would barely be able to sense the change in timing due to knock sensors or cam sensor.

Maleficio

Do the drops in power have an immediacy, a mechanical feel, or a slow rise and fall in power?

My oxygen sensor connector was shorting and dumping the signal to ground causing the computer to go in and out of closed loop. The effect was dramatic, and felt mechanical, immediate, and was more noticeable when pushing the revs higher in the lower gears.

Maleficio

I actually am thinking about this backward. When the ECU goes into open loop, you will feel an increase in power because the air/fuel mix will suddenly turn rich, and economy will drop.

You're experienced loss of power, the opposite of what I was referring to. Sorry.

It does sound like you have some resistance occurring to either the fuel pressure or air flow.