recently I've replaced both fuel pumps/ relays, gas filter, tuned, fixed vacuum leaks on my 930 and the beast seems very happy. But... at steady gas pedal pressure between 3000 and 4000 rpm in 2nd and 3rd gear, the car doesn't purr along steady; it surges. by that I mean it seems to get faster than slower, minutely, rapidly, like a vibration front to back.

I know the car is much more responsive now than before this work was done but I don't think this phenomenon is a result of this work. maybe the sloppiness before masked this observation; I don't know.

is this "bucking motion" fuel pressure changes? is there a regulator?
or, is this some feedback loop between the 02 sensor - DME adjusting mixture?


I don't notice it in 4th, but then again that's 80-95mph and I don't want to be caught 'cruising' at that speed in the double nickel... It was bad enough being nailed for 75 behind a semi in the middle lane!!! these 930s say 'ticket me' while parked!!!

 

Have your mixture checked and adjusted. Typically, a lean condition causes surging at steady throttle, but running too rich can cause it too. If your mixture was previously set to compensate for vacuum leaks, it may be rich now that the leaks are fixed.

Ed

 

I would look for the CO to be around 3.5% (before cat if you have one still) on a gas analyzer higher# is richer lower is leaner as they can both cause surges. these are kinda complicated with the boost recirc system that the CIS system uses. waste gate ok? regulators functioning? this is a hard one to diag over the internet!

 

for anyone that was interested in the solution to the surging at steady pedal pressure...

brought it back to the wrench (EPE, Natick-MA). Jerry put a meter on it to measure the duty cycle electrics. we saw how the O2 was signalling to adjust and fine tune the fuel pressure to adjust the mixture to the preset. he felt that it was a bit lean so he richened it a click or 2.

I test drove and it was a dramatic difference, but still a bit of a bucking at steady pedal in 1st around 3-4 grand and in 2nd, not nearly as bad, unnoticable in 3rd/4th.

went back and he nudged the mixture up another click. test drove it and that was the sweet setting... came back to the shop and reset the idle... perfect.