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Super Jenius
 
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Join Date: May 2002
Location: Syracuse, NY
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930 Hesitation/Stutter

Gentlemen, I need your help.

The situation is a 1979 930, twin plugs, Electromotive ignition, TiAL wastegate and Kokeln intercooler/exhaust.

Car starts immediately and runs strong; however, as we get to the 4000 rpm area, it feels like it loses a bit of power, and then... stutters or judders a bit. If you put your foot into it, it will come right out of this bouncy/jerky zone, but you can't cruise at 4000 rpm area b/c it's uncomfortable. Once you're above about 4500, it goes away, so it's not like an increasingly bad judder cause by bad wires or what have you.

This is not Turbo lag. I start to get boost at about 2800 rpm, and I know what lag feels like. This is something else. It kind of *feels like* fuel is stopping and starting (or being cut out and in) a couple of times a second. I have a HALmeter on the car, and it doesn't show a change in lambda when this happens (but it might not be "fast" enough to detect these pulses).

What I've done (my mechanic has done, mostly) thus far:

Checked all 12 plugs -- they look good.
Checked both fuel pumps -- both have good pressure and flow
Changed fuel filter
Changed fuel line to distributor (it was worn and weak)
Generally tried to tune the car... it idles solid and, other than the 4000 rpm stutter, runs well.

So... a few thoughts/guesses?
The boost pressure sender is not working (the dash gauge stays pegged at 1 when the car is on -- I have a separate boost gauge/controller that I use. Could the boost pressure sender's failure cause such a problem? Is there some sort of integral fuel cut-out that the pressure sender might be causing?

There is another sender "next to" the boost pressure sender, whose function is a mystery to me. If anybody can enlighten me as to that little doohickey's purpose, and what problems it might cause, I'd appreciate it.

Any other ideas/theories would be greatly appreciated. This is the last "issue"with this car, and I'd enjoy it a whole lot more if I could get rid of this stutter/judder.

As always, thanks in advance.

Best,

JP

Note -- also posted in the 911 Tech Forum.
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2003 SuperCharged Frontier ../.. 1979 930 ../.. 1989 BMW 325iX ../.. 1988 BMW M5 ../.. 1973 BMW 2002 ../..1969 Alfa Boattail Spyder ../.. 1961 Morris Mini Cooper ../..2002 Aprilia RSV Mille ../.. 1985 Moto Guzzi LMIII cafe ../.. 2005 Kawasaki Brute Force 750
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