engine diagnosis help needed

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Dec 6, 2004 | 11:31 AM

jlambert

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From: Ann Arbor Michigan

engine diagnosis help needed

I need some help diagnosing a 928, nobody who looks at it has the slightest idea what the problem is. Here are the symptoms I've noticed.
-'85 S3
-significant idle wander at startup (200-1800rpm cycles)
-3mph wander in cruise control
-throttle surging, sometimes even shuddering under acceleration
-low on power
and starting today on my way to work in the rain the engine started periodically running extremely rough - way down on power and became hard to drive. it felt like the plugs were fouled but being somewhat intermittant I couldn't imagine that.

I've gone through looking for vacuum leaks, manifold is vacuum looked fine to me. None of the mechanics have been able to find the problem either.

Any help would be desperately apreciated!

Thanks

Joe

Dec 6, 2004 | 01:12 PM

John Boggiano

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From: Cheshire, England

Joe,

I'm moving your query to the 928 forum - I'm sure those guys will be able to help.

The 'Help' forum is ment for help with Rennlsit itself.

Dec 6, 2004 | 01:43 PM

Fastest928

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From: California

Check vacuum, sounds like a leak....maybe intake gaskets?

Marc

Dec 6, 2004 | 01:57 PM

Jim bailey - 928 International

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From: Anaheim California

Quite possibly it was running on 4 cylinders when you were experiencing the very low on power and rough. Any issue with either of the two ignition systems ...coils ,coil wires, caps rotors, ignition switching (secondary stage) units could cause such a dramatic loss of power and roughness. The hot wire mass air flow sensor may have degraded so that you air fuel ratio is way off as well . It is also possible that the L H injection brain is not processing information correctly. Unfortunately more questions than answers.

Dec 6, 2004 | 02:06 PM

Shane

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From: Rochester, WA

Quote:

Originally Posted by marc@DEVEK

Check vacuum, sounds like a leak....maybe intake gaskets?

Marc

Sounds like you have unmetered air getting into the engine. As Marc says check all your intake boots, my '86 ran like this once when the boot had slipped loose on the intake. If you are having trouble keeping the boots from slipping off when you tighten the clamps, try cleaning as much of the oil as possible from the boots and roughing up the clamping surface on the intake with some fine sand paper.

Dec 6, 2004 | 08:08 PM

mark kibort

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Joined: Feb 2003

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From: saratoga, ca

timing belt slip? false air, cracked rotor caps? clogged fuel injector? Plugs?, spark?

mk

Dec 7, 2004 | 12:28 AM

atari_st

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From: Norfolk, VA

Sounds like an issue I had with my 86. Could be Mass Air Flow sensor. I had the exact same issues...with the exception of the cruise....that could be an issue with the cruise control brain. My car would run fine.....then run like crap...then would run normal. I ended up fouling out the plugs trying to keep the thing running. When I disconnected the Mass Aif Flow, the car would run...when it was attached it would not run unless I nursed the gas peddle. I did all the test in the shop manuals and the MAF tested OK.....yet it was still bad.

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Dec 8, 2004 | 10:04 AM

Mark

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From: Mountains of GA!

Quote:

Originally Posted by jlambert

I'm going to take the car to the dealer next week and specifically ask them to look at the MAS.

I would suggest that 98% of the Porsche 'DEALERS' haven't got a clue on a 928! You need to find an independent shop that KNOWS 928's. Hopefully one of the R'listers in your area can chime in with some suggestions for a wrench.