924Board.org

neophite · Joined: 11 Sep 2008 Posts: 16 Location: georgia

Please help with my misfiring problem. It will miss at any speed but particularly when I let off the clutch after upshifting. I have also noticed that at idle the rpms will sometimes drop considerably and then come back up. I have replaced the plugs, fuel filter, o2 sensor,and dme relay. My plug wires, distributor cap, and rotor are not that old. Please tell me where I need to go from here. As always, your help is greatly appreciated.

Haven · Joined: 18 Jan 2011 Posts: 31 Location: Townsend, TN

I am by no means an expert on these engines, as I just bought a 924S recenty myself.

But... it sounds like it could possibly be a vacuum leak problem or maybe the reference mark sensor, which basically acts like a camshaft position sensor.

If your reference sensor is bad, then your engine doesn't know when to fire and it can cause the symptoms you described.

I'm sure someone with more knowledge will chime in before long.

Good luck with it!
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1987 Porsche 924S Auto

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

You might check the AFM for wear on the contact "tracks." Also, these things are getting to the age where a vacuum refresh is becoming mandatory (all lines, plus AOS seals). See 944Online for a good kit for this project.

I would also recommend refreshing the feed, return, and "jumper" fuel lines (before the car catches on fire and burns to the ground). See RennBay for these items.

Neither of the above are cheap procedures, but it's a Porsche!

Good luck!
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

neophite · Joined: 11 Sep 2008 Posts: 16 Location: georgia

Thanks guys, now I have an idea of where to start. I will replace and test when my budget allows it. Again, Thanks

masterdave · Joined: 10 Dec 2008 Posts: 183 Location: Harrison NY

i had a similar problem and it was the fuel pump ground, the power was getting to it fine but the pump ground was so dirty it was not working properly and the pump was running at like half power. it is very simple to check fuel flow,first disconnect the battery, simply remove the nut on the head of the fuel rail, connect a piece of rubber hose to it, run it to a measuring cup, jump i think terminals 80 and 32b, on the DME relay. If you dont have the Haynes manual GET ONE, they are a huge help. then reconnect the battery this will kick the pump on and fuel will flow in to the measuring cup, i want to say its 500ml every 30sec or min, but the haynes manual will tell you what value you should be getting. If you are not getting proper flow you may have either a bad pump or bad pump ground. I would also advise checking the injectors, they ban be checked with a simple multi meter, again the haynes value will have the right measurement. I happen to be away at school and dont have my manual other wise i would look up all these values. if you have access to an oscilloscope you can check the out put of the DME to the injectors, it should be a square wave at 12 volts.

Paul · Posted: Thu Feb 03, 2011 12:43 pm Post subject:

Check your engine grounds.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

neophite · Joined: 11 Sep 2008 Posts: 16 Location: georgia

I will do the tests. Thanks

neophite · Joined: 11 Sep 2008 Posts: 16 Location: georgia

I am ready to drive this car off a cliff. As suggested, I replaced the engine grounds (which I assume are the 2 negative battery cables), cleaned what I think is the fuel pump ground (really could not tell but it was a brown wire from the pump that attached and a female connector),
checked fuel out put, replaced plug wires and rotor. Still cuts out but not as much. I have noticed it misfires a lot worse on cold or rainey days which makes me believe it is electrical. Suggestions PLEASE!

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

There is a cluster of grounds at the back of the intake manifold that should be disassembled, cleaned, and reassembled with antioxidant.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

headstrong2740 · Joined: 29 Apr 2010 Posts: 111 Location: Moline IL

Hey was i not mistaken... but is there not a coil on the passanger fender well that runs to the center of the distributor cap... Could be worth having it tested and see if it is within spec. I know autozone can test most of these.

neophite · Joined: 11 Sep 2008 Posts: 16 Location: georgia

Thanks for the quick replies. I will look for the cluster of grounds but don't recall seeing them. It may well be the case that I don't know what I am looking for. I will also have the coil tested.
Thanks

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Don't recall exactly, but it may be in a lovely spot under the heater control valve.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Paul · Posted: Tue Apr 05, 2011 12:21 pm Post subject:

Yes under the heater valve on the bell housing near the reference mark sensors.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Paul · Posted: Tue Apr 05, 2011 12:26 pm Post subject:

_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

neophite · Joined: 11 Sep 2008 Posts: 16 Location: georgia

Many thanks for the photos. Now I will have to figure out how to get to them this weekend.