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HDMacaw · Joined: 04 Jul 2011 Posts: 24 Location: Tennessee

I have a 1987 924s that just turned 100K and is exhibiting an odd symptom.
When I pop it into neutral as I approach a stop (appx 2000rpm) and let off the accelerator pedal the engine speed will drop down to about 500rpm and then come back up to its normal smooth idle. When it does this the cars shakes a bit until the idle rises to normal which only takes a second. Other then that the car runs smooth as glass with plenty of zip and is getting about 27mpg.
It's got new timing/balance belts with new idler pulleys, new plugs, wires, distributor cap, rotor button, and air filter. I've also tried different grades of fuel including non ethanol none of which seemed to make a difference.

Any suggestions anyone?

fiat22turbo · Joined: 18 Jan 2006 Posts: 4040 Location: Portland, OR

Clean the throttle body and the idle bypass system.

If they are dirty from oil, etc buildup, they won't work too well and could cause your issue. Worst case, you can eliminate that as an issue.

I'm not sure what else it could be as many cars use an air bypass system to keep the motor running on decel. Whether that bypass is the throttle blade or a separate idle circuit.
_________________
Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)