my 944 has a bit of a lumpy idle. you can feel the car vibrating at idle which is a little annoying.

i have also noticed that if you rev the engine then release the throttle in neutal the revs hang for a second or two then drop below idle speed and the engine labours and vibrates even more until the idle stabliser can bring the revs back up to normal.

can anyone advise if this is normal or suggest a solution?

i am pretty sure i have no vacuum leaks.

thanks in advance.

kenny
1991 944 turbo, guru chips

 

have you tried doing basic maintenance ie plugs, wires, cap and rotor etc? i had somewhat of a similar gremlin but after the aforementioned preventive maint. it went away.
i'm sure a lot of the gurus have a more detailed insight. good luck
i also cleaned out the throttle body and intake man. and re did the vacuum hoses.

 

I'd take a look at the idle by-pass screw in the throttle body as well as the throttle plate mechanical stop.

The ISV will have trouble maintaining a proper idle if the throttle body is out of whack.

Connect a Dwell/Tach to the coil, start the engine, let it come up to temperature and then bridge pins B and C on the diagnostic connector. This bridge will disable the ISV so the engine is idleing on the idle by-pass. Adjust the idle by-pass to get the correct engine RPM and then remove the bridge.

 

when i by passed the diag port, nothing happened, i mean no rise nor fall nor stumble on idle, does this mean that the idle is fine or something is jacked up? if so which part?
tia

 

How about a tune up? Are the motor mounts OK? Injectors clean?

 

Check your balance shafts as well

 

thanks for all the responses.

plugs, leads, cap & rotor are fairly new

i have bridged b&c on the diagnostic connector at the idle is roughly correct according to the tach in the dash. i will have to check this with a proper tach

the injectors have just been overhauled

i have checked the balance shafts and they are fine, she is quite smooth above idle rpm

what is the best way to check the motor mounts?

thanks again, kenny

 

so, can anyone advise on the best method to check motor mounts?

i haven't noticed excess free movement of the engine.

thanks again kenny

 

When the mounts fail, you'll actually have less free movement, because they collapse and the engine sits directly on the crossmember. I think paragon has a tech article with how to test them on its site. If you have excess vibration at idle, chances are they're bad.

 

I have the same exact problem. My car has just got a full tune. All the vacume hoses are new, the injectors, plugs are clean. However my wires are old. In any case I think it is the idle stabalizer in my car, but I have not run any diagnostics...If I come up with the problem source Ill let you know.

 

Oh and my motor mounts are new.

 

Idle stabilizer for sale!!! Almost new, check the rennlist classifieds under parts/951. Motor mounts will not affect how your car revs, just losts of noisy vibration.

 

"i have also noticed that if you rev the engine then release the throttle in neutal the revs hang for a second or two then drop below idle speed and the engine labours and vibrates even more until the idle stabliser can bring the revs back up to normal."

Is your car warmed up when this happens? Idle-speed that falls below spec and is brought back up a fraction of a second later by the idle-stabilizer is a sign that the idle-adjuster screw in the throttle-body is screwed in too far. With the car fully-warmed up, 100% of idle air should come from the throttle-bypass screw and 0% from the stablizer. So unscrew the idle-screw until revs drops right down to idle speed and no lower.

Also some people screw in the idle-adjuster thinking their idle-speed is too high. But you need to use a real diagnostic tool to measure idle, not the dash tach. That's because it usually reads 200rpms too high. Correct 850rpm idle will look like 1050rpm, that's OK, you're really not idling that high.

I was trying to correct that apparently high-idle speed for the longest time thinking it was above 1000rpm. Then I hooked up a digital display and found that I was trying to force a low 650rpm idle speed to show up on the dash at 850rpm..

 

thanks Danno,

the problem is worst when the car is cold.

i'll see if i can get a proper tach and adjust the idle screw.

 

Me too, when its cold it idles so bad that if I dont keep on the gas it will stall. Once it is warm idle will be high but not fluctuate