Idle RPM's

Bia

True Classic
Does anyone know what an 81' FI car should idle at? My car seems to idle high, around 2000 RPM's, even when the car is warm. Is this normal?
Thanks
Bia
 

ng_randolph

Bjorn H
800 - 900 RPM according to the manual. Don't trust the tachometer in the dash if you try to adjust the idle, it is not accurate enough for this.
 

Bia

True Classic
800 - 900 RPM according to the manual. Don't trust the tachometer in the dash if you try to adjust the idle, it is not accurate enough for this.
Is there a way to adjust the idle? I figured that because it was fuel injected it was all controled by the ECU.
 

ng_randolph

Bjorn H
The ECU has virtually no control over the idle speed, and yes, it can be adjusted. See page 10.95 of the FSM (attached). As mentioned earlier, use a proper tachometer for this, not the one in the dash. If you can't get the idle within spec using the idle screw, check the throttle stop screw on the the throttle body as outlined on page 10.104 of the FSM. And if you still have a problem, check that all air / vacuum hoses and fittings are present and in good order.

Edit:
What the FSM does not say is that you have to loosen the lock nut before adjusting the idle screw. After adjustment, tighten the lock nut again, and check that the idle is still in spec. Repeat as necessary.
 

Attachments

  • FSM_102.26_Page10-95.pdf
    14.5 KB · Views: 11
  • FSM_102.26_Page10-145.pdf
    479.4 KB · Views: 11
Last edited:

Bia

True Classic
The ECU has virtually no control over the idle speed, and yes, it can be adjusted. See page 10.95 of the FSM (attached). As mentioned earlier, use a proper tachometer for this, not the one in the dash. If you can't get the idle within spec using the idle screw, check the throttle stop screw on the the throttle body as outlined on page 10.104 of the FSM. And if you still have a problem, check that all air / vacuum hoses and fittings are present and in good order.

Edit:
What the FSM does not say is that you have to loosen the lock nut before adjusting the idle screw. After adjustment, tighten the lock nut again, and check that the idle is still in spec. Repeat as necessary.
Thanks Bjorn much appreciated.
 

SuperTopo

True Classic
99% of the time this means your bushings for the throttle butterfly are worn and sloppy, leading to a big vacuum leak through there causing high idle. That's where I place my bet...
 

fiatfactory

Steve Cecchele
Actually I would say 90% of the time it's because the auxiliary (idle air) bypass valve isn't closing off when it gets hot, that will bump the idle up considerably too....

looks like this

worn bushes on the throttle shaft would be easy enough to see, you should be able to grab the linkage at the butterfly and move it back and forth a considerable amount, with a corresponding change in idle speed.

SteveC
 

Dan Sarandrea (Phila)

Waitin' On Parts...
Actually I would say 90% of the time it's because the auxiliary (idle air) bypass valve isn't closing off when it gets hot, that will bump the idle up considerably too....

looks like this

worn bushes on the throttle shaft would be easy enough to see, you should be able to grab the linkage at the butterfly and move it back and forth a considerable amount, with a corresponding change in idle speed.

SteveC
+1

Worn bushes might allow enough of a leak to account for 200 extra RPMs, but for 1100 extra RPMS you need a bigger hole.

First, remove (easy enough, two big hose clamps and a few smaller ones) and closely examine the black rubbery snorkel between the AFM and the snout of the FI plenum. These have been known to develop cracks in the corrugated/accordion areas.

Other than the intake snorkel, the next biggest hole is as Steve C noted, the aux air system.

Much smaller and thus less likely "false air" leaks can come from the direct vacuum taps into the plenum like the A/C fast idle solenoid and/or the vac line to the A/C controls reserve tank (if equipped), the fuel pressure regulator, the fuel vapor cannister, and the crankcase vent (check the quality of the dipstick seal).
 

carl

True Classic
Spray break cleaner over all those parts while the engine is running, increased idle speed from the cleaner will help you find the air leak, if there is one. My first look is always for linkage too tight and pulling on the throttle.
 

SuperTopo

True Classic
Actually I would say 90% of the time it's because the auxiliary (idle air) bypass valve isn't closing off when it gets hot, that will bump the idle up considerably too....

looks like this

worn bushes on the throttle shaft would be easy enough to see, you should be able to grab the linkage at the butterfly and move it back and forth a considerable amount, with a corresponding change in idle speed.

SteveC
Agreed, super easy to see. I usually just wiggle it by hand and immediately know if it's worn. Most are. Then I slop grease around there and immediately see the idle come back to normal. I've gotten cars past smog by greasing the bushing so it'll idle at the right level long enough to pass. I've yet to see a bypass valve fail. I've seen MANY worn butterfly bushes causing high idle.

I accept the challenge that the 2000 rpm idle is a little high for it to be ONLY due to worn bushings. Typically it's around 1500 rpm max that I've seen. So maybe there's a combination of failures here...
 
Top