LS 400 Idle

[redcarrara]

Hi, I am new here, I have a 90 LS 400 that has developed a wandering idle. It actually cannot idle sometimes and stalls. This started on deceleration off exit ramps. It now happens randomly. I have turned the idle screw up so the car does not stall. Now it either idles at 1000rpm or 1500rpm. It has been to 4 mechanics including the dealer. It will not read or indicate any codes. We replaced the alternator and the car was good for about a month, now it does it all the time. The car has 210,000mi and actually is very good. Is it the O2 sensor, fuel pump, mass air meter or engine cpu? It has been doing this for about 2 months now. I have had the car for 100,000mi , please help, I love the car.
Thanks in advance

 

[Zuffen]

Check and clean the ISCV.

Do a search here for ISCV and it should fix the problem.

You'll them have to re-set your idle speed.

 

[redcarrara]

iscv

I looked at the ISCV which I think is at the front of the intake plenum. I unplugged it and it did not change the idel phenomenon at all. It still idels low when cold, or spontaniously and then later idles faster. I think something is telling the ISCV to change the idle speed. What controlls the ISCV? Is there a temp sensor or is it the engine CPU? There are no idiot lights going on the dash. Please help!!!!!!!

 

[STA]

hey where is the idle screw on the engine as my bro has one of these cars and idles too low. also a pic of the spot would be great.

Cheers
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[redcarrara]

The idle screw is under a cover on the back right of the engine. The cover is held on by 3 or 4 different nuts. Look for the throtle cables going into it. Then the actual throtle setting is a small allen screw and a lock nut thing. If you cannot find it I will photo it for you.
FYI all my problems were a bad engine CPU, the ISCV was not the problem, I cleaned it and nothing changed, so I concluded that it was the computer telling the ISCV what to do. The computer is under the glove box. I got one on ebay that works great. Thanks for everyones help. 5 mechanics could not figure it out, including the dealer!

 

[redcarrara]

I am sorry the cover is at the back center or left of the engine as you look at it.

 

[jibbby]

Your TPS sensor could be faulty....(Throttle position sensor).... ECU may be throwing bad signals as well. However, I think the check engine light would flash on a ECU problem of that magnitude...Check the TPS sensor....

 

[STA]

The idle screw is under a cover on the back right of the engine. The cover is held on by 3 or 4 different nuts. Look for the throtle cables going into it. Then the actual throtle setting is a small allen screw and a lock nut thing. If you cannot find it I will photo it for you.
FYI all my problems were a bad engine CPU, the ISCV was not the problem, I cleaned it and nothing changed, so I concluded that it was the computer telling the ISCV what to do. The computer is under the glove box. I got one on ebay that works great. Thanks for everyones help. 5 mechanics could not figure it out, including the dealer!

I have three of those allen scews under there which one is it?

 

[jibbby]

If your idle is bouncing and or wandering I don't think adjusting the idle screw manually will solve the problem...You will still experience the wandering and or bouncing idle but with just a higher set idle...

Did you inspect and fully clean out the throttle body... A dirty or faulty throttle body gives you a bouncing idle too...