quick ? about setting minimum idle..very specific

[RedRod90]

I've never asked,and don't remember ever seeing the answer.When setting minimum idle speed..should the car be at operating temperature,or cold,as in it has sat all night? I'm having trouble keeping the car at idle first start in the morning,in fact it will not idle at all,then it's fine the rest of the day.Also,how accurate is the factory tach in a '90? It does not seem very accurate to me.
Thanx
Ronnie

 

[vette079]

You probably have the 1990 Vette tach no work disease, (feared by the whole 1990 vette owner-society).
That´s why I have a 1991.
It can be the instrument cluster itself or something called tach-filter wich is mounted either on the rear left bank right under the dizzy connected with a white and a (?) brown wire to the dizzy, or taped on the big wiring harness under the wiper motor (small 2" aftermarket black box).
I´m not 100 point sure but I think I´ve read somewhere that it is a 2k ohm resistor, but don´t nail me on that one.

Idle, check the IAC and the TPS first.
The two top terminals on the TPS should be checked with a DVM.
Engine off, red DVM prong on top terminal, black on middle terminal.
DVM on 20 volts.
Idle should read 0.54 volts.
Steadily raise the accelerator via the TB.
Volts should steadily increase to 4.6 at WOT when at full throttle.
Don´t disconnect the connector from the TPS, you should be able to pry the DVM prongs through the isolation.
Check the 0.54 at idle first, could be off a little.

IAC, disconnect the connector at the IAC, put the DVM prongs on the two top terminals in the IAC, DVM on 2K ohm, it should read between 40 and 80 ohm if I´m not mistaking.
Again, don´t nail me on the 40-80 ohm here.

Sure someone out there knows a bit more about that.

 

[redvett]

When setting the Min idle you must have a scanner to read IAC counts & engine must be at 195F in "D" or "R" IAC with A/C off should be 15 - 30 counts.

 

[UNCLEBILL]

You probably have the 1990 Vette tach no work disease, (feared by the whole 1990 vette owner-society).
That´s why I have a 1991.
It can be the instrument cluster itself or something called tach-filter wich is mounted either on the rear left bank right under the dizzy connected with a white and a (?) brown wire to the dizzy, or taped on the big wiring harness under the wiper motor (small 2" aftermarket black box).
I´m not 100 point sure but I think I´ve read somewhere that it is a 2k ohm resistor, but don´t nail me on that one.

Idle, check the IAC and the TPS first.
The two top terminals on the TPS should be checked with a DVM.
Engine off, red DVM prong on top terminal, black on middle terminal.
DVM on 20 volts.
Idle should read 0.54 volts.
Steadily raise the accelerator via the TB.
Volts should steadily increase to 4.6 at WOT when at full throttle.
Don´t disconnect the connector from the TPS, you should be able to pry the DVM prongs through the isolation.
Check the 0.54 at idle first, could be off a little.

IAC, disconnect the connector at the IAC, put the DVM prongs on the two top terminals in the IAC, DVM on 2K ohm, it should read between 40 and 80 ohm if I´m not mistaking.
Again, don´t nail me on the 40-80 ohm here.

Sure someone out there knows a bit more about that.

Im going to throw a bit of a curve here. The TPS setting for 1990-91 ONLY is .36 to .96 volts per the FSM. Also per G. Killebrew mini "Questions and answers"; the 90-91 are non adjustable and normal setting is .72 volts.
When I checked mine (1990); it showed .78 volts. I was having a problem of when cold in morning or late at nite;it seemed that my power wasnt there..no stumbling..just wasnt there till warm up. The best I could adjust was to .66 volts. I drilled the bolt holes bigger (.06" bigger in dia.) and got it down to .58. Ran better then dropped down to .56...started to feel sluggish again. My Vette is stored now but will bump it up to .65 volts and try out next year. I didnt realize till after the difference of voltage for 1990-91 till I read my FSM and G.K. write up after the above try outs.
I dont know why the difference for the 1990-1 years.......

 

[JAKE]

If you have an adjustable TPS it's set with the ENGINE OFF but ignition ON.

To set the minimum idle speed on a L98, first fire the engine and allow it to idle until it warms up and enters CLOSED LOOP. Wait about five minutes.

Shut down the engine and disconnect the IAC. Refire the engine. You may have to play with the throttle on the TB to get the engine to idle with the IAC disconnected, but it should after several seconds of fiddling.

Using a short, I believe, torx head tool turn the idle speed screw. To access the screw you may have to remove a factory installed plug unless someone has already done so. The screw is on the windshield side of the TB, above the throttle/cruise control cables.

Adjust the idle speed. From memory I believe the factory manual says to set idle speed at 450 RPM but I've found 550 RPM to work better. Once that's done, shut down the engine.

Now you have to re-set the TPS because adjusting the idle speed changes the TPS voltage reading, that is unless you have the NON-ADJUSTABLE TPS that came on later L98 engines. I forgot what year GM began that.

On LT1/4 engines, all that is controlled by the PCM, which differs from the ECM, so it's not manually adjustable.

Also, you can reset the IAC to ZERO counts by using the ole' "paper-clip in the ALDL" trick.

I wrote this from memory, so to do a double-check on me, you can go to the forums.corvetteforum.com site and look in the TechTips section for the posted procedure.

Jake

 

[vette079]

Im going to throw a bit of a curve here. The TPS setting for 1990-91 ONLY is .36 to .96 volts per the FSM. Also per G. Killebrew mini "Questions and answers"; the 90-91 are non adjustable and normal setting is .72 volts.
When I checked mine (1990); it showed .78 volts. I was having a problem of when cold in morning or late at nite;it seemed that my power wasnt there..no stumbling..just wasnt there till warm up. The best I could adjust was to .66 volts. I drilled the bolt holes bigger (.06" bigger in dia.) and got it down to .58. Ran better then dropped down to .56...started to feel sluggish again. My Vette is stored now but will bump it up to .65 volts and try out next year. I didnt realize till after the difference of voltage for 1990-91 till I read my FSM and G.K. write up after the above try outs.
I dont know why the difference for the 1990-1 years.......

I´m with you. I thought it was only the 91 that had the 0.3-0.9 volts TPS unadjustable.
Didn´t do my homework I guess (not the first and probably not the last timeotfl).
I did something similar to mine, I loosened the torx screws a little and noticed there was a little play and could adjust it without drilling.
On the other hand, drilling out the holes does help to adjust if needed/wanted.
Maybe something for me to consider.

 

[vette079]

If you have an adjustable TPS it's set with the ENGINE OFF but ignition ON.

To set the minimum idle speed on a L98, first fire the engine and allow it to idle until it warms up and enters CLOSED LOOP. Wait about five minutes.

Shut down the engine and disconnect the IAC. Refire the engine. You may have to play with the throttle on the TB to get the engine to idle with the IAC disconnected, but it should after several seconds of fiddling.

Using a short, I believe, torx head tool turn the idle speed screw. To access the screw you may have to remove a factory installed plug unless someone has already done so. The screw is on the windshield side of the TB, above the throttle/cruise control cables.

Adjust the idle speed. From memory I believe the factory manual says to set idle speed at 450 RPM but I've found 550 RPM to work better. Once that's done, shut down the engine.

Now you have to re-set the TPS because adjusting the idle speed changes the TPS voltage reading, that is unless you have the NON-ADJUSTABLE TPS that came on later L98 engines. I forgot what year GM began that.

On LT1/4 engines, all that is controlled by the PCM, which differs from the ECM, so it's not manually adjustable.

Also, you can reset the IAC to ZERO counts by using the ole' "paper-clip in the ALDL" trick.

I wrote this from memory, so to do a double-check on me, you can go to the forums.corvetteforum.com site and look in the TechTips section for the posted procedure.

Jake

Your right on that one, I recall now that I have it bookmarked somewhere.