: Idle speed control problem? 90 Deville 4.5

10-24-10, 03:28 AM
Hi everyone, I'm having problems again with my 90 Deville 4.5. The engine seems to be pushing as I slow down, the ISC plunger is holding the throttle open too long. This is called coastdown sail on in the FSM. We have done the idle learn procedure with no results, and my scanner shows that the TPS is reading correctly and no codes. Using the on board diagnostics shows that the ISC is extending fairly well but is slow to retract. I'm thinking it is the ISC but we have put so much money in this car recently I would like to hear from you guys before I spend more. Is there anything else that could cause the ISC to act this way? I'm a bit jaded at this point I guess because it seems with these cars the obvious is not always the answer it can be something else overreacting and driving the offending control to malfunction. Thanks in advance for any help!

10-24-10, 06:40 AM
Sailng is almost always a misadjusted TPS but as long as the TPS was set to the correct value with the ISC plunger NOT TOUCHING the throttle lever, then I would blame the ISC motor.

Provided that you have never messed with the 'min air screw' located on the TB housing itself......

10-25-10, 10:49 PM
Thank you Sevillian for the answer, I did have this TB off a while back to check for carbon, it was clean. No adjustments to the min air screw, TPS or ISC were made at the time and it did run ok afterwards. The ISC is holding the throttle open now so that it never touches the min air screw. Even during the onbaord diagnostic test where you can manually retract and extend the ISC it did not retract to allow contact with the min air screw. So since no adustments have been made and it is now is not or is slow retracting I'm thinking the ISC is bad. I guess it could be disconnected to see how it acts. We had to disconnect the AC compressor on this car would that have this effect on idle?

10-26-10, 11:48 AM
When you command the ISC to retract, it should go in far enough that the engine barely runs, like 450 to 500 RPMs because the throttle should be resting on the min. idle screw at that point. Once you command the ISC to retract, quickly disconnect the ISC wire harness. Shut off engine. Then screw in the ISC shaft to obtain a 30 to 40 thous air gap from the throttle arm. Reconnect the wire harness and restart the car. Report back.

the recluse
10-30-10, 01:11 AM
The ISC is holding the throttle open now so that it never touches the min air screw.

It is suppose to.

Even during the onbaord diagnostic test where you can manually retract and extend the ISC it did not retract to allow contact with the min air screw.

Did you follow full procedure? I know once you get into diagnostic mode you tell the system to retract the ISC plunger, then you can see if it's actually moving or not. Unfortunately, I forget the exact procedure. Will look into...

10-30-10, 02:26 AM
Again, once commanded thru on board diagnostics to retract, you need to disconnect the wiring to the ISC after it stops retracting. Then adjust the ISC shaft to allow the throttle to rest on the min idle screw. Once resting, continue to adjust the shaft to get a clearance. Then shut off engine. Then reconnect its wiring connector. In my experience the retracting takes awhile so hold the button till you are sure it has retracted fully. If after the clearance has been set it still sails on, I would suggest you verify base TPS setting per the factory service manual.

10-31-10, 02:46 AM
What do you do if you want to make you car idle higher like around 700-800 RPM's ?

10-31-10, 09:23 AM
With further testing I find the ISC is retracting after all ( manually retracting the ISC via the on board diagnostiics) and allowing the throttle to rest on the min idle screw, with well over .030/.040 gap. The TPS is in the correct range per FSM. Road testing however, releasing the gas pedal the car will maintain 25/35 mph without touching the gas pedal... no coast down or very little for blocks. As you brake to a stop the rpms are in the 1200 rpm range dropping to the normal 600/700 after stopping (like pressing the brake and accelerator at the same time). Disconnecting the ISC (not easy) causes the ISC to extend fully with eng rpm jumps to around 3000rpm. As to an answer for quins90lac, I can see that you could change the base idle by adjusting the min idle screw, as long as the ISC is not interfering. However I have not changed any adjustments on this car and the ISC at this point seems to hold the throttle open too long, or has a delay in coast down.

10-31-10, 11:22 AM
Well over .030? Then adjust the shaft to get to a specific .030. The TPS reading should be at zero at min. idle thru the on board display. Any reading above zero will cause this sail on. Is the base timing at spec? advanced timing can affect it. A weak throttle return spring can cause it as well. My bet is on a misadjusted TPS

the recluse
10-31-10, 12:09 PM
Well over .030? Then adjust the shaft to get to a specific .030.

:hitstick: I agree. If it was out of spec, why wouldn't you fix it? :helpless:

My bet is on a misadjusted TPS

I agree here also, however, my ICS gave me the same fit and after adjusting it and the TPS; it turned out BOTH were for crap. Run full procedures on both.

As a side note, some manuals tell you to disconnect the green clip on the alternator when running this procedure...

11-02-10, 12:15 AM
Well the ISC is allowing the throttle to rest on the min air screw when retracted via onboard diagnostics. The TPS voltage readings are good (according to the FSM) for retracted and full extend ISC settings via onboard diagnostics. When the ISC is retracted the gap between the ISC and throttle is a bit more than carnuts spec of .030 to .040 but to adjust the ISC out seems to be the opposite thing to do if the problem is that the ISC is holding the throttle out too long on coastdown. The FSM says to ground the green wire from the alternator to check and set the TPS and ISC which I did not do. Grounding the alternator on a running engine??? This FSM (90 Deville) has been wrong in the past...anyone know for sure if it is OK to do that?

11-02-10, 11:52 PM
I spent some time testing on this today and came up with the following.The FSM specs for the TPS voltage while seated on the min air screw are 0.45 to 0.55 (reads 0.47) while the ISC is retracted and between 1.15 and 1.20 vts (reads 1.18) while extended so the TPS seems to be set correctly. Using the on board diagnostics P.0.1 throttle angle is -.6 which according to another post on here is too low if I understand it correctly, nothing in the FSM that I can find on that however. The idle while on the min air screw is a bit high at 700 rpm (should be 525rpm) so that will get adjusted as soon as I can find a tool to do so. The other thing that bothers me is that the ISC will extend in less than 10 seconds but it takes over 80 seconds for it to retract fully. I know to check the TPS after I adjust min idle speed but how about that slow retracting ISC, anyone have any ideas on that? BTW I did not mess with the min idle screw, but it may have been adjusted while in a shop to diagnose other problems a while back, idk. Any thoughts on this will be appreciated.

11-03-10, 02:23 AM
Adjust the minimum idle screw to lower the base idle to somewhere around 525 RPM. then recheck the TPS to see if its closer to zero in P.O.1. - .1 is ok. If the minimum idle is 700 all other settings will be wrong. You might have to readjust the TPS once base idle is lowered. Slow ISC retraction is OK because in normal use the ISC never fully retracts. We are making headway!

11-03-10, 04:10 PM
Sounds like a plan to me carnut, thanks. They sure don't make that min idle screw easy to get to. I bought a flex driver today hopefully it will allow me to do this adjustment.

11-03-10, 05:30 PM
I used a 1/4 drive ratchet with an extension or a 1/4 drive nut driver. Its ackward on purpose! they didn't want us altering the emission setting in the driveway. At least you are not drilling out a plug over the adjustment screws on a carb!