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1991 Seville


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Please tell me what is out of adjustment when driving on flat road, shift to neutral, and the RPM jump up to about 2000. I have mounted a video camera at the ISC plunger/throttle area, and it is plain that the plunger holds the throttle open until the vehicle comes to a near complete stop (by either coasting or braking). At about 3-5 MPH the idle will begin to drop, and go down to about 700. Then I can accelerate up to 8-10 MPH, go to neutral, and back it goes up to 2000. Needless to say, I can feel the extra pull when initially braking. Earlier the TPS was replaced, and adjusted to .2 and a step in that adjustment is to retract the ISC motor. Initially I thought I could adjust the screw, but this does not help. However, it will always idle at 700 after warmup like it should. Trouble is only coasting..and I am using way too much gas. Halp please. Al

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Sounds like the closed throttle switch in the ISC motor assembly has failed. There is a TON of information in the archives on this.

There is a way to test the switch via onboard diagnostics and using the status of one of the climate control indicator lamps. That is somewhere in the archives also.

Edited by KHE

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Please tell me what is out of adjustment when driving on flat road, shift to neutral, and the RPM jump up to about 2000. I have mounted a video camera at the ISC plunger/throttle area, and it is plain that the plunger holds the throttle open until the vehicle comes to a near complete stop (by either coasting or braking). At about 3-5 MPH the idle will begin to drop, and go down to about 700. Then I can accelerate up to 8-10 MPH, go to neutral, and back it goes up to 2000. Needless to say, I can feel the extra pull when initially braking. Earlier the TPS was replaced, and adjusted to .2 and a step in that adjustment is to retract the ISC motor. Initially I thought I could adjust the screw, but this does not help. However, it will always idle at 700 after warmup like it should. Trouble is only coasting..and I am using way too much gas. Halp please. Al

Have you done all the adjustments yourself?

When you install new TPS (what was wrong with old one?), you retract the ISC motor plunger and let the throttle lever rest at the so called minimum air screw. After adjusting that screw so that you would get around 0 is all you need to do. Th eold adjustment of ISC motor should have been fine. Looks lik eyour initial problem was rather faulty ISC motor than TPS. In ten years I went through three ISC motors on my 1991 Seville.

The saddest thing in life is wasted talent

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Barry95, did a nice tutorial on cleaning the ISC motor with photos and showing the contacts, see it here. KHE is correct the closed throttle contacts get dirty and need cleaning or the plunger does not retract on closed throttle

http://caddyinfo.ipb...?showtopic=7605

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Thanks KHE: I am new here and do not see a reference to "archives." The system will let me look at the last 200 posts, but the subject ISC motor only comes up on a couple of them. Can you help me get to "Archived" please. Al

Do a search on "ISC" or "Idle Speed Control" up at the top left - below the Cadillac picture and above the "terms of usage/view new content" buttons. You should see a lot of posts on the ISC. Body by Fisher also posted a good link on the ISC.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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ISC adjustment (bbob)

Forget the idle learn procedure. It doesn't apply in this case.

First clean the throttle body bores below the blades thoroughly if you haven't already. Good time to clean out the EGR tubes if you haven't done those either.

With the engine idling, hold the throttle lever open slightly by hand to get the engine to rev to 2000 RPM or so. Depress the plunger on the idle speed motor with your finger. It should start retracting. Hold the idle speed up with the throttle lever and keep depressing the plunger so that the idle speed motor retracts fully. When it is fully retracted, freeze it by disconnecting the wire connector. It helps to have the lock undone ahead of time so you can yank the wires off without theidle speed control motor extending any.

Return the throttle lever to idle. This is the closed bore idle position. There should be about .030 clearance between the plunger and the throttle lever. Adjust the plunger by turning it to get about .030. The engine should be idling about 450-500 RPM warmed up with the AC off with the gap between the plunger and the throttle lever. If not, or if the RPM will not come down you need to find out why. Check the throttle cable to make sure it is slack and allowing the throttle to go to the closed bore position. Check the cruise control link. It, too, should have a slight amount of slack in it when the throttle is in the closed bore position. If not, disconnect the cruise link and reattach so that there is some slack in the link when the engine idles down to 450-500 at the closed bore position.

If all this is ok so far then you should be in good shape. I doubt that the TPS is off or needs adjusting. You can check it as described but it is probably fine.

It sounds like something may be holding the throttle on occassion so as to keep the throttle lever from contacting the idle speed control motor plunger and closing the nose switch or the closed throttle switch....that click you feel when you first depress the ISC plunger. If that switch is not making when the throttle returns to idle then the idle speed control will not work...much like you describe.

Enter into the diagnostics mode with the engine idling. After the codes display the system should show PCM? Press the HI button to get into the PCM mode. Now look at the climate control panel. See the "LO" icon? That signals the status of the closed throttle switch. When the LO is lit the switch is closed. It should lite up to signal the switch is closing anytime you release the throttle. Watch it while driving. If the closed throttle switch does not close when you are releasing the throttle then something is restricting the throttle movement or the idle speed control motor plunger is sticky and/or the switch inside is defective.

Make the checks as described and report back....

The saddest thing in life is wasted talent

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Adallak: Thanks,I will follow your instruction sheet Saturday, AM. Today I removed, disassembled,cleaned and lubed, and reassembled and remounted the ISC. Grease was very dry. Switch was cleaned and ohm=meter tested thru the 8 inch cable. Switch is fine. At engine start it was very smooth, but the RPM accceleration started before I got out of my driveway. So I drove it about 20 miles,it gets better mileage than yesterday, but the idle while coasting is still 1700. Bacl in my 'hood I rolled from a stop, in neutral, no touch the gas, down a slight down-angle. As soon as it hit 5 MPH, it idle up to 1700, like before. I took a video of the ISC/plunger while in motion, and one can see at the end when the roll gets back down to 5 MPH, the plunger will move, and allow the throttle to go back to zero (as the RPM descends to 700 at vehicle stop. Manual retraction movement of the plunger is very easy now with the reworked ISC motor.

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Adallak: Thanks,I will follow your instruction sheet Saturday, AM. Today I removed, disassembled,cleaned and lubed, and reassembled and remounted the ISC. Grease was very dry. Switch was cleaned and ohm=meter tested thru the 8 inch cable. Switch is fine. At engine start it was very smooth, but the RPM accceleration started before I got out of my driveway. So I drove it about 20 miles,it gets better mileage than yesterday, but the idle while coasting is still 1700. Bacl in my 'hood I rolled from a stop, in neutral, no touch the gas, down a slight down-angle. As soon as it hit 5 MPH, it idle up to 1700, like before. I took a video of the ISC/plunger while in motion, and one can see at the end when the roll gets back down to 5 MPH, the plunger will move, and allow the throttle to go back to zero (as the RPM descends to 700 at vehicle stop. Manual retraction movement of the plunger is very easy now with the reworked ISC motor.

This is very strange... When engine is at operational temperature, with plunger retracted completely and the throttle lever resting against so called minimum air screw, what RPM do you get? Should be around 500.

The saddest thing in life is wasted talent

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Thanks, sorry I was away for awhile. 1. When I put in the new Throttle Position Sensor last week (first thing done other than new plugs, because of code 046) I did the electric plunger retraction(not with my finger), and when the cable was unplugged I remember the drop in RPM(because the procedure speaks of the possibility of stalling), and then adjusted the TPS to .20. The idle was low, I guess around the stated 450-500. Now, however, when throttle closed to the min screw, it is always aroung 650-700, and the RPM do not change with the A/C on?? The cable has slack and the cruise cable is disconnected(by me), so help me figure how the RPM is higher when the screw has never been adjusted? In fact, the screw is pretty well hidden by the plunger, and it appears that one would have to remove the ISC motor to get a tool down there.(FYI--yesterday I disassembled, cleaned, greased the ISC motor along with cleaning the switch points and ohm-metered continuity of the switch back thru the short cable that crosses over toward the area where the cruise vaccuum device is.)

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Thanks, sorry I was away for awhile. 1. When I put in the new Throttle Position Sensor last week (first thing done other than new plugs, because of code 046) I did the electric plunger retraction(not with my finger), and when the cable was unplugged I remember the drop in RPM(because the procedure speaks of the possibility of stalling), and then adjusted the TPS to .20. The idle was low, I guess around the stated 450-500. Now, however, when throttle closed to the min screw, it is always aroung 650-700, and the RPM do not change with the A/C on?? The cable has slack and the cruise cable is disconnected(by me), so help me figure how the RPM is higher when the screw has never been adjusted? In fact, the screw is pretty well hidden by the plunger, and it appears that one would have to remove the ISC motor to get a tool down there.(FYI--yesterday I disassembled, cleaned, greased the ISC motor along with cleaning the switch points and ohm-metered continuity of the switch back thru the short cable that crosses over toward the area where the cruise vaccuum device is.)

Hard to say. Are you sire the throttle lever rests against the minimum air screw (there is no gap)?

I would clean the throttle bores and the back sides of the butterflies with a carb cleaner and disconnect the battery for a minute to reset factory settings for clean throttle body. Recheck RPM after that procedure. Always perform tests when the engine at operational temperature.

The saddest thing in life is wasted talent

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Thanks for hanging with me. I did a good spray into the TB and into the "throats." Since this started after (but not immediately after) I replaced the TPS. So I thought I would reset it. Last week I set it at +.20. But recently at idle I have seen a -0.2. So I redid it to +.2 having electrically retracted the plunger first, and unpluging it. Then I reset the TPS. When I started it, the veh will idle now closer to 850, not the 700 I have always seen. (Do you have any idea what the TB would indicate at about 2500 RPM?---would it be more than +5.0 or always less.)

The throttle always rests on the screw when the plunger is retracted. But it has always been 700-750 RPM, not the 450-500...that can only happen by turning the screw, which appears to require the ISC to be removed so a screwdriver can turn the screw. By the way, the Cad dealer service advisor said 700 is the number, but the forums always say 450-500....any idea if the rep might have misunderstood?? Seems odd since the car came to me in Tenn and the local Cad people have never seen it, so someone else is also doing 700 I guess. Advise please?

In previous posts I mentioned the video I did while driving (thanks for wood clamps and duct tape!) and when slowing for a stop I can hear the RPM drop and see the plunger retract. Would this not confirm that the switch is operating, if it were not I assume it would never retract at all?

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Thanks for hanging with me. I did a good spray into the TB and into the "throats." Since this started after (but not immediately after) I replaced the TPS. So I thought I would reset it. Last week I set it at +.20. But recently at idle I have seen a -0.2. So I redid it to +.2 having electrically retracted the plunger first, and unpluging it. Then I reset the TPS. When I started it, the veh will idle now closer to 850, not the 700 I have always seen. (Do you have any idea what the TB would indicate at about 2500 RPM?---would it be more than +5.0 or always less.)

The throttle always rests on the screw when the plunger is retracted. But it has always been 700-750 RPM, not the 450-500...that can only happen by turning the screw, which appears to require the ISC to be removed so a screwdriver can turn the screw. By the way, the Cad dealer service advisor said 700 is the number, but the forums always say 450-500....any idea if the rep might have misunderstood?? Seems odd since the car came to me in Tenn and the local Cad people have never seen it, so someone else is also doing 700 I guess. Advise please?

In previous posts I mentioned the video I did while driving (thanks for wood clamps and duct tape!) and when slowing for a stop I can hear the RPM drop and see the plunger retract. Would this not confirm that the switch is operating, if it were not I assume it would never retract at all?

700 is way high for minimum air setting. The FSM (factory service manual) recommended 450. If the engine has high mileage, it may be 500 but that's about it. The RPM should be as low as possible, but high enough so the engine would not stall. Forget 700, that's just wrong.

Do not worry about -0.2 0r +0.2 . You can never get it at 0.0 and it i snot that important. Just disconnect the battery for a minute, bring it to operational temperature, retract the plunger, let the throttle lever rest against min air screw and read the RPM. If it is not close to 500, remove the ISC motor readjust the minimum air screw. MAKE SURE THE TPS IS SECURED PROPERLY.

The saddest thing in life is wasted talent

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Adallak: Thanks, wife & I have to go visit family this PM. I'll remove the ISC tomorrow,adjust for 475 (veh only has 66,000 miles!! I got it 5 years ago with 37,000 on it), look at the TPS value and correct if necessary. when the plunger is retracted and unplugged, and I adjust for .030, should I take battery cable loose before or after reconnecting ISC plug, or does it matter? Can you even guess why the California mech's would set for 700? In order for the RPM to increase on its own, the screw would have to turn "in" against the spring tension, so it seems that it would had to have been done deliberately?? Oh well, I did say the word "California" didn't I, never mind!

I will report back on Monday. Thanks, Al

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Adallak: Thanks, wife & I have to go visit family this PM. I'll remove the ISC tomorrow,adjust for 475 (veh only has 66,000 miles!! I got it 5 years ago with 37,000 on it), look at the TPS value and correct if necessary. when the plunger is retracted and unplugged, and I adjust for .030, should I take battery cable loose before or after reconnecting ISC plug, or does it matter? Can you even guess why the California mech's would set for 700? In order for the RPM to increase on its own, the screw would have to turn "in" against the spring tension, so it seems that it would had to have been done deliberately?? Oh well, I did say the word "California" didn't I, never mind!

I will report back on Monday. Thanks, Al

Disconnect battery for a minute before you adjust minimum air.

BTW, there is a way to manually retract the plunger. Turn engine on, press on the plunger to activate the internal switch, keep the pressure until the plunger retracts completely and stops. At that point disconnect the connector of ISC motor.

The saddest thing in life is wasted talent

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Thanks for the help. Some good news for a change. Today...I removed the ISC motor, got up to op temp, screwed out the min idle screw to pull idle RPM to about 500; retracted the ISC plunger electrically, then additional retraction with the finger to clear the throttle at idle; unpowered (freeze)the ISC; adjusted for about .030, and with engine off, readjust TPS to -.20. Then turned ign off, replugged the ISC, then removed a battery lead for 15 min. At start was pretty smooth, idle in neutral does go up to 1200 (but not 1900-2000). Inst MPG much closer to my recollections as before. Can not feel the high RPM pulling when braking, because it is lower. Got up to highway (60) and coasted in Drive, on gentle downgrade Inst MPG actually showed 70 a couple of times, but usually 60-65. Much happier camper here today! Think I will recheck the gap tomorrow. By the way, yesterday I had unclipped the cable at the Cruise servo when trying to verify problem not in the Cruise, and I just realized I have not reconnected it yet. Just before I got home I got Serv Eng light, and see an E 048, which is EGR. Today I had to pull the small vaccuum tube off the EGR to make room to unscrew/screw adjust the TPS, but I put the tube back on before driving. I did however remove and reattach the EGR itself last week when I was replacing the TPS because you cannot back out one TPS screw because it hits the base of the EGR. I did not spray into the bottom of the EGR, and probably should have. Friend has the correct distr wrench to get under the EGR. I think I will go see him tomorrow and spray it out good from the bottom, and remount to see if the code comes back. Will replace is needed. I am guessing my mileage has easily doubled based on ECON numbers above. I will report back Monday. Thanks, Al

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Thanks for the help. Some good news for a change. Today...I removed the ISC motor, got up to op temp, screwed out the min idle screw to pull idle RPM to about 500; retracted the ISC plunger electrically, then additional retraction with the finger to clear the throttle at idle; unpowered (freeze)the ISC; adjusted for about .030, and with engine off, readjust TPS to -.20. Then turned ign off, replugged the ISC, then removed a battery lead for 15 min. At start was pretty smooth, idle in neutral does go up to 1200 (but not 1900-2000). Inst MPG much closer to my recollections as before. Can not feel the high RPM pulling when braking, because it is lower. Got up to highway (60) and coasted in Drive, on gentle downgrade Inst MPG actually showed 70 a couple of times, but usually 60-65. Much happier camper here today! Think I will recheck the gap tomorrow. By the way, yesterday I had unclipped the cable at the Cruise servo when trying to verify problem not in the Cruise, and I just realized I have not reconnected it yet. Just before I got home I got Serv Eng light, and see an E 048, which is EGR. Today I had to pull the small vaccuum tube off the EGR to make room to unscrew/screw adjust the TPS, but I put the tube back on before driving. I did however remove and reattach the EGR itself last week when I was replacing the TPS because you cannot back out one TPS screw because it hits the base of the EGR. I did not spray into the bottom of the EGR, and probably should have. Friend has the correct distr wrench to get under the EGR. I think I will go see him tomorrow and spray it out good from the bottom, and remount to see if the code comes back. Will replace is needed. I am guessing my mileage has easily doubled based on ECON numbers above. I will report back Monday. Thanks, Al

70 MPG INS while costing down is normal. Before doing anything to EGR valve (it is pain to remove), remove air duct t access the TB. Lift the butterflies and secure thier wide open position with a piece of cork. You will see two EGR tubes sticking out under the butterfly. They may be partially clogged with carbon, if you never cleaned thew. Spray some carb cleaner and rod them with a piece of coat hanger. Do not worry about chinks of carbon falling down. That will most likely solve 048 code. Do nos spray any solvents on the rubber diaphragm of EGR valve, it will be destroyed.

Make sure diaphragm holds vacuum. Depress the diaphragm of EGR valve with fingers all the way and plug the vacuum input with finger. The diaphragm should not move back.

The saddest thing in life is wasted talent

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