Ok I just bought a '92 Coupe Deville I am having two problems. Problem one the car idles rough in and out of gear and likes to stall at red lights and in park etc. Sometimes it does not idle rough and stall and sometimes it does. This is not my first early 90's Cadillac so I have complete intentions on cleaning the throttle body PCV valve etc... Problem two needs to be solved first though. The car will not start sometimes. This is especially troublesome when it stalls at a red light. I can cleary here the solenoid click when the key is turned but aboslutely no evidence of the starter motor turning. This still allows the battery to run down over time though. I have only had the car one day and this has happened three times. The last time thankfully was in my driveway. I was letting the car run to get to normal temp to check the tranny fluid which thankfully was nice and clean. After this the car would not start and has not started for a few hours now. The battery is so drained I can't even check for codes. I am hoping there will be enough charge by morning to check the codes otherwise i will need to pull the battery to charge it and that will clear the codes. Right now i am leaning toward either a solenoid, battery or starter. Am I on the right track? I have also read that the ground cable from the starter to the frame may need to be cleaned and moved. Is this a possiblity? Also I did notice once or twice the starter hesitating while cranking. I may also be worth mentioning that when i bought the car it needed to be jumped in order to start. It did sart quickly and started right back up after only a short 10- 15 mile test drive. Thanks for any help in advance.

OK I tried to check the codes this morning and the battery is completely shot. This leads me to believe it is the batter. I have it on a charger now and will be going to the auto parts store as soon as it is charged to have it checked and more than likely replaced. Could this cause all the problems with the car? I hope so.

After careful inspection of the title this car is not a 92 but 90
OK just got back from the battery store and the battery was weak but good and did take and hold a charge. Starting problem seems better of course this is only after 20 or 30 min of driving.

Error codes before battery charge

e52 Reset
e53 Dist Signal Interrupt
e96 Torque converter overstress

Error codes after battery charge

E21 Shorted TPS Signal
E26 Shorted Throttle Switch Signal

So now I am leading toward a new TPS and probably a new distrib cap rotor and coil.

Also i noticed that idle is steady and smooth in and out of gear on cold start for about 5 min. or 5 mi which ever comes first.

Any suggestions comments or ideas are gladly accepted.

thanks in advance

The throttle switch is inside the ISC motor. I'd check to make sure that's functioning properly first. If the throttle switch isn't closing, the idle will be rough. These ISC motors go out all the time. It's the black motor with a plunger coming out of it at the front of the throttle body. Try that first. If the ISC isn't working your car will definitely stall if the A/C compressor turns on and the idle speed isn't increased, or if you suddenly turn the wheel.

In other words, before replacing the TPS, check the ISC motor. When you shut the car off, the gas pedal should move up and down 3 times.

OK the gas pedal does nothing when I turn the car off so I guess I need to change the ISC motor. I have done this before but can someone give me the instructions on how to reset the ISC motor. Something about holding the plunger down etc.. I just don't remember

Awadeki is right, the throttle position SWITCH is inside the ISC, and if it starts going wonky you can have lots of problems. Replace the ISC and see how things go from there.

The Haynes book has a description of the adjustment process. Replacing the ISC itself is pretty simple, but the adjustment instructions are more intricate than I want to transcribe here. It's pages 4-15 through 4-16 if you want to go look at the parts store.

Dist signal interrupt is responsible for stalling. You may have bad wiring to the distributor, bad ignition control motor, bad hall effect switch, etc. If all the cables, contacts, grounds, battery is fine, replace the starter. Probably the ISC motor needs to be replaced too. I went through three ISC motors,

Dist signal interrupt is responsible for stalling. You may have bad wiring to the distributor, bad ignition control motor, bad hall effect switch, etc. If all the cables, contacts, grounds, battery is fine, replace the starter. Probably the ISC motor needs to be replaced too. I went through three ISC motors,


I am going to replace the battery terminals tommorow and move the ground or at least get my wire brush out and do a little scrubbing. Ever since the battery charge at the auto parts store this morning no more starter issues so i am going to keep my fingers crossed and hope that was the problem. To be honest it probably was the battery was really in bad shape.

So I need to check the wiring to the distributor and if the problem still persists I should change the ICM and the hall effect switch and more than likely the ISC motor, correct?

If your closed throttle switch isn't working (i.e. the "Off" indicator doesn't light when you take you foot off of the gas pedal in diagnostic mode), then you definitively need to replace your ISC.

HUF is right, distributor issues can cause the sudden stalling. An ISC-induced stall wouldn't feel like you just shut the key off. It'd be more like the engine was under to heavy of a load for the current idle speed, and slowly died.

OK I do believe the ISC needs to be replaced but let me explain the stalling issue in greater detail. When i start the car in the morning it runs perfect not missing no unstable idle nothing out of the ordinary. After driving about 20 miles or so the car runs fine at any speed other than an idle but at idle the engine surges misses slightly and the idle bounces up and down until the engine finally cuts out. It does seem that the more I drive the car with premium fuel and a bottle or Marvel Mystery Oil the more stable the idle becomes and the longer it remains stable. Also worth mentioning I noticed that the PCV valve likes to pop out of the valve cover from time to time. I believe this is the wrong PCV valve and plan to rectify this ASAP. Could all this just point to the need for a tuneup? Plugs, wires, cap and rotor should probably make a huge difference... I hope

OK I do believe the ISC needs to be replaced

The ISC may be bad, but before replacing it, monitor its internal switch as Awadecki indicated (i.e. thru the Diagnostics).

Additionally, test the ISC by the following:

1. With the engine running, get under the hood and manually increase the engine speed to 1.5k - 2k rpm by moving the throttle cam.

2. With your other hand, push the ISC's plunger back in to close the internal switch (it will move a very small amount...3-5mm?). With the ISC's plunger pushed in, the plunger should start to retract. Let it retract until it is all the way in. Now, release the throttle cam.....engine rpm will decrease to ~400rpm....the ISC should extend to increase the engine speed back to a normal idle around 600rpm.

If the ISC does not retract or extend as indicated above, then the ISC is likely bad.

How do I monitor the ISC through the on board diagnostics? I have looked but can't find any info.

Hold "Off" and "Warmer" on the climate control panel for about 2 seconds. All lights will flash on, and the service engine lights will illuminate. Watch the climate control panel. When you take your foot off the gas, the "Off" indicator should illuminate, when you depress the pedal, it should go off. This monitors the closed throttle switch in the ISC.

When done, shut the car off, or just press the "Auto" button to exit diagnostic mode.

OK the ISC seems OK. I did change the ICM it makes a huge difference but I really need a new cap and rotor. All the terminals are black, chipped etc... To top it off I dropped one of the damn bolts to the rotor. Oh well another trip to the auto parts store. Gas mileage went from 16-17 MPG to 19-20 MPG and I am certain these numbers will only increase with the addition of the new cap and rotor (wires and plugs probably won't hurt either) Thank you so much to everyone for all the help.

OK the ISC seems OK. I did change the ICM it makes a huge difference but I really need a new cap and rotor. All the terminals are black, chipped etc... To top it off I dropped one of the damn bolts to the rotor. Oh well another trip to the auto parts store. Gas mileage went from 16-17 MPG to 19-20 MPG and I am certain these numbers will only increase with the addition of the new cap and rotor (wires and plugs probably won't hurt either) Thank you so much to everyone for all the help.

Sounds good. Do the easy stuff first. Get a new cap and rotor. See if it helps. Replacing the spark plugs on a 4.9 is not very fun because of access problem. If you go for new plugs, twist the boots really well before pulling them off. I did the rear ones by laying across the engine. There is more space around rear plugs, you just do not see them and have to act by touch.

Well one piece at a time as i can afford it SO far NEW ICM, and rotor. Still stalling... Next week new cap wires and plugs... I hope.

All the terminals are black, chipped etc

For a short term fix, you can try cleaning the cap's terminals w/ emory cloth or emery board (worst case, use sandpaper)....but it does sound like the cap really needs to be replaced, as you mentioned.

Heres the thing. Since I put in the rotor and the ICM the computer throws code e23 and runs worse than ever... I did find a busted vacuum hose and fixed that. Idle smoothed out a bit. I did clean the cap that helped some too... But whats the e23 code EST Signal? Timing? but why now with a new ICM?

Hello, n00b hthorn here...first post, FWIW...I once had a DeVille with an otherwise OK and < 130,000-mi 4.5. It would sometimes fail to start, and it gave the Distributor Signal Interrupt code. Replacing the battery (shorted cell) solved this. In your case, it sounds as though you have a combination of problems, one of which could be a vacuum leak. You can (carefully) use a can of ether or brakleen to troubleshoot for it (engine on of course), or you can simply replace all the vacuum hoses, and you could be on the right track with the PCV valve. Sorry I don't have the Haynes manual handy so I can't look up the e23 code...I have heard about defective power steering control switches causing stalling problems. If this switch fails, the idle is not increased enough to compensate for the additional load imposed by turning the wheel at idle; the engine fights to stay on a bit, then loses its struggle and dies...