View Full Version : ISC Adjustment Procedure for OBD1 How To

07-08-06, 08:26 AM
I have 1995 Eldorado/Northstar engine with OBD1.
I have replaced the ISC and there are NO codes.

Currently the plunger is fully retracted and the screw is set at ~.03 gap from the throttle arm and NOT plugged in. This means that the throttle plate is FULLY closed inside the throttle body and the engine will not run in this conditon.

I need a step by step procedure to properly adjust the throttle ICS.

07-08-06, 10:42 AM
You have already adjusted it. Plug it in and start it. There is an idle learn proceedure (that I do not recall and is quite lengthy as I recall) but if you just drive it for a day or so it will learn it on it's own.

07-08-06, 11:08 AM
Thank you. I did find and follow the procedure and it is running great!

I had lots of issues and now that I know the solutions I am writing a synopsis of my finding so that others like me, the fairly knowledgable, do it youself type can fix issues like this.

If I were like my business associate, I would suggest that he take his car to a professional... it would be cheaper for him. If I were counting my time on this for my living, I would starve. So I think I will keep my day job and enjoy the fixing process for its own merits.

I will post my findings shortly. :thumbsup:

07-08-06, 02:19 PM
Repair Report

1995 Cadillac Eldorado 4.6 Northstar engine, 135k miles.

Normal maintenance has been preformed and at 100k miles the plugs and wire were replaced.

The fuel rail was replaced by Cadillac in a recall.

Other maintenance items that have been done: Battery replaced, alternator replaced, water pump and hoses replaced, EGR valve and throttle position sensor, belts, idlers and other simple items that all are considered normal.
The starter was replaced at about 120k miles. This is not a simple procedure in that the intake manifold must be removed. Virtually the entire top of the engine must be removed to replace the starter. I replaced the intake seals with new seals and I replaced all the o-rings on the injectors. I also cleaned all airways in the intake and in the port areas where the intake fits.

Over a period of time the fuel mileage has slowly declined as well as power and performance. A noticeable roughness at idle and during heavy pulling (climbing hills) began. Then the car began to surge and idles at a high rpm with dangerous power loss.

Vacuum lines and all hoses were checked and cracked ones replace. The throttle body and butterfly were cleaned using a cotton towel dampened very slightly with gasoline. I determined that I could change practically every ignition system part for less than a trip to the dealership to repair the problem. So I replaced, MAP sensor, ignition control module, a coil with an open secondary, and the ISC (idle speed control motor). After replacing all these items the car ran very well except that it would not idle below 1000 rpm. This is a common problem after replacing the ISC where the PCM does not know where the idle point is in regard to the TPS. It must “relearn” the position. The trick to properly setting the ISC has to do with several factors.

How the idle system works on the Northstar. This engine has a “base” or “minimum” idle speed adjustment screw. It is hard to find and harder to adjust. Looking into the throttle body (TB) with the number 8 cylinder on your left and closest to you is my reference for locating the minimum idle screw. It is located on the right side of the TB near the top right TB mounting bolt. To see the screw you must have the throttle fully open.

Note: The ISC is NOT plugged in until specifically directed.

Adjusting the minimum idle speed. Remove the ISC or fully retract the ISC plunger and unplug it so that it will not operate. Make sure the throttle is clean and does not stick when closed. Start the engine.
Depress the accelerator very slightly and start engine.
Get it running at about 1000 rpm, steady.
Slowly back of the throttle until it begins to idle on its own.
If it stalls out, repeat the process several times until you are sure that it won’t idle on its own. If you can not get the engine to idle or if the idle speed is below 450 rpms then you must make the minimum idle speed adjustment.
There are two ways to adjust the speed (three if you count taking the whole TB off) The easiest way is to cheat and put a temporary fix in place. Put is some spacer material over the screw like a few thicknesses of a business card and stick it there with some glue. Or you might be able as I was to actually turn the screw about ¾ of a turn. I removed the one TB mounting bolt and used a pair of electrician’s pliers (have a knurled clamp surface) and was able to turn the screw.
So the final result of this procedure is to get the engine to run on its own at 450 rpms. Next we will install/connect the ISC.

ISC Install/Adjust Procedure. Knowing what to expect is about as important as the procedure steps. The ISC controls the idle speed. When you start your car the PCM/ISC will set the engine to idle at about 750 for cold operation and in anticipation of you putting the car in gear which would stall the engine if it didn’t raise the engine speed. As the engine warms or you raise the engine speed to more than 1500 – 2000 rpms the ISC will retract. Inside the ISC is a small micro-switch that is activated by the physical pushing back in of the plunger. The PCM is monitoring the engine speed and also detects the micro-switch action. It is the combination of engine speed and micro-switch that cause the plunger to retract.
OK. Now we know what to expect we can adjust it. Earlier we set the minimum speed to 450 rpms now we can properly set the ISC. Now install the fully retracted ISC and turn the adjustment screw in the ISC (not the minimum idle speed screw) until it is about 3 business card thicknesses (book says .03 inch) away from the throttle control, not touching it.

Next connect the ISC control wires to the ISC.

Start the engine, enter diagnostics mode. Turn the engine off wait 20 – 30 seconds.
Repeat this three times.
Start the engine and allow the engine to warm up completely. It should be at normal idle speed at about 550 rpms. Hold brake and put in Drive, continue to hold brake. Then turn on climate control system and let it run for a while. Turn off the climate control and return the car to park. Shut down engine.

At this point your car should crank and idle properly.

07-08-06, 10:11 PM
Nice description of what you did and thanks for taking the time to explain and the results. I come here because I may have a problem and the first thing I do is a search on the subject and a lot of the time I can find the answer I'm looking for. On the other hand, I'll find a lot of posts where someone is asking about a problem, they may get an answer and something to try, soometimes multiple suggestions and then that's the end of the post. You assume 1 of those suggestions corrected the problem and then again, maybe not. It would be nice if people would respond back once they correct the problem stating the solution!

Anyhow, I have a 94 STS, "Z rated" with the entire package, runs like a champ. Lately it's been idling a little high now and then, a little tap on the ISC and it seems to be okay. What I'm curious about is the way it acts when I rev the engine. If the car is in neutral or park and I give it a little rev, maybe half a pedal, about 3k rpm, when it comes down it drops to 1k rpm rather quickly and then it sort of pauses at 1k rpm. The pause is almost like you were holding your foot on the gas pedal right at 1k rpm for about 1-2 seconds. After this pause like symptom at 1k rpm it slowly drops to about 600 rpm, maybe 3 seconds to drop from 1k to 600 rpm's. So, I rev the engine to about 3k rpm's, it drops to 1k rpm's rather quickly, pauses at 1k rpm's for about 2 seonds and then slowly drops to 600 rpm's. Is that pause at 1k rpm's normal, seems like my car has been doing that for as long as I can remember?

Thanks in advance!

07-09-06, 09:41 AM
By way of disclaimer, I didn't design the software in the control system but from all that I can glean from everything I have read up to this point and my experimentation, that is a normal condition. I am a systems engineer much of my job is to discover the ways systems work so I carefully observe the operation and note points of interest.

I am sure the software (firmware actually) has continued to develop and that the system is set up so that "one size fits all butts". Most people start the car cold put it in gear and drive away. Or they start the car hot put it in gear and drive away. As simple as that is there are radically different datasets for the car. In my youth you the driver were concerned with the cranking process, pat the foot-feed acouple time, adjust the choke and push the start button. Then you monkeyed with the choke until it got warm. Now days the computer does all the monkey work.

So now the PCM "Positive Control Monkey" runs your car for you. He guesses that if he runs the car at about 750 rpm when you crank it cold you won't choke it down when you put the thing in gear. He also seems to think that he can slow it down after a few minutes to about 450. I have no clue why he likes to let the engine walk down the rpm ramp when I let off the gas. But driving down the road at say 50 and let off the engine remains near the same rpm. I don't know if that is because the PCM is keeping it there of if there is a rear pump in the transaxel doing a little engine breaking. I really don't feel any engine breaking happening so I think that maybe the PCM is keeping the rpm close to the highway speed so that the engine doesn't lunge when you re-apply the gas which is great for highway speed driving.

I think its normal.

07-09-06, 10:29 AM
Why don't you post the "How To" portion of your post in the Tech Tips section.

07-09-06, 01:58 PM

Thanks for your answer, by the way, your description for the entire slow down process from 50 sounds like my car. The rpm's drop very little when you let off the gas but like you said that is probably the way it was programmed so that when you step on the gas again there isn't a big ramp up in rpm's to get back to where you were when you let off for a second.

My car has always acted like that and the drop off in rpm's from 1k down to 600 has always been a little slow compared to an older carburated car. Your analogy on the thinking behind the cold and warm start ups is probably pretty accurate!

Thanks again....

07-10-06, 08:24 AM
THERE'S A "HOW TO" SECTION?? Where is it hiding? :confused:

:rant2: Why don't they put it on the TOP menu line?... Then I wouldn't have to read all these wonderfull messages. :thumbsup:

Wow! I could have written an artilcle on how to replace the alternator without removing the radiator. And another on replacing the starter. Changing the rear breaks pads was a real dozie, I had no idea how to get the piston back into the caliper on the rear. I do now! I even replaced the window motor, that's worth an article.:stirpot:

Thanks for the suggestion and thanks for the comment. I appreciate the fact that you think my article deserves that respect, thank you. :)

07-10-06, 09:28 AM
Go to the bottom of this page. See where it says Northstar Performance and Technical Discussion? Click on the down button to the right. Tech Tips section is right above the Northstar section.