1990 Deville 141k runs slightly rough, high idle, maintains speed on highway by itsel
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HT4100 4.1, 4.5, 4.9 Discussion, 1990 Deville 141k runs slightly rough, high idle, maintains speed on highway by itsel in Cadillac Engine Technical Discussion; Thank you for reading, I did a considerable amount of research before doing any work, but now that I have ...
  1. #1
    daveinmaryland is offline Cadillac Owners Member
    Automobile(s): None - 1990 Deville
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    1990 Deville 141k runs slightly rough, high idle, maintains speed on highway by itsel

    Thank you for reading, I did a considerable amount of research before doing any work, but now that I have done a few things, the car performance has not considerably improved, so, I decided to join, and place my own list of woes online.

    The vehicle, 1990 Deville with 141,000 miles. The car belongs to my step daughter, she lives two hours away. My newlywed wife offered my assistance on the vehicle. I am an electrician by trade, but, do my own vehicle maintenance, I know my limits. The car was in the (ex) family since initial ownership. I was informed that the car has not been driven much in the last five years (garaged) until it was put on the road about six months ago. The car was said to have a tune up, not sure what all that entailed. I don't have a lot of access to the car, but, try to work on it, when I do see the Sdaughter. She has limited funds, so, I would like to help in any way I can.

    Please bear with me, as I try to recreate what I have done. Initially, it ran rough, seemed to have a high idle, and maintained speed on the road, all by itself. I learned how to read the codes, and found a few codes, (of which I have forgot all of what they were), but one was E26. Having some experience with a Oldsmobile throttle block engine in my past life, I was confident and replaced the TPS, and the engine seemed to run differently, can't say it was better.

    The car went back with my Sdaughter, and recently returned. I now have it for a few days, so, I read the codes and found E49, E98, EE49. So, after some searching, and frankly getting confused, I purchased and replace the ISC motor. And again, the car runs differently, however, not as good as I would like. I did the "lift one spark plug wire at a time trick", from the [I]coil pack[I] ??, and found cylinder 8 (back side, closest to driver side, is that correct?) to not affect the running like the other seven did. So, I removed the spark plug, a bosch platinum, was sooted, but not the worst I have seen in my life, the threads did not look like the plug was loose and was losing compression through a loose plug. I cleaned the plug, and carefully reinstalled it, I checked the spark plug wire for resistance compared to a similar length plug wire and it was similar. So, restart, and nothing really changed. I drove the vehicle, and the rough running occurs all the time, regardless of speed. The car has a great deal of acceleration. Also, the vehicle seems to attempt to maintain speed on the highway, all by itself. I get up to 65, remove my foot from the accelerator, and it took three miles to slow to 45 (one piece here, this was much worse prior to the ISC motor replacement). Also, another tidbit. When warmed up, the idle speeds seems to change every minute or minute and a half, there is a noticeable electrical "click", and the engine RPM changes, for about 6 seconds, then returns to where it was. I do know how to read the codes on the car, but, do not know how to reset them. So, here I am, placing my ignorance and inexperience at the alter of the Cadillac knowledgeable.

    I would appreciate any help/guidance.

    #1 - rough idle, spark plug seems good, wire seems good................

    #2 - code E49, E98, EE49.

    #3 - engine self speed maintain

    Thanks in advance. Happy New Year.

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  3. #2
    cadillac_al is offline Cadillac Owners Connoisseur
    Automobile(s): Cadillac 78 Seville, 78 Coupe de Ville, 92 Sedan de Ville
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    Re: 1990 Deville 141k runs slightly rough, high idle, maintains speed on highway by i

    Welcome to the forum. Try searching here for ISC adjustment and get yours adjusted right and I think that will solve most of your problems. Whenever i pull plug wires on a running engine there always seems to be one that doesn't make a difference. I don't think I would worry about it. While you are working around the TB take a look down past the throttle blades and see if the EGR tubes are plugged up.

  4. #3
    needstires is offline Cadillac Owners Member
    Join Date
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    Quote Originally Posted by daveinmaryland
    Thank you for reading, I did a considerable amount of research before doing any work, but now that I have done a few things, the car performance has not considerably improved, so, I decided to join, and place my own list of woes online.

    The vehicle, 1990 Deville with 141,000 miles. The car belongs to my step daughter, she lives two hours away. My newlywed wife offered my assistance on the vehicle. I am an electrician by trade, but, do my own vehicle maintenance, I know my limits. The car was in the (ex) family since initial ownership. I was informed that the car has not been driven much in the last five years (garaged) until it was put on the road about six months ago. The car was said to have a tune up, not sure what all that entailed. I don't have a lot of access to the car, but, try to work on it, when I do see the Sdaughter. She has limited funds, so, I would like to help in any way I can.

    Please bear with me, as I try to recreate what I have done. Initially, it ran rough, seemed to have a high idle, and maintained speed on the road, all by itself. I learned how to read the codes, and found a few codes, (of which I have forgot all of what they were), but one was E26. Having some experience with a Oldsmobile throttle block engine in my past life, I was confident and replaced the TPS, and the engine seemed to run differently, can't say it was better.

    The car went back with my Sdaughter, and recently returned. I now have it for a few days, so, I read the codes and found E49, E98, EE49. So, after some searching, and frankly getting confused, I purchased and replace the ISC motor. And again, the car runs differently, however, not as good as I would like. I did the "lift one spark plug wire at a time trick", from the coil pack ??, and found cylinder 8 (back side, closest to driver side, is that correct?) to not affect the running like the other seven did. So, I removed the spark plug, a bosch platinum, was sooted, but not the worst I have seen in my life, the threads did not look like the plug was loose and was losing compression through a loose plug. I cleaned the plug, and carefully reinstalled it, I checked the spark plug wire for resistance compared to a similar length plug wire and it was similar. So, restart, and nothing really changed. I drove the vehicle, and the rough running occurs all the time, regardless of speed. The car has a great deal of acceleration. Also, the vehicle seems to attempt to maintain speed on the highway, all by itself. I get up to 65, remove my foot from the accelerator, and it took three miles to slow to 45 (one piece here, this was much worse prior to the ISC motor replacement). Also, another tidbit. When warmed up, the idle speeds seems to change every minute or minute and a half, there is a noticeable electrical "click", and the engine RPM changes, for about 6 seconds, then returns to where it was. I do know how to read the codes on the car, but, do not know how to reset them. So, here I am, placing my ignorance and inexperience at the alter of the Cadillac knowledgeable.

    I would appreciate any help/guidance.

    #1 - rough idle, spark plug seems good, wire seems good................

    #2 - code E49, E98, EE49.

    #3 - engine self speed maintain

    Thanks in advance. Happy New Year.
    Sounds like tps sensor problem it can cause high idle and it sets code ee98 when shifting from park to drive or reverse . Tps problems drive these cars crazy replace it and set it with the instructions that ome with it get a AC deco part aftermarket junk doesn't agree with dfi systems the other code is AIR they use electronic diverter air valves .could be the cause it uses feed back from the o2 sensors to switch incoming air to the cat converter . Take care of the TPS replace it or if you have a scope you can graph it you will most likely a drop off ov the voltage Ignacio The computer looks at that and has no idea what o do o it raises the idle so it doesn't stall .just make sure you set the tps and iac motor to spects these engines are very very sensitive to this . By the way I lived in havre de grace last year . But came back to NJ.

    ----------

    Replace he TPS and set it up with the directions that come with it only use an AC Delco unit that's setting he code 98 because the idle it too high going from park TP drive and also reverse. The other code is the electronic diverter valve that switches air to the cat converter . The tps voltage can also be causing that codes set. Make sure you set the tps and idle speed controller correctly .

  5. #4
    drewsdeville is offline Cadillac Owners Master
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    Re: 1990 Deville 141k runs slightly rough, high idle, maintains speed on highway by i

    Starting fresh here:

    I wouldn't worry about correcting the idle or curing codes until you get #8 cylinder to fire. That's top priority. If you are certain you have spark, and this happened suddenly, suspect a fuel injector problem. In general, these specific injectors don't plug up or fail mechanically - rather the injector coil fails, causing a short or open circuit. Use your VOM and check the resistance between the two blade connectors on the injector. You should find a reading somewhere between 12-18ohm's. Any resistance out of range, infinite, zero, or otherwise, points to a failed injector. It's also a good idea to compare #8's resistance to a few other cylinders - they should all be within close proximity to each other. For example, if all injectors show 12ohms except #8, which shows 18ohms, suspect a problem even though it's in range.

    The e098 code, as mentioned, may be related to an ISC or TPS problem. Are you aware that the TPS is adjustable, and there is a specific, delicate procedure to set adjustment when installing? If not, you should be able to find it with a quick search. It's been discussed quite a few times here.

    e049 can also, to a lesser extent, cause driveability problems if the A.I.R system fails in a specific manner. See here:
    http://www.cadillacforums.com/forums...ml#post2408050

  6. #5
    brougham is offline Cadillac Owners Master
    Automobile(s): 1991 Cadillac Brougham D'Elegance 5.7 Litre
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    Re: 1990 Deville 141k runs slightly rough, high idle, maintains speed on highway by i

    Not being able to see what the car is actually doing, are you sure the click you hear with the speed increase isn't something simple like AC coming on?
    The high idle and maintaining speed on it's own is most common issues with a bad ISC but can easily be caused by other problems too which is more likely in this case since you already replaced it.

  7. #6
    carnut is online now Cadillac Owners Master
    Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
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    Re: 1990 Deville 141k runs slightly rough, high idle, maintains speed on highway by i

    BOSCH plugs are not correct for this car. Replace with A/C Delco's as recommended as your first step. The click is the compressor cycling on/off and the ISC is increasing the idle to carry the extra load on the engine. You have at least the ISC misadjusted. If the TPS is set too high, it will cause an ISC to be out of range. You MUST adjust both to spec percisely as the factory manual dictates.

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