1990 4.5 Deville rough idle/miss
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HT4100 4.1, 4.5, 4.9 Discussion, 1990 4.5 Deville rough idle/miss in Cadillac Engine Technical Discussion; I recently bought a 1990 Cadillac Sedan Deville with 80,000 miles. When I bought the car, it had a rough ...
  1. #1
    sixtires is offline Cadillac Owners Member
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    Red face 1990 4.5 Deville rough idle/miss

    I recently bought a 1990 Cadillac Sedan Deville with 80,000 miles. When I bought the car, it had a rough idle/miss. The guy I bought the car from had it tuned up with new plugs, wires, distributor cap, new filters, etc. Still it had a miss. The check engine telltale was on also. When I ran the codes, I got an E48.

    So, I removed the EGR valved and cleaned it thoroughly. It seemed in pretty good shape, but the tubes under the throttle plate were completely clogged with carbon. I cleaned those and blew all the passages out with compressed air. Also, I cleaned the bottom of the throttle plates (they were pretty gummed up also). I cleared the codes on the ECM and nothing else has turned up. However, it still has a rough idle--especially when cold. After I've come off of a freeway run at 70mph or so, it runs great until the next morning. I've run a bottle of injector cleaner through the tank also, but that didn't seem to help matters.

    Any ideas where to turn on this problem?

    Marko

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    Stealth's Avatar
    Stealth is offline Cadillac Owners Enthusiast
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    to start, check the MAP sensor, the ISC, and also the connectors for both. EGR/EVR valves checked?

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    Katshot's Avatar
    Katshot is offline Cadillac Owners Master
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    Fatten the mixture very slowly with some sort of additional fuel like propane and see if the idle smooths out. If so, the engine is running a little lean and you're getting a lean miss in at least one cylinder, or there's an ignition secondary system fault that gets covered up with a fatter mix. Either way, I would suggest to get the car hooked up to a scope with a tech that knows how to read one. He will quickly be able to diagnose the miss, or at least narrow it down to a A/F problem or ignition problem.

  5. #4
    CadillacJack76's Avatar
    CadillacJack76 is offline Cadillac Owners Fanatic
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    I have owned my 90 Coupe since 92 and it is the most reliable vehicle I have ever owned, I just hit 198k a couple of days ago, this car has always been top notched maintained and you will not find another one with the same mileage that is that clean, anyway, it also has had a rough idle since the day it rolled out of Cadillac, I don't know how rough your idle is but mine sometimes seems to have a miss, it comes and goes as soon as I hit the gas it smooths right out, I had to change (not clean) my EGR valve once because it was setting a trouble code and causing a rough idle, the fuel pump can also cause it, but the question I have for you is, how is your altenator, and how are the grounds? That stuff can make a huge difference with computer controlled vehicles. If your altenator's output is off by even 1 volt, it could trip up the computer in turn effecting your idle, anyway, I just figured I would offer my thoughts, lemme know how you make out.

    Jack

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    Bubb is offline Cadillac Owners Member
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    Re: 1990 4.5 Deville rough idle/miss

    I have the same problem,changed everything ,even distributor.Sad ,It only just turned 120 km on the odometer,not even broke in yet. since the throttle stuck ,I have had all these problems since.
    My next task is to replace the timing chain,I am sure it stretched the linkage.hopefully that will solve it once and for all.
    If not I plan on yarding the engine and rebuild it
    At my wits end with it,love the car but hate the problems I am having with it lately
    This is last my car I purchased from Sarasota Florida 4 years ago.I had 3 of the same year and model .I have always owned some form of Cadillac.
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    dennis93coupe is offline Cadillac Owners Connoisseur
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    Re: 1990 4.5 Deville rough idle/miss

    Have you run a compression test yet? I'd do a few more things before dropping the engine out. Run it at night in the dark, maybe have a bad plug wire. Have you checked the fuel pressure yet? Get yourself a FSM off ebay and do all of the diagnostics.

  8. #7
    justin4yu is offline Cadillac Owners Member
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    Re: 1990 4.5 Deville rough idle/miss

    i would check the noise of the injectors with a stethoscope and make sure they are all pulsing in sync. If not change the computer out.

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