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Thread: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

  1. #1
    devillekelly is offline Cadillac Owners Member
    Automobile(s): 1990 deville
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    compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    Just acquired a 1990 Deville with a 4.5L I have a chilton manual which covers the car, but does not give a number for compression cylinder pressure. Can I run a compression test in the usual manner? And what is a good ballpark number? It has a bad misfire even though the PO said a "tuneup" was just done (to the tune of $500, and the "mechanic" said it was in fine shape now). There is no smoke coming from the car and the oil looks clean and normal. Thought I'd start by checking for a bad plug(s) or wire(s) and then do a compression test... if I don't find the culprit, where shall I look next?
    Thanks in advance for any constructive input!
    Kelly


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    onebadsuv's Avatar
    onebadsuv is offline Cadillac Owners Member
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    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    Greetings - 1990 4.5L Deville engine has the port fuel infection (injection) system . When You pull the plugs , check to see which are black or sooty looking , as they indicate which cylinders misfire ... bad Injectors foul plugs quickly

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    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    Most engines are in the 120-150 compression range. A $500 tune up should include AC Delco spark plug wires or some other high quality name like MSD or Moroso. If they are just cheap wires they will always be suspect. I always run some Techron in any new used car i buy. It is the only snake oil I ever use or recommend. It's always nice to verify plug gaps too because there is a lot of misprints out there. I think these cars are supposed to be .045"

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    carnut is offline Cadillac Owners Master
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    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    I would unplug each injector first to locate the misfiring cylinder. Just like pulling one plug wire at a time to localize the cylinder. Compression tests are done AFTER electrical culprits are ruled out! I'm thinking faulty injector or cracked spark plug.

  6. #5
    ht4100's Avatar
    ht4100 is offline Cadillac Owners Fanatic
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    Re: compression test on 1990 4.5L ? constant misfire, idling, under load, coasting...

    my 4.9 had 145-150 psi, You could do a compression to rule that out if good. If a tune up has been done, plugs, wires, cap and rotor, fuel filter, etc. I would suspect fuel injectors by your description though, you can ohm test the injectors with a voltmeter and see if they are the specified range.


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