Coupe Deville 4.5 Rough Running
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HT4100 4.1, 4.5, 4.9 Discussion, Coupe Deville 4.5 Rough Running in Cadillac Engine Technical Discussion; Hello All. So I purchased my 1989 Coupe Deville... with known issues. I drove it home. The ride itself is ...
  1. #1
    88_Deville_Executive is offline Cadillac Owners Member
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    Smile Coupe Deville 4.5 Rough Running

    Hello All.

    So I purchased my 1989 Coupe Deville... with known issues. I drove it home. The ride itself is nice, but very rough driving when accelerating and its also alittle poor on the accereration. I drove on the freeway. When the gas is off the pedal, obviously is quiet and smooth then.

    When I had tried to start it a few days before when it was really cold out... it did take quite a few tries to start it.

    Anyways,

    Brought it home and checked things alittle more. I checked the throttle body and its clean. The EGR passages are also clear and clean from what I can tell. I checked the EGR and can flex the valve with some pressure, but nothing major. I have not checked the injectors on the throttle body to see if they are spraying ok yet. I did notice that once the car warms up - I heard this really odd - almost digital sounding thing - sorta like a modem makes when you conn. to the internet, but very light and faint. Not really sure what that was but it just kept going. The car seems fairly quiet but does have a seemingly noticable miss or something? No codes as of yet that I can pull.

    Any thoughts of what I might check next with my specific issue? I know the basic to semi moderate so if there is something to check - can you give me an idea of what/where to check too?

    Thanks for any info.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: Coupe Deville 4.5 Rough Running

    Hopefully the compression is not low on that cylinder and causing a miss/poor starting. Check to see if it's burning a lot of oil, if not, then the compression may be OK. Buy some Seafoam and dump it down the TBI. That will certainly help. Check the MAP sensor vacuum hose, too.

  4. #3
    88_Deville_Executive is offline Cadillac Owners Member
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    Re: Coupe Deville 4.5 Rough Running

    I did check the spray from the injectors this morning and they look fine. Nothing noticable. Exhaust around tip (I mentioned before in a previous article) is black.

  5. #4
    Edahall's Avatar
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    Re: Coupe Deville 4.5 Rough Running

    Does it run rough at idle? If so, remove one spark plug wire at a time while the engine is idling to isolate which cylinder is not firing. If you remove a wire and you feel no difference, this is the cylinder that is not firing. It could be a bad spark plug or bad wire. Also, you might want to put on a new cap and rotor. It's a low cost item.

    It's probably not an EGR issue since the problem would manifest itself when you're off the throttle.

    It's rare to have problems with the injectors on these engines with TBI.

  6. #5
    HUF's Avatar
    HUF
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    Re: Coupe Deville 4.5 Rough Running

    Quote Originally Posted by Edahall View Post
    Does it run rough at idle? If so, remove one spark plug wire at a time while the engine is idling to isolate which cylinder is not firing. If you remove a wire and you feel no difference, this is the cylinder that is not firing. It could be a bad spark plug or bad wire. Also, you might want to put on a new cap and rotor. It's a low cost item.

    It's probably not an EGR issue since the problem would manifest itself when you're off the throttle.

    It's rare to have problems with the injectors on these engines with TBI.
    I agree with Edahall - the problem is most likely ignition related.

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    88_Deville_Executive is offline Cadillac Owners Member
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    Smile Re: Coupe Deville 4.5 Rough Running

    Ok. Sounds good. I will see about doing that next here tonight. The bright side is - I have my current GOOD running car that I can use for temp. check parts if I needed to.. but again - depends cause I don't wanna make my good one bad too.

    I appreciate all your help with this. To be honest.. the reason for this forum was why I decided to take on my little project / future commuter car!

    Will check these and hopefully have more information soon.

  8. #7
    88_Deville_Executive is offline Cadillac Owners Member
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    Re: Coupe Deville 4.5 Rough Running

    So tonight I got home. Did a few additional tests. I started up the rig and one by one tested each spark plug wire and each time one was disconn. it would run worse.. plug it in - runs back to original crummy - but still better when it was disconn. So, I then took out the MAP from my working rig and stuck it in there as well. Did not really notice anything different there. Still idled quiet for the most part... only really notice the issue when I took it for s short spin. Really rough. Got back, I then went ahead and unplugged the battery for 30 sec or so - no different.

    So... while all this was going on. And wondering if related, but when I was inside.. I turned on the defrost.. what happens? I hear this muffled pop, the a/c must have attempted to kick on - blows the fuse and of course all the dash displays run on the same 25 AMP fuse so it all goes. I shut off the car and also smelled some sulfer like smell??? But not certain. I assume that when I turned on the a/c it caused some strain on the car that already had an issue during idle.. and caused some type of backfire???

    Anything new to hint or what I might try now?

    Thanks

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    Re: Coupe Deville 4.5 Rough Running

    The A/C blowing out the fuse is probably a separate issue. You probably have a bad coil in the A/C clutch which is burning out the fuses.

    Have you replaced the cap and rotor? I would definitely start there first and it's not expensive.

  10. #9
    88_Deville_Executive is offline Cadillac Owners Member
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    Re: Coupe Deville 4.5 Rough Running

    Not to sound dumb here, but for all the experience I DO have, I have never replaced the cap/rotor. I took autoshop back in HS but after playing with engines on some carts.. and causing a nice backfire it has caused me to be leary of those things. Can you give me an idea of the process and how to do it?

    Also, if one was concerned that it could be a cat issue, is there ways to determine or know? When I was debating of getting it originally, the owner said that if you hold a dollar bill up to the muffler that it should have a temp. vaccuum for a sec between each puff out the tailpipe. Which it did not. Anyways. In regards to the above question. Any info there would be helpful too. :-)

    Thanks.

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    Re: Coupe Deville 4.5 Rough Running

    The hardest part in replacing the cap & rotor is getting the wires back on to the right spot. Make sure you label them. Other than that, it's very easy to replace. If you replace a light bulb, you probably can do this job. There are 3 or 4 screws around the cap that you have to push down and rotate to release them. Then the rotor is just a couple of screws.

    If the cat is clogged, the transmission also won't want to shift. It'll only shift at excessive high RPM's. The CAT is not causing your problems.

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    88_Deville_Executive is offline Cadillac Owners Member
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    Re: Coupe Deville 4.5 Rough Running

    Ok Great.

    I did check out some parts so will probably see about getting that tonight. I will post my results. And yea, the car is shifting ok that I can tell... not having to really get the rpms up to make it do that.

  13. #12
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Coupe Deville 4.5 Rough Running

    Cap & rotor is very easy to do...just transfer one wire at a time to the new cap, and make sure you put the rotor on in the same direction in which it came off. You might also want to check to make sure the spark plug wires are connected to the correct terminals, and your ignition timing. You're also going to have to move the old coil into the new cap. In fact, a bad coil would cause your backfiring/lack of power too, so maybe you should pick one up. They're about $15. How new are the spark plugs/wires? They might need to be replaced, too.

    Are you sure the EGR diaphram moves freely? Take a wrench and hit the top of the valve about 5 times--you won't hurt it. This will free a stuck diaphram. Then start the car...if it runs better, the EGR needs to be replaced.

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    88_Deville_Executive is offline Cadillac Owners Member
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    Re: Coupe Deville 4.5 Rough Running

    So tonight I picked up a new cap and rotor. Installed it on the car. Once warmed up it ran alittle better but still not good. The cap and rotor already on it did not look that bad. It I do not romp on the car it will be ok just sorta cruising, but if I step on the gas thats when it really hesitates now. I did notice more that the exhaust smells sorta like sulfer???

    Anyways... I can try to replace the coil on the top too. I noticed when I had moved the coil, one of the terminals was sorta black. I used a small dremmel and cleaned it off...

    Anything else to check too?

    Thx

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    NickySantoro is offline Cadillac Owners Fanatic
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    Re: Coupe Deville 4.5 Rough Running

    The problem sounds like a classic case of breaking up under load. How old are the plugs and wires?

  16. #15
    Edahall's Avatar
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    Re: Coupe Deville 4.5 Rough Running

    Also, make sure you are using AC Delco plugs & wires. There have been reports of problems with competing plugs & wires. I will add though that I am using MSD high performance wires on my 88 Eldorado and have not had any problems. These are very high quality wires but you have to custom make them yourself.

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