1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough
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HT4100 4.1, 4.5, 4.9 Discussion, 1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough in Cadillac Engine Technical Discussion; Hello, I have a 1986 Cadillac Fleetwood 75 with the notorious ht4100 living up to its reputation.. The problem is ...
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    Caddylimo86 is offline Awaiting Email Confirmation
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    1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough

    Hello,
    I have a 1986 Cadillac Fleetwood 75 with the notorious ht4100 living up to its reputation.. The problem is that the car runs very rough. It starts just fine but it has a very rough idle, take off, and acceleration under any load. Also, it's getting poor mpg at 13 mpg. I replaced the plugs, wires, and coil with all performance parts made by accel. It seemed to have just put a band aid on it for a few months. It slowly got worse until the point where it shakes the whole car at idle. More specifically, it rattles the whole car when in 4th gear around 45mph under any load. As soon as I step on the peddle some more to make it downshift to third, it stops shaking the car but still doesn't run well. I know this car is not a big powerplant, but I have noticed a slow power loss over the year that I have had it. But now it barely has as much power as it used to. I do have the chilton's repair manual for this car but it's not specific. I'm aware of the on board diagnostics but i'm not too sure what good readings are as it doesn't tell me what a good reading is in the book. Could anyone relay some tests that I can possibly do to start digging in to what may be the problem?
    Thanks,
    Quentin

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    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: 1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough

    to Engine forum

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    carnut is offline Cadillac Owners Master
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    Re: 1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough

    Perform a vacuum gauge test. Google what different readings mean on any engine. This engine can have a broken rocker arm support which will cause improper valvetrain action. With it running, pull one wire off at a time at the distributor cap to isolate the cylinder. USE insulated pliers!

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    warnereg is offline Cadillac Owners Member
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    Re: 1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough

    First off what codes are present.

    How many miles.

    What color is the exhaust.

    What does the oil look like (any white creamy stuff, foreign material, ect?)

    Id have to say that it sounds like a vacuum problem. Manifold vacuum should be -18 hg. Check your vacuum lines for dry rot. If they are get yourself some vacuum line and loan out a vacuum guage from autozone. Replacing vacuum lines is a no brainer, just do one at a time. One easy test before getting into all that is to set the parking break and shift into reverse or drive, the parking brake should release right away, if it takes a few seconds or doesnt release at all there is most likely a vacuum leak. Also pop the hood and listen for any hissing sounds and listen under the dash as well.

    Also if your vacuum readings are low your catalytic converter could be clogged which would restrict exhaust gas flow causing the motor to run poorly, which by the way you described the power loss over time could be the catalytic clogging up, which would cause all sorts of problems.

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    carnut is offline Cadillac Owners Master
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    Re: 1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough

    A vacuum leak on a fuel injected engine makes it idle too high not rough. Also a cat. converter will normally restrict top end acceleration but rarely idle rough. Yes a vacuum gauge will determine a plugged exhaust. Start with the basics: is it firing on all cylinders, is the vacuum gauge reading about 18 and steady?

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    eaglebus40 is offline Cadillac Owners Member
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    Re: 1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough

    Hello,
    In reading your 1986 situation it reminded me of an almost identical performance issue that I had with a 1984 Fleetwood with a 4100. Turns out that in my case the fuel pressure was too high. I did what the manuals say not to do. I Adjusted the regulator within the throttle body to minimum spring pressure. This reduced the fuel pressure from the starting point of over 13 PSIG to around 11 PSIG. This may not be your problem, but check the fuel pressure. Caution: If you try this, remove the fuel pump fuse and run the engine to reduce fuel pressure. Also be extremely cautious of the regulator spring pressure it will tire your hand muscles very quickly. Good luck!

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    carnut is offline Cadillac Owners Master
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    Re: 1986 Cadillac Fleetwood 75 Ht4100 Running Very Rough

    3 month old post and the OP never responded. Another example of a taker not a giver. Or he died!!

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