Question about 1991 Cadillac DeVille

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When my 1991 Deville 4.9 Liter Engine is cold it runs great. But after it warms up it misses, and is noisy and gets horrible gas mileage, and I only run premium. The EGR valve moves up and down freely, and I just replaced the EGR Solenoid. HELP!!!

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  • Hey Bo Oct 01, 2010

    I just had the plugs replaced about 2 months ago thinking this was the problem, it didn't help. The mechanic said the wires were good according to his engine anaylizer. But why all the noise when it gets hot, almost sounds like rod or main bearings but not quite that noisy. I've tried blocking the EGR off and that doesn't help at all, if anything it's worse. Thank you for the rapid response, it is sincerely appreciated.

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The combination of heat and missing tells me that you have an ignition problem. Warm the engine as much as possible and take a water mister and mist the exhaust manifold with the water mist at each cylinder where the exhaust comes out of the cylinder. The cylinder(s) that doesn't evaporate as quickly as all the others is the one that's missing (no combustion = no heat out the exhaust for that cylinder). The next thing to do is to pull the spark plug. examine it for dirt built up around the electrode. If it's dirty then just replace all your plugs. If the plugs look good ( not worn - no contaminates around the electrode) then suspect plug wires. If all the cylinders evaporated the water pretty much the same, then your coil wire to the distributor cap or the cap and/or rotor itself is faulty. Examine the rotor cap and rotor for thin black lines running along the inside. If you find evidence of arcing (the lines inside) then replace the cap and rotor at the same time. The data you gave though sounds more like plugs or plug wires if I were to guess... For best results you should plan on replacing plugs/wires/rotor/distributor cap at the maintenance intervals suggested in the owners manual. But now that you're having these issues you should have some kind of concrete evidence before replacing these items - that is unless you've never replaced them ever before. In which case it's high time to have replaced them.

Posted on Oct 01, 2010

  • michaelwitt Oct 01, 2010

    I'm sorry, but I misunderstood about the EGR. I took it to mean that you'd eliminated the EGR from the equation. You can easily test the EGR with vacuum to verify it is opening. If when vacuum is applied it opens and then closes when there's no vacuum then you're good. There's also an EGR switch that will apply vacuum to the EGR when water reaches operating temp. EGR should not move freely by hand. It is spring loaded closed. Vacuum applied to the diaphragm opens it. If the diaphragm has a hole in it that will prevent opening (and would be a vacuum leak - that'd cause your missing issue right there).

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