Engine misfire help?

I have a 1997 Cadillac Seville STS with 128,000 miles on it I have a constant engine misfire enough to create a small vibration while driving at high speeds and enough to fail smog and make my gas mileage drop down to half of what it should be and at the same time every once in a while the check wngine light comes on for a faulty o2 sensor but it doesnt come on for anything else I replace the fuel pump fuel filter all the spark plugs and wires and it seems that I have a small amount of oil in the cylinders that I found out when checking the spark plugs the spark plugs had oil on them I believe that to be causing the misfire but I can't find a oil leak and I can't figure out how it's getting into the engine cylinders

3 Answers

  • 3 years ago
    Favorite Answer

    Worn piston rings or valve seals can allow too much oil to leak into the combustion chamber, leading to oil fouling of the spark plugs. Any fouling is likely to cause at least a little bluish exhaust smoke. Replacing the valve guide seals might help and it's not too big of a job. If the rings are shot, however, that's major overhaul work.

    Also have a compression test done on the cylinders. If the compression rings are shot, the oil rings are worn out too. If the exhaust valves don't seal properly, leading to low compression, that could lead to a false conclusion about the rings so you need to look at the exhaust color, but leaky valves can cause misfire issues too. On such an old vehicle, even though it doesn't have huge mileage, bad valve stem seals are a common issue as the rubber hardens with age and heat, so that's what I'd focus on first.

    To do the trouble shooting on the valve seals notice if you are getting blue smoke at idle when the engine is cold. At idle the manifold pressure is high and what happens is the oil gets sucked past a bad valve seal because of the high vacuum. If you can't tell at idle, the best way to diagnose it is to create high manifold pressure by driving down a steep grade with your foot off the accelerator pedal and have someone follow you to observe the exhaust color. .

    Sometimes the valve guides can be bad too and this will cause blue smoke continually during engine operation. Quite often, a valve guide problem will involve only one cylinder and you will know because only one plug gets fouled. If this is the case, remove the valve cover on that side and then pull the rocker arms and shafts on the offending cylinder(s) with a good pair of plastic jaw pliers to see if you can get perceptible sideways movement on the valve stems. If they seem tight, go with with the stem seals as being the problem.

  • Anonymous
    3 years ago

    See a mechanic and check O2 sensor/s operation with a scanner and check for any other codes. O2 sensor/s might require replacement or there is something wrong with the O2 sensor wiring system. Valve seals and/or valve guides might be bad or it could be just a bad valve cover gasket and bad spark plug seals. Check for bluish smoke on cold starts that disappears after a while. Mechanic will need to get the compression checked, do a leakdown test and check the head gasket. Cylinder head or engine might require a rebuild.

  • Joe
    Lv 6
    3 years ago

    I'm not 100% sure, but oil leaking into the bore sounds like a head gasket fault. It could potentially be spitting it up the side of the piston rings if worn or under too much crank case pressure. There are a few possibilities, but I'm guessing it's not going to be an easy fix. You're probably better taking it to someone who specialises in your particular brand of vehicle.

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