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Followup To
Question -
Hello again Bill - Sorry to bug you again, but I'm still working on this problem. Turns out I changed plug #5 and the Check engine light went off, but the car still is running rough and balking under load, such as going up an incline. So the problem is still there. I'm taking it in for some diagnostic work.
Any ideas on why it would still run rough, but not have the check engine illuminated?
Thanks - Mike
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Hi Bill - Thanks a bunch for your answer. I feel better about this problem. I will check the #5 plug again after 5K miles. (Note I still have the original plug and upon close visual inspection I can't find any cracks or defects.)
Would you recommend any diagnostic work now to check for leaky valve seals or gaskets or whatever?
Does your comment on the blown head gasket mean that if it were blown I would be getting engine coolant in the crankcase? (This does not appear to be happening at this time.)
Mike
Followup To
Question -
Hi Bill - Thanks for your interest. This is not a Cadillac question, but I hope you can help me with your general knowledge and expertise. There were no other experts available for my car. I have a 2000 merc grand marquis with a 4.6l engine. 41000 miles. It has run great till the other day when the check engine light went on and the car was noticeably misfiring. I was 500 miles away from home and decided to drive home without diagnosing or fixing the problem. I had the problem diagnosed at Auto Zone as "#5 cylinder misfiring". I checked the plug and it had built up considerable spotty grayish residue. I changed the plug and after several cold start/driving cycles the light went off. Problem solved.
But I do have a couple questions that I hope you can answer for me-
1. Could I have damaged the engine by driving 500 miles with the misfire occurring
2. What may have caused the plug to go bad. These are 100K mile plugs per Ford. (I checked the #1 plug and it was okay.) What would be a logical test to find the root cause of the build up on the plug. Could there be a blown head gasket?
Thanks - Mike";
Answer -
Hi Mike, A sparkplug that has light tan or grey on the tip is considered normal. Seeing how much build up and compairing it to all the other sparkplugs would determine if it is isolated to just 1 cylinder or a problem that affects all cylinders needs to be looked at. Since you removed #1 plug and it looked normal and assuming all the plugs but #5 were the same it might be safe to assume that the plug failed internally since installing a new plug cured the problem....I wouldn't think that you created any engine damage by running the plug that way because the engine runs normal now and it would have been pretty bad on several cylinders to cause any real problems. Most of my experriance has been with GM vehicles and they use platinum plugs that should be good for 100,000 miles but they need to get replaced from time to time because a platinum disc falls off the electrode finger and makes the engine run rough due to the larger gap or the inner carbon core gets a higher resistance than the others due to high cylinder temps and that is what I suspect happened to you. Just monitor the engine and if the light turns on or starts to missfire remove #5 plug and evaluate the color and maybe you will need to do some further investigation into that cylinder.A blown head gasket would seem unlikely because the engine would run rough, you would need to add coolant, the engine temp gage would show hot or overheating and in some cases the plug end would have a green tint to it. Hope that helps. Bill
Answer -
Hi Mike, I would just run the engine and see what happens. Then if the engine starts to act up again then I would start the testing process. With the blown head gasket the coolant could leak outside the block, inside a cylinder or into the crankcase and some engines are more prown to one vs the others. Bill
AnswerHi Mike, Changing the spark plug must have made a differance otherwise the misfire code would still be there. Fuel pressure needs to be checked as it might be a little lower than it should be and when you need more power it can't get it. Restricted fuel filter or weak fuel fump. If and oxygen sensor isn't allowing the engine to swing to full rich that could do it. Carbon in the throttlebody will cause a rough idle but should still accellerate normally. Hopefully you have a sharp tech that knows Fords quirks. Bill
