Thread: Engine Miss Puzzler...

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I just retired from this industry, and I am leaning toward what R Pope is saying. When you have two adjoining cylinders missing at only idle, it could be telling you that there is a little coolant getting into each cylinder. The reason you only notice it at idle is because at higher rpms the engine can burn off the water easier and the plugs won't be fouled so easily. I had a 3 liter Mercruiser that only fouled # 4 cylinder at idle, at more rpms it ran great. Turned out to be a cracked exhaust manifold.

I would suggest a leakdown test and also checking the exhaust manifolds and risers to see if they are bad. Sometimes a leakdown won't even show it as the water only gets sucked in under compression, like when the engine is running.

Also look for tell tale rusting on those two plugs. Could be something else, but I am betting a new head gasket or manifolds will be the fix. Here in Florida manifolds only last about 3 years typically due to the salt water, so we change these and head gaskets quite a bit. Let us know what you find out.

Don