3.0 Missing on cylinder 3

I have a 1988 Mercruiser 3.0. It seems to be running well for the most part, but when hooking up a timing light I can tell that it is missing occasionally on cylinder #3 only. Timing is close (8btdc, will adjust to 6 but that's not the problem), dwell is good at 30, and I have pulled all the spark plugs and then look good. Well black meaning rich, but I have not had a good run on the lake yet, had to replace water pump, which went well.

It did overheat when on the lake, ran it up for about a minute, maybe 2, got hot, idled down and shut it off. When replacing the water pump the plastic tube was melted but the water pump housing wasn't warped, seals all seemed OK, and no water in the drive oil. Impeller was toast. But really only ran it for a minute or two, hoping it didn't damage anything major. No water in the oil.

Have not checked compression yet which I know is going to be the first thing you ask for! Will get a gauge later today and test it.

If it is something like a sticking valve or lifter is there anyway to test that with a gauge? Any other thoughts on what to check for?

Re: 3.0 Missing on cylinder 3

Faulty plug, or plug wire will also cause it not to fire.
Swap the plug to a different cylinder and see if it changes, do the same with the plug wire (just not at the same time or you won't know which one is the problem)

Re: 3.0 Missing on cylinder 3

Black plugs yup. definatly rich, but thats at an idle correct? put in new correctly gapped plugs, clean the flame arrester. try again see what happens. most motors dont like to sit around at idle speeds. good luck.

Re: 3.0 Missing on cylinder 3

Also look at the rotor and the inside of the distro cap. If you have a meter you can ohm out the plugwires. Another favorite test of mine is to use a cheapie test light with a bulb inside (NOT an expensive led one). With the engine running hold the end of the light and drag the probe end up and down the wires, if the light starts to flicker or the engine starts to stutter then the wires are leaking and bad.

Re: 3.0 Missing on cylinder 3

hoping it didn't damage anything major.

Click to expand...

Ayuh,... Ya might wanta Check the exhaust Shutter in the down-pipe...
It's usually the 1st thing to get Burnt up...

Re: 3.0 Missing on cylinder 3

Thanks guys. Seems to be running better as it goes. I am going to take it out for a test run tonight and see how things go, clear the lead out so to speak. Will check the flapper as well.