Engine Miss Fire

I have a 1993 Mercruiser 3.0 XL that seems to be running on 3 cylinders when first started. After it has run of three or four minutes, it picks up the fourth cylinder. Is this likely a fuel delivery problem or a spark problem? Also, the engine idles fairly smoothly at 1000 rpms, but when I throttle back to about 800 rpms the engine idles very roughly. This is based on the onboard tach, not a test instrument. Should the 3.0 XL idle smoothly at 600 to 800 rpm?

Re: Engine Miss Fire

Crownline 186 said:

I have a 1993 Mercruiser 3.0 XL that seems to be running on 3 cylinders when first started. After it has run of three or four minutes, it picks up the fourth cylinder. Is this likely a fuel delivery problem or a spark problem? Also, the engine idles fairly smoothly at 1000 rpms, but when I throttle back to about 800 rpms the engine idles very roughly. This is based on the onboard tach, not a test instrument. Should the 3.0 XL idle smoothly at 600 to 800 rpm?

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hello,

as a rule, an engine should run smooth at those rpms... unless it has a real hot cam in it. if it has had a recent tune up, prob time to do a compression test and ck combustion chamber/cyl's condtion. be sure both ends of plug wires clean - brite.

regards,
lakester

Re: Engine Miss Fire

I am not an expert but one way to determine if it is a spark problem is to pull an individual spark plug wire off while it is idling. (Make sure you use an insulated pulling tool or you could get quite a shock) If nothing changes, (no noticeable difference in its idle) then that plug is not firing. Then determine if the problem is the spark plug, the wire, distributor cap or rotor. If all that is good you have to look at a potential cylinder problem. Spark problems tend to disappear when an engine is run at a higher rpm.

Jerry

Re: Engine Miss Fire

Thanks for the replys to my question. Do you have any experience with the life expectancy of the ignition wires. The coil and plug wires are originals to 1993, but the boat only has 590 hours on it. Actually the same question applies to the distributor cap. There is no distributor rotor on this engine. Again thanks ........

Re: Engine Miss Fire

Spark. new plugs & wires.

Re: Engine Miss Fire

Crownline 186 said:

There is no distributor rotor on this engine. ....

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Re: Engine Miss Fire

Pull the #4 plug and compare it to the others.If it`s white you most likely are ingesting water into #4 exaust from a hole in the riser.

Re: Engine Miss Fire

Bt Doctur said:

Pull the #4 plug and compare it to the others.If it`s white you most likely are ingesting water into #4 exaust from a hole in the riser.

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Bt Doctur ..... I need a little help with your suggestion to pull the #4 plug .. I'm guessing the the plug will only appear white if I look at it after running the engine for a short time and shutting it down before the engine begins firing on all 4 cylinders .... is this correct? ...... also, I hate to admit it but I do not know exactly what you are talking about when you say the "riser" ..... water entering the #4 cylinder would have to come from a cooling water passage way in the block, a passage way in the head, or, I'm guessing again, via the exhaust valve since the cooling water and exhaust mingle prior to being discharged overboard, correct? ....... thanks for the imput ...... I'm learning...