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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10964
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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My engine runs fine then it seems I lose cylinders. If you

Customer Question

My engine runs fine then it seems I lose cylinders. If you can please give me a call.************
Submitted: 10 months ago.
Category: Boat
Expert:  Jason replied 10 months ago.

Good morning, my name is ***** ***** am I speaking with?

All Mercurys are cataloged by serial number. Can I start by getting that number.

Customer: replied 10 months ago.
Mark, SN: OG144781
Expert:  Jason replied 10 months ago.

Hi Mark,

Got the serial number. What kinds of questions did you have for me?

Expert:  Jason replied 10 months ago.

I'm sorry but I don't do calls unless I know what is going on first. How is it I can help you?

Customer: replied 10 months ago.
My outboard will run great at times then I will lose I think 1 to 2 cylinder but if I keep running it will clean up and take off. I have checked spark, fuel pressure, trigger ohm test
Expert:  Jason replied 10 months ago.

How intermittent is the issue?

Customer: replied 10 months ago.
It can run good for a long time, yesterday it ran good when I put the boat in, but after fishing for 4 hrs. the engine did not run right till I almost got back to the ramp
Expert:  Jason replied 10 months ago.

What you would need to do is get a timing light. Then, duplicate the problem. Once the engine is running poorly, pop the cover off, hook the timing light up to each plug wire one at a time and see if the timing light shows flash or not. Flash means spark, no flash means no spark. Is that something you can do?

Customer: replied 10 months ago.
During my testing I have always had spark, but 1 time my plugs were dry on 5 & 6 cylinders, the next day checked spark and plugs again and they were all wet.
Expert:  Jason replied 10 months ago.

Did you test it when the engine was acting up, or when it was running poorly?

Customer: replied 10 months ago.
Not every time but when I found the above problem it was running poorly and I had spark
Expert:  Jason replied 10 months ago.

If you had spark when it was running poorly then this is going to be a fuel related issue. Is there a fuel water separating filter mounted anyplace on the boat between the outboard and fuel tank?

Expert:  Jason replied 10 months ago.

Are you still with me?

Customer: replied 10 months ago.
it does have a water separating filter on the engine which I change yearly and I checked fuel pressure which it had 36psi at idle.
Customer: replied 10 months ago.
Customer: replied 10 months ago.
I will be away from my computer for about an hour.
Expert:  Tan replied 10 months ago.

Hi Mark my name is Tan.

What we are going to have to do is find out if you are having an injector failure.

This will require swapping the injectors around. Then running the boat does the misfire move from the cylinders that were misfiring before?

Since we have spark we know it is an injector issue. But I would like to verify this as an issue.

The injectors are firing in pairs which is triggered by the ignition circuits. Now cylinder # 5 will cause # 1&2 cylinders to not fire. So we must have good spark on # 5. If you have good spark on 5 then we have either injector or ECM issue.

Does this make sense?

Customer: replied 10 months ago.
Tan it does make sense. I have changed the switch boxes. Would this give me 2 dry plugs on 5 & 6?
Expert:  Tan replied 10 months ago.

Nope that would be the Injection ECM. What would cause loss of injectors firing would be no spark on # 5 cylinder. Since you have spark on that cylinder that leaves injectors or ECM. I don't see 2 injectors going out on the same top cylinders side by side as both one and 2 fire from the same pulse. Leading me to believe the ECM is bad.

The other reason I suspect is you said cylinders 5 & 6 were dry which is fired by cylinder # 3. There again you have spark on # 3 & 5 leading back to the ECM.

Customer: replied 10 months ago.
Have you ever seen the electrical connection to the ECM be the problem, I ask because I haven't looked at it?
Expert:  Tan replied 10 months ago.

Anykind of electrical connection is possible cause for an issue. Check the pins and the plug on the harness and the ECM.

Customer: replied 10 months ago.
It has seemed like the ECM to me too when it come back it is like you flipped a switch, but I am going to check spark 1 more time.
Expert:  Tan replied 10 months ago.

Ok Mark Keep me posted. Watch the spark real close on # 3 and 5 as those are the circuits that fire 5&6, 1&2 injectors.

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