Motor not firing on all cylinders

[shankos]

I have a mercruiser 350 mag MPI tow sport. The motor is not firing on all cylinders. unburnt fuel is coming out the exhausts.

The spark plugs are two months old. The dizzy cap is two months old. I checked all the leads and they have valid ohm readings. It was running perfecly a week ago and this week it has stopped.

I checked the timing by hand and TDC lines up with the dizzy point for cylinder 1. However once you fire the motor using a timing light it does not line up at all on the harmonic balancer. Moving the dizzy I can make the motor purr on all cylinders for about 10 seconds then it stops again.

Oil pressure is normal according to the gauge. I dont believe i have a fuel problem as this would not cause the timing to be out. I have not pulled the plugs out but the fact that the timing is out and i can make it purr for 10 secs makes me not believe they are the issue.

So at present i am thinking something is wrong with the ignition coil causing it to not fire on time. Would this be a correct assumption seeing the plugs, leads and dizzy cap are ok. I was thinking of replacing the coil but they are not cheap so was looking for advice

 

[shankos]

Re: Motor not firing on all cylinders

also the dizzy does not have an ignition module on the side of it. There is electronic plug on the side of the dizzy but it is unused apparently.

I am getting spark coming out of the dizzy on all leads.

 

[achris]

Re: Motor not firing on all cylinders

Your problem is not the coil. As this is an MPI engine you will need to have a scan tool or diagnostic software hooked up to read any trouble codes. The most likely cause (without looking at the trouble codes) is that you may have a block or partially blocked injector. For that you would need to test the injectors AFTER you read any trouble codes.

It could also be a faulty knock sensor, which is retarding the timing in an attempt to reduce 'damage' to the engine due to knocking....

YOU NEED TO READ THE TROUBLE CODES.

The timing is moved and set by the ECU with information from many sensors around the engine, which is why when you move the 'dissy' the timing reverts back to where the ECU thinks it should be....

Chris.......

 

[shankos]

Re: Motor not firing on all cylinders

Unfortunately I do not have a scan tool so I cant use one without taking it to someone who does like a shop. So I cannot get the codes.

The fuel filter is only two months old and if i had a fueling issue then i dont see why moving the dizzy would allow the engine to fire smoothly again.

The knock sensor could be a possibility. I will see if I can test if it is working.

Apparently i can test the coil for a resistance ohm reading to determine if it is ok. So i was going to try that but need to find out what ohm values i need.

 

[Don S]

Re: Motor not firing on all cylinders

You do not adjust the distributor like that on an MPI engine. You need to set the timing per the manual, then the computer controls the timing. Have you done a compression test? A full tuneup?
What is the Mercruiser SN of your engine, all MPI systems are not the same

 

[shankos]

Re: Motor not firing on all cylinders

compression test was done two months ago and all was ok. I have not redone one since then.

I have only checked all the ignition system as that seemed to be the problem to me as it was not firing on time for some reason.

 

[shankos]

Re: Motor not firing on all cylinders

...

 

[Don S]

Re: Motor not firing on all cylinders

compression test was done two months ago and all was ok. I have not redone one since then.

When you did the compression test, was the engine having the current problem, or has the problem come up since the compression test?

 

[shankos]

Re: Motor not firing on all cylinders

No problem only started occuring this week. It had been running fine every week up untill now since the compression test was done. the boat is usually run at least once a week as it is summer.

 

[Don S]

Re: Motor not firing on all cylinders

Then you need to do a new compression test to see if it shows a problem on a particular cylinder. Post the results.

 

[Bondo]

Re: Motor not firing on all cylinders

Unfortunately I do not have a scan tool so I cant use one without taking it to someone who does like a shop. So I cannot get the codes.

Ayuh,.... The way yer missin' with everything, Without a Clue as to exactly What yer doin',...

I suggest ya get it to a Mechanic, while it's still runnin', before You do some serious Damage to it....

It's an MPI motor, 'n yer Way outa yer league...

 

[shankos]

Re: Motor not firing on all cylinders

Ayuh,.... The way yer missin' with everything, Without a Clue as to exactly What yer doin',...

I suggest ya get it to a Mechanic, while it's still runnin', before You do some serioNJ
us Damage to it....

It's an MPI motor, 'ner Way outa yer league...

Thanks for the tip but i wasnt after a take it to a mechanic quote but some usefull information.

 

[Don S]

Re: Motor not firing on all cylinders

Thanks for the tip but i wasnt after a take it to a mechanic quote but some usefull information.

Then get the proper OEM service manual #36 here http://boatinfo.no/lib/mercruiser/manuals/mercruiser36.html#/0

You will also need some diagnostic tools. Here is about the least expensive full featured diagnostic computer you can get http://www.rinda.com/marine/mtspec.htm

You will also need a DVM, fuel pressure gauges for EFI engines.

Without the tools, you will just be throwing parts at it until you hit on the problem (If you do).
There are a lot of things that can cause your problem.