Logo
FULL SITE MENU

Merc 3-cyl crazy ignition problem

T

Tom in AZ

New member
Hello,
I purchased this 1998 40 hp pretty cheap. Owner says running bad but good compression. No prob. I checked the timing. Numbers didn't even show up in the window for the timing light! I moved the light to all other plugs and no numbers show in the window, but all plugs verified firing. #1 verified at TDC when timing marks aligned. I pulled the plastic cover off so I could light up the entire flywheel. #1 fires at the #2 position consistently. #2 and #3 fire correctly. I chased the wires and everything is correct, but #1 always fires when #2 fires and not when its supposed to. That is very strange! This problem doesn't show in any trouble-shooting manual or even a thread that I can find! Anyone seen this one before?
Tom in AZ
 
To be more clear, I replaced all three CDMs after confirming #3 was bad. This improved the way the engine ran, but did not fix the low power and exhaust backfire. In order for it to be CDM#1, it would have to consistently fire 120 deg late after receiving a trigger signal, every time. I don't think this is a credible failure mode for a CDM. The trigger is next to be replaced after I get the flywheel off (its stuck really bad, but the tips offered in this forum will help). Still, a strange problem. I've heard of triggers going bad and randomly firing the CDMs when they are not supposed to, but not like this...
 
You could put a timing light on it to see what's happening. You'd have to mark the flywheel in 3 places, 120 degrees apart, and select a convenient spot for a pointer. If the mark stays steady--no problem--but if it jumps around all over the place...

Jeff
 
I guess my first post was not clear. I am using a timing light and have confirmed #1 always fires at #2. #2 and #3 fire where they are supposed to. No jumping around. Rock steady at those positions and good solid spark.
 
Ok. It is OG745812. I will check the flywheel carefully if I can ever get it off. Its soaking in a mix of acetone and ATF right now. I'm pulling out all the best tricks. Cold freeze on the shaft and big hammer on the puller on Monday!
 
Thanks! I ordered it from eBay. Should be there when I get back to Scottsdale on Monday. I thought a little heat, WD-40, and the automotive puller with Grade 8 bolts would be enough since its not rusted, but not this one!
 
Tom, for what it is worth, that is the only style of puller that should be used on that type Mercury engine. The others can damage the flywheel. I have also noticed that hammering is not good, either. Please let us know how you did.
 
I got the flywheel off after much effort even with all the right tools and techniques. It was really stuck! Everything looked liked it came from the factory yesterday! No damage, cracks, or corrosion. I replaced the trigger and #1 now fires where its supposed to. The timing is correct. I guess that failure mode is possible for a trigger. But....
 
I took the boat out for a test today. I still don't have much power. It will only go 25 mph. Here is a list of things that I have checked: fuel pump and filter replaced. Carbs looked great when I pulled the bowls to check the needles and float settings. It has a new tank and fuel line with no signs of air leaking in upstream of the fuel pump. I know the plugs have good spark. CDMs replaced. This is a 13 ft Boston Whaler and it should go much faster than that. The prop is the same 13 pitch I have used on that boat before with a different motor, also a 40 hp. It is not damaged.

The motor sounds like its bogging down. If I didn't know all three plugs were firing I would say it is missing on one cylinder, but there is nothing else to check. Just wondering, I think this motor was run for a long time with #1 firing 120 deg late. There seems to be a lot of carbon build-up in the exhaust system. Is it possible that the exhaust is completely blocked on that port? I hope there is something else to check before I have to pull the powerhead! Is there anything? Is this even possible? I'm grasping at straws now...
 
You need to check the voltage on the trigger, if voltage is high on one lead, it will fire the coil early,

have seen this before, hole will have spark, just does not run on it , running out of time.
I would need to look up the speck, 10 volt sound right

 
I changed to BP8H-N--10 plugs gapped at 0.036. That helped a little. I'm getting a little more rpm and 28 mph, trimmed as high as I can without propoising. The engine is still bogging down like it can't breath. WOT timing is 24 deg. I would like to measure it running in the water but that's too insane. It actually bogs down more and loses rpm at full throttle. It runs a little faster just before full throttle. Without any new inspiration I guess I will pull the powerhead and check the exhaust path.
 
Did you miss this? One last thing to check.



"You need to check the voltage on the trigger, if voltage is high on one lead, it will fire the coil early,
have seen this before, hole will have spark, just does not run on it , running out of time.
I would need to look up the spec, 10 volt sound right"
 
The trigger has four leads. One for ground and one for each cylinder. So I presume I need to check voltage on each lead while cranking?
 
You still need to do a WOT plug color check. You have the plugs in there to do it. Be sure to run the HELL out of it, wide open, for at least 30 seconds before immediately shutting the engine down and off--no idling permitted. I know it's miserable to work with such smoking hot plugs, but t must be down.

Jeff
 
Ok. I will get back to the lake on Monday after work. AZ keeps Daylight time year round so I should have just enough time to do a WOT run across the lake and shut off the motor before it idles down. I will also check the voltage on all three leads going to the trigger. I guess with ignition on, static, plug-out cranking, and while running at idle.
Tom
 
All, thanks for the help! I have never started a thread before, but it seemed necessary since this trigger failure was very unusual. Your help was great, especially in getting the flywheel puller. I had no idea I could get the right one so cheap and I would have been stuck without it. I ran power tune through the motor to clean out the carbon and now it runs normally. Still not great, but not so bad that I have to pull the powerhead. The scenario now makes sense: unusual trigger failure, owner ignores it for years running on a pontoon boat at low speed, then sells the motor cheap when the #3 CDM goes bad and it barely runs at all. The motor is badly clogged with carbon, especially on the #1 port. I fixed the trigger problem but the carbon still remained and is slowly burning off. The plugs look normal after a hard run. A little more power tune and some more hours with the motor should burn off the residuals. This is now a great motor that will take me fishing for years!
 
Tom in AZ said:
I fixed the trigger problem but the carbon still remained and is slowly burning off. The plugs look normal after a hard run. A little more power tune and some more hours with the motor should burn off the residuals. This is now a great motor that will take me fishing for years!
Click to expand...


So the trigger voltage was high?

Decarb that engine slowly(over time) or it will be over shortly.
 
I tried to measure the voltage on each of the three leads while running and stopped. I got hardly any voltage at all. I wouldn't have known what to do if it was high and causing the problem. Changing the trigger fixed the issue. Just for curiousity, what do you do if the voltage on one of the three leads is high?
 
Throw some Seafoam carbon cleaner in the fuel. Or Yamaha Ring-Free, OMC Carbon-Guard, etc.

Besides doing the shock treatment, having the additive in the fuel gives a continual, gentler cleaning effect. If you can find non-alcohol fuel in your area, use it. Your motor will run better & your fuel system will thank you!
 
Tom in AZ said:
I tried to measure the voltage on each of the three leads while running and stopped. I got hardly any voltage at all. I wouldn't have known what to do if it was high and causing the problem. Changing the trigger fixed the issue. Just for curiousity, what do you do if the voltage on one of the three leads is high?
Click to expand...

"You need to check the voltage on the trigger, if voltage is high on one lead, it will fire the coil early,
have seen this before, hole will have spark, just does not run on it , running out of time.
I would need to look up the spec, 10 volt sound right"
 
quote_icon.png
Originally Posted by Tom in AZ
I tried to measure the voltage on each of the three leads while running and stopped. I got hardly any voltage at all. I wouldn't have known what to do if it was high and causing the problem. Changing the trigger fixed the issue. Just for curiosity, what do you do if the voltage on one of the three leads is high?


Docksidemarineservices said:
"You need to check the voltage on the trigger, if voltage is high on one lead, it will fire the coil early,
have seen this before, hole will have spark, just does not run on it , running out of time.
I would need to look up the spec, 10 volt sound right"
Click to expand...


Lets try this again, Missing a sentence. If voltage is high on one of the leads you replace the trigger.
 
Back
Top