Intermittent no fire issues 1991 7.4l

aross2566

aross2566

Member
Apr 29, 2015
59
Live in Ohio,boat on Dale Hollow Lake
Boat Info
She's Some Kind Of Wonderful Too.
Engines
Twin 454 V-Drives.
Hi guys. This is gonna be long thread. I think. June of this year I had my motors tuned up. New wires,plugs, rotor and cap. All were merc parts. Actually gained 200 more rpms after tune-up. Run perfect till Labor Day. Started one morning and my port motor was running awful. Wasn't missing. It was like a plug wire came off. After crawling around in there for a bit. I found number 4 wasn't firing at all. Used a timing light to discover this. I shut it down. About hour later restarted and it run perfect. So I took off. No problem all day. Next morning did the same thing. I HAD to go dump my toilet. About 1/4 mile in idle, it smoothed out. Again, run perfect all day. That's what it's been doing every time we go down. We go every two weeks. I bought new plugs and wires. I replaced the #4 wire and plug. No fire. I went in town and bought another cap. Here's the strange thing. I put the new cap on and the no fire went to #3. 4 was firing fine. I put the old cap back on and it ran perfect. Next morning no fire issues again. Once it clears up, it runs fine till the next day. I have left the slip and pushed the rpms up while it's doing this and sometimes I hear a poofing sound. I've chased this for two months... I'm at a total loss. I'm closed down for the winter now. Any suggestions, I won't be able to try till next April. But it sure would help me get thru the winter if I had a better idea of where to look. Thanks guys for your input.
 
You might have a valve failing to close in that cylinder. That would mean no power stroke. I'd do a compression test and if it's low, then a leak down test.
 
You should have taken advantage of the "twins" and swapped caps from one motor to the other.
 
Thanks for the input. The cap on there was new. I replaced it with another new cap. Didn't solve problem. Switched caps back and issue went away. Doesn't make any sense. Jimmy, if the valve is sticking, shouldn't it still fire? I would think I should still have juice to the plug.
 
Could you give us the year, engine model and type of ignition system in order to aide with the diagnosis.
 
Cylinders 3 & 4 are on opposite sides of the block from each other, but they are right next to each other in the firing order. (1-8-4-3-6-5-7-2)

Not sure what you have under the cap, but check the components under the cap, make sure there isn't anything there that could be allowing the spark to ground out as the rotor rotates near #4 & #3, maybe a loose wire, broken insulation on a wire, debris or anything out of the ordinary.
 
my guess is either the ignition sensor in the distributor or a failing ignition control module......I assume this boat has the Mercury Thunderbolt IV or V ignition system....if the ignition control module is the problem they are VERY expensive IF you can find one....I had one fail on a previous boat....I changed out the entire ignition system with a Delco EST Marine Ignition System which included the distributor for around $400 which is less than the cost of the Mercury ignition control module alone....no more issues after that....

good luck with troubleshooting this problem....intermittent ignition problems can be hard to track down....it took me two years to finally find out what was wrong with my boat...the only way I found it was the ignition module finally died completely and the engine would not start...it was only acting up once in a while and left me stranded 3 times where I had to be towed in....

cliff
 
Thanks guys...the pick up sensor was replaced in June this year. It died. It is the Thunderbolt IV. Its a 1991 7.4L Serial number OD434231. I tried swapping the ignition module from the starboard to the port. But the connector wouldn't connect. They look identical. When I looked inside the cap, I could see where the spark was hitting the contacts. They all looked identical in the aspect of where the fire was hitting.
 
I wonder if a worn distributor gear could cause this......

cliff
 
Several complete TB4's on ebay for short money. Wouldn't hurt to have spare parts.
 
aross2566 said:
I checked the shaft. There is no back and forth play. Its pretty tight.
Click to expand...


did you try to rotate the rotor by hand?......I believe the shaft could be tight (no side to side play) but the distributor gear or cam gear could still be worn....

cliff
 
Yes I did. I pulled and pushed every way possible. It's pretty tight. Although the boats 26 years old. The motors only have 850 hours on them. The guy I bought the boat from was very anal about maintenance. He and the original owner kept every invoice for work done. The oil was change every year. I pull the dipstick now after a season and the oil still looks clean. I say all that just thinking could it wear that much with low hours and proper maintenance?
 
Doubt it, but ignition parts fail. Is the sensor wheel tight ? Try another sensor assembly.
 
could the ignition coil be marginal and not supplying enough spark some of the time for all 8 cylinders when it heats up?....then after a cool down things return to normal for a while....

cliff
 
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Not sure what you mean by sensor wheel. If you mean rotor button. Yes its very tight. Cant get it off. The pick-up sensor is new. What I've been told, its like a light bulb. Its either on or off. No in between. The coil appears to be doing the opposite. When its cold the motor runs bad. When it heats up its runs good. But that is the second comment about the coil. I'm gonna replace the coil in the spring when I open her open. I stopped working on motors 30 years ago. My trouble shooting skills are poor at best now. Plus I'm too damn fat to crawl around those motors anymore. Keep the info coming guys. Greatly appreciate all your input. I'm making a list for the spring.
 
Punch in your motor number on Mercuryparts.com. Look at the exploded view of your distributor. The wheel is under your rotor. How old is your rotor? Why can't you get it off? The wheel triggers the spark
 
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