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MERC 7.4L Problem
I am having some trouble with a mercruiser 7.4L motor. the motor starts and idles fine, it even runs fine to a point.
the motor will idle all day, but once you are moving, anywhere above roughly 3000 rpm, every so often, the motor basicaly cuts out for a split second and then just goes back to running, the motor is running very well, then that split second it essentially stops, but picks right back up and keeps running good. that is my problem. it was doing this before I changed some parts and did a few things, figured that would fix this problem along with the other problems I was having, but I was wrong, so recently I have replaced the cap, rotor, ignition sensor, plugs, and plug wires, the carb was also rebuilt- that was due to ethanol/water problems.. I checked a few things, main ground is tight and secure, battery leads are secure and positive blocks are secure. what could this be? what can i check? thanks, does not mater whether water is rough or glass it happens both ways. thanks for responses in advance |
Well you checked all the stuff i was thinking what about fuel filter and pump is the pump electric?
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no mechanical fuel pump, which incidentally was replaced at the end of last year.. i basically had a ton of small problems that all masked each other, now just left with this one.
I thought about it some more, and I guess it could be fuel, but it is too perfect in my eyes to be fuel. it does the same exact thing whether its at 3000 or 4000 or 4500, and it does it in smooth water, rough water, anything. cuts for that split second and is running perfect again, the prop stops turning too because the boat feels like you are hitting the brakes... i was thinking, could it be something with my kill switch or shift interrupter, not really even sure about the 2nd option, it is a bravo 1 drive, not sure if they have that like the alpha's. mechanics around here are really backed up, and just trying to figure something out on my own. |
Do you have a rev limiter could be acting funny weird that it does it at 3000rpms and then acts fine what kind of ignition
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Change your coil. I had something similar on my 454 Mag.
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it is thunderbolt V, also, changed the coil last year, at the end of the season as part of my whole trouble shooting thing, figured it was old and cost was very cheap so i just did it. lol I am sorry I forgot to mention that, as you can see, as far as ignition goes, I have changed everything except the module for the ignition, what about the rev limiter, how would i know whether my motor has it or not?
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Originally Posted by firehawkcat
(Post 3165914)
Do you have a rev limiter could be acting funny weird that it does it at 3000rpms and then acts fine what kind of ignition
Same symptoms, we would be cruising along, then the motor would cough and pick right back up. |
Take a couple of pics of your ignitiion. Most TBV's have rev limiters.
Originally Posted by Timeless61
(Post 3165935)
it is thunderbolt V, also, changed the coil last year, at the end of the season as part of my whole trouble shooting thing, figured it was old and cost was very cheap so i just did it. lol I am sorry I forgot to mention that, as you can see, as far as ignition goes, I have changed everything except the module for the ignition, what about the rev limiter, how would i know whether my motor has it or not?
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images are not showing up for some reason, i will put up a link. but, I ran the boat on the hose today and fooled with a lot of things while it was running, and it will not do what it does in the water on the hose... i was wiggling every wire for the ignition at the motor including the cannon plug, all connections are good, only does it while actually using the boat. if that tells anyone anything.
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I have the same problem with my 7.4 L with Thunderbolt V ignition. After changing everything and putting on a new coil it still cut off. Soon after it backfired and would not start again. I started swaping parts with the other motor and found it to be a bad ignition control module. I hope that is not your problem as the part is pricey. I am looking for a used one now. Part number 806961-2. It is shown in your first picture.
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yea, kind of funny, but thats what i thought it was all along but figured i would do the like big tuneup of small cheaper parts first, this week we are going to hook it up to controls and take the rest of the boat out of it, and if it does it, i guess i have my answer... i don't know what my cost would be on it, but from when i looked it up in midas it was pretty expensive
did yours only do it under load actually running the boat or even on a hose on land? |
While underway the port motor would shut down without warning. It restarted easily and I made it back to port. Once in the slip I tried to restart but there was a huge backfire and the motor would not start. I did all the tests for bad fuel, clogged filter, and bad gas but determined it was ignition. I even purchased a new coil but that did not fix the problem. I started switching parts and finally determined it was the ignition control module. I have found the prices range from $485 to $425.
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How about ignition switch or connectors on under dash wiring. Also cannon plug on the engine. Good move to eliminate these things for the next test. Also a remote fuel tank can help sometimes also.
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cannon plug is good, actually, while on land, i was wiggling everything i could, and could not get it to do it, appears it is only while under load... but like i said we will remove the "boat" from the test.
bajasport, when you said yours cuts off and then you have to restart, mine, does not turn off, just cuts out for a split second, and then continues running. very weird |
Its pretty easy to pull the can plug and run a hot wire to the ign , and use a remote starter to turn it over. That will eliminate the boat.
I have seen the tach do things like you are describing, the reason it needs to be drove it the tach is getting bounced around when you are running the boat on the water and not so much on land. But first eliminate the boat that will narrow it down. Good luck |
actually, as an update, something was happening with my key switch, but we also replaced the tach as it was being a little funny at times.
boat finally has no more ignition/electrical issues |
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