7.4L Merc. stumbling at High RPM

[eblazic]

I have a 1989 7.4L mercruiser that started acting up this season. Originally, the problem would occur when the boat was cold about 75% of the time. Once I throttled above 3k RPM, the engine would stumble and appeared to be fuel starved. I would let the boat sit for 2-3 hours and it would run fine across all RPM ranges. I changed the fuel/water filter which did not help the problem.

Now the problem is getting worse. THe problem occurs 100% of the time. The stumbling is so bad I can not get the boat above 2k RPM and could not get on plane. I had to idle into the dock. The boat never stalls and is never hard to start. I thought I smelled excess fuel the last time when I was out and had a stumbling condition.

I am thinking this is electrical but am looking for some advice on where to begin. The plugs, wires, and distibutor cap are about 5 years old.

 

[Limited-Time]

Re: 7.4L Merc. stumbling at High RPM

..........................................I am thinking this is electrical but am looking for some advice on where to begin. The plugs, wires, and distributor cap are about 5 years old.

That would be where I start............a COMPLETE tune up

 

[Shaun78]

Re: 7.4L Merc. stumbling at High RPM

I am thinking that you have a crack in your distributor cap....replace the cap and rotor and if its new, it still could be cracked so I would check it

 

[eblazic]

Re: 7.4L Merc. stumbling at High RPM

I replaced the distributor cap, wires. To rule out bad fuel, I also drained the fuel tank to ensure old gas was not causing the issue. The fuel/water seperator/filter was also replaced. No luck. In fact, the problem is now getting worse as the boat will periodically stall. Starts no w/ no problem.

I noticed another problem which I'm not sure is related. Periodically, my RPM gauge indicated 1K greater than what the engine actually is running. Other times, it shows the correct RPM value.

Someone indicated this could be the ignition module and/or the coil. Is there any value in going after these parts?

 

[Mach 1 2350]

Re: 7.4L Merc. stumbling at High RPM

Having the exact same problem with my 1988 454 mag. Your definatly getting a lean miss condition. My buddy had the same thing happen on his 88 350 Mag ended up being a faulty fuel pump, so thats what im trying. Putting the boat in tonight ill post the results

 

[Eliminator]

Re: 7.4L Merc. stumbling at High RPM

I would put a timing light on it and make sure you are getting mechanical advance! Sounds like it was starting to seize up and now the distributor advance weights are totally seized in the non advanced position. Pull the cap off and twist your rotor, should be able to turn it about 1/2" and when you let go it should return to the position it started. Seizing advance weights can also give you symptoms like starting fine sometimes and bucking against the starter others.

 

[bruceb58]

Re: 7.4L Merc. stumbling at High RPM

Just as a test, disconnect the tach. A bad tach could cuase the problem you are having. Its an easy test so it won't cost you much time to try it.

 

[eblazic]

Re: 7.4L Merc. stumbling at High RPM

The problem turned out to a be a failing fuel pump. Replaced pump - boat works fine now.