Mercruiser 4.3L - Stumbles at 3000RPM

[nola mike]

I can order the pressure gauge for $22, or I see that an OEM fuel pump is like $150, or $25 for some Chinese knockoff. I wonder if I just get the Chinese one for the same price as the inline fuel gauge. It should run for at least long enough for me to confirm or rule out the pump issue. That way I don't have to find a way to splice that gauge into the very short and mostly metal fuel line.

The Carter OEM pumps can be had for like $70. But I went through 3 of them before getting one that put out the correct fuel pressure. Possible you'd be replacing a bad part with a bad part.

 

[Maverick7]

Did you check to see if you're getting a stream from the accelerator pump when you stab the throttle?

And I'm confused about the symptoms. It now only hesitates on muffs?

Last, did the symptoms get immediately worse after changing the plugs?

No I didnt, its just me so Im not sure how to increase the throttle at the helm and also look at whats happening in the carb.

As for as the muffs question goes: I dont live on a lake and the boat doesnt live in the water. The issue started last night with the boat on the water. Tonight, I was just trying to diagnose things in my driveway, with the boat on muffs.

 

[Maverick7]

Another possibly I just thought about after hours of youtube.

Any thoughts on it being a bad ignition coil? I glanced at the ignition coil thats in there now. It does have a slight film of oil on it, could be from anything I suppose over the years but is it possible the ignition coil has gone bad?

 

[Scott Danforth]

It's almost never the coil. Coils work or they don't. They are not ever problematic at a specific RPM in the middle of the range

 

[Scott Danforth]

I would have pulled the carb apart by now and been out a few bucks for gaskets.

regarding fuel pressure, its either there or not. most intermittent issues are not fuel supply related as the fuel bowl in the carb has enough fuel in it to run for a few minutes.

the fact the motor runs at higher RPM's indicates its not the fuel supply.

a carb has 3 circuits.
the idle circuit (usually good to about 800 RPM)
the intermediate circuit or off-idle circuit (usually good from 800-2500 RPM
the high speed circuit

my guess is you have debris in the main metering jet which is masked at the higher rpms and you just dont notice it as it will be more prevalent at the lower end of the high-speed circuit RPM range.

 

[ESGWheel]

Agree to not cut into metal fuel lines > again I was thinking u had mechanical and thus rubber hose feed. Take a look at your set up and see if there is an easy way to test fuel pressure. Regardless, yes, you can get a cheap fuel pump and try that and if all works out decide if you get a second cheap one as a backup or go with OEM and keep the cheap one as backup.

When I had issues with my electric (two stage so low + high pressure for MPI) I did go with cheapo while I got my OEM one squared away and had no issues. The OEM one is back on line and I have the cheapo as a backup.

Be aware that part changing out vs. doing diagnostics is known as the ‘parts cannon’ ie keep firing parts at it until finally get the right one changed out and all works. In your case, with the low cost / risk of a cheapo fuel pump, I would give it a go. Please post results.

 

[Scott Danforth]

they make an inline test fitting.

 

[Maverick7]

SOLVED

Just wanted to update all of you on the resolution to this issue.

This ended up being a bad Ignition Sensor Assembly (part no 87-892150k2).

Thanks for all the incite to the issue.

 

[Maverick7]

Well another update.

Boat ran fine for about 30 minutes after replacing that ignition sensor. Then it crapped out again, same exact symptoms. While idling back to the dock, I happened to notice the battery voltage gauge flickering almost in synchrony with the rpm gauge. Still thought it was a bad sensor install or something. Dove a bit deeper, and found the true culprit. Many hours on the boat after some splicing/soldering, tape, sleeve, rerouting, and a zip tie. And she runs like a top.

Best guess is that purple wire got shuffled out of the way for a bit while installing/testing different parts, giving me a false sense of a fix, but it eventually fell back onto the manifold.

 

[nola mike]

Well another update.

Boat ran fine for about 30 minutes after replacing that ignition sensor. Then it crapped out again, same exact symptoms. While idling back to the dock, I happened to notice the battery voltage gauge flickering almost in synchrony with the rpm gauge. Still thought it was a bad sensor install or something. Dove a bit deeper, and found the true culprit. Many hours on the boat after some splicing/soldering, tape, sleeve, rerouting, and a zip tie. And she runs like a top.

Best guess is that purple wire got shuffled out of the way for a bit while installing/testing different parts, giving me a false sense of a fix, but it eventually fell back onto the manifold.

Glad you found it. I've had that same fail, with those same symptoms. Intermittent electrical issues suck.

 

[Maverick7]

Glad you found it. I've had that same fail, with those same symptoms. Intermittent electrical issues suck.

What’s funny is I should have recognized it as electrical in nature must sooner. Not only that, I found it by dumb luck and not any sort of real diagnostic method.

After researching this, seems like it’s a very common issue.