4.3 MPI poor performance WOT

[MosbyMPI]

Good morning I have a new to me 2008 Glastron with a 4.3 MPI. Had 334 hours when I got it now I am at about 339 hours. Issue has been when it's cold it misfires under load and once warmed up at wide open throttle. Once warm it just doesn't seem to pull as well as it should getting up on plane. I used to have a 19' fourwinns with a 4.3 TKS so I know roughly what I should be expecting.

I ran it for about an hour then changed the fuel filter. Next I found the vacuum line was disconnected at the fuel cooler, reconnected now. I have also added 1 can of Seafoam marine and a half can of regular Seafoam. I have ordered new plugs cap and rotor and will install them next week as these ones look fine but appear like they could be original. At low Rpm when warm <3500 it runs awesome, it is only when you really get into it you can feel a misfires/hesitation.

I suspect my fuel is old so I'm trying to burn off as much as I can before filling back up with premium and another couple cans of seafoam. I suspects this is a fuel issue and hopefully fresh fuel a bit more seafoam and new plugs will do the trick. Does it seem like I'm barking up the right tree?

Or is there something simple I could be overlooking?

Boats at the lake so I only get to try it on the weekend.

Thanks for any advice, wisdom and encouragement you may have.

 

[makomark]

old fuel can be a problem...so removing what you can, from the tank, will only help.

I'd go easy on the seafoam...

Would also be a great idea to measure the fuel pressure across the operating range, under load, from idle to WOT...

 

[Dieter]

Good morning I have a new to me 2008 Glastron with a 4.3 MPI. Had 334 hours when I got it now I am at about 339 hours. Issue has been when it's cold it misfires under load and once warmed up at wide open throttle. Once warm it just doesn't seem to pull as well as it should getting up on plane. I used to have a 19' fourwinns with a 4.3 TKS so I know roughly what I should be expecting.

I ran it for about an hour then changed the fuel filter. Next I found the vacuum line was disconnected at the fuel cooler, reconnected now. I have also added 1 can of Seafoam marine and a half can of regular Seafoam. I have ordered new plugs cap and rotor and will install them next week as these ones look fine but appear like they could be original. At low Rpm when warm <3500 it runs awesome, it is only when you really get into it you can feel a misfires/hesitation.

I suspect my fuel is old so I'm trying to burn off as much as I can before filling back up with premium and another couple cans of seafoam. I suspects this is a fuel issue and hopefully fresh fuel a bit more seafoam and new plugs will do the trick. Does it seem like I'm barking up the right tree?

Or is there something simple I could be overlooking?

Boats at the lake so I only get to try it on the weekend.

Thanks for any advice, wisdom and encouragement you may have.

Take the fuel filter apart and empty contents and look for contamination

Also many times the crab style dist cap on mpis will cause mis fire


What do the plugs look like ?

 

[MosbyMPI]

old fuel can be a problem...so removing what you can, from the tank, will only help.

I'd go easy on the seafoam...

Would also be a great idea to measure the fuel pressure across the operating range, under load, from idle to WOT...

Definitely that will be my next step after mostly new fuel. I think the fuel pressure regulator is in the fuel cooler on this one as the fuel return off the rail is capped off from factory.

Ya I guess I shouldnt curb my frustration with seafoam

 

[MosbyMPI]

Take the fuel filter apart and empty contents and look for contamination

Also many times the crab style dist cap on mpis will cause mis fire


What do the plugs look like ?

I just had it out, once it cools I will pull a couple plugs again to see how they look. A couple weeks ago they were reasonable beige ish not really wet or anything but you could tell by overall they are likely original.

Yes cap is also a suspicion. I haven't pulled the cap to look yet because I figure I will wait until I have a new one to swap in and save myself a Re&Re, I'm hard working but lazy when I can

I do have the old fuel filter I can pull it apart and see what I can find.

 

[MosbyMPI]

Definitely that will be my next step after mostly new fuel. I think the fuel pressure regulator is in the fuel cooler on this one as the fuel return off the rail is capped off from factory.

Ya I guess I shouldnt curb my frustration with seafoam

I just had it out, once it cools I will pull a couple plugs again to see how they look. A couple weeks ago they were reasonable beige ish not really wet or anything but you could tell by overall they are likely original.

Yes cap is also a suspicion. I haven't pulled the cap to look yet because I figure I will wait until I have a new one to swap in and save myself a Re&Re, I'm hard working but lazy when I can

I do have the old fuel filter I can pull it apart and see what I can find. It looks reasonable no large debris bit of discoloration but nothing unexpected for a maybe 2-3 year old filter.

 

[Dieter]

I just had it out, once it cools I will pull a couple plugs again to see how they look. A couple weeks ago they were reasonable beige ish not really wet or anything but you could tell by overall they are likely original.

Yes cap is also a suspicion. I haven't pulled the cap to look yet because I figure I will wait until I have a new one to swap in and save myself a Re&Re, I'm hard working but lazy when I can

I do have the old fuel filter I can pull it apart and see what I can find.

Just dump the contents of the fuel filter. If no sediment or moisture separation you do not have a fuel contamination issue

 

[MosbyMPI]

Distributor cap and rotor was corroded. The cap appeared to be original but it had aluminum not brass contacts. Also swapped out the plugs, again they appeared original. Finally put in 15 gallons of fresh marine fuel. Now runs with no issues .

I used a premium cap and rotor with brass contacts for a 2005 Chev Blazer with a 4.3 for my replacement cap and rotor. (Much cheaper than Quicksilver and I m not sure there could be a downside if you go premium brass). Plugs the same but went with NGK Ruthenium instead of Iridium for kicks. NGK is my normal go to for plugs.

Anyway thanks, it was just a boat that was short on maintenance items, and old fuel.

 

[makomark]

good deal!

 

[sandkicker]

I never get a third season out of a Thunderbolt Cap and Rotor.. Swap them annually IF i am flush, otherwise every other year.

 

[MosbyMPI]

I never get a third season out of a Thunderbolt Cap and Rotor.. Swap them annually IF i am flush, otherwise every other year.

The one I took out certainly looked long overdue.

It was marked a marine cap but had aluminum contacts so the corrosion was fairly significant.