7.4 MPI low power
 

Hi all. My Baja H2x seems to be low on power. Takes a long time to get on plane and struggles to get to 45 mph. I had a intermittent banging noise which I isolated to the prop hub. Replaced it and the banging noise is gone. I checked and replaced the plugs. 6 of the plugs looked good. plugs from Cylinders 6 and 8 were black. Looked fouled. I also found a ground wire had worn through and was touching the block. Fixed that. After a test run it seems a little better. I pulled the plugs on cylinders 2,4,6,8 and compared them. 2,4 looked as they should. 6,8 were completely clean like they haven't been used. I have cap,rotor and plug wires ordered. Waiting for them to get here. Any suggestions as to y those 2 plugs looked new after the test run?

Thank you
Mark

It can be hard to tell if you are not a spark plug reader type person. Just pulled my plugs from my junk earlier this year. Where in motor for 6yrs plus. Looked like they hardly ran.

We need more diagnostics....compression good ? fuel psi good ? If been many years, do what you are doing by replacing all ignition components - plugs (get OE Marine delco's), cap/rotor (Mercruiser) and good quality wires - I won't say because always debated. LOL. Clean out your flame arrestor which no one does, including many marina's, for some ungodly reason.

I dont want to assume,,,,, is the a new to you boat, or a boat you have had that developed this problem?

If its new to you , I would suggest dropping some pitch in prop and target 4600 rpm WOT.

If new problem, then you are on the right track starting with a full tune up and go from there.

Thank you for the help. It's a new problem. 1st full season with the boat. My tuneup stuff is finally here. Gonna put that stuff on. Do the compression test and fuel pressure test today. Finally have time. Another question. Does the boat have a "limp mode" like cars? If it senses an issue. It will cut the power?
Thank you again
Mark

On a side note. I think I am supposed to be upset it's broke. I'm not because I like turning wrenches and trouble shooting. I've always enjoyed building and wrenching my cars more than I like driving them. Weird I guess.

LIKE TO WRENCH RATHER THAN DRIVE THEM -- someone within this group needs to have a long talk with you-- I really do understand your passion.

Please let us know what you found -- I likewise have a 7.4 MPI and I'm always interesting in learning what other people experience. Regarding your flame arrestor - Believe it or not - I clean ours with warm soapy water at each fill -- considering the volume of air that goes thru this it is VERY cheap maintenance.

All the best

3pointstar

The other thing is if they are totally clean you could be getting water in those cylinders.

I didn't get lucky with just a tuneup. I have 2 adjacent cylinders 6 and 8 with perfectly clean plugs. Those 2 cylinders also showed low compression when doing dry test. Stayed the same with oil in the cylinders. Fuel pressure is good. Plugs are firing. Looks like a bad head gasket. Anyone have thoughts on my conclusion of head gaskets being the issue?
I appreciate all your help.
Thank you
Mark

Op,

How many hours? The VST filters on mine were 2/3 and full plugged with junk at 235 hrs. (1996 model 7.4LX MPI's) causing a bogging issue at 3k rpm. Also the VST diaphragms were cracked, looked dry rotted.

It's has 120 hrs. I looked in my manuals and I can't find any mention of the VST. This sounds dumb. But where is it? Lol.

Likely is a head gasket. Sounds like you're steam cleaning the plugs. Low compression. Get the heads off.

I've done lots of automotive head gaskets. By the looks of the repair manual. It looks to be the same thing. Other than the marine accessories. Am I correct in thinking that?
seems like it may be a lot easier yanking the motor. Is it usually done that way?
And a bonus is I get to justify buying the specific tools I will need!!!!!! Lol. Love that part.

Yup. It's a Chevrolet engine. The low compression tells you something is going on internally for sure. Pray it's only a head gasket. Pull it apart check it out. You will likely see it when you pull the head.

Thank you. I appreciate your help. Makes it easier to wrap my mind around.

I WIN!!!!!!!!!!!! Take that ya broken ass boat!!!! Head gaskets done. Injectors fixed. Runs like a champ. Thank you all for your help. I really appreciate it.
Thank you
Mark

I am soooooo happy it broke. It has not run like this since I got it. It's a completely different boat. Like night and day. It's on plane in seconds. Torque like a big block should have. First time the wife grabbed the holy **** handles. Love it. Hell yeah!!!!! Thank you. Thank you. Thank you!!!! I appreciate all the help.

If you have one,Top back starboard side. Green/yellow/clear hoses coming off it. Vapor saturation tank, only used a short time before cool fuel was out. Glad you're running strong now.

did u send the injectors for flow testing&cleaning?

not sure but i think 96 was the last year for vst.

Believe you're right. Mines a '97 boat rigged late '96. Gen VI 502 Mag, VST, V belts. Had the Gil's on 16 years. Starting to get a little nervous wondering what they look like inside.

Yeah I had the injectors serviced. They were loaded with garbage when I looked at em. Turned the engine over after I had the plenum off to see what they were doin. Mostly curiosity. Had a couple that looked like they were questionable.

the reason i asked is i have seen several mpi engines with detonated pistons and blown head gaskets due to bad injectors.

at this point i would replace the fuel line from the tank to the fuel filter,when they get old they can delaminate on the inside of the hose.the ethanal in today,s fuel speeds up this process.

Good idea. Thank you. Are there inline fuel filters before the water/fuel seperator filters on these boats?

What was the fix ? I see you started another thread for something different but stated it runs 'awesome now."

Hi SB. I ended up doing the head gaskets. Had the Heads checked out. Machine shop cleaned em up and said they were fine. I also found the fuel injectors were plugged up. Had those serviced. And did a tuneup. While it was apart I could get to things easier and gave things a once over. Found a ground wire that had worn through and fixed that.
Thank you
Mark