7.4 MPI Bogging under load
 

I am the original owner of a 1998 boat with a 7.4 MPI. Over the years the power / speed has dropped a couple of MPH. In August last year I got hurt Wakeboarding and the boat sat up a couple of months. When I took it out in the Oct / Nov time frame it had a noticeable power loss but no real symptoms as to why. In previous years I replaced the cap, rotor, oil/filter and sea pump but that was it. This season I replaced the plugs, performed a compression test (with good results) and replaced the fuel / water separator. The engine now starts and idles PERFECT (just like new). The problem comes in when moving off idle. The engine sounds and feels as if it is dropping a cylinder or two. Its nothing horrible but you can hear and feel a miss now. After on-plane it is not as noticeable but it’s down about 750rpm and 4 – 6 more mph. Common sense tells me it a fuel / injector problem. I am going to check the fuel pressure next but wanted to ask your thoughts on the matter.

Thank you in advance!

Danny Hightower
Dallas, Texas

there are a number of things, but id vote for wires crossed, bad plug or bad wire.

If you have never replaced the plug wires, thats where I would start.

Dave

I third the ignition wires. An injector is a possibility as well. An easy test that I've done in the past is to spray the wires with water and that will almost guarantee that you'll get an arc if they are leaky. You also need to check the end connections for tightness and condition. If the wires are original, they are about due anyway.

Rene

I would also check fuel pressure, I had a regulator go bad and do exactly what you describe.

start w/ wires

all above are good places to start, if that dont take care of it , you might look at the fuel injectors if it has had gas sitting in them for long periods of time without starting the engine, the screens in the top of the injectors clog, Tyler Crocket just flowed and cleaned all my injectors a month ago, one was so bad it had to be replaced,

Plug Wires
 

Thanks for the feedback.

Although the plug wires look perfect I dont have a problem changing them. Do you have recomendations on replacement wires? I mean are you guys saying buy Mercruiser ones?

The wires can look good and still be bad. You can check the resistance with an ohm meter- 300 to 400 ohms per inch is normal. If theres any doubt how old they are, check the date code on the wires. It's a four digit number like 2Q97, meaning second quarter of 1997.

If you are thinking about aftermarket plug wires, make sure the ones you get provide enough EMI supression for a computer controlled engine.

Dave

I ran ito this once.