Bad Compressions on 2000 7.4L MPI - Maxum Boat Owners Club

ni7irs · 05-20-2013, 02:31 PM

So, my boat seemed to lose a lot of power last summer. I did a brief compression test this winter while getting it de-winterized and the numbers were almost all at 50. I did not have the throttle wide open and the motor was quite cold. Also, I did not shoot oil down the cylinders for the wet test.

Assuming the worst and the numbers do not meet the 100 psi minimum, what are my options? Can I install larger rings and try and get a few more seasons? If I have to go with a new motor, where should I get it and can I increase the horsepower? This is a 2000 2800 scr.

Many thanks for info!

mmwjr · 05-20-2013, 05:41 PM

First of all sorry to hear you have low compression. Next you need to determine why it is low before trying a short term fix. Any water in the oil (milky looking), this would say the head gaskets are blown. Could be as easy as new head gaskets or as bad as cracked heads. If the rings are worn then using replacing them may help but you need to check the cylinder bore for wear to see if new ring will help. If worn to the point that oversize rings are needed then the pistons will have to much play and may end up breaking. In this case either rebuild or replace.

Regarding a new engine are you looking to go with a whole new package (pan to arrestor), new long block, or remanufactured long block? Lots of places on line to choose from but be sure to account for shipping as any of these are heavy and cost accordingly.

ss3964spd · 05-20-2013, 09:25 PM

50 across the board? I could see maybe one or two being down that much but almost all of them? If you have the numbers per cylinder handy post them here.

Before breaking a sweat and breaking out your wallet you really do need to run the compression test again. Warming up the engine would be ideal but I think having the throttle blade open is a must on an MPI engine.

How many hours are on this 454?

Dan

mmwjr · 05-21-2013, 12:44 PM

I agree with Dan that having all 8 cylinders down that low is very rare. Having a blow head gasket that I mentioned before would give you no compression and usually only impacts 2 adjacent cylinders just can not understand how they are all low. To be that low yet still have some compression would most likely mean stuck rings but even so I have never heard that happending in all 8. As Dan suggest I would redo the compression test, you may want to spend a few bucks and get a new guage just to rule that out or check the compression on another engine just to verify your guage isn't broke (only going up to 50 and sticking).

ni7irs · 05-21-2013, 02:55 PM

Yes, the consensus seems to be that the engine needs to be warmed up and these numbers are completely incorrect. I will do another test after i get it started and running a bit.

last summer, I lost a lot of power and I guess I jumped to conclusions. I am wondering if the shutters came apart because I no longer get any smoke or exhaust through the prop whereas before I used to get most of it through the prop. For history purposes, the lower unit was replaced right before the last season. Is there a chance that the mating could have been screwed up?

mmwjr · 05-21-2013, 05:39 PM

While your compression may end up being OK, pending retest it sounds like you have some type of issue if you lost a lot of power as you state. So as Dan asked how many hours are on the engine? What is your definition of lossing a lot of power?

I don't believe changing the lower unit can result in a mis match of the exhaust port. What shape is the exhaust bellows in (lower bellows), if it has a large hole or missing the exhaust will exit here and not thru the prop.

BTY how much smoke were you seeing, really shouldn't be seeing much. may be a hint of power loss. What color is the smoke?

Robert2300 · 05-24-2013, 04:24 AM

I would check fuel filter, injectors, spark plugs and plug wires. You would be surprised how much power loss you get from one misfiring cylinder or a cylinder starving for fuel.