mats

Hi
I own a 25 foot Baja which originally had a 1988 7.4 L bravo One in it. In 1999 I changed it for a rebuilt 1996 502 MPI(it was rebored to a 509). Since then I have had all sorts of problems.
1.lack of power. The boat speed increased only 2 mph(going from 4600 to 4800 rpm using the same prop). Checking the sparkplugs the engine seem to run lean. The electrodes on the plugs are basically white. Further checking the compression it is ranging from 115 to 140 psi on the port bank and 155 to 165 psi on the starbord bank.
2. Oil consumption. The engine consumes more than a quart an hour at cruising speed(3000 rpm). However, checking the exhaust, the oil appears to be consumed by the starboard bank (sooty residue in the starboard exhaustpipe), the one with good compression. Any good ideas?

Mats

Vinny P

The first thing I would do is throw away the compression guage. Get a leak down guage and run a check with it on all the cylinders. This is the only real way to check the condition of the rings, cylinders and valves. With that said, who did the re-programming after the rebuild?? Was it lean from the first day of running? How many hours did you run it while lean? What else was done to the motor on the rebuild besides the overbore? Cam, heads, compression, injectors?

rjcardinal

One thing to remember is that the 502 will use more oil than the 7.4 because it has forged pistons. The cast pistons in the 7.4 dont expand as much as the forged pistons of the 502 do. Because of this the piston clearances on a 7.4 is smaller because you dont have to leave room for expansion.

If you think you are having a lean condition with the reprogrammed ecu borrow a stock one and run it. The stock ecu is fairly rich and should be good enough to give you a baseline.

Good luck and post what you find.

Ron

Offshore Addiction

sounds like your on borrowed time with that motor,leak down would be good,I'll betcha somebody screwed those heads up,you need to level your compression off before you go to the lean problem,your oil consumption is either wore out valve guides,or sloppy rings.misseated intake valves will cause a lean condition,and it may only be at WOT,did somebody mess with the computer?

mats

More clues;
1. The computer is stock. I have tested with another one, a Vortech engineering smartcharge programmed one. but the result was similar. maybe gained a mph or two
2. the lean condition seems to appear when running at higher rpms above 4000. I have changed the mechanical fuel pump, the water separator filter and checked the fuel filter in the VST tank. I have also tried to run it from another(portable) gas tank with the same result.
2.5 When advancing the throttle, basically nothing happens the last third of the travel
3.The rebuilder made a leakdown test and found it to be "OK". I don´t know the numbers.
4. The engine has GIL exhaust and a Vortech K&N flamearrestor, everything else is stock(I think)
5. The engine now has 80 hours on it since the rebuild. Performance and oil consumption have been basically the same all the time

blown formula

"If" the leak down was good (done by the suspect mechanic!), I would check the injectors themselves for trash/blockage in the screens.... this is easy and takes about an hour to do. This can melt an engine quickly!! You may or not be lucky. If a piston was damaged badly, you would have oil everywhere except the oil pan!! no mistaking that. Maybe the cylinders were never honed by the mechanic, they were glazed so the rings never seated. You'll know once the heads are off!

Vinny P

You need to get those leak down numbers and post them here!!! Don't trust the guy who rebuilt the motor for accurate numbers. I would bet he will be trying to cover his ass. Check them yourself or get a third party to do it for you.

Offshore Addiction

checkmate and formula are right,I just rebuild a V-6 that another mechanic had just rebuilt less than a 100 hrs ago,same simptoms,exactly the same.....turned out the guy had only wire wheeled and hand lapped the valves,(now dont laugh guys)he tore the short block down and only reinstalled the rings back on severly beat up pistons,and stuck a crank kit in it. the pistons had been destroyed by water,broken and beat up ring lands,and he only put new rings in it.......customer was in shock.....he just stood there shaking his head......the motor had a large comp variance across the board,and used a quart of oil an hour and the mechanic told him it was still breaking in!!!!!!!

just want to add,the poor customer had to carry a case of oil with him if he took it out for a two days in a row!!!!!

Vinny P

It is unfortunate, but that is what some so called engine builders do. Rip off the unknowing people with lines of B.S. As for me , I trust nobody to build my motors. And if you can't build your own, all I can say is only trust a very select few people to put their hands inside of your motor.