502 MPI problems, help
 

1999 502 MPI. Engine looses power at around 3000 rpm. rpm according to tach goes down 1000 rpm. after a few seconds goes back up to 3000 pulls strong up to 4500 then backs down to 3500. The time it takes for the tach to move up or down 1000 rpm is only half a second so I wonder if the actual engine rpm can change that quick. If I hammer the throttle at 2500 rpm, the tach goes down to 2000 but the engine slowly accelerates the boat???
Is it fuel or ignition related?? It feels like I loose some cylinders . All the plugs look OK.

Re: 502 MPI problems, help
 

You need to hook a diagnostic computer to it. The engine is trying to tell you something. If the computer finds an issue such as knock it will pull timing away and go into a "limp" mode. You could have the problems that you are thinking about, but a diagnostic computer could save you a lot of money and time trying to find the issues.

Re: 502 MPI problems, help
 

Do you think it goes into knock retard? Long story short; Is it possible that using a 30 psi fuel pressure regulator instead of a 43 psi regulator can cause it to behave like this?

Re: 502 MPI problems, help
 

Check your fuel pressure, you are due for a fuel pump ! also change your water seperator filter.

Re: 502 MPI problems, help
 

You betcha !!!!

Re: 502 MPI problems, help
 

that sounds like where your problem is....or as stated above.....You Betcha

Re: 502 MPI problems, help
 

Thanks for your input. The story is that the engine was retrofitted with an adjustable regulator. I then decided to put an OEM regulator on it but we were not sure which one to take since the engine visually looks like an 1998 (1998 injectors, ECM, etc) allthough the serial number indicates 1999. The fuel pressure changed between the years.

Re: 502 MPI problems, help
 

Changed the fuel pressure regulator to a 43 psi. I think this is the correct one. The plugs look somewhat more normal but no big difference. However, the problem persists!? The engine intermittently looses rpm at around 3000. No trouble codes. What could it be?? I will try to get hold of a scantool and a fuel pressure guage.

Re: 502 MPI problems, help
 

Not to through a monkey wrench at you, but is the exhaust original?
The problem you are having sounds a lot like I've experienced with a leaky riser. Water ingestion can make an engine run like what you are describing.

Re: 502 MPI problems, help
 

The exhaust is Gil. However it may of course leak. Please describe what happens if I have a leak. Please observe that all the plugs look OK.