502 Mag MPI Performance Issue

[Bucketlist]

I have a 1998 502 mag mpi engine with about 500 hrs on it. The engine is completely stock and serviced regularly by the same reputable shop. It has been used almost exclusively in fresh water lakes. The engine exhibits intermittent performance issues. When the problem happens the engine is down on power during throttle settings from about 80 percent to WOT AND the air temperatures are always above 95 degrees. When the engine starts to exhibit power loss it feels like it is not getting adequate fuel supply. The constant clue to this problem is air temps above 95. When the engine loses power the engine does not seem to be missing or surging. It just feels like it loses about 100 hp when it happens. Problem occurs the same with either full or near empty fuel tank, with fresh 87 or 91 octane gas purchased from many different fuel stations. This problem has existed for at least the last 200 hrs of use and has gradually gotten worse and more severe. When engine starts to lose power i can only get between 4100 to 4400 RPM at WOT. When the engine is running properly i can get 4900 to 5100 RPM.

When air temps are below 95 this problem generally does not occur. Also, when the engine is first started cold, the engine performance is perfect and as the air temps increase during the day the problem starts to intermittently occur. Engine idles good.

Inline fuel and water separating fuel filters have been replaced, gas tank venting system checked and is ok, seawater and circulation pumps replaced, engine water flow checked and ok, compression and leak down tests normal for hours on engine. Plugs and plug wires have been replaced at 400 hrs. Bilge area has adequate exterior air venting in and out.

 

[Pete104]

Re: 502 Mag MPI Performance Issue

Take & have scanned to see what going on in the ECM.

Have you tried getting more air in the engine compartment? You could still have a bad wire or plug. Just 1 would do it! What about the cap & rotor? The cap is suseptive to corrosion on the contacts.Also check the fuel pressure. You didn't leave a serial number but you might have a lift pump on the engine also. Vaccum test it!

 

[Bondo]

Re: 502 Mag MPI Performance Issue

I have a 1998 502 mag mpi engine with about 500 hrs on it. The engine is completely stock and serviced regularly by the same reputable shop. It has been used almost exclusively in fresh water lakes. The engine exhibits intermittent performance issues. When the problem happens the engine is down on power during throttle settings from about 80 percent to WOT AND the air temperatures are always above 95 degrees. When the engine starts to exhibit power loss it feels like it is not getting adequate fuel supply. The constant clue to this problem is air temps above 95. When the engine loses power the engine does not seem to be missing or surging. It just feels like it loses about 100 hp when it happens. Problem occurs the same with either full or near empty fuel tank, with fresh 87 or 91 octane gas purchased from many different fuel stations. This problem has existed for at least the last 200 hrs of use and has gradually gotten worse and more severe. When engine starts to lose power i can only get between 4100 to 4400 RPM at WOT. When the engine is running properly i can get 4900 to 5100 RPM.

When air temps are below 95 this problem generally does not occur. Also, when the engine is first started cold, the engine performance is perfect and as the air temps increase during the day the problem starts to intermittently occur. Engine idles good.

Inline fuel and water separating fuel filters have been replaced, gas tank venting system checked and is ok, seawater and circulation pumps replaced, engine water flow checked and ok, compression and leak down tests normal for hours on engine. Plugs and plug wires have been replaced at 400 hrs. Bilge area has adequate exterior air venting in and out.

Ayuh,... Welcome Aboard,.... Sounds like you've done alota Checkin',...

Trouble with those 'puter controlled motors,... Ya gotta Scan 'em with a 'puter, or yer wrenchin' totally Blind...

 

[tpenfield]

Re: 502 Mag MPI Performance Issue

You know, when it is 95 degrees or above out, I tend to run a bit slower too. Like the others have said, probably an ECM issue, may actually be sensing the input air temp.

try running the blowers (if you don't already) during normal operation and see it that extra air going through the engine compartment makes a difference.

 

[Bucketlist]

Re: 502 Mag MPI Performance Issue

Thanks for everyones replies. The probelm is not inadequate air flow in and out of the engine compartment. The air intakes and exhaust grilles are identical to when it left the factory and this problem has only developed over the last 200 opearting hours. So the first 300 hundred hours of operation the engine performed flawlessly. So i am very confident it is not an air flow problem. Also the ecm as been checked for codes and none were found.

 

[Bucketlist]

Re: 502 Mag MPI Performance Issue

Another thought i have.........could the manifold absolute air pressure MAP sensor intermittently fail under hotter air temps? If this sensor fails i believe that it could reduce the fuel flow output of the fuel pump. Again, the ECM did not show any codes during a recent lake test. But then again the problem is intermittent and seems to develop with above 95 degree air temps.

Also, since the fuel pump is original with about 500 hrs on it, could the pump motor be starting to fail during the higher air temps as well? Again, when the engine is first started and the air temps are below 95 , the engine runs great? My experience with pumps are that when the fail they quit all at once. Ha anyone had a pump gradually fail as described above?

 

[Pete104]

Re: 502 Mag MPI Performance Issue

Just because someone plugged in the diagnostic computer doesn't mean that it will tell you what's wrong. A fail code yes & change that sender/sensor, clear the codes and you are done. The data that the ECM collects & displayed must be studied.
The only time I would put into this (if I had a serial #), is to make absolute sure the fuel system in it's entirity is beyond acceptable. Pumps can give up at any point in time, there is no experation date. You could have a bad anti-siphon valve or even delaminated fuel line. CHECK THE FUEL SYSTEM.

The CDS has the ability to take a snap shot of what the ECM is recieving when it does act up. The dealer hooks up the connections, you go boating & when you aren't happy with the performance- hit the button & that info is stored.