502 stumbles

[Mark W]

Got a friend w/a 502 that suddenly develoved a problem. He says it starts and idles great but when he begins to throttle up the motor stumbles and dies. It starts right back up and idles perfectly but upon throttle up, same problem. Hes got no computer and I told him I'd try to findout if someone could at least give him a place to start looking. I don't own a boat but recently built a ffr cobra and and the forum there was really a big help for me. Help me show this guy why he needs to get a computer and throw me a few ideas.<br /><br />Thanks in advance<br />Mark

 

[Mark W]

Re: 502 stumbles

Oh yeah,,,This is an EFI engine.

 

[Dhadley]

Re: 502 stumbles

Whose 502 EFI? Mercruiser, Crusader, Flagship, Marine Power ect? What octane are you using vs what they recommend?

 

[Mark W]

Re: 502 stumbles

It's a Merc and I'm sure he's using the best fuel they have at the marina. He's got some kind of Fountain that is apparently pretty rare. He's also a mechanic but, like me, is old school and not up to date on efi.

 

[JB]

Re: 502 stumbles

Howdy Mark. Tell your pal this: I spent 50 years wrenching, writing about and teaching about electronics and machinery of any kind you can name before I retired, and have messed with them for several years since. I have learned more practical stuff in the past couple of months from guys like ODDD1, DHadley, 12footer, corm, and many others than in any 2 month period in my career. If there is a way for grizzled old timers like us to get, and stay, current . . .and have a ball doing it . . .this board is it.<br /><br />On your friend's 502 EFI: I would start by testing the throttle position sensor and the sensor that measures intake air volume (or mass). After that . . . the ECM

.<br /><br />Good luck.<br />JB

 

[Dhadley]

Re: 502 stumbles

I agree with J B, check the TPS (sometimes called TPI) and all basics first. Now -- and this is very important -- DO NOT use any higher octane than Merc says to. The electronics are programed to burn that octane. With the added oxygenites in the fuel today the electronics will sense the different fuel and try to compensate. This results in a variety of running problems from driveability to parts failure. If your friend insists on running a higher octane, contact someone like Flagship Marine Engine Company and see if you can get the ECU reprogramed for the higher octane. But remember, you must then stick to that octane.<br />Personaly I would simply run what Merc says. You'll be surprised how good that motor will run. Good luck!

 

[ODDD1]

Re: 502 stumbles

Mark , tell your buddy not to let that pile of electronics on top of his motor get the best of him....90% of the people I see that think thet got a efi prob forget there is a motor under all that stuff.....first thing...pull the water sep filter off an dump it in a pan......you got water in there? if you do it'll run like you said....after this..sounds like the motors going lean doesn't it? did engine management take a dump or cant it get the fuel it needs?<br />Next 2 steps a test of fuel pressure at the rail.should be over 30psi even when motor is screwwin up or you got a pump goin down. after that a scan tool is needed to run trouble codes..same one GM uses [its their computer] the cheapo one with the blinky light is like 30 bucks..and a manual to interperet the codes...good luck...if you nail this down yer buddy will think you DA MAN! LOL