Leathl_WS6

I just bought a boat with twin 502 MPI's It has fuel Pressure guages on the dash but I only see 6 or 7 PSI of fuel Pressure. The guages only go to 15 psi. This can't be right aren't MPI's susposed run at 30 or 40 PSI. Does he have the wrong pumps installed?

The motors have fuel pumps that ride on the seawater pumps.

Thanks Rob

Jerry B

Your engines have 2 fuel pumps. The mechanical pumps on the sea water pump and electric pumps to boost the pressure for the injection system. Your gages must be installed between the 2 pumps. I think the pressure at the fuel rail is close to 40 PSI.
Hope this helps.

robyw1

I don't believe they could even start on 7-PSI. Something is not right for sure. Where are these gauges tied into and why is this boat running 15-PSI gauges on an MPI setup? This could be dangerous. It's possible that the former owner didn't know any better and installed those gauges for looks. They are probably so F#&%ed up from too much pressure that they are grossly inaccurate.

Roby

Jerry B

Your engines have two fuel pumps. The pump on the sea water pump and an electric pump to boost the pressure for the injection system. You fuel pressure gages must be installed between the two pumps. 5 to 7 PSI is about normal for the mechanical pump and pressure at the fuel rail should be somewhere around 40 PSI.
Hope this helps.

Leathl_WS6

Ok where would the second pumps be located and what would they look like. Would they be before or after the seapump fuel pumps? Inline pumps like a booster pump? These motors were put in an 82 boat and the guy knew little about boats or motors for that matter. The guages should be near the rail right like after the last injector? And should be 60psi guages right?

Also what kind of octane should I be running in theses motors. I had some problems over the weekend and messed one motor up bad. I'm trying to figure out why so I dont screw up the other one.

Thanks Rob

Jerry B

The earlier MPI systems used a VST, " Vapor Seporator Tank" . I dont know what year they changed to the cool fuel system but my 1993 Fountain has the VST. The electric pump is inside this tank. It is located at the rear of the engine above the left valve cover. It is easy to identify. Look for some fuel lines and a wire lead for the internal pump. I'm not sure on the location of the pump on the cool fuel system but they are both after the mechanical pump. I just bought fuel gages for mine and I have to get adapters to attach to fuel rail. I did buy 60 psi gages.
As far as fuel octain goes the compression is pretty low on these engines so regular fuel should be ok. However a higher octain fuel may help prevent vapor lock so I've heard.
There are many folks on this board that know much more than I about these systems. I have been having a small problem with one engine on mine so I have learned from a manuel that I bought . Still dont know if I have a pressure problem or something else but there are lots of post here about vapor lock on the 454 & 502 MPI engines.
Good luck, Jerry.

Pismo10

My 1996 502 Mag MPI still has the VST. Relax, 5-7psi is normal.

Pismo10

My 1996 502 Mag MPI still has the VST. Relax, 5-7psi is normal. How did you mess up one motor bad?

Leathl_WS6

Don't know yet. I'm going to pull it sunday. Knocking bad and really smells. There is white oil coming out of the breather and plugs 5 and 7 are melted pretty bad. Looks like I detonated 5 and 7 I really dont know how or why yet. The 5 -7 is normal I had a chance to look at the setup and the guages are connected before the second pump. But as for relaxing thats gonna be hard seeing how I'm going to need a new 502.

I'll keep you posted

32storm

The 502 MPI just runs on regular gas.....don't waste your mine on the higher octane. You'll just get more black crap on your transom.