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Discussion Starter · #1 ·
Got a 97 502 mag mpi with VTS fuel system.

Had the mechanical fuel pump replaced last year, finally splashed last weekend. Runs good up to 3000 rpms then cuts out, no sputter or backfire or pops. Seems like it is starving for gas.Flutters all the way down to 1500 and back up when I try to throttle up.

Replaced fuel/water separator, didn't see any water. stored with 40 gallons of treated gas, topped off with 30 gallons of super 2 weeks ago. Seems to have gotten better after running for awhile, got it up to around 4k before puting it back on the trailer, burned about 1/2 tank up. still cuts out badly.

Is there any other inline filters or screens that I can clean out? Does it have a antisyphon check valve on a mpi motor?

Thanks

Mike
 

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Detonation sensor pulling timing back possibly? Good fresh gas in it? After all you have done on the fuel side, I am wondering about spark/ignition...

Good Luck
 

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Discussion Starter · #3 ·
Thanks, but feels like its running out of gas, then when the demand is gone it runs fine.I would think a timing issue would cause a misfire,therfore a backfire or stumble. Try to get a fuel pressure reading , and go from there. Anyone have the Vapor separator apart? can there be a clogged screen in there?
 

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I would install a cheap fuel pressure gauge connected between the mpi set-up and fuel pump to see if fuel pressure falls off a cliff where you're having the problem.
 

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wtfo said:
I would install a cheap fuel pressure gauge connected between the mpi set-up and fuel pump to see if fuel pressure falls off a cliff where you're having the problem.
good point, I would start at the source, pull the fuel tank pick up and make sure it is clean. had a boat last week that had poor full throttle performance. Found a large piece of crud lodged in the fuel tank pick up screen. might just be the problem.
snivley
 

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when inlet pressure drops to a certain point, fuel pump cavitates, fuel pressure drops hard and doesn't start to come back up until cavitation stops due to inlet pressure building back up... Had almost the identical problem at a little higher rpms when repowered with a lot more engine... in my case I replaced the pick-up, check-valve, fittings, lines, etc... in front of the fuel pump with larger. Since yours worked with the same set-up, could well be as simple as crap in the inlet screen...
 

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Discussion Starter · #7 ·
Thanks for the advice, took apart the VST assembly, seemed to have alot of crud on the electric pump screen, cleaned it up and PRESTO! problem solved.


Now I think the IAC is junk, doesnt want to idle now.
 

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The 502 MPI Injectors have screens on their inlets too - may want to check these too. A few of us have burnt down a 502MPI due to plugged injector screens. :dead:

These motors also have crap vacuum lines connected to fuel psi regulator. Very normal for them to fall off or create vacuum leak which would mean higher fuel psi at idle making for rough idle. IAC could have problem too - need to diagnose this.
 

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Discussion Starter · #9 ·
Thanks, will go over it next weekend check vacume lines and injectors. Try to get a new IAC also, had a feelin it was going bad sometimes stuck on high idle, now it just stalls.
 

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Edited: All 502MPI's appear to have replaceable injector inlet screens but these 502MPI's Serial#0L085433 THRU 0M024999 ????? I dunno, seems weird. Maybe typo ?

Take a look here and to find injectors
ck on "Intake manifold and Plenum" after clicking on the Motor/Serial # of course.
http://www.mercruiserparts.com/selectModels3.asp?type_id=8

Remember, the engine can be a yr or 2 older than the boat, so go by engine's info - not boat.
 
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