454 Mag MPI fuel problem
 

I have the 385-hp 454 MPI.I have had problems starting the engine after running it hard.The first time I start the engine,it fires right up and runs great until it is shut off and sits for a while.I turn the engine on and it stays on for a few seconds then dies.It almost sounds like if it's running out of gas.The problem is worst at altitude.

Re: 454 Mag MPI fuel problem
 

Do you rev your motor up in nuetral? Ive heard that it does something to the ECU. You may need to reset it, I don't know if you can do it without a scan tool or not. You may want to have it serviced and have them check it for any codes.
How many hours? Ever changed the plugs? Fuel water seperator?
Somebody will chime in about the codes or the ECU.

Re: 454 Mag MPI fuel problem
 

Check the IAC motor, it controls the idle. I have the same motor with the same problem, found out that the IAC was sticking closed. Ishopmarine.com was the cheapest around.

Re: 454 Mag MPI fuel problem
 

Could be any number of different things. Depending on the year of your motor, it could be a vapor lock condition typical of the VST tank EFI system. It could also be a clogged VST tank screen. As others mentioned, the easiest way to diagnose is with a scan tool. Let us know what you find out.

Re: 454 Mag MPI fuel problem
 

Ttt

Re: 454 Mag MPI fuel problem
 

Magic Medicine is having a starting / idle problem with his 502 MPI. Sometimes it starts and idles like a top, and others it is very finickey. When it is acting up there is a swoosh noise coming from the engine compartment. It is changing in pitch like it is trying to compensate for something to keep the motor running. Almost every time that swoosh noise is made the motor dies. Is the swoosh noise the IAC motor? Is it bad or is there possibly a sensor there causing it to act up?

Re: 454 Mag MPI fuel problem
 

The "swoosh" is the IAC. It does not mean the IAC is bad. It means the IAC is attempting to keep the engine running at the desired idle speed, but there is a problem that is causing severe fluctuations in idle speed, so severe at times that even with the IAC wide open (swoosh sound) the engine still dies.

In other words, the IAC "swoosh" is a symptom, not the problem. Time to check for trouble codes, check for proper fuel pressure, and start figuring out what the problem actually is.

Dave

Re: 454 Mag MPI fuel problem
 

I have an appointment to run it on the Merc scan tool tomorrow. The Marina is going to charge me $45 to check it out. I didn't think that was a bad price since I did not buy the boat from them.

Any Ideas of what could be causing the idle fluctation???

Re: 454 Mag MPI fuel problem
 

Great Price!!!!!!!!

Re: 454 Mag MPI fuel problem
 

I agree with Dave. Sounds like a vapor lock. Next time it happens, check the fuel rail shrader valve. If air comes out, it is a vapor lock. Bleed all the air, and it should fire up. If you have the VST, that is where I would go next.

Re: 454 Mag MPI fuel problem
 

Had my motor checked on the merc scan tool and no codes came up, none where stored in the memory. Talked to the mechanic who ran the test and he recommended replacing the IAC motor, a couple of the other techs recommended the same as well. I bought a new one for $80 buck and installed it, the boat fired up on the hose fine and started with authority numerous times. The marina bid not even charge me for the test, I guess that is a fringe benefit for being a former employee. I guess time will tell if this is the permanent fix, but the engine sounded alot healthier at idle and start up. :D

Re: 454 Mag MPI fuel problem
 

I had similar problem and it ended up being the fuel pressure regulator. I did replace the IAC and got it from automotive supply house for a lot less than the "marine" version. Exact same GM part numbers.

My pressure regulator and rusted and was binding the diagphram. THe rust ( and the GM part number) were covered by the pain the manufacturer had applied. It had almost rusted thru when it started causing problems and we tested everything and changed the IAC before we discovered the rust bubles in paint.

Re: 454 Mag MPI fuel problem
 

:eek: Thats an even better price!! :D

Re: 454 Mag MPI fuel problem
 

Well it did it again. Althougth this time it did not even want to idle and suck air, it just plain wouldn't fire over. I let the boat sit for an hour and it fired up just fine. I am begining to think that it is vapor lock. Where is the fuel rail shrader valve??

What is the VST? Where is it on the motor? The motor is a 1999.

Re: 454 Mag MPI fuel problem
 

well, sorry to hear that you still have the problem. The shrader valve should be in the front of the intake area just under the center of the plenum. Right around the t-stat housing. If your motor has a VST tank, this will be located on the odd bank side at the rear. (port) It will have 2 hose going to it and 2 wires. It will be on top just next to the distributer.

Good luck, keep us informed.

John :D

Re: 454 Mag MPI fuel problem
 

The vst is a vapor separator tank it acts kind of like a fuel bowl in a carburetor in the chamber is a fuel pump, float, needle valve the idea is to bleed out any air in the fuel line so the motor doesn't vapor lock and has a contsant suppy of fuel.

Re: 454 Mag MPI fuel problem
 

Thanks for all the info guys!!! I post what I find.

Re: 454 Mag MPI fuel problem
 

There is some good information about vapor lock in Mercruiser Service Bulletin 99-7. A pdf file of this bulletin is available at another site. Not sure if its ok to post link here.

Dave

Re: 454 Mag MPI fuel problem
 

go ahead and do it , its ok if its for the help of a fellow boater

Re: 454 Mag MPI fuel problem
 

Mercruiser Service Bulletin 99-07

http://www.boatfix.com/merc/Bullet/99/99_07.pdf

Dave

Re: 454 Mag MPI fuel problem
 

That is some good info. Thanks, I see a few things that I will try.