1996 454 385 hp hesitates at about 30 mph

[steinjo]

454 starts, idles, and runs fine up to about 25 mph. It then will periodically hesitate for about 1 second at a time and goes completely quiet as if someone hit the kill switch. Began a couple of months ago at around 45 mph and is getting worse. Now it sounds as if it may stall if I do not throttle back.

 

[waterone1@aol.com]

Re: 1996 454 385 hp hesitates at about 30 mph

When was the last time the engine had a complete tune-up ? Have the fuel filters been changed ? Does it have fresh gas, or is this a first outing with gas from last year ? Could you confirm if this is carburated or fuel injected ?

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

It's fuel injected. Just added 15 gallons of new 93 octane to tank without draining old and also added some fuel injection cleaner. Have not had a complete tune-up in about 3 years. Changed the fuel filter last year.

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

Wasn't the fuel vent... Any other ideas anyone?

 

[Dunaruna]

Re: 1996 454 385 hp hesitates at about 30 mph

Any warning buzzers? Have the codes been read?<br /><br />You need to eliminate the obvious and do a complete tune first, plugs, filters etc. Then you can start checking the not so obvious, fuel pump pressure, module etc.<br /><br />Aldo

 

[waterone1@aol.com]

Re: 1996 454 385 hp hesitates at about 30 mph

Assuming that all gauges are showing normal (oil pressure, temp., etc.) I would start by replacing the water seperator. If that doesn't do it, I would run down the gas in the tank (at lower rpm's)and refill with fresh gas. If running out the gas at lower rpm's doesn't suit you, you could go ahead and replace the spark plugs, dist. cap and rotor (might solve the problem, and after 3 years they are due to be replaced). If after all of this you still have the problem, a test of fuel pressure at the fuel rail would be in order. I have seen all kinds of problems and symptoms caused by bad or stale gas, including exactly what you have, the tune-up stuff is needed anyway, but before I spent any serious time or money on real diagnostics, I would eliminate the fuel from the possible list.

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

You can see what I've done below but I've been told that my next step would be to rebuild the vapor seperator on the electric fuel pump. I've heard it's not too hard. Anybody know how to look that kit up and a good place / website to purchase? <br /><br />I've changed the fuel / water seperator, plugs, wires, cap and rotor. Ran fuel intake out of a tank of "good gas". Found a loose vaccum hose which moved the hesitation up from 35mph to 50mph.

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

You can see what I've done above but I've been told that my next step would be to rebuild the vapor seperator on the electric fuel pump. I've heard it's not too hard. Anybody know how to look that kit up and a good place / website to purchase? Any tricks to rebuilding it? Contains a float, needle valve, and screen from what I've been told.

 

[waterone1@aol.com]

Re: 1996 454 385 hp hesitates at about 30 mph

Could you post your engine serial number ? Is this engine equipped with the "cool fuel" system ? If so, the cooler is not servicable and is a replacement item (about $150.00). I've never heard of one failing though, the pump is a different situation, lots of those fail. Have you run the boat under load with a fuel pressure gauge attached to the fuel rail ?

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

Engine serial: 0F802536<br /><br />Not fuel cooled.<br /><br />Our next step is to test the fuel pressure on the rail. The gauge we have has a much bigger female end than the nipple on the end of the rail. What type of gauge do you suggest? I'm looking to get this Actron Fuel tester kit but do not know if it will fit. Also do you know what the kit to rebuild the vapor separator would be technically called? Maybe even a part number?<br /><br />Thanks waterone1

 

[waterone1@aol.com]

Re: 1996 454 385 hp hesitates at about 30 mph

River, I looked up the fuel vapor seperator, there are four different rebuild kits for different sections of this unit(ranging in price from $45.00 to over $300). I have never worked on one of these, and could not venture to guess which kit you might need. I would suggest you purchase a factory service manual (not clymers or seloc)and follow the diagnoses chart. I recently had the same problem you did with the fuel pressure gauge. The quality one I have had for a few years had too big of a fitting and I looked everywhere, yes everywhere for an adapter....they do not exist. The actron unit that is available from Advance or Auto Zone had the right fittng and seams to work fine ($35.00).

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

Bought the Actron fuel pressure gauge from Advance last night. Haven't tested yet.<br /><br />The Clymer service manual is fairly detailed on the rebuild. Do you think that the factory manual would be that much better? Where do I purchase one?<br /><br />Also, where did you find the rebuild kits? I looked on Mercury's site but they just had each part as a seperate item number. Would love to see those kits.<br /><br />If anyone else has experience with this VST the help would be greatly appreciated.

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

Need to buy some kits to rebuild the Vapor Separator Tank on this 1996 454 Magnum MPI. One of you experts that we love, which kits are mandatory when doing this (eg. O-ring kit, float kit, diaphragm kit, etc.) and which are optional based on your experiences?<br /><br />Also, someone mentioned that the fuel pump in this system goes often. Is that the mechanical one down by the fuel / water separator filter or the electric one in the VST?<br /><br />Thanks

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

Still need help. I've replaced the mechanical lift pump on this 454 Magnum EFI MPI and found a loose vacuum hose. Helped get it from 35 to 55 mph before the hesitation occured but it's still there...<br /><br />When doing a fuel pressure check off the end of the rail while the hesitation occurs the pressure drops momentarily from 34 psi to around 20 - 25 psi.<br /><br />Did everything from clean the screen, new antisiphon valve, and fuel / water separator. Think it's something in the Vapor Separator Tank like the electric pump, diaphram, float, etc. Anyone ever rebuilt one of these VST's or think of something else it could be? The electric fuel pump is expensive.<br /><br />One other thing. It's supposed to run at 34 - 36 psi but it's coming in at around 31 - 32. Don't know if that matters.

 

[imported_scott_m]

Re: 1996 454 385 hp hesitates at about 30 mph

Have you checked electrical connections to your fuel pump(s)? Bad ground/corrosion could be at work here causing intermittent power problems to the pump.<br /><br />What about your oil pressure switch? My understanding is the fuel injected motors run a low/no oil pressure switch to shut down the fuel pump in the event of a sudden engine failure/stop. Could a faulty oil pressure switch be showing 0 and telling the fuel pump to shut down?

 

[steinjo]

Re: 1996 454 385 hp hesitates at about 30 mph

Could these faults only cause the hesitation at high speeds?

 

[imported_scott_m]

Re: 1996 454 385 hp hesitates at about 30 mph

Sure....higher RPM, higher demand for fuel, higher electrical demand for the pump.