Engine mis-firing at speed - advice please

[bdgregory]

This is a new problem with my 2004 20V. I had it tuned up before last season (2016), and put maybe 6 hours on it all last season. This year, when I took it out it started out fine, came up to plane and cruising at ~30MPH (~3000 rpm) and the motor starts to gasp, then gradually loses power.

Assuming it was dirty injectors, I added Sea Foam to the tank, as well as some HEET. Then I applied a can of Spray Sea Foam to the throttle intake per the mfg instructions. After letting it sit for 10 minutes hot per the instructions, I took it out to drive, and after a while it was running well. No gasp/choking, no loss of power. It was running fine again. Then, after about 30 minutes of running fine, the problem returned.

I've been going on the assumption that my gas from last season is the problem. Yes, it has Ethynol in it. I'm trying to burn off the remainder of the last year tank before I add new/fresh gas.

My question: Is the problem likely the gas? Is there something else I can check or try?

Add'l info: The motor (Which is a Marine Power 335hp fuel injected) starts fine, idles fine, but after accelerating, it chokes out. If I run along at idle for a few minutes, I can accelerate to planing again, and it runs fine for 30 seconds or so, then chokes out. I can to this all day long it seems.

[cbath]

When you had it tuned up, were the distributor cap and rotor replaced or just plugs? I would suggest getting a fuel pressure gage and see what your fuel pressure is at the rail when it acts up. Kind of sounds as though you are lacking pressure or volume of fuel at high demand.


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[bdgregory]

thanks for the response. I'm pretty sure the tuneup included distributor/wires/etc, but I need to dig the invoice out to be positive. I don't have a pressure gauge, but will try to buy, or rent one today.

By the way - do you know what the pressure spec is for my engine?

[freeheel4life]

Not sure on MP engine, but I imagine 50-60 psi. 48 at the lowest...but just a guess. Obviously don't forget to test under a load. Neutral in driveway on a fake-a-lake won't cut it.

[bdgregory]

I found my engine manual and it says 39 PSI. Does that sound like an idle pressure, or is this constant?

[cbath]

39 sounds low to me for an MPI engine. Typically will see those in the 48-55 psi range. Anything below 30 and will give the symptoms you're describing.


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[IDBoating]

I found my engine manual and it says 39 PSI. Does that sound like an idle pressure, or is this constant?

Fuel rail pressure should be relatively constant whether at idle or under heavy load. The high pressure fuel pump is electric, and thus not dependent upon engine RPM's to determine pressure. It should always generate an excess of pressure which is then regulated by a pressure control valve on the fuel rail.

freeheel4life is correct, you have to test pressure under heavy load on the lake. An engine on the trailer consumes almost no fuel no matter what RPM's since you're not really asking it to do heavy work. You've got to get the fuel flow way up to know if the HPFP and regulator are working properly, and the best way to do that is load up the engine.

[bdgregory]

UPDATE:

I had a service tech out - Fuel pressure is the problem. After talking to the Marine Power Service Support line to see about ordering a pump (and describing the symptoms), we checked the pressure regulator too. It appears that's the culprit, so I have one on order. I'm hoping that's the end of it since I assume the fuel pump will be many $ to buy.

[IDBoating]

Yep, HPFP's are notoriously expensive. The one for our earlier Mercury engine, which is shared by all sorts of marine and automotive applications and thus ships in high volume, was still ~$250 and that was me buying it from a discounter, not buying through a dealer. I believe the dealer's price was about 2X that.

[TeamAllen]

Thanks for the update. I hope it gets fixed soon!

[bdgregory]

another update:

I installed the new pressure regulator and the problem is not fully resolved. It's better - more intermittent, but still comes back. I'm thinking I need a new fuel pump, but suspecting buying one from Marine Power will be super expensive (the pressure reg was $159). Is there another lower cost source? Attached is a pic of my pump. fuel pump.jpg

[TeamAllen]

That thing looks nasty. Looks like it was/is used in salt water.

[IDBoating]

Idea: If the HPFP is connected solely by hoses (e.g. no physical mount is involved in handling fuel), you could move away from this pump design and use a more generic marine pump instead. Might be less expensive, more convenient, etc. The pressure regulator is going to maintain the proper fuel rail pressure so the pump just needs to exceed that pressure and the flow requirements.

[bdgregory]

Final Update: (hopefully)

It looks like I will not need to spring for a $1000+ fuel pump. After installing the regulator, I added 5 gallons of gas to the remainder of last years gas in the tank (guess ~ 8-10 gallons) along with some Star Tron fuel treatment. I ran it for a bit that day and it didn't help initially. On a whim, I unplugged the electrical connection to the fuel pump and gave it a shot of contact cleaner and put it up for the evening. As noted above, this boat was in salt water and many of the electrical connections had serious oxidation. The next day I took it for a spin and it ran perfectly. Took it for a spin this morning and still perfect. Strong and no sign of hesitation. I don't know if it was the contact cleaner or the Star Tron, but it's running great now.

Thanks to you guys for the assistance . . .

[IDBoating]

Sounds like cleaning ALL of your engine's electrical contacts might be time well spent. Fuel pump today, something else tomorrow. Better to correct those problems on YOUR schedule rather than chance....