fuel pump issue?

[smykowski]

Hi all, I'm a bit new to this, so please pardon my ignorance in advance.

My boat has a Mercruiser Gen VI 7.4LX MPI motor in it. Starts right up, idles fine, no knocks or noises, no smoke or smell of things burning, etc. The problem is the boat won't go much past 3500rpm under load. When I get to that rpm, it kind of hesitates, surges a little bit, goes a bit faster, then slows back down. No hiccuping or cutting out, almost like it bogs a little here and there. I've done a ton of searching and people who've had similar symptoms always point to the fuel pump. The fuel filter/separator was replaced last week, injectors have been cleaned, the ignition module was replaced just to rule that out (thankfully, none of this on my dime). Could the fuel pump be causing this? Can I do this myself? Is it smart to do this myself? I'm sure there's a bunch of info needed (apologies again), but I'd like to get this thing running like it should be.

Thanks!

 

[Don S]

Re: fuel pump issue?

What's the full model number or serial number of your engine.

 

[Levithan]

Re: fuel pump issue?

Definetley sounds like a fuel issue.
The only way to know for sure is to put a fuel pressure gauge on it.
Pressures should be approx. 30-40psi at idle and upto 50-60psi(not 100% on the numbers as I don't have specs on me right now) under 3/4 wot if this is an injected engine(sounds like it since you said it has a pcm and you had the injectors cleaned.

 

[smykowski]

Re: fuel pump issue?

Thanks guys, sorry for the delayed response...got caught in a meeting =/. Serial for the motor is 0K151578. I'd like to check the fuel pressure, but as I understand it, my boat has that cool fuel system, so from tracing the fuel lines, it looks like the pump is practically under the motor.

 

[Don S]

Re: fuel pump issue?

There was two different fuel systems on that engine, and why I asked for a serial number. the change from the VST was at 0K147350.

Here is from the manual on testing the fuel pump system.

View attachment 16 pump tests.pdf

 

[smykowski]

Re: fuel pump issue?

Thanks Don, I'll take a close look at it when I get home. I'm assuming that the fuel pressure service tap is more or less a Schrader valve to connect a gauge?

 

[Don S]

Re: fuel pump issue?

Thanks Don, I'll take a close look at it when I get home. I'm assuming that the fuel pressure service tap is more or less a Schrader valve to connect a gauge?

Yes, that's all it is.

 

[smykowski]

Re: fuel pump issue?

ok, it looks like there's a brass plug in the front of the fuel rail, I'll check it out tomorrow, ran out of daylight and the skeeters are vicious tonight.

 

[smykowski]

Re: fuel pump issue?

fuel pressure seems to check out, although this issue happens only under load, and I'm stuck on the trailer for the time being. I'll have to get another reading when it acts up out on the water. I was wondering, do you guys think this could be caused by a clogged pickup screen in the tank? I haven't checked that yet, but I have a suspicion. The previous owner didn't use the boat all that much (430 hours over a 16 year period = just under 27hrs per year), I wonder if that damn ethanol has caused some kind of sludge buildup on the screen. Thoughts?

 

[dgg190]

Re: fuel pump issue?

has it never gone over 3,500rpm for you? If so. Check your compression. All cylinders should be within 10 psi of each other. I would suggest that one or more cylinders will have low compression and therefore low power output, i.e. you are not making enough horsepower to go any faster than 3,500rpm.

 

[smykowski]

Re: fuel pump issue?

it'll go over 3500rpm in neutral all day, and it does go over 3500 when cruising, but it'll surge a little between 3500-4200

 

[BornToRide92]

Re: fuel pump issue?

Hey guys (this is my first post btw)

I'm having the same problem on my merc 260/v8 5.7.
Runs great all the way till i go above 3800 rpms and then i lose power till i back off the throttle.:mad-new:

I am replacing-
-fuel/water seperator filter
-Mechnical fuel pump
-carb inlet fuel filter
-spark plugs

I also took the carb off and cleaned as much as i could without doing a rebuild on the carb.

Am i headed the right direction?

 

[smykowski]

Re: fuel pump issue?

Principles are the same more or less, my setup is efi.

Fuel filter/water separator is a good start, it's a common culprit. Dump the contents into a clear container to see what your fuel looks like.
I've seen lots of posts about the inlet screen on the carb getting gunked up. You said you removed the carb, so I'll cautiously assume you've cleaned it . Spark plugs, I would CHECK them before I blindly replace them. The issue seems to be more fuel related than spark/ignition related. My plugs were replaced a month ago obviously had zero effect on correcting the issue.
I would check the screen in the pickup tube inside your gas tank to make sure it's not gunked up either. Make sure any check valves/anti-siphon valves/vent tubes are working correctly and clear of any obstruction. More experienced people will hopefully add their input, but from doing research, these are some of the common causes for what we're experiencing.

 

[Don S]

Re: fuel pump issue?

Hey guys (this is my first post btw)

I'm having the same problem on my merc 260/v8 5.7. ..................................
............................................................
Am i headed the right direction?

You need to start a new thread of your own BTR92, this thread belongs to smykowski.

 

[BornToRide92]

Re: fuel pump issue?

You need to start a new thread of your own BTR92, this thread belongs to smykowski.

Im having a similar problem as he is having.

I have ordered all the parts already and will put them on this weekend or next week. If these parts fix the issue then he could try the same thing and maybe it will help him too.

That is why i posted into his thread. Thanks

 

[smykowski]

Re: fuel pump issue?

Update:

I think the issue is resolved. Fuel pressure and volume check out fine. I finally got the pickup tube out of the tank and checked it to make sure it wasn't loose (pulling in air), all good there. The anti-siphon valve was a bit on the stiff side, for testing purposes, I knocked the little ball out of it and my fuel pump really quieted down, so it was apparently laboring to grab fuel out of the tank. Unfortunately, the labored sound was the only sound I've ever heard it make, so it sounded "normal" to me. I also noticed that the fuel line going from the tank to the filter/separator was in good shape, but it was a thin-walled line. My theory is the anti-siphon valve was causing the issue, and the increased suction by the fuel pump was also causing the line to collapse and pinch closed, amplifying the problem. I'm going to get some real fuel line with the thick walls so it wont collapse, then take it out this weekend. I'm glad I didn't start throwing $$$$$ parts at it. =) I'll let you know how this weekend goes.

 

[smykowski]

Re: fuel pump issue?

I should also note that I've heard of stories with boats that have the captain's call exhaust where the internal exhaust flapper can break loose and get stuck in the exhaust bellows in the outdrive, in other words, it would run fine using the thru-hull exhaust but would act in a similar fashion to what I was experiencing if I was directing the exhaust through the prop hub due to the exhaust being blocked. Just a little FYI those looking for issues in not so common places.

 

[smykowski]

Re: fuel pump issue?

well unfortunately that didn't fix the problem. Not sure what to look for next. Same symptoms, boat will idle and run at low speed just fine, but when I get up to about 3500rpm, it kind of hits a wall...won't rev any higher, obviously no increase in speed either. Again, seems like it's starving for fuel or something. Any thoughts?

 

[BornToRide92]

Re: fuel pump issue?

well unfortunately that didn't fix the problem. Not sure what to look for next. Same symptoms, boat will idle and run at low speed just fine, but when I get up to about 3500rpm, it kind of hits a wall...won't rev any higher, obviously no increase in speed either. Again, seems like it's starving for fuel or something. Any thoughts?

Same problem here, as soon as I replace my parts on my boat and test it out. (If it fixed it) I will let you know what I did

 

[smykowski]

Re: fuel pump issue?

Well, finally got to the bottom of this. Plugs were not right (I didn't put these in), I had 2 plug wires that had torn boots (i got tazed when I touched them), but what REALLY fixed the problem was the FPR down by the the fuel pump was giving an inconsistent reading, after doing some testing to eliminate the FPR from equation, the fuel pump also needed to be replaced. Seemed the more I dug, the more problems I've found. I'm happy to report that the boat moves pretty well now, tops out around 56mph.