2000 mercrusier 7.4 misfiring

[alldodge]

IMO your still to high on fuel pressure. Removing power from the ecm may cause it to relearn the motor, but as before your pressure is to high

 

[RaceCarRich]

Wow!

 

[thegoat86]

My gauge came from autozone. I do feel there may be a 5 PSI +/- differential. I looked for the regulator today and it seems to be under the intake. Do I need to remove the intake to get to it or is there a way to test the regulator without?

 

[thegoat86]

I am ultra impressed with the performance today. Top speed at WOT used to be around 46 knots @ 3800RPM. I hope this isn't the precursor to it puking its guts all over the bottom of my hull. I had an old truck motor run good and hot before it went south..... Boat has 460 hrs on the ticker.

 

[alldodge]

Your motor shouldn't be exceeding 45 psi max, so your well above that. You should be able to turn 4800 rpm so your under by 1000 rpm but 46 knots (52 mph) isn't far off from where it should be. My 23.5 foot Rinker hits 60 mph at 4800 rpm.

Could be your speed or rpm is off a bit which would accommodate the differences.

Again: what do you mean by when the injectors fire?

 

[R055]

Your rpms are low at wot, test with a shop tachometer maybe your tachometer is off.

 

[thegoat86]

I will get the tach tested and replace if needed. Even in my driveway I cant get the motor to rev over 4k.

AD, I was poking around on here and see you have tried to help on this issue before. There is a thread from 2014 "7.4 MPI High Fuel pressure" That you helped out on. There doesn't seem to have been a resolution. This is my identical situation. I am curious about the return line screen being a culprit for the high fuel pressure....

53 knots measured by GPS plotter. My speedometer is plugged at the drive. (another recurring issue I don't want to keep messing with. )

 

[thegoat86]

I found some info on the screen location. If you go to this site;

http://www.cpperformance.com/s-1895-fuel-pump-and-fuel-cooler.aspx

They have an exploded view of the fuel system for a similar motor. The regulator is on the cooler. Filter screen is under the regulator. My cooler is behind the motor mount on the left side of the engine. But accessible. I can get the cover off. I will test regulator with a vacuum gauge while running and report results. I will also check the screen for debris.

 

[alldodge]

Dang, I thought I already posted this, some how it never went and was saved. Go figure, you found the info

The screen (item 16) is under the regulator (item 15) and is mounted on top the fuel cooler.

http://www.mercruiserparts.com/bam/s...31918/2705/110

The regulator (item 5) listed which is under the plenum is not a regulator, its a fuel damper, which is used to smooth out pulses coming from the pump.

http://www.mercruiserparts.com/bam/s...31918/2705/130

Note: I transposed numbers in previous post 53 knots = 61 mph your doing good

 

[thegoat86]

My buddy believes the 800 RPM loss may due to my aggressive prop. 22P bravo I four blade.

 

[R055]

A prop shouldn't make you loose 1000 rpms.

 

[alldodge]

My buddy believes the 800 RPM loss may due to my aggressive prop. 22P bravo I four blade.

What was the previous prop?

 

[thegoat86]

I don't know previous prop.

I bought this boat used in 2014 end of year. The previous owner told me Reinelle came from Washington state, and only had 5.7 mercruiser motor option. He ordered this boat custom with the 7.4, had larger ribs installed for the motor to mount to, and matched the drive to the motor. This could be all BS, but I cant check because the company changed hands and only does smaller bowrider type boats. So its hard for me to get info on this particular boat.

As well, I have a stumbling block as the motor does not have a serial number. It only comes with a transom number on the motor shield. And as far back as I remember, the boat never revved over 5k. Always around 4800 + - when I am in the water. I revved it today on muffs and it did hit 5k. But wont go over. Governed?

I just recently got a plotter with GPS speedo, I never knew how fast I was going because the speedo hole in the drive always gets plugged with sand and silt. And I got tired of trying to keep it clear. I am satisfied with 53 knots and don't think I need to go any faster. I only open it up that fast on the river when its nice and smooth. 20 to 25 knots in 2 foot seas in the bay.

 

[alldodge]

I revved it today on muffs and it did hit 5k. But wont go over. Governed?

If its a TB5 ignition it can have a rev limiter, but to keep your motor running well and not let it come apart on you, don't let it go above 4800 rpm. 5K on that motor can have some bad effects on it

 

[thegoat86]

Ok. Makes sense. I didn't hold the rev long. Just a pop on the throttle to see what it would do.

My previous post is incorrect. I meant to say it never revved over 4k RPM underway in the water. When I was just sitting, or on muffs it would rev out of gear to 4800 - 5k

I am having some difficulty getting my hands in behind the motor mount to get at the screws that hold the regulator on the cooler. I will report back when its out my findings. I rechecked the pressure. It still swings wildly between 45 and 65 psi. When I turn the key to on before it starts, it will pump to 60 then settle at 45. You mentioned a fuel damper under the plenum, this would have nothing to do with high pressure correct? Maybe why the gauge is swinging??

 

[alldodge]

You mentioned a fuel damper under the plenum, this would have nothing to do with high pressure correct? Maybe why the gauge is swinging??

The damper keeps the pulses down, not a swing but a bump type of the needle. This should have nothing to do with high pressure, the regulator has control of it

 

[thegoat86]

Sooooooo..... AD, your thread from 2014 found the culprit. It was the screen under the regulator. Once I took it out, I tested the fuel regulator with a vacuum gauge, it dropped under light vacuum. Once I hooked the vacuum line back up, the fuel pressure gauge was steady at 38-39 PSI. My performance issue is gone and the stumble is gone. If you haven't already, maybe you should put a link to that thread on here. It was very helpful. I will report the performance when we get back from fireworks on the bay tonight.

 

[alldodge]

Glad to have assisted and hope your back up, but the that thread it was Fun Times that mentioned about the screen
http://forums.iboats.com/forum/engi...-outdrives/7852577-7-4-mpi-high-fuel-pressure

 

[thegoat86]

The bay was kinda choppy, but boat ran like a top. When I started it on muffs this morning to check the fuel pressure before I started the work, it was running badly again. After the screen removal, motor ran great. No problems. I wonder why they would put a screen there. Seems like the filter would catch anything large. The screen looked clean to my eye, but after it popped out it was lost to the underhull gods. So I couldn't further inspect it. Still no change in WOT while underway, 4k is it. Same top end 52-53 knots at 3800 rpm. BTW fireworks were awesome at channelside in tampa. Thanks for the help. This is a great forum to get help on. Hoping to learn some stuff and help others as my journeys continue.:usa2:

 

[alldodge]

Since your rpm is the same and your running good, next thing is to reduce the prop pitch to gain higher rpm. What prop do you have now?