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brwn234
05-26-2015, 10:25 PM
I've got a 2000 7.4 MPI that won't start. I'm leaning toward a fuel issue. Here's what's going on:

Uncovered it from the shrink wrap earlier this spring. Put it on the hose and could barely keep it running and when it did the rpm would rise and fall like it had a big cam. I heard the IAC opening and closing like crazy so I thought that may not be operating properly. Frantically threw a new IAC in it trying to get on the water. Ran a bit rough, but ran. Got to to the lake and nothing.

If I cycle the fuel pump by turning the ignition on and off several times without bumping the starter it will fire for a brief moment. The pump is definitely spinning but at what pressure I dunno. I've checked simple things like my Racer fuel filter, drained and topped with fresh fuel. Replaced the worn vacuum line on the fuel regulator. Previously when I'd open the fuel filter id need to purge any air from the lines using the port just behind the thermostat housing. There's no air in the lines, I'm pretty sure.

Just today I bought a fuel pressure test kit. it was kinda late so I couldn't start it up but turned the ignition switch on to cycle the fuel pump. 35-45 psi several times. Where do I go next? I was almost completely convinced it's a fuel issue but not so much anymore. Tomorrow I'll attempt to start it again while monitoring the pressure. How can I determine if it's a pump vs a regulator issue?

Thanks

mattyboy
05-26-2015, 11:15 PM
oil pressure safety switch? shouldn't the fuel pump only pump when you are cranking then it should only pump when there is oil pressure?

brwn234
05-26-2015, 11:26 PM
Are the oil pressure safety switch and sender for the gauge separate?

The gauge is reading pressure

mattyboy
05-27-2015, 07:00 AM
that's probably a question for a merc guy with my electric fuel pump I have no other electronics so I have an OP sender and an OP switch the OP switch sends voltage to the fuel pump when I am cranking then when I release the key there is no voltage until oil pressure builds. on an EFI boat that function may be handled by the ECM. If you have clips on your test leads of your digital multimeter I would check for voltage at the fuel pump while cranking.

don't some of the merc boats have an electric safety valve in the fuel line from the tank to the motor. I seem to remember hearing a buzz from that when the key is turned to on when we put the 383 in the the sting.

brwn234
05-27-2015, 08:42 AM
I'm defenitley heading that way. Thanks for the lead. Of course someone is borrowing my Merc manual right now but I think I can figure it out with google and a parts diagram. I'll let you know what i find

Pismo
05-27-2015, 01:56 PM
Best diagnostic trick out there - Spray a little Gumout/carb cleaner(not ether/starting fluid which pings badly) down the flame arrestor. If it is fuel issue it will run perfectly for a few seconds, if not it is ignition. Works on everything from lawnmower on up.

VetteLT193
05-27-2015, 02:36 PM
research mercruiser mpi "cool fuel" system.

Tidbart
05-29-2015, 10:29 AM
What exactly did you mean by, drained and topped with fresh fuel? Did you drain the tank, or the fuel filter? This sounds like a bad fuel case.
That port behind the thermostat housing is the on the end of the fuel rail. It is for checking the fuel pressure with a gauge, not for purging air. I would start with checking the pressure, which you did and it looks good. Next i would be getting a new source of fuel, like a temp gas tank with fresh fuel.

Bob

brwn234
05-29-2015, 09:07 PM
So today I got smart and pulled the MEFI codes using the OBD port on top of the motor. I pulled two codes 14 and 34. 14 - coolant sensor voltage high (low) 34 - Mass Absolute Pressure Sensor low. Can he MAP sensor be pulled wo removing the intake manifold?

TXDONZI
05-30-2015, 03:04 AM
I have the same issue can I scan it with a regular OBD II scanner?

RedDog
05-30-2015, 05:32 PM
I had a similar problem several years back. Turned out to be a faulty fuse in the ECU (?) box. It wasn't blown but must have had a hair line crack in the element that would intermittently separate and then remake contact. Not sure which one it was but replaced all of them and the problem hasn't recurred since.

RedDog
05-30-2015, 05:35 PM
I had a similar problem several years back. Turned out to be a faulty fuse in the ECU (?) box. It wasn't blown but must have had a hair line crack in the element that would intermittently separate and then remake contact. Not sure which one it was but replaced all of them and the problem hasn't recurred since.

I'm referring to the blade type fuses in the box mounted between the exhaust manifold and the intake

Tidbart
05-31-2015, 02:12 PM
So today I got smart and pulled the MEFI codes using the OBD port on top of the motor. I pulled two codes 14 and 34. 14 - coolant sensor voltage high (low) 34 - Mass Absolute Pressure Sensor low. Can he MAP sensor be pulled wo removing the intake manifold?

Neither of these will stop the engine from starting, and just because a code was set, that doesn't mean that the sensor is bad. Don't chase sensors. Get a manual and start going through the troubleshooting section, 5C.

B

TXDONZI
06-09-2015, 09:22 PM
I've got a 2000 7.4 MPI that won't start. I'm leaning toward a fuel issue. Here's what's going on:

Uncovered it from the shrink wrap earlier this spring. Put it on the hose and could barely keep it running and when it did the rpm would rise and fall like it had a big cam. I heard the IAC opening and closing like crazy so I thought that may not be operating properly. Frantically threw a new IAC in it trying to get on the water. Ran a bit rough, but ran. Got to to the lake and nothing.

If I cycle the fuel pump by turning the ignition on and off several times without bumping the starter it will fire for a brief moment. The pump is definitely spinning but at what pressure I dunno. I've checked simple things like my Racer fuel filter, drained and topped with fresh fuel. Replaced the worn vacuum line on the fuel regulator. Previously when I'd open the fuel filter id need to purge any air from the lines using the port just behind the thermostat housing. There's no air in the lines, I'm pretty sure.

Just today I bought a fuel pressure test kit. it was kinda late so I couldn't start it up but turned the ignition switch on to cycle the fuel pump. 35-45 psi several times. Where do I go next? I was almost completely convinced it's a fuel issue but not so much anymore. Tomorrow I'll attempt to start it again while monitoring the pressure. How can I determine if it's a pump vs a regulator issue?

Thanks

What was the out come?

brwn234
06-12-2015, 03:29 PM
I haven't had much time to work on it lately. I'm currently getting ready to move and have a bunch of other stuff goin on. This is where I'm at now;

I removed the MAP sensor, cleaned it off and reinstalled it. the boat will run at idle now but very rough and will die with any throttle. The fuel pressure will jump up to 80psi when the pump is priming and then rest at 35 when it stops. when running the pressure bounces around 80.

This pressure when the pump is running seems very high. Could this be due to a regulator issue or possibly a partial line blockage?

Oh yeah, reset the codes and they do not come back.

BUIZILLA
06-12-2015, 03:54 PM
there is a small screen underneath the regulator in the cool fuel rail, take it out, it's a b1tch to get to................... :wrench: :bonk: :nilly:

report your findings :yes:

brwn234
06-12-2015, 08:11 PM
Is it in a parts diagram or something? Behind where the regulator mounts to the fuel rail. I can't picture it sorry


https://www.mercruiserparts.com/Show_Pictures3.asp?dnbr=809994 98&ivar=images/COMMON/20147.png&inbr=2705&bnbr=110&bdesc=FUEL+PUMP+AND+FUEL+COOLER

Number 16?

BUIZILLA
06-12-2015, 09:47 PM
http://www.mercruiserparts.com/Show_Pictures3.asp?dnbr=809994%2098&ivar=images/COMMON/20147.png&inbr=2705&bnbr=110&bdesc=FUEL+PUMP+AND+FUEL+COOLER

16 would be it... it's down in the rail about 1/2" or so, pick it out with a pick or needle, if it's plugged or restricted that's your problem

brwn234
06-13-2015, 04:44 PM
Well, I destroyed it getting it out because it was pressed in there so tight. I can't figure out which way it flows to know what side the blockage would be on. There really wasn't much of anything. The bottom side(the side I could see when I removed it appeared to have some very very light sediment on it. But nothing that I would think would be of concern. I guess I order another screen now.

BUIZILLA
06-13-2015, 10:06 PM
leave it out...

brwn234
06-14-2015, 12:41 AM
I'm pretty sure the fuel flow goes down through that screen so I wouldn't necessarily of had to remove it to see any blockage. oh well. I'm wondering if the regulator is bad and not opening or allowing more fuel with engine vacuum. Or maybe its as simple as a vacuum line. I checked those first but maybe I over looked something.

BUIZILLA
06-14-2015, 07:08 AM
your problem is unstable and elevated pressure, ANYTHING causing even a slight restriction downwind of the regulator can cause that... it doesn't take much....

not sure why it's even there in the first place, since the return fuel goes back to the fuel filter anyways, which if it hasn't been changed in awhile can cause the same thing

I, and a LOT of others over the years, pretty astute mechanics and surveyors alike, have found that screen to either be a single source or contributing source. If it doesn't solve the immediate issue, then chalk that item off the list :)

there is another regulator on the port side rear of the main fuel rail, it ties in with the vacuum signal with the lower regulator as well, all good things to check

TXDONZI
07-04-2016, 03:22 PM
your problem is unstable and elevated pressure, ANYTHING causing even a slight restriction downwind of the regulator can cause that... it doesn't take much....

not sure why it's even there in the first place, since the return fuel goes back to the fuel filter anyways, which if it hasn't been changed in awhile can cause the same thing

I, and a LOT of others over the years, pretty astute mechanics and surveyors alike, have found that screen to either be a single source or contributing source. If it doesn't solve the immediate issue, then chalk that item off the list :)

there is another regulator on the port side rear of the main fuel rail, it ties in with the vacuum signal with the lower regulator as well, all good things to check


How do you get the cover off with the motor mount right in front of it?

TXDONZI
07-04-2016, 03:23 PM
I'm pretty sure the fuel flow goes down through that screen so I wouldn't necessarily of had to remove it to see any blockage. oh well. I'm wondering if the regulator is bad and not opening or allowing more fuel with engine vacuum. Or maybe its as simple as a vacuum line. I checked those first but maybe I over looked something.


What was the outcome...did you get your issue resolved?

txtaz
07-06-2016, 07:22 AM
How do you get the cover off with the motor mount right in front of it?

I just did mine. I replaced the pump and regulator just because they were 16 years old and I was working on the motor at the time. I don't want to touch those again for a long time.

Pull the port exhaust manifold. You need to remove the intake plenum and disconnect the fuel line so you have slack. Remove the two water cooling lines to the cool fuel. Remove two bolts holding the cool fuel to the block. Pull the assembly forward to remove.

You can get an airtex fuel pump (~$100) which is the same as the mercruiser.

TXDONZI
07-07-2016, 01:07 PM
I just did mine. I replaced the pump and regulator just because they were 16 years old and I was working on the motor at the time. I don't want to touch those again for a long time.

Pull the port exhaust manifold. You need to remove the intake plenum and disconnect the fuel line so you have slack. Remove the two water cooling lines to the cool fuel. Remove two bolts holding the cool fuel to the block. Pull the assembly forward to remove.

You can get an airtex fuel pump (~$100) which is the same as the mercruiser.

Thank you for the details....I think. LOL

This sounds like a real pain like mentioned above. I was thinking of just pulling the mount and stringer bracket since that would only be 6 bolts......