Mercruiser 7.4 top end issues

[Glastron Fisher]

so long story short i pulled my engine two seasons ago due to a cracked block. Got a new used engine with forged internals and put her back together, drained and refueled. Won’t go up on plane, hits 3,000-3,500 RPMS and is boggy, if i throw a few guys up in the bow or go out solo it will get on plane and max out at 4,000. I did a pull with it bogging down for about 30 seconds and plugs are so white they look brand new(fresh with engine swap). Except cylinder 6, which is golden brown. i checked fuel pressure and it’s fluctuating from like 40-50 pretty rapidly and checked compression and everything’s at 140ISH. Pulled codes and I’m getting 21 and 23 but both sensors tested fine. Intake temp sensor tested at 1.7K ohms which puts it at just over 100* and TPS tested at .65 closed and 4.56 open. Also tested the wires going back to the PCM and all is well. I’m going to check for intake leaks and replace my fuel filter to see what that does but I’m stumped as to why it’s throwing these two codes even tho they both tested fine. Any advice? Any way to verify that codes have been cleared from the system? I opened throttle to 100% multiple times and they still return

 

[alldodge]

You have a 7.4 what? Serial number helps a bunch

Not so sure open throttle 100% clears codes. Even if it did, they remain and issue so they show back up.

fuel pressure and it’s fluctuating from like 40-50 pretty rapidly

No matter what motor, it should not be bouncing, and not that high (50 psi)

 

[Glastron Fisher]

7.4L Chevy 454 mercruiser. Also fuel pressure key on drops to like 25PSI and stays there. Not sure if it’s supposed to stay up but it’s not

 

[Bondo]

I did a pull with it bogging down for about 30 seconds and plugs are so white they look brand new(fresh with engine swap).

Ayuh,....... That, says it's runnin' way to lean,......

 

[alldodge]

7.4L Chevy 454 mercruiser. Also fuel pressure key on drops to like 25PSI and stays there. Not sure if it’s supposed to stay up but it’s not

Yep, all are 7.4 or 454 are chey's, need it narrowed down a bit more

 

[Scott Danforth]

the 454 has been in boats in 3 major block generations and about 30 induction versions and 3 of them are MPI

so.... what do you have?

 

[Glastron Fisher]

the 454 has been in boats in 3 major block generations and about 30 induction versions and 3 of them are MPI

so.... what do you have?

It’s a gen 6 MPI i believe it’s the 310HP motor. Not sure what else could narrow it down for ya

 

[Glastron Fisher]

Ayuh,....... That, says it's runnin' way to lean,......

Yeah i know that, just trying to figure out why. Fuel pressure when you key on goes up to 45 then drops to 25 and holds. While running it fluctuates by about 10PSI PRETTY RAPIDLY. i did find a small vacuum leak, just enough to hear the engine stumble a little bit when sprayed with brake cleaner so I’m getting another set of intake gaskets today. I don’t think that small of a vacuum leak could cause that bad of a lean issue but i guess we will see tonight. We’re supposed to take it out for the fireworks tomorrow so I’m hoping i can get it fixed before then.

 

[Scott Danforth]

that should be an L29 motor. your serial number would help

fuel pressure is off. its a 3-bars system, so your fuel pressure should be a steady 43-45 psi

 

[Glastron Fisher]

that should be an L29 motor. your serial number would help

fuel pressure is off. its a 3-bars system, so your fuel pressure should be a steady 43-45 psi

Motor is not original so I’ll have to get the serial number off the old block when i get home. Where would i start with it fluctuating this bad? Regulator on the rail or regulator on the pump? I’ve cleaned the screen on the pump before due to an over pressure issue but I’m not sure where to start with this fluctuating issue

 

[alldodge]

If its an L29 then the TB is in the center of the intake manifold, and is TBI and 37 psi. If Gen VI then pressure is at 43 psi and cool fuel. If fuel system was over pressured then this can damage the reg

 

[muc]

Forget the fuel system for now. First you must address the codes.
A TP code is important, without a functioning throttle position sensor the engine will not run right. It’s one of the most important sensors.
Codes 23 and 21 have a relationship on their ground. It sounds like the ECM has a open ground on circuit 813.

This is is just guessing until we know exactly what system you have. The serial number from the engine the electrical and fuel system came off of will do that. Or if the engine was registered, the drive or transom S/N can work.

You say that there are 2 fuel pressure regulators. The one on the rail is a dampener and the one on the pump is the regulator.

 

[Glastron Fisher]

If its an L29 then the TB is in the center of the intake manifold, and is TBI and 37 psi. If Gen VI then pressure is at 43 psi and cool fuel. If fuel system was over pressured then this can damage the reg

It has cool fuel, was overpressurized a few seasons ago and had been fine after that.

 

[Glastron Fisher]

Forget the fuel system for now. First you must address the codes.
A TP code is important, without a functioning throttle position sensor the engine will not run right. It’s one of the most important sensors.
Codes 23 and 21 have a relationship on their ground. It sounds like the ECM has a open ground on circuit 813.

This is is just guessing until we know exactly what system you have. The serial number from the engine the electrical and fuel system came off of will do that. Or if the engine was registered, the drive or transom S/N can work.

You say that there are 2 fuel pressure regulators. The one on the rail is a dampener and the one on the pump is the regulator.

Do you have access to wiring diagrams? It’s a gen VI with cool fuel and both sensors tested good, if i had a wiring diagram i could locate that ground. Are they grounded at the ECU or is it a ground on the block? There was a couple ground wires on the backside of the block but both sensors had a good ground at the plugins

 

[alldodge]

Pics

 

[muc]

Do you have access to wiring diagrams? It’s a gen VI with cool fuel and both sensors tested good, if i had a wiring diagram i could locate that ground. Are they grounded at the ECU or is it a ground on the block? There was a couple ground wires on the backside of the block but both sensors had a good ground at the plugins

I’m guessing you should be looking in manual #23
Wiring diagrams and troubleshooting will be there.
They ground through the ECM, you need to make sure that the harness isn’t grounded internally.

 

[Glastron Fisher]

Pics

Appreciate it!! Everything tested fine, even undid the harness and both grounds are fine and both sensors tested fine. Not sure why it’s still throwing those codes

 

[alldodge]

Appreciate it!! Everything tested fine, even undid the harness and both grounds are fine and both sensors tested fine. Not sure why it’s still throwing those codes

Don't know the inner workings of the MEFI3 (how its wired) but do you have a ground connection on J2-3 to ground?

 

[Glastron Fisher]

Don't know the inner workings of the MEFI3 (how its wired) but do you have a ground connection on J2-3 to ground?

Yes we had good ground, i put new intake gaskets to fix the leak i had and with messing with the wires it stopped throwing the codes but I’m back out on the water and still the same thing, wondering if a valve adjustment could do it? It’s a little noisy up top. Gonna double check while I’m out here and see if i have any codes again

 

[Glastron Fisher]

So i pulled codes again out on the water and got 14, 22, 23, and 34. I’m guessing that I’m chasing an electrical issue around with the PCM at this point