Absolutely frustrated with this Mercruiser engine

[Hoplite808]

Has your Bravo been serviced throughout this process? Have you confirmed you've got the correct prop on her?

The drive unit is less than a year old now. Completely rebuilt and installed by a professional. The props are the OEM ones. The outdrive is absolutely not the issue.

 

[PlayDate]

You'd be surprised at what little growth or even algae slime it takes to extremely effect performance. That along with the low compression #'s you posted surely need to be rectified before moving forward with diagnosis.
Complete, comprehensive and proper diagnosis is 85% of the job. Any body can throw parts at it costing ton's of money unnecessarily. I wouldn't even hook my scanner to it with those low compression numbers. Any fuel injection system is worthless without a good base engine.

Mitch,

I discounted the compression numbers at this stage. That just raises a bunch of other questions regarding how the compression test was done. I agree that they are lower than the 150 I believe is the spec for this engine.....but the engine hasn't run right for some time and it has 700 hours on it. If memory serves me right this was a 305-310 HP engine moving a heavy boat.

Hooking up a scanner would answer a lot of basic information and possibly identify where the problem lies. If that is off the table....then we are back to guessing where the problem might be.

Since we are guessing it seems like the sequence of things begins with the OP buying the boat with an engine problem. It then seems to shift to possible water intrusion necessitating replacing the exhaust manifolds and reconditioning the heads which was a very reasonable approach to take for water intrusion.

However.....that did not solve the operating problem. So subsequent new parts were applied to try to eliminate the problem.

I also believe the OP has replaced all the usual suspects which then begs the question...."what is wrong with this thing"?

If someone brought this to you or I ......we would start over. We would do a compression and leak down test and move on from there. That way any valve sealing issues (remember that issue on another thread) or ring issues are known up front. From there it is hooking up a scanner and see what it says. Also, I would have a separate portable tank with fresh fuel to feed it.

It is a pain to start over......but looking at the work that has been done......it would be worth it to walk through it again before you junk the engine.

 

[Skybolt]

@PlayDate & @techmitch To me is sounds like pulling the timing chain cover would be worth a try and check that the chain hasn't jumped a tooth.

But to your point John, at this point you need to have someone that knows what they are doing and not know what was done to be able to diagnose this correctly. And a scanner is definitely needed to see what is going on with timing/injectors.

 

[techmitch]

Mitch,

I discounted the compression numbers at this stage. That just raises a bunch of other questions regarding how the compression test was done. I agree that they are lower than the 150 I believe is the spec for this engine.....but the engine hasn't run right for some time and it has 700 hours on it. If memory serves me right this was a 305-310 HP engine moving a heavy boat.

Hooking up a scanner would answer a lot of basic information and possibly identify where the problem lies. If that is off the table....then we are back to guessing where the problem might be.

Since we are guessing it seems like the sequence of things begins with the OP buying the boat with an engine problem. It then seems to shift to possible water intrusion necessitating replacing the exhaust manifolds and reconditioning the heads which was a very reasonable approach to take for water intrusion.

However.....that did not solve the operating problem. So subsequent new parts were applied to try to eliminate the problem.

I also believe the OP has replaced all the usual suspects which then begs the question...."what is wrong with this thing"?

If someone brought this to you or I ......we would start over. We would do a compression and leak down test and move on from there. That way any valve sealing issues (remember that issue on another thread) or ring issues are known up front. From there it is hooking up a scanner and see what it says. Also, I would have a separate portable tank with fresh fuel to feed it.

It is a pain to start over......but looking at the work that has been done......it would be worth it to walk through it again before you junk the engine.

@PlayDate & @techmitch To me is sounds like pulling the timing chain cover would be worth a try and check that the chain hasn't jumped a tooth.

But to your point John, at this point you need to have someone that knows what they are doing and not know what was done to be able to diagnose this correctly. And a scanner is definitely needed to see what is going on with timing/injectors.

With compression that low, until the root cause of that is found, rectified and the compression restored to at least close to spec., I'm out.

 

[scoflaw]

Agree the compression is low for a fresh set of heads, maybe the valves aren't adjusted correctly or worst.
The VST on those motors are problematic, you got pressure but do you have volume? Have you taken it apart?
Probably a very old Mefi-1 ecm and they don't last forever. They also offer up very little info.
Injectors are probably original and they don't last forever as well.
Put a timing light to it and see if it's advancing.

Does the engine sound smooth when it quits at 2000 rpm?

 

[Hoplite808]

Agree the compression is low for a fresh set of heads, maybe the valves aren't adjusted correctly or worst.
The VST on those motors are problematic, you got pressure but do you have volume? Have you taken it apart?
Probably a very old Mefi-1 ecm and they don't last forever. They also offer up very little info.
Injectors are probably original and they don't last forever as well.
Put a timing light to it and see if it's advancing.

Does the engine sound smooth when it quits at 2000 rpm?

The SELOC manual I have says 125 is spec for compression with a dry test with all the plugs out and engine cold. So I don’t know why a 5 psi drop is considered too low. The valve seating issue you recalled was fixed with a Jegs kit to convert it to adjustable valves. The engine sounds fine throughout. No knocking or pinging throttle response is smooth and strong until about 2000 RPMs under load and then it’s like it’s not getting enough fuel. You can feel it wants to go faster but something somehow is keeping the engine from accelerating further. I’ve found a place that said they can test the injectors today so I’m going to pull them and try that next. Also, I did put a timing light on it and it is advancing fine as far as I can tell.

 

[scoflaw]

So did you open up the VST? If those flexible fuel lines are original replace them they can be an issue as well

 

[Hoplite808]

So did you open up the VST? If those flexible fuel lines are original replace them they can be an issue as well

I rebuilt the VST entirely. Measuring fuel pressure at the rail it stays at about 37psi constantly. I know for a fact that fuel in sufficient quantities, quality, and pressure is getting to the injectors. I know spark is occurring in all 8 cylinders and it appears to be doing so in the right order and with the appropriate amount of advance from TDC. This is why it’s frustrating me. Everything I know about engines says it should run fine, but then it doesn’t.

 

[ttmott]

I have used RC Injection for years on injector servicing, cleaning, calibration, and matching. I agree that your next move should be getting them cleaned, tested, and calibrated. https://www.rcfuelinjection.com/
It sounds like it is running smoothly up to a point where load and rpm is needed then it falls on it's face and hard starting. Correct?
If the fuel injectors are not putting out a good spray pattern it will be hard to start - engine is lean.
If the ignition is all over the place; there is spark but not at the correct time, it could be the crank position sensor. But, it sounds like once the engine is running it is smooth and pretty much fault free.

Injectors....

Oh Yea - one other thing; check the wiring for the injectors. I believe there are two banks of B+ that powers them. The ECM triggers the B-. So check the positive leads for good high current wiring, fuses, and connectors.

 

[Skybolt]

The SELOC manual I have says 125 is spec for compression with a dry test with all the plugs out and engine cold. So I don’t know why a 5 psi drop is considered too low. ...

With the compression test(s), +/- 10% is standard so 5psi is actually great. But the avg compression for pretty much all 350's is 150 Psi. @ 8.5:1 ratio. Some were as high as 180psi. with a higher ratio.

 

[techmitch]

With the compression test(s), +/- 10% is standard so 5psi is actually great. But the avg compression for pretty much all 350's is 150 Psi. @ 8.5:1 ratio. Some were as high as 180psi. with a higher ratio.

My thoughts exactly!

 

[Skybolt]

My thoughts exactly!

The fact that all are ~120PSI tells me to look at the cam placement. That's not to say other things are going on, but with compression like that across the board means something else is going on.

 

[aerobat77]

I have been trying to fix this engine for over a year now with no success. I’ve tried to find a professional mechanic to fix it and the only two I could get to come look at it both just said they don’t know what’s wrong with it and good luck. It’s a 1996 Mercruiser 7.4LX MPI (Gen V) with a Bravo 3 stern drive mounted in a 270 Sundancer.

So far I’ve replaced the cylinder heads, exhaust manifolds, the spark plugs, spark plug wires, cap, rotor, distributor, ignition coil, ignition module, air temp sensor, MAP sensor, IAC, throttle position sensor, mechanical fuel pump, the electric pump in the vapor separator tank, the alternator, raw water impeller and housing, the heat exchangers, thermostat and housing, engine temp sender, oil pressure sender, the entire outdrive, and repaired or replaced wiring. I have quadruple checked the timing and fuel pressure. All is set exactly as it should. Fuel in the tank has been flushed and replaced with new. Oil is fresh and pressure is good. Engine coolant temperature is fine. Compression in all 8 cylinders is 120 psi and testing for spark at all 8 cylinders showed no problems.

You need to get more purposeful on this since some items you replaced have close to nothing with engine lack of power and its just throwing money on it hoping for a miracle .

Compression we can argue but even 120 psi are not the answer a 7.4 mpi struggles to push a 270 past 2000 rpm,s . Compression is imho fine .

If it does not misfire badly spark and timing is ~ correct.

You really need to check with a scanner if it does not go into guardian mode / has recent fault codes before you replace anything more .

After this i would check for clogged injectors and really check again the fuel pump for paint clogging .

I assume you checked WOT on the throttle lever is really full throttle on the engine ? ( linkage is ok ?)

 

[ColoSR]

My 6.2l MPI has two fuel pumps, a helper pump for vapor lock and the main electric fuel pump in the cool fuel 2 unit. The main pump failed so I couldn’t get over 2000 rpm under load. Do you have 2 pumps by chance?

 

[ColoSR]

This guy on Amazon said that the mechanical pump was the helper, the main efi pump is in a canister in the vapor separator tank.

https://www.amazon.com/ask/questions/Tx3DH9UM1HZ16OS/?

 

[Scott215]

You said you installed an adjustable valve train do you know that they rockers are not over torqued and taking up all of the lash in the hydraulic lifter. Just a thought

 

[dvx216]

I have been trying to fix this engine for over a year now with no success. I’ve tried to find a professional mechanic to fix it and the only two I could get to come look at it both just said they don’t know what’s wrong with it and good luck. It’s a 1996 Mercruiser 7.4LX MPI (Gen V) with a Bravo 3 stern drive mounted in a 270 Sundancer.

So far I’ve replaced the cylinder heads, exhaust manifolds, the spark plugs, spark plug wires, cap, rotor, distributor, ignition coil, ignition module, air temp sensor, MAP sensor, IAC, throttle position sensor, mechanical fuel pump, the electric pump in the vapor separator tank, the alternator, raw water impeller and housing, the heat exchangers, thermostat and housing, engine temp sender, oil pressure sender, the entire outdrive, and repaired or replaced wiring. I have quadruple checked the timing and fuel pressure. All is set exactly as it should. Fuel in the tank has been flushed and replaced with new. Oil is fresh and pressure is good. Engine coolant temperature is fine. Compression in all 8 cylinders is 120 psi and testing for spark at all 8 cylinders showed no problems.

But still it has a hard time starting when cold and it will not throttle past 2000 rpm’s under load. Fuel pressure remains at 37 at the rail when under load and adjusting the timing either more retarded or advanced makes no difference. If I had to guess I’d say it almost feels like it’s running to lean but the plugs look normal.

I am at a complete loss as to what’s wrong with this motor. Has anyone ever run into something like this? If so what was the issue and how did you fix it?

Give Chris a call at Lakeland Auto and Marine in Port Clinton Ohio.

 

[Hoplite808]

@aerobat77 There is no misfiring and timing shows dead on with my timing light. Throttle linkage is fine. The scanner isn’t really useful with limited data from the mefi1 I have. I can check for codes by counting beeps with my continuity tester when I jump the leads on the scanner plug and it’s showing no codes. Also I believe this setup is pre-guardian mode.

@ColoSR Both pumps are new and pressure shows 37 psi at the rail.

@Scott215 When the heads were rebuilt the machinist had to regrind several valves to get them to seat again. Unfortunately this changed the geometry and the factory setup was holding those valves open.

@dvx216 Thank you but I doubt he’ll be able to come to Hawaii to fix it for me.

I’m heading down now to yank all the injectors and then take them to get tested. I’ll give an update when I know more.

 

[KevinC]

@aerobat77 There is no misfiring and timing shows dead on with my timing light. Throttle linkage is fine. The scanner isn’t really useful with limited data from the mefi1 I have. I can check for codes by counting beeps with my continuity tester when I jump the leads on the scanner plug and it’s showing no codes. Also I believe this setup is pre-guardian mode.

@ColoSR Both pumps are new and pressure shows 37 psi at the rail.

@Scott215 When the heads were rebuilt the machinist had to regrind several valves to get them to seat again. Unfortunately this changed the geometry and the factory setup was holding those valves open.

@dvx216 Thank you but I doubt he’ll be able to come to Hawaii to fix it for me.

I’m heading down now to yank all the injectors and then take them to get tested. I’ll give an update when I know more.

We worked through a similar issue with a 320DA. Boat ran fine but bogged down at 3300rpm. After running down multiple issues my friend finally pulled the 16 injectors and sent them out. Both sides showed failures. One side 5 of 8 were bad. After rebuilding injectors and putting them back boat ran great. Hopefully this will be the case for you.

-Kevin

 

[Hoplite808]

Well, good news, I guess. The shop got back to me today about the injector tests. 3 of the injectors have a short when cold and another 4 of the 8 have a short when they are hot. So I gotta go find 8 new injectors. Anyone know where I can find a set that won’t break the bank?