Absolutely frustrated with this Mercruiser engine

[Hoplite808]

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?

 

[atrick]

With everything that you've done I'll take a shot in the dark and suggest the cam shaft timing is off one tooth.

 

[KevinC]

Intake manifold leak??

-Kevin

 

[scoflaw]

Does the timing advance when you rev it? I don't like 120 and if that gauge is off maybe it's 100.

 

[Scott215]

You are missing something right on front of you. I know I sound crass but it's the stupid things that get you

 

[PlayDate]

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?

I'm just curious....this is a MPI engine....how is it being diagnosed? Did any of the techs hook up a Rinda or Mercruiser Diagnostic Scanner to it? That is the starting point for most mechanics to see what the ECM sees.

I appreciate that you must be frustrated after so much work.....but what is not clear is why all the parts were replaced in the first place which is why asking others on CSR can be equally frustrating.

As to the end result of it ......the engine is hard to start cold and won't go beyond 2,000 rpm.......that certainly sounds like a fuel related issue. Can you post the engine number and how many hours are on the engine?

Also you mention you replaced a mechanical fuel pump.....is that correct?

 

[Hoplite808]

I'm just curious....this is a MPI engine....how is it being diagnosed? Did any of the techs hook up a Rinda or Mercruiser Diagnostic Scanner to it? That is the starting point for most mechanics to see what the ECM sees.

I appreciate that you must be frustrated after so much work.....but what is not clear is why all the parts were replaced in the first place which is why asking others on CSR can be equally frustrating.

As to the end result of it ......the engine is hard to start cold and won't go beyond 2,000 rpm.......that certainly sounds like a fuel related issue. Can you post the engine number and how many hours are on the engine?

Also you mention you replaced a mechanical fuel pump.....is that correct?

Yes it is an MPI engine. Both Mechanics that came did not have access to a scanner for this particular engine, and neither do I. The engine Serial Number is OF764186.

The exhaust manifolds had visible cracks and were replaced due to that.
I found several valves sticking so rebuild and replaced the heads.
Repaired and replaced the intake manifold, checked for leaks after and none found.
Sensors were replaced as guesses to the possible issue plus damage to some connectors.
Thermostat housing and whatnot was replaced because of corrosion.
Fuel pressure was found low so rebuilt fuel separator and replaced mechanical pump just to be sure.
Changed the raw water pump because I was unsure of it's age.
Out drive replaced because gimbal bearing was bad and crack found in outdrive housing while replacing that.
All the ignition components changed as unknown age of previous parts and guessing it could be related to mechanical issues.
Alternator changed due to bearing failure.
Starter motor changed due to old one failing.

 

[Lazy Daze]

Have you had the fuel injectors rebuilt? Around here, it's about $20/injector. It's a good thing to do, regardless, given the age.

 

[PlayDate]

Yes it is an MPI engine. Both Mechanics that came did not have access to a scanner for this particular engine, and neither do I. The engine Serial Number is OF764186.

The exhaust manifolds had visible cracks and were replaced due to that.
I found several valves sticking so rebuild and replaced the heads.
Repaired and replaced the intake manifold, checked for leaks after and none found.
Sensors were replaced as guesses to the possible issue plus damage to some connectors.
Thermostat housing and whatnot was replaced because of corrosion.
Fuel pressure was found low so rebuilt fuel separator and replaced mechanical pump just to be sure.
Changed the raw water pump because I was unsure of it's age.
Out drive replaced because gimbal bearing was bad and crack found in outdrive housing while replacing that.
All the ignition components changed as unknown age of previous parts and guessing it could be related to mechanical issues.
Alternator changed due to bearing failure.
Starter motor changed due to old one failing.

Give me a day or so to look up a few things on your engine. In the meantime…you came to right place. It may take a little while but the brain trust at CSR is pretty good for big block gas engines.

 

[Golfman25]

Consider getting yourself a rinda scanner. Kind of flying blind without it. Seems expensive but considering you’ve replaced thousands of dollars worth of parts, maybe not.
https://www.rinda.com/

 

[Bill Curtis]

I haven't seen anything mentioned about the ECM. At this point, it may be prudent to send it out and have it checked.
Ghost problems are usually real simple, but hard to pin down, or electronic in nature.

As stated, the simple Rinda diagnostic tool or the full fledged computer based one in needed here. Kind of eliminates the guess work and money wasted in parts swapping.

Can't remember off top of head, but there are a couple of companies that will test the ECM and even repair / re-program if needed.

 

[PlayDate]

Yes it is an MPI engine. Both Mechanics that came did not have access to a scanner for this particular engine, and neither do I. The engine Serial Number is OF764186.

The exhaust manifolds had visible cracks and were replaced due to that.
I found several valves sticking so rebuild and replaced the heads.
Repaired and replaced the intake manifold, checked for leaks after and none found.
Sensors were replaced as guesses to the possible issue plus damage to some connectors.
Thermostat housing and whatnot was replaced because of corrosion.
Fuel pressure was found low so rebuilt fuel separator and replaced mechanical pump just to be sure.
Changed the raw water pump because I was unsure of it's age.
Out drive replaced because gimbal bearing was bad and crack found in outdrive housing while replacing that.
All the ignition components changed as unknown age of previous parts and guessing it could be related to mechanical issues.
Alternator changed due to bearing failure.
Starter motor changed due to old one failing.

A few more questions:

1) How many hours are on the engine?
2) Has it ever run perfectly and how long ago?
3) Where did the replacement heads come from?

It seems as though both the ignition and mechanical issues have been addressed one way or another. Mechanical issues should be present across the rpm spectrum. You have replaced most of the ignition components. Assuming those are working properly we are left with the fuel system and what the ECM sees.

We are at a major disadvantage without a Rinda scanner which can tell you what the ECM sees for timing, sensors and codes. Yes......it would have been great if Mercruiser would have used straight OBDC protocols but they didn't which means somebody has to spend $500-600 to buy one.

In regards to the fuel system......IF the gas quality and pump pressure are okay......then you are left with the fuel injectors. As previously mentioned, getting those cleaned and tested eliminates them as possibility. If they are clogged.....the engine will have difficulty in starting and will not rev with a load.

 

[Stee6043]

Silly question...you said it won't throttle past 2,000 under load. Can you get 4,400+/-rpm WOT without a load on it?

 

[370Dancer]

As Playdate will attest, after a year of messing with my 1999 MAG MPI it turned out to be a spun bearing on the main crankshaft. Mine would start ok cold, but would always fall off RPMs as the bearing heated up and expanded. Then would almost never restart hot, or would quit at idle speed, like docking in a crosswind....... I could idle around at up to 2000 rpm all day. Not saying this is your issue. Just adding to what can cause mysterious symptoms.

 

[Hoplite808]

A few more questions:

1) How many hours are on the engine?
2) Has it ever run perfectly and how long ago?
3) Where did the replacement heads come from?

It seems as though both the ignition and mechanical issues have been addressed one way or another. Mechanical issues should be present across the rpm spectrum. You have replaced most of the ignition components. Assuming those are working properly we are left with the fuel system and what the ECM sees.

We are at a major disadvantage without a Rinda scanner which can tell you what the ECM sees for timing, sensors and codes. Yes......it would have been great if Mercruiser would have used straight OBDC protocols but they didn't which means somebody has to spend $500-600 to buy one.

In regards to the fuel system......IF the gas quality and pump pressure are okay......then you are left with the fuel injectors. As previously mentioned, getting those cleaned and tested eliminates them as possibility. If they are clogged.....the engine will have difficulty in starting and will not rev with a load.

1) About 700 hours
2) At some point in history yes just not after I completed the purchase.
3) Heads were rebuilt by a local machine shop.

Not sure where I could send the injectors locally for service and new injectors are nearly $300 each and the engine requires eight of them.

https://www.mercruiserparts.com/805225a1-injector-kit-fuel

When I had it all apart last time I did spray them out with a can of cleaner while triggering them with a 9v battery. They all seemed to spray well enough then. I did take a chance on some cheap injectors from China via Amazon and the engine wouldn’t run at all on those.

 

[Hoplite808]

Silly question...you said it won't throttle past 2,000 under load. Can you get 4,400+/-rpm WOT without a load on it?

Yes, if I leave it in neutral it sounds fine and revs all the way to redline. Under load it runs perfectly till about 2000 RPM and then can’t develop any more power. The boat ends up with a top speed of about 9-10 knots.

 

[techmitch]

Yes, if I leave it in neutral it sounds fine and revs all the way to redline. Under load it runs perfectly till about 2000 RPM and then can’t develop any more power. The boat ends up with a top speed of about 9-10 knots.

How long has the boat been in the water. Is the bottom, outdrive and prop perfectly clean?

 

[Hoplite808]

How long has the boat been in the water. Is the bottom, outdrive and prop perfectly clean?

Bottom was repainted two years ago with trilux 33.

Hauled it out last summer and pressure washed the bottom and touched up the bottom paint while the outdrive was getting replaced.

I freedive every other month or so and clean the growth off the bottom and outdrive also so it’s definitely not a drag issue from growth.

 

[techmitch]

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.

 

[Stee6043]

Yes, if I leave it in neutral it sounds fine and revs all the way to redline. Under load it runs perfectly till about 2000 RPM and then can’t develop any more power. The boat ends up with a top speed of about 9-10 knots.

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