Mercruiser 7.4L MPI slowing down

[wingless]

My Mercruiser 454 MAG MPI Horizon engine cover is different than your 7.4L Bravo MPI 454 engine. I don’t know how to remove your cover.

There should not be special screws. It might even pull of w spring clips.

The freeze spray cans are all very similar. I select based on availability.

There is “lots” of thermal mass. The cooling will either reveal bad part or not. Little chance of freeze damage.

[Scottd21]

Originally Posted by wingless

My Mercruiser 454 MAG MPI Horizon engine cover is different than your 7.4L Bravo MPI 454 engine. I don’t know how to remove your cover.

There should not be special screws. It might even pull of w spring clips.

The freeze spray cans are all very similar. I select based on availability.

There is “lots” of thermal mass. The cooling will either reveal bad part or not. Little chance of freeze damage.

Cool thanks. Just in case I have trouble can I still check the manifold and risers for warming without opening the engine cover or is that only exposed through opening cover?

[wingless]

You’re welcome.

Bring to operating temperature, engines running, verify hand holding exhaust possible.

Manifolds, risers and elbows all exposed.

[Scottd21]

Originally Posted by wingless

You’re welcome.

Bring to operating temperature, engines running, verify hand holding exhaust possible.

Manifolds, risers and elbows all exposed.

Sounds like a plan I’m taking it Monday!

[Scottd21]

What about using a thermal gun so that I could measure all the possible parts to see what’s heating up? I just saw that Amazon has them for $20 - just point it at object and it tells you the temperature.

[wingless]

Sure.

IMO that is overkill.

Most people have a hand, plus a spare, always handy.

[wingless]

Originally Posted by Scottd21

Sounds like a plan I’m taking it Monday!

Good luck!

[Scottd21]

I ran the boat on Monday and traveled 45 miles (much of the trip at 2500 rpm and the latter part was to test the issue at 3100) and it did not occur! I ran it at 2500 rpm because I wanted to confirm that it does not happen at that rpm (with longer test times than mechanic) as mechanic had suggested but also wanted to be closer to the dock if issue occurred and later tested it at 3100 closer to the dock .

I did not notice any difference in hand temperature from the begging to end of the trip as far as the manifold and risers in the front of my engine. The engine sounded and ran great all the way at both 2500 and 3100 rpm. If there is a problem would that be hot or just warm?

Then the mechanic tried it and he was going to spray CRC if it occurred to see if it’s ECM and he ran it for over an hour (with cover on) and it did not occur! This is like the old story where you bring your car to the mechanic and the noise stops. It has gotten a little bit cooler in R.I. lately so that may be it. I’m awaiting for their second test for ECM with mechanic. Is there another way to test ECM besides it heating up? It apparently isn’t throwing any computer codes but I would think it would show logs of it doing something so if it had no logs or info then somethings wrong. Or can Mercruiser do tests on ECM maybe?

Is there anything else that it could be that could be cooling off though that could cause a false positive in this test if opening hatch cooled it off (making it look like it was the ECM)?

I guess what I’m asking is if it is a thermal issue (certainly appears to be) what parts could only heat up at 3100 to 3000 rpm after a half hour of running apparently when it’s warm enough outside but not heat up at 4000 or 2500 rpm if that rings a bell.

If would be interesting to see if it did it with hatch up all the way or engine cover off but the fact that it stopped doing it even with cover closed (perhaps weather related) makes those tests harder to confirm because it adds another variable of the outside weather.

Again the two things that we do know is that for the first time it (did not) do it with boat-in-a-box hooked up with hatch open, for first time with boat wiring it happened but when it occurred opening hatch fixed it right away for the first time so I would that that would narrow it down.

Scott

[wingless]

Thanks for the detailed update. Yes, it is disappointing to be unable to completely analyze the problem.

The ability to continuously hand hold the exhaust is just a quick test of those parts, not eliminating the requirement for annual disassembly / inspection / periodic exhaust component replacement.

The lower local ambient temperature, combined w/ no problem, is also pointing at the suspected thermal issue.

The local RI temperature today is forecast for mid 80s. The window for analyzing the problem is closing. Soon the water temperature will start dropping too. Fortunately the RI water temperature is peaking now.

The RI ambient temperatures in July were in the 80s to 90s, now they are in the 70s to 80s.

Pick a day like today where the forecast is for 80s to attempt for problem replication.

Run it hard in a safe condition. More heat is generated at higher RPM.

The reported problem was the boat would be unable to attain normal RPM. If the problem goes to a different condition, preventing operation, then fall back to the subscribed towing service, if required.

It will be disappointing if the problem resolution cannot be resolved for a year.

The FSM has lots of information in the Fault Chart section that may be helpful. Many of those would show up as fault codes on the DDT. Here is an example for Guardian Strategy faults from that table: "Block pressure, map sensor, oil pressure, port and starboard exhaust temperature, engine coolant temperature or overspeed readings are out of normal ranges"

[wingless]

Originally Posted by Scottd21

I ran the boat on Monday

The highest Monday temperature got into the low 80s at 2PM, w/ the lowest temperature in the low 70s.

[Scottd21]

Originally Posted by wingless

Thanks for the detailed update. Yes, it is disappointing to be unable to completely analyze the problem.

The ability to continuously hand hold the exhaust is just a quick test of those parts, not eliminating the requirement for annual disassembly / inspection / periodic exhaust component replacement.

The lower local ambient temperature, combined w/ no problem, is also pointing at the suspected thermal issue.

The local RI temperature today is forecast for mid 80s. The window for analyzing the problem is closing. Soon the water temperature will start dropping too. Fortunately the RI water temperature is peaking now.

The RI ambient temperatures in July were in the 80s to 90s, now they are in the 70s to 80s.

Pick a day like today where the forecast is for 80s to attempt for problem replication.

Run it hard in a safe condition. More heat is generated at higher RPM.

The reported problem was the boat would be unable to attain normal RPM. If the problem goes to a different condition, preventing operation, then fall back to the subscribed towing service, if required.

It will be disappointing if the problem resolution cannot be resolved for a year.

The FSM has lots of information in the Fault Chart section that may be helpful. Many of those would show up as fault codes on the DDT. Here is an example for Guardian Strategy faults from that table: "Block pressure, map sensor, oil pressure, port and starboard exhaust temperature, engine coolant temperature or overspeed readings are out of normal ranges"

Yes I made sure he runs it in afternoon today when it’s 85+ and they will try higher Rpms (have to stay in 3000 range though). I don’t think there’s any threat of it stopping (they’ve run it for hours) it just slows down as if the computer thinks this is what you want.

Does the diagnostic tool show what it’s actually recording though even when there’s not an issue? Can you see what the temp is even though it’s within normal conditions? Can it see the history of results?

Because to me if the ECM want working it wouldn’t be recording that info. I would think there would be more troubleshooting that they or Mercruiser could do to determine if it’s ECM. It’s a $1,700 part only available through OBD diagnostics and they don’t offer return on ECMs.

Thanks

Scott

[wingless]

Yes, the DDT permits viewing / controlling live, real-time data, plus viewing / clearing any stored faults.

Why stay at 3,000 rpm? Doesn't the engine permit max rpm to 4,600 rpm?

Another thought, maybe the fuel level is dropping, decreasing the total mass, making lower load being pushed by the engines. What about topping off the fuel level and the water storage, so the boat is pushing the maximum mass?

[Scottd21]

The engine can run to 4500 (I think that’s the recommended limit) but it does not occur in the 4000s! That’s the mystery in that it only occurs in 3000 range which is the ideal cruising speed (the sweet spot/most efficient) for the engine. 2500 and 4000 range should run fine indefinitely.

That’s a good idea about the mass -
That could be why it did not occur also. I should know today because he filled it with 55 gallons and is testing it again.

[Scottd21]

Originally Posted by wingless

Yes, the DDT permits viewing / controlling live, real-time data, plus viewing / clearing any stored faults.

Why stay at 3,000 rpm? Doesn't the engine permit max rpm to 4,600 rpm?

Another thought, maybe the fuel level is dropping, decreasing the total mass, making lower load being pushed by the engines. What about topping off the fuel level and the water storage, so the boat is pushing the maximum mass?

Mechanic took it out yesterday and when it occurred he sprayed ECM he said it did nothing. Today he took it for a run and when he opened engine hatch it cleared it again so it’s purely thermal/ventilation. He said there is an ignition control that is inside the distributor box (he sprayed everything Down with freeze spray but he said that’s built inside box so he’s going to try replacing that as a last resort). If that doesn’t fix it he believes that I would need to install a vent to vent the engine, right at bottom of the hatch.
I don’t know how difficult that would be to do.

[wingless]

Is the hit-or-miss mechanic doing better than you can do? Is he charging hourly to run your boat for this effort?

The reported effort / results are at the DIY level, unless I'm missing some "magic".

The mechanic "progress" would make me INSANE (more insane than usual).

The boat needs to be operated under the problem conditions, then those conditions changed to discover when the problem fails. Doing things like changing throttle position is just one example.

Opening the hatch once the problem occurs, to cool the engine so the problem clears, is not very useful. Soaking the engine in the problem condition, so it doesn't clear once the hatch opens would permit using freeze spray on a component to better understand the problem. The systems that could be targeted are: the ECM; the distributor base and the exhaust temperature sensor.

Has it been confirmed that the operational exhaust may be continuously hand-held? Has the raw water impeller been confirmed to be good? Have the raw water passages been back flushed? Has the raw water intake screen / strainer been cleaned?

Did the mechanic operate w/ the DDT connected, or connect afterwards to determine if fault codes existed? If so, what faults were detected. Note the DDT tool has been discontinued although used examples pop up on eBay.

The correct FSM is invaluable when analyzing the problem. As was stated, there are diagnostic tables to pinpoint the possible causes.

All this beats the heck out of the current "shotgun" process.

Your boat should be capable of proper operation w/o extra cooling added. IMO adding the proposed extra cooling modification is just another shotgun repair attempt. I would guess the implementation by this "mechanic" would result in totally unacceptable audible noise in the cockpit. First locate the problem, then determine the solution.

Note, my southern Florida boat now has a custom thermostatically-controlled heat exhaust system that I designed, fabricated and installed to resolve a vapor lock problem that occurred after stopping for an hour, then being unable to restart until the fuel rail cooled. Now my heat exhaust blowers kick on and off, maintaining my engine room temperature, I now never get vapor lock and can always restart my engines. My implementation adds ZERO extra audible noise in the cockpit. This heat soak vapor lock was never a problem when operating in New England.