Mercruiser 7.4L MPI slowing down

[BruceS]

Well after reading most of this I still question if you really know exactly where the manifolds, risers & exhaust is located?
It definitely isn't those rubber hoses in front of the motor!!

[Scottd21]

Originally Posted by BruceS

Well after reading most of this I still question if you really know exactly where the manifolds, risers & exhaust is located?
It definitely isn't those rubber hoses in front of the motor!!

I did feel those hoses also but I believe the manifold and risers are on left and right side towards the middle (attaching photos). I had no clue where they were but I YouTubed a video showing them being installed.

I still haven’t been able to find the ECM though. I am told that I have to remove the engine cover, but my mechanic said that my engine doesn’t have a real cover. In the last picture I see two wires. I assume the one on the left is the PCM connection where I can connect a diagnostic device, but I have no idea what the middle cable is and why it’s just sitting there in the middle.

Thanks,

Scott

[wingless]

Originally Posted by Scottd21

In the last picture I see two wires. I assume the one on the left is the PCM connection where I can connect a diagnostic device, but I have no idea what the middle cable is and why it’s just sitting there in the middle.

The image shows the flame arrestor is dirty and should be cleaned. On mine, I just remove the part and use garden hose water and soapy cleaner, like Simple Green, then shake dry and air dry before replacement. It is restored to shiny new / clean appearance.

My intake plenum is removed and cleaned at that time, same also for the throttle body and all the vacuum hoses. On my engine, this was the undocumented hidden trick to ensure continuous proper operation. I find that performing this maintenance on an annual interval works great. Prior to determining this service I was struggling to maintain proper operation.

That detached loose connector looks like the IAC connector. If correct, then it MUST be plugged into the IAC or the engine cannot idle properly. The position and length also show that it could plug into the IAC.

[wingless]

Originally Posted by wingless

The image shows the flame arrestor is dirty and should be cleaned. On mine, I just remove the part and use garden hose water and soapy cleaner, like Simple Green, then shake dry and air dry before replacement. It is restored to shiny new / clean appearance.

My intake plenum is removed and cleaned at that time, same also for the throttle body and all the vacuum hoses. On my engine, this was the undocumented hidden trick to ensure continuous proper operation. I find that performing this maintenance on an annual interval works great. Prior to determining this service I was struggling to maintain proper operation.

Here is an example of an owner struggling to determine the problems resulting from failing to maintain a clean intake plenum, then FINALLY discovering the problem / solution. This is the reason I remove / clean / inspect / replace my plenum on an annual maintenance.

My engines perform properly since I discovered this undocumented service.

[Scottd21]

Hey I thought I had updated this thread but things got hectic towards the end of the year. I appreciate all of your help on this. As we're getting closer to the spring and the boat is out of the water Im wondering if there is anything I should look into since I'm still concerned.

I got my boat back at end of August and the issue never occurred however I am not 100% confident that it is resolved. Since the issue occurred after running a half hour to an hour above 3100 RPM as it got later in the season it was harder to maintain that speed as I had family out in the boat (who were dying to go out all in summer - it was out for 3 weeks) and it was getting rough and sometimes wet out there.

The mechanic had last done a test on ECM by spraying freeze spray on it when it occurred and he said it did nothing. The last thing he said was that it could be a coil inside the distributor box and he replaced that. He stated if it occurs after that he doesn't know what else it could be but I could resolve the issue by installing a vent (I believe in the hatch) and he didn't think the boat was well ventilated.

I would think that Sea Ray would have taken that into consideration when they designed it and it was also suggested (thanks Wingless) that adding a vent would introduce unwanted noise into the cockpit.

So now the boat is out of the water and I have reviewed this thread in its entirety. Again I did not notice any of the manifolds getting too hot but again the issue did not occur at the end of the season and it did get cold.

It had been suggested that running the blower could possibly resolve it. Thats the part that I still don't understand. If I am running the boat with the blower turned off isn't hot air still getting expelled through the hoses and the blower designed to remove gas fumes from the bottom of the engine compartment?

I do like the idea that Wingless had used which was a thermostat controlled 12V fan and I do have a wireless thermometer that I could keep in engine compartment to test out the engine temp as well.

I know that the problem could very well be resolved but I am preparing for the worst but hoping for the best so just seeing what options I could look into if its still a possible issue and maybe there are things that I could check in the spring while the boat is out of the water. The blower does make a squeaky noise when its first turned on, maybe there's some type of ventilation issue?

Thanks,

Scott

[Cheechako]

So... end of season... did you have a bunch of people in the boat? It makes a huge difference in power... I was surprised by that myself.
I dont think ventilation is an issue, if the temperature gauge isn't hot it doesn't matter. Unless you are saying it cant get enough air intake?

It's been too long and too many posts to go back, but you're sure your exhaust manifolds are clogged right? That's common on well used older boats.

[Scottd21]

Originally Posted by Cheechako

So... end of season... did you have a bunch of people in the boat? It makes a huge difference in power... I was surprised by that myself.
I dont think ventilation is an issue, if the temperature gauge isn't hot it doesn't matter. Unless you are saying it cant get enough air intake?

It's been too long and too many posts to go back, but you're sure your exhaust manifolds are clogged right? That's common on well used older boats.

Hi Cheechako,

Well yes I did have more people towards end of the season in the boat but there were a couple of times where there were only a few people. The issue did occur earlier in the season with quite a few people and also with just a couple of people (3 including myself). Are you saying that its more likely to happen with more people? Wouldnt it correspond with the RPM though? I mean I could understand if the rpm increased due to heavier weight load but the issue is still happening at 3100 RPM range.

If it's not a ventilation issue then would you say its just something overheating? When it slows down opening the engine hatch clears it within minutes (except one time it didn't because it was warmer outside).

I am not sure if the exhaust manifolds are clogged (would thin ECM would have thrown an overheat code since it measures that temp) or how I would determine that. Those are the types of things that I'm wondering about. Maybe I should have a mechanic take it apart and clean it in the spring.

Thanks,

Scott

[Cheechako]

Originally Posted by Scottd21

Hi Cheechako,

Well yes I did have more people towards end of the season in the boat but there were a couple of times where there were only a few people. The issue did occur earlier in the season with quite a few people and also with just a couple of people (3 including myself). Are you saying that its more likely to happen with more people? Wouldnt it correspond with the RPM though? I mean I could understand if the rpm increased due to heavier weight load but the issue is still happening at 3100 RPM range.

If it's not a ventilation issue then would you say its just something overheating? When it slows down opening the engine hatch clears it within minutes (except one time it didn't because it was warmer outside).

I am not sure if the exhaust manifolds are clogged (would thin ECM would have thrown an overheat code since it measures that temp) or how I would determine that. Those are the types of things that I'm wondering about. Maybe I should have a mechanic take it apart and clean it in the spring.

Thanks,

Scott

I'm just too far behind on all the details. Maybe if you can put a short synopsis of the problems and trials to solve them, more people will respond. Difficult to catch up at this point.

"When it slows down opening the engine hatch clears it within minutes (except one time it didn't because it was warmer outside).
I am not sure if the exhaust manifolds are clogged (would thin ECM would have thrown an overheat code since it measures that temp) or how I would determine that. "
How many electronic controls are on the boat? is it a newer boat with those? Maybe there is something in the "black box" causing it when ambient heat rises?
On a normal "old school" engine, it doesn't care about much but it's internal operating temp.
Is the engine room vented so it can get air?

My last powerboat was a 24 foot with a 305 GM engine. Two people on board was much different in power than 4 people on board for sure. Surprising.

Does your fuel line from the tank have a backflow check valve? They are required for outboards but some makers just put them in anyway. I ran into an issue once with those not supplying enough fuel to the engine when it warmed up, on an engine with a "black box".

[jimbunyard]

Here's a guess that you won't be able to check until the weather warms, but it might explain why it only happens (happened?) at around 3100 rpm and not faster or slower.

If, as appears to be the case, the engine is raw water cooled, it is at least possible that the temperature sensor is sending an incorrect value to the ECM, because of it either being faulty, or because of some idiosyncrasy in the dynamics of the coolant flow at 3100 rpm, which is causing the computer to send the wrong amount of fuel or timing to the injector or distributor. Because the system thinks it's doing what it's supposed to do, no codes will show.

Just a wild guess though.

[Scottd21]

Originally Posted by Cheechako

I'm just too far behind on all the details. Maybe if you can put a short synopsis of the problems and trials to solve them, more people will respond. Difficult to catch up at this point.

"When it slows down opening the engine hatch clears it within minutes (except one time it didn't because it was warmer outside).
I am not sure if the exhaust manifolds are clogged (would thin ECM would have thrown an overheat code since it measures that temp) or how I would determine that. "
How many electronic controls are on the boat? is it a newer boat with those? Maybe there is something in the "black box" causing it when ambient heat rises?
On a normal "old school" engine, it doesn't care about much but it's internal operating temp.
Is the engine room vented so it can get air?

My last powerboat was a 24 foot with a 305 GM engine. Two people on board was much different in power than 4 people on board for sure. Surprising.

Does your fuel line from the tank have a backflow check valve? They are required for outboards but some makers just put them in anyway. I ran into an issue once with those not supplying enough fuel to the engine when it warmed up, on an engine with a "black box".

I agree that's a great long thread! It's a 2001 Sea Ray bowrider. It has an ECM that can store lots of information such as manifold temperature, guardian alarm (apparently my MEFI version doesn't even have a guardian alarm though), and other types of sensors and warnings. As far as electronics it has the basic gauges (temp, oil etc) at the helm but nothing fancy. I am thinking about adding a Fox Marine gateway device that will allow me to view ECM information on my phone via bluetooth and to my chartplotter not only for this issue but may be helpful in the future.

Yes I will give you a basic summary of events:


Boat slows down after running 30 minutes to an hour at RPM above 3100. When it slows it downs down to 2500 and wont go faster no matter how far throttle is advanced and stays at 2500 all the way home.

Mechanic believes its likely fuel tank vent, clears it out - Issue still occurs
He checks ECM codes as he runs and when it fails it doesn't throw any codes. Checks and adjusts oil pressure to no avail. Believes it may be MAP sensor, replaced it and still having trouble.
He hooks up a portable gas tank - Issue still occurs
He hooks up a boat-in-a-box - basically it lets him bypass boats wiring harness to rule out electrical issues - for first time issue does not occur.

When he was hooking up boat-in-a-box the hatch had to be opened so he suspected a thermal issue. He goes back to original boat wiring and runs it until it happens again and when it does he opens hatch. Within minutes for the first time the issue clears (normally it would be stuck at 2500 for 1/2 hour or more at that point I would be heading home not sure if its going to stall).

He tries replacing a few more parts before he concludes it might be ECM but wants to test it first. Mefiburn company that rebuilds ECMs say that he should run boat and if there's an issue with ECM getting warm if he sprays it with freeze spray that will confirm if its a problem with ECM. He sprays it and it does nothing and tried spraying other parts. At this point he says there's a coil that is inside the distributor box that could be the cause and he replaces the coil. At this point its getting colder and rougher and harder to test but have not noticed issue yet still uncertain.

As far as the engine room being vented there are two vent hoses on port side and 2 on starboard under hatch. Should it be vented in any other way except through the exhaust going out of the stern? He also bypassed the possibility of a fuel issue when he hooked up a portable tank and bypassed all electrical systems with his boat-in-a-box.

So opening the hatch is the only thing that is fixing it and it's not a fuel or electrical issue.

[Scottd21]

Originally Posted by jimbunyard

Here's a guess that you won't be able to check until the weather warms, but it might explain why it only happens (happened?) at around 3100 rpm and not faster or slower.

If, as appears to be the case, the engine is raw water cooled, it is at least possible that the temperature sensor is sending an incorrect value to the ECM, because of it either being faulty, or because of some idiosyncrasy in the dynamics of the coolant flow at 3100 rpm, which is causing the computer to send the wrong amount of fuel or timing to the injector or distributor. Because the system thinks it's doing what it's supposed to do, no codes will show.

Just a wild guess though.

Hey Jim,

They actually replaced the MAP sensor but maybe it could be a temperature sensor. That should show in the ECM logs though if the temperature reported was always constant etc but maybe it was at a range that was not throwing any alarms. I think that's worth looking into, I appreciate the suggestion.

Scott