Mercruiser 7.4L MPI slowing down

[Scottd21]

Originally Posted by wingless

From the image, it looks like the boat has two blowers, mounted up high, out of the image frame, with the two intake hoses on the port side and the two exhaust hoses poking through the transom on the starboard side.

That’s not my boat. Tired posting picture of mine but this forum won’t let me without a URL. Look at the two hoses on the starboard side - the one furthest to the right looks like it goes all the way down below the engine. That is how all four of my hoses are so there is no hose that’s up higher for the heat like the other three are. They all go below engine.

[wingless]

Originally Posted by Scottd21

That’s not my boat. Tired posting picture of mine but this forum won’t let me without a URL. Look at the two hoses on the starboard side - the one furthest to the right looks like it goes all the way down below the engine. That is how all four of my hoses are so there is no hose that’s up higher for the heat like the other three are. They all go below engine.

Here is the ScottD21 engine compartment image, not the image I found on-line / posted for a similar boat

Note that the image shows what appears to be packing foam, probably added for sound reduction. Assuming that is correct, then the foam is unsafe in that application because of the flammability of the material.

That type of packing foam is one of the main issues w/ the 2003 Station nightclub fire that killed 100.

If correct, then that packing foam should be removed from the boat.

Please provide a better image(s) showing the existing blower system when possible.

Note the flame arrestor (engine air cleaner) appears dirty and should be removed / cleaned / replaced.

[Scottd21]

Originally Posted by wingless

Here is the ScottD21 engine compartment image, not the image I found on-line / posted for a similar boat

Note that the image shows what appears to be packing foam, probably added for sound reduction. Assuming that is correct, then the foam is unsafe in that application because of the flammability of the material.

That type of packing foam is one of the main issues w/ the 2003 Station nightclub fire that killed 100.

If correct, then that packing foam should be removed from the boat.

Please provide a better image(s) showing the existing blower system when possible.

Note the flame arrestor (engine air cleaner) appears dirty and should be removed / cleaned / replaced.

Yes unfortunately the station fire hit very close to home (that was Rhode Island) and a kid I knew growing up was a victim. Interestingly I had thought about adding sound proofing but that’s the first thing that came to mind. Are there any safer alternatives to that I’m surprised it’s there unless the previous owner just added it.

I see about the flame arrestor, the previous owner had a water leak so there are some rusted parts in the front of engine as well but I’m going to have that treated before end of season so could do that then.

I’ll get you a clearer image sorry when I’m down there, it was taken from a video clip but still if all hoses are under the engine it seems like a poor setup for ventilation although safer for bilge fumes

[wingless]

Yes, there are safe / heavy / expensive / flame rated sound proofing sheets and seals available, like those already installed by the factory.

That packing material foam was installed by a PO.

[Scottd21]

Oh yes West Marine sells one for about $130 it’s flame resistant (not fireproof) but that’s good enough. I was thinking that foam that the previous owner installed could also be insulating the engine to a certain degree (not in a good way) as well.

Today the mechanic changed the ignition module.
Think it could be the ICM? I did find some forum posts where people had lack of power after 20 minutes or so and attributed it to the ignition control.

He did not have time to test but I think this about ends my dealing with the mechanic. This is either going to work or I’ll be trying to figure it out on my own.

I still don’t understand about the bilge blowers versus intake and exhaust sorry. I thought the intake hose was bringing fresh air into the engine and exhaust expelling it when the engine is running even with the bilge pump shut off and I thought the bilge was for getting fumes out of the bilge. If so where does the added benefit of having the blower on come from (is it going to blow more hot air out of the compartment) and bring more new air in? What if it’s 90 degrees outside would it still help (there seems to be a relationship between outside temp and how long it takes to appear and how quickly it clears). If running blower helps couldn’t I just leave it on all of the time so it never happens?

Trying to learn about this stuff because if it still occurs it sounds like the solution is just finding a way to keep it cool although the part that is heating up could be defective.

But what is left that could cause loss of power after running fine for half hour and then fixed itself until it stops working again.

Checked:
ECM
Coil
Spark plugs
Fuel tank eliminated
Ignition module
Map sensor

Could it be a failed temp sensor not reporting the correct temp to ECM?

[wingless]

Again, hopefully all the "good" parts that have been swapped out have been retained for potential usage as spares in-case a real failure occurs.

The manufacturer builds the boat w/ a thick / effective layer of high-quality insulation and weatherstripping to control engine noise to an acceptable level. Controlling noise also contains heat, but that is addressed separately, if required.

The engine compartment does not require a forced air inlet, just unrestricted appropriately-sized vents. The manufacturer already provided those vents.

Air is removed from the engine compartment through the engine, during combustion and from the forced air blowers.

The boat is required to have properly-installed and properly-operating blowers to exhaust potential exhaust fumes from below engines, not only for standards but for safety. Those should be verified to exist and be properly intact / operational. The factory installed a blower module on mine that blinks the switch light if a blower fails.

The correct factory service manual (FSM) is a required tool for ownership.

[ross.bendigo]

I don't know if you fixed the problem; it's late here and I've run out of attention span to read all the replies.
My brother in law had some sort of boat that had two of these in back. I think they were reversed and ran through v drives.
He also had a similar problem. One motor would run fine. The other would lose revs and often wouldn't do better than idle.
I did some research and found a mention of clogged fuel cooling elements. I think that was it, it was about four years ago and a sixty-five year old brain only has so much ram left
As I recall, I disconnected the water inlet into the fuel cooler and jammed a plastic pipe up it to clear out any muck in there. Which it did, and the motor worked fine after that.
The Merc mechanic, who was all set to replace everything that could be replaced, ( diagnosis by dollars style ), admitted that the brother in law's mooring spot had a lot of ground mud which may have been sucked in when he idled at the mooring, and this blockage restricted the fuel flow. Perhaps the fuel was too hot and you got the bane of all early Holdens in Australian temperatures ... vapour lock, which of course, comes and goes and is a bugger to diagnose because it's always gone when the motor cools down enough to look at.
I seem to recall that the water supply to the fuel cooler was a white coloured plastic pipe at the bottom of the motor. Front or aft I can't quite recall.
It was a simple fix, once you knew where to look, and I was chuffed to so simply help someone who's always been a help to me.

[ross.bendigo]

PS to my previous post. I just read through what you've done and vapour lock inside the fuel system somewhere still sounds logical. I recall that they needed a fuel cooler because the motor had no airflow in the confined engine bay. Lifting the cover may supply enough airflow on the exterior of the fuel system piping to temporarily clear the vapour lock, even if the cooler was non-functional.
As a pure supposition, the vapour pressure may mimic actual fuel pressure so a pressure gauge would still read correctly but the fuel flow would remind me to get a prostate check
Dunno. Just a thought.
PPS I think that the white pipe was the water outlet from the cooler, not the inlet. Same difference either way.

[wingless]

Welcome to the forum.

If the problem is vapor lock, my forced air heat extractors resolved my unable-to-restart heat soak vapor lock condition.

The Mercruiser solution is this supplemental fuel pump as-detailed in this technical document.

The symptoms detailed in the technical document are very different than those experienced by the OP.

The symptom for the OP is the loss of the ability to maintain cruising RPM.

These are the vapor lock symptoms described in the technical document:


Under certain conditions, engines may experience a ‘vapor lock’ condition. The three most common complaints that vapor locking cause are:

1. The engine starts. When the throttle is advanced, the engine quits running and will not restart.
2. If the engine does restart, it quits when advancing the throttle to get the boat up on plane or to pull up a water skier.
3. After running the boat and shutting the engine off for 1 to 3 hours, the engine does not want to restart.

My experience and understanding of vapor lock is that problem is Boolean, the engine either works or it doesn't, the fuel can be pumped as a liquid and cannot be pumped as a vapor, not loss of full throttle problem.

[wingless]

Originally Posted by wingless

Welcome to the forum.

If the problem is vapor lock, my forced air heat extractors resolved my unable-to-restart heat soak vapor lock condition.

These are the vapor lock symptoms described in the technical document:


Under certain conditions, engines may experience a ‘vapor lock’ condition. The three most common complaints that vapor locking cause are:

1. The engine starts. When the throttle is advanced, the engine quits running and will not restart.
2. If the engine does restart, it quits when advancing the throttle to get the boat up on plane or to pull up a water skier.
3. After running the boat and shutting the engine off for 1 to 3 hours, the engine does not want to restart.

My experience and understanding of vapor lock is that problem is Boolean, the engine either works or it doesn't, the fuel can be pumped as a liquid and cannot be pumped as a vapor, not loss of full throttle problem.

My experience with heat soak vapor lock is that restoration to normal operation takes an extended amount of time.

That makes sense because of the large thermal mass.

The OP had instances where normal operation was restored almost immediately once the engine compartment was exposed to “cool” air.

[Scottd21]

Originally Posted by ross.bendigo

I don't know if you fixed the problem; it's late here and I've run out of attention span to read all the replies.
My brother in law had some sort of boat that had two of these in back. I think they were reversed and ran through v drives.
He also had a similar problem. One motor would run fine. The other would lose revs and often wouldn't do better than idle.
I did some research and found a mention of clogged fuel cooling elements. I think that was it, it was about four years ago and a sixty-five year old brain only has so much ram left
As I recall, I disconnected the water inlet into the fuel cooler and jammed a plastic pipe up it to clear out any muck in there. Which it did, and the motor worked fine after that.
The Merc mechanic, who was all set to replace everything that could be replaced, ( diagnosis by dollars style ), admitted that the brother in law's mooring spot had a lot of ground mud which may have been sucked in when he idled at the mooring, and this blockage restricted the fuel flow. Perhaps the fuel was too hot and you got the bane of all early Holdens in Australian temperatures ... vapour lock, which of course, comes and goes and is a bugger to diagnose because it's always gone when the motor cools down enough to look at.
I seem to recall that the water supply to the fuel cooler was a white coloured plastic pipe at the bottom of the motor. Front or aft I can't quite recall.
It was a simple fix, once you knew where to look, and I was chuffed to so simply help someone who's always been a help to me.

Hey Ross,

I appreciate you taking the time to post this. That’s interesting info and was thinking it might be fuel system initially, but they did hook up a portable fuel tank to rule that out.

Thanks,

Scott

[Scottd21]

Originally Posted by wingless

My experience with heat soak vapor lock is that restoration to normal operation takes an extended amount of time.

That makes sense because of the large thermal mass.

The OP had instances where normal operation was restored almost immediately once the engine compartment was exposed to “cool” air.

I have the boat back and ran it Monday and it ran great. There was a small craft advisory so I couldn’t run it that long at 3100 rpm, but so far have run it 40 minutes without issue. Because it’s getting cooler outside its harder to ensure that it’s fixed 100%, but I’m going to try running it again for as long as possible this weekend.

Last week I found this website called justanswer.com where professionals such as mechanics answer questions and figured I’d give it a shot (certainly much more reasonable than the price of a mechanic) and they said:

“A failing ignition coil would fit that symptom. It's very common for a failing ignition coil to work when cold, fail when hot, and then work when cold again.” He went on showing me how to test it with a volt meter.

My mechanic (who I am done with) replaced a coil and also the ignition module, but this online mechanic referred to part #18 (ignition coil) as the one that is suspect. It shows part as $150 whereas the mechanic only replaced part number #8 which is $32. He did replace the whole module which shows as part #9 ($250), but it appears that he used an after market part since he charged me $100 less for it. I’m having trouble telling from the illustration and was wondering if the ignition coil (#18) is included in the ignition module that was replaced. Otherwise if this other mechanic is right then he replaced the wrong coil - the pick up coil.

Here is the parts site:
https://www.mercruiserparts.com/bam/.../31918/2705/80

Thanks,

Scott

[wingless]

The weather forecast is warmer again on Monday and Tuesday.

If the coil is suspect, then why not heat it carefully / safely to determine if this recreates the problem?

Were the original parts retained during the part replacement cycle? Those used parts can be helpful in the future. I have many used parts that are still good on-board for spare part usage if ever required.

[Scottd21]

Originally Posted by wingless

The weather forecast is warmer again on Monday and Tuesday.

If the coil is suspect, then why not heat it carefully / safely to determine if this recreates the problem?

Were the original parts retained during the part replacement cycle? Those used parts can be helpful in the future. I have many used parts that are still good on-board for spare part usage if ever required.

Well the coil supposedly was in the distributor so I don’t know about getting to it - I don’t want to make anything worse. No unfortunately he did not leave me with replaced parts.

Can you hook up a laptop to engine instead of needing to use the DDT tool which is hard to find? I suppose I would need a special adapter but can’t seem to find a PCM555 to USB but I am showing software out there that can read it. If so I could look at the file and see what it shows.

Scott

[Scottd21]

Originally Posted by wingless

The weather forecast is warmer again on Monday and Tuesday.

If the coil is suspect, then why not heat it carefully / safely to determine if this recreates the problem?

Were the original parts retained during the part replacement cycle? Those used parts can be helpful in the future. I have many used parts that are still good on-board for spare part usage if ever required.

Hey there,

Sorry I’ve been meaning to update. The mechanic had ruled out the ECM because when it occurred it didn’t do anything when he cooled it off and he replaced the ignition coil in the distributor box (hoping it’s right one because there are apparently two different ones)

The problem has not occurred since the mechanic did his last repair. The difficulty is that the temperature has cooled off and often it was too rough to run it at 3100 rpm for that long.

As was suggested I did put my hand on the engine parts - risers and manifold and they are warm but not hot.

I may have to wait until next year to know for sure that it’s resolved but it sounds and runs great and I did have it at 3100 for about 45 minutes a few weeks ago.

The mechanic said that if this didn’t fix it then it’s a ventilation issue but I would think that Sea Ray would have accounted for that in their design. If it were that perhaps I could install some type of cooling fan.

thanks

Scott