Mercruiser 7.4L MPI slowing down

[Scottd21]

Originally Posted by wingless

Good update w/ lots of good information!

The TPS is "easy" to test on a bench and is reported by the DDT.

What about operation w/ the cover off, to determine if it is a thermal issue or not? What about holding a hand on the operational exhaust to determine if that is good or not?

Hey wingless,

That’s good to know about the TPS. Today I just met with the mechanic and they have the boat at the marina for now so I’m letting them troubleshoot it. I’m going to give them another shot now that I have a better understanding of what they did.

I’m trying to understand wouldn’t the computer throw a code if it was a thermal issue and doesn’t it record exhaust temperature as well? Also since the fuel pressure is good would you say that rules out a fuel system issue? I guess they want to check it anyway with a fuel tank. What else could it possibly be some type of electrical issue?

I’m really wondering too if shutting it off would temporarily fix it. It wouldn’t be the end of the world to run it for an hour and the shut the motor off for a minute completely clearing ECU memory and then it might be fine. Is this all that’s happening when I start boat each time I go out?

[wingless]

Originally Posted by Sdools

It’s a 454 Mag with Bravo III stern drive.

Mercruiser Engine HP: 310 (some online materials suggested that this has a higher HP, not sure which is correct)
Engine Model: 7.4L MPI
Hours: 520
Engine Serial Number: 0M010279

Originally Posted by Scottd21

They also mentioned the possibility of a TPS (throttle position sensor) issue, but that costs $1,400 to replace so they are going to test with portable tank and boat-in-a-box first.

Originally Posted by wingless

The TPS is "easy" to test on a bench and is reported by the DDT.

This site shows all the parts for your engine.

This is the Mercruiser 8M0097035 TPS Sensor for your engine. The list cost is $89.17.

As was stated, this part is "easy" to bench test w/ FSM information and a multimeter.

Originally Posted by Scottd21

I assume replacing the ECM would be costly too (if the part is even available).

Both the re-manufactured Mercruiser 861736R1 ECM and the Mercruiser 861720T2 ECM are no longer available.

[wingless]

Originally Posted by Scottd21

I’m trying to understand wouldn’t the computer throw a code if it was a thermal issue and doesn’t it record exhaust temperature as well?

The reported problem is: "After about an hour when I’m cruising all of a sudden the boat slows down". That indicates a thermal problem.

Operation w/ the cover removed, for more than an hour, will confirm a thermal issue or discount that theory.

If the boat runs great for two hours, then the cover is replaced and the problem returns, thermal issue confirmed.

If the problem appears w/ the cover removed then thermal problem is a stretch.

Originally Posted by Scottd21

Also since the fuel pressure is good would you say that rules out a fuel system issue? I guess they want to check it anyway with a fuel tank.

They operated the boat for an hour to observe the max RPM decrease while measuring the unchanged before / during fuel pressure?

Originally Posted by Scottd21

I’m really wondering too if shutting it off would temporarily fix it. It wouldn’t be the end of the world to run it for an hour and the shut the motor off for a minute completely clearing ECU memory and then it might be fine.

This is very unlikely, but easy to test, assuming it the conditions are safe for a temporary engine shutdown.


Again, verify the exhaust may be continuously hand held while the operating engine is up to temperature.

[Scottd21]

Originally Posted by wingless

This site shows all the parts for your engine.

This is the Mercruiser 8M0097035 TPS Sensor for your engine. The list cost is $89.17
.

Hi I did tell him that I found the part on Amazon but he said that is not the correct part. Unfortunately my part is part of the throttle body assembly. The B3 in mine was added later so I don’t know if my boat being customized created a discrepancy in that parts list.

Part number 802622a1 is the whole throttle assembly and is no longer available from Mercury but he said there could be a used or aftermarket one available.

Also yes he did observe the fuel pressure gauge when the issue manifested itself (there were 2 mechanics in the boat).

I will ask them about the cover (Ive never taken it off). I’ve always run with the hatch down so could always try it with hatch open too. I didn’t realize it would make that much of a difference.

Wow so I guess I’m out of luck if the ECM needs replacing unless it’s available in after market parts.

[Scottd21]

Originally Posted by wingless

The TPS is "easy" to test on a bench and is reported by the DDT.

So can the mechanic easily replace the TPS sensor without having to change out the whole throttle body assembly? If so I’m not sure why he gave me the assembly part (we both agreed to do the tank and wiring test first) but it was sort of a side thought so he may have looked it up fast.

[wingless]

Originally Posted by Scottd21

Hi I did tell him that I found the part on Amazon but he said that is not the correct part. Unfortunately my part is part of the throttle body assembly. The B3 in mine was added later so I don’t know if my boat being customized created a discrepancy in that parts list.

Part number 802622a1 is the whole throttle assembly and is no longer available from Mercury but he said there could be a used or aftermarket one available.

The parts listing link I provided is for the engine only, based on the engine serial number.

The throttle position sensor is always screwed onto the throttle body. It "tells" the engine computer how much throttle is applied.

The stern drive has it's own parts listing and is bolted onto the rear of the boat. The stern drive also requires the correct parts during service.

The attached FSM image shows the throttle body on your boat.

The throttle body parts listing lists the throttle position sensor as Not Shown, meaning there is no numbered bubble pointing to a graphical representation of that part, but I think that part is shown attached to the throttle body. It looks to be at the end of the #10 extension line.

Originally Posted by Scottd21

Wow so I guess I’m out of luck if the ECM needs replacing unless it’s available in after market parts.

That replacement part will probably never be required.

[wingless]

Hopefully the faith that continues to be heaped onto these mechanics will bear fruit before the available funds are depleted.

All of the steps I've identified are possible for DIY at no cost.

Hoping for success would make me insane (more insane than usual).

Good luck!

[Scottd21]

Originally Posted by wingless

Hopefully the faith that continues to be heaped onto these mechanics will bear fruit before the available funds are depleted.

All of the steps I've identified are possible for DIY at no cost.

Hoping for success would make me insane (more insane than usual).

Good luck!

I hope so too thanks. I was ready to pull the plug on this but giving it one last shot since I’ve gone this far with it. By the way is it easy to remove the cover in these engines, I’ve always had outboards.

[Scottd21]

Originally Posted by wingless

Hopefully the faith that continues to be heaped onto these mechanics will bear fruit before the available funds are depleted.

All of the steps I've identified are possible for DIY at no cost.

Hoping for success would make me insane (more insane than usual).

Good luck!

I have some news. The mechanic ran it with a portable fuel tank connected and issue still occurred. He than hooked up a boat-in-a-box and the issue DID NOT OCCUR (for first time!). When the boat-in-a-box was hooked up they had to open the engine hatch so the hatch had been opened when it worked so they suspected AS YOU DID that something is heating up.

They went back to the original boat wiring and ran it first with hatch down and waited for issue to occur. When it occurred they opened the hatch and it cleared it! So we have confirmation that it is a thermal issue as you put it.

I did ask them about the exhaust and risers and they ruled that out. They said that its either a coil or the ECM heating up. They are going to replace the coil first and hopefully it’s not the ECM.

I would have done that myself as far as testing but as far as replacing a coil or possibly the ECM (which I believe has to be reprogrammed) I’d rather have the mechanic do until I figure this stuff out on my own.

Now that we know it’s a thermal issue please let me know if you have any other suggestions or ideas.

Thanks,

Scott

[wingless]

Thanks for the update.

That makes perfect sense based on the reported failure mode.

It is possible to narrow the root cause using directed heat and cold.

Careful application w/ a heat gun to induce the problem, followed by freeze spray to eliminate the problem is an alternative to part swapping. If going this route, be VERY careful because it is “easy” to overheat w/ the heat gun.

If it was my boat I would verify that I could hold my hand on the operational exhaust.

[Scottd21]

Originally Posted by wingless

Thanks for the update.

That makes perfect sense based on the reported failure mode.

It is possible to narrow the root cause using directed heat and cold.

Careful application w/ a heat gun to induce the problem, followed by freeze spray to eliminate the problem is an alternative to part swapping. If going this route, be VERY careful because it is “easy” to overheat w/ the heat gun.

If it was my boat I would verify that I could hold my hand on the operational exhaust.

Okay is that the manifold risers on the engine itself that you’d hold your hand on?

Thank you

[wingless]

Yes, manifolds, risers and elbows are all raw water cooled and good parts can be held, bad parts are too hot.

This is just one quick test.

The annual disassembly inspection is also important.

[Scottd21]

This is crazy I know, but replacing the coil did not fix it.

At this point the mechanic thinks the only thing that it could be is the ECM. I did ask about exhaust and ventilation and he said that he ran it again with hatch closed until it occurred but this time when he opened the hatch it did not clear issue - apparently it was pretty warm outside (so if it was the air getting in there it would have cleared it so it appears to be a thermal issue.)

He spoke to Mercruiser also and they said it’s symptomatic of a bad ECM. Mercruiser doesn’t have that available but MEFIburn.com rebuilds them and my mechanic said he spoke to mefiburn and the rep told him to pour cold water over the ECM and he would know right away if that’s the issue. I have heard of using Freeze it on the ECM, but could water just short it out (water and electronics)? It sounds like the mechanic might not be able to test it over the weekend, is this something that I could try myself? I can see what the ECM looks like from the Mercury parts site with my engine SN, but it doesn’t show where ECM is. Is that easy to get to? I have no idea where it is. If I could find it I could test that myself to try and speed things up.

Thanks,

Scott

[wingless]

Wow!

It would be great if these "mechanics" returned all your fine / usable original parts so spares would be on-hand for a real failure.

Yes, this problem isn't rocket science. Before throwing a $1,600 or $1,700 rebuilt part at the boat, plus labor charged at professional rates, grabbing the wheel back is long overdue.

Snag two cans of freeze spray and a bucket of ice water, then go for a ride w the cover off. (Sounds like a broken record...)

Get everything up to operating temperature. Verify ability continuously hand hold exhaust parts. Note ECM attached to exhaust. If this fails, testing done.

Replace cover. When problem returns, don't immediately open cover because cooler ambient air might toggle fault back to normal.

Best case is one person operating boat while second dumps freeze spray onto ECM.

If problem gets fixed, then repeat enough times to rationalize that planned huge expense.

The cold water could work by dumping, but the red hot exhaust parts won't like that thermal shock. I would instead use a towel saturated in ice water to transfer cold thermal mass to a directed location.

[Scottd21]

Originally Posted by wingless

Wow!

It would be great if these "mechanics" returned all your fine / usable original parts so spares would be on-hand for a real failure.

Yes, this problem isn't rocket science. Before throwing a $1,600 or $1,700 rebuilt part at the boat, plus labor charged at professional rates, grabbing the wheel back is long overdue.

Snag two cans of freeze spray and a bucket of ice water, then go for a ride w the cover off. (Sounds like a broken record...)

Get everything up to operating temperature. Verify ability continuously hand hold exhaust parts. Note ECM attached to exhaust. If this fails, testing done.

Replace cover. When problem returns, don't immediately open cover because cooler ambient air might toggle fault back to normal.

Best case is one person operating boat while second dumps freeze spray onto ECM.

If problem gets fixed, then repeat enough times to rationalize that planned huge expense.

The cold water could work by dumping, but the red hot exhaust parts won't like that thermal shock. I would instead use a towel saturated in ice water to transfer cold thermal mass to a directed location.

Do you need special tools to remove engine cover? That’s why I never found the ECM! Haha. Are they machine screws? Also some of those freeze sprays say they cool to -65 is that possibility too cold? Is there a certain freeze spray that you recommend?

Thank you

Scott