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Old 26-07-2019, 21:13   #1
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Mercruiser 7.4L MPI slowing down

Hi last season I bought a 2001 Sea Ray bowrider boat and from the very beginning I have been having an issue. I run the boat for about an hour between 25-32 mph upwards to 3100 rpm and it runs beautifully. The engine sounds great and the temperature is good right at 170. After about an hour when I’m cruising all of a sudden the boat slows down to about 20 mph and then about 18 mph. When this happens I have tried to use the throttle and even almost to full throttle the boat is only running at 2500 rpm and going a max of 18 mph. This happens the whole ride back to the dock and even though its running only 18 mph it sounds good and engine temp is good.

I dropped this off at a Mercruiser authorized mechanic 2 weeks ago. At first they suspected that it’s the fuel tank vent. They asked me to open the fuel cap and if I hear a swoosh and it clears the issue then thats the issue. I tried that, I did not hear a swoosh of air but it seem to clear the issue temporarily. They told me it was the fuel tank vent and replaced it.

I picked up the boat and ran, it sounded great again and this time it only took about a half hour and it slowed down again. I brought it back to the marina, they were thinking that its the fuel filter. They replaced that but it did not clear it.

Next they hooked up a computer to it and it did a recording. They studied the computer results and believed that the issue was with the map sensor. They said they had a specialist from Mercruiser look at it and they thought the same thing. The map sensor cost $250 to replace and I gave my consent and they replaced it. They took it again for a spin but said unfortunately it did not resolve the issue.

Today they told me that they changed spark plugs, distributor cap, and rotor to eliminate the possibility and tested it again, but the problem still exists. So far we have fuel tank vent, fuel filter, map sensor, spark plugs, distributor cap and rotor replaced but it’s still occurring.

This is starting to become a costly repair and wondering if anyone has any thoughts or ideas as to what could be causing this and would there be a more effective way of troubleshooting this such as replacing the parts that could be related to this. I had an engine compression and oil analysis done last year and it was all good. I also had the boat wintered and engine commissioned. They said the engine compression test came back great and the mechanic who looked at my boat in the spring said he found virtually no issues.

The fact that it only occurs after a 1/2 hour to an hour sounds to me like something is heating up and it sounded too me like a sensor issue but the map sensor was replaced and it’s still occurring.

Any opinions or suggestions are appreciated.

Thanks,

Scott
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Old 27-07-2019, 04:38   #2
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Re: Mercruiser 7.4L MPI slowing down

Welcome to the forum.

Which 2001 Sea Ray bowrider model has the 7.4L Mercruiser MPI? Is this the Horizon engine? I was unable to locate the model w/ the 7.4L engine.

My 2000 380DA w/ 454 MAG MPI Horizon engines had zero heat issues when operated in New England, but I always had heat soak vapor lock when operated in southern Florida bath water, in my case preventing restart after shutting down for an hour.

My solution was to add additional high-volume thermostatically controlled marine blowers, sucking from near the fuel rail, to automatically exhaust heat and to automatically maintain the engine compartment temperature.

There have been zero restart issues on my boat after implementing this change.

Another option is adding the Mercruiser 862264A7 supplemental fuel pump kit, shown here.

https://www.amazon.com/dp/B005IGSW0G...v_ov_lig_dp_it

People with this kit also report complete success.

There are lots of other essential maintenance items required for continued proper operation on my engines, but please first provide the above better definition of your boat.
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Old 27-07-2019, 04:57   #3
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Re: Mercruiser 7.4L MPI slowing down

Here is the technical document supporting usage of the Mercruiser 862264A7 supplemental fuel pump kit.


http://www.engineprofessional.com/TB/TB041916-1.pdf
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Old 27-07-2019, 07:49   #4
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Re: Mercruiser 7.4L MPI slowing down

Hey wingless,

Thanks for your reply. I have a 2001 Sea Ray 260 BR.
Here is a link to a spec sheet on it:

https://www.searay.com/boat_graphics...731/8501_f.pdf

I just want to clarify I am not having any issues with engine temperature (stays at 170) nor am I having trouble starting or it stopping. The only issue that I am having is that when running in the 3000 Rpm for about an hour it slows to 2500 and won’t go any faster once it does that. Also the mechanic said that it doesn’t happen at 4000 rpm only at 3000 as well.

Thanks

Scott
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Old 27-07-2019, 08:11   #5
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Re: Mercruiser 7.4L MPI slowing down

Hey wingless,

Thanks for your reply. It’s a 2001 Sea Ray bowrider 260 signature series. Here is a link to spec sheet:

https://www.searay.com/boat_graphics...731/8501_f.pdf

I just want to clarify that I am not having trouble with engine overheating. The engine remains right at 170 even when issue is occurring. I am not having any issues with starting it or it stopping either. It’s only after running it an hour or so at 3000 rpm that it slows to 2500 and won’t go faster once it does that. Also the mechanic told me that it does not occur at 4000 rpm so if I were to completely floor it apparently it wouldn’t happen. That’s why it makes me think it’s some type of sensor or computer thing but I’m lost because we tried replacing sensor and it’s still occurring.

Thanks,

Scott
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Old 27-07-2019, 12:29   #6
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Re: Mercruiser 7.4L MPI slowing down

Scott,

Thanks for the specification sheet link, that was very helpful.

Our boats have very similar engines, mine w/ two vee drive 454 MAG MPI Horizon engines, yours w/ a single Bravo I 7.4L MPI stern drive.



What part of the country is the boat being operated? What are the water / air temperatures?

Has operation w/ the engine cover removed been tried? Any effect?

Has operation w/ the bilge blowers running continuously been tried? Any effect?

Has a thermocouple been placed near the engine to monitor the engine compartment temperature when all is good and when the problem exists? What are the results?

For maintaining my boat I use my Quicksilver DDT. It is very handy for interrogating the engines.





The other handy tool is a fuel pressure gauge. If that can be carefully secured to the fuel rail before, then during the problem, the fuel pressure reading may be indicative of the problem source. A fuel pressure gauge is an essential tool.

My assumption is that the flame arrestor is clean, as is the intake plenum and throttle body. Mine is disassembled for an annual cleaning to keep everything happy.

Also assuming raw water cooled exhaust. Is the boat operated in salt water? If so, have the parts been unbolted, gaskets broken open, for internal passage inspection? The raw water passages must be clean and there MUST be sufficient SOLID wall thickness between the raw water and exhaust gas passages.

On a salt water boat the internal inspection must be performed annually.
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Old 27-07-2019, 14:01   #7
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Re: Mercruiser 7.4L MPI slowing down

Quote:
Originally Posted by wingless View Post
Also assuming raw water cooled exhaust. Is the boat operated in salt water? If so, have the parts been unbolted, gaskets broken open, for internal passage inspection? The raw water passages must be clean and there MUST be sufficient SOLID wall thickness between the raw water and exhaust gas passages.

On a salt water boat the internal inspection must be performed annually.
If the exhaust is acceptable, then it should be possible to continuously hold the exterior of the manifold, riser and elbow while the engine is running at operating temperature.

If those parts are too hot to hold then the interior is likely unacceptable, or there is insufficient raw water flow.
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Old 27-07-2019, 21:22   #8
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Re: Mercruiser 7.4L MPI slowing down

Hey wingless.

That’s good to know. I actually have it with the Bravo 3 stern drive.
It’s being operated in Rhode Island. Specifically Narragansett Bay which is brackish/salt water. Water temp: 75 avg Air Temp: 86 avg

I did not try to remove cover since temp gauge stayed at 170. I did not try running blower. I don’t think mechanic hooked up a thermocouple.

That’s interesting about Quicksilver, where does that hookup to your engine and what does that monitor? In your opinion does the computer give all the information needed that would be able to determine where the issue is at? I mean if the throttle is almost fully open and the RPM is at 2500 the computer must be showing something, it’s either not realizing that the throttle is down or its trying and something else is slowing it down.

About the fuel pressure gauge the mechanic did mention that he hooked that up and also tweaked the fuel pressure on the engine to his liking. It was after running the fuel pressure gauge that he recommended changing the fuel filter.

About the flame arrestor, the engine looks great and sounds great. The compression test came back excellent and I had it commissioned where they change the oil and filter in the winter and spring. It has about 520 hours on it.

In regards to the cooling system interestingly the previous owner spent about $4,000 on a new cooling system because there was a water leak and supposedly anti freeze was getting into bilge and boat was overheating. The same marina that is doing this work also installed the cooling system. I have not had any issues with overheating.

Thanks for your help. As far as obtaining computer information what type of standard does our engine use to connect its computer to a device like yours because I believe our engine was prior to NMEA2000.
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Old 28-07-2019, 05:46   #9
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Re: Mercruiser 7.4L MPI slowing down

The Quicksilver DDT connects to the Mercruiser proprietary PCM555 / ECM 555 diagnostic bus connector, NOT NMEA2000.

This FSM flow chart shows the available DDT functionality.





The steps I've identified are correct for problem resolution. There is a thermal problem interfering w/ normal operation. It takes a while for the temperature to rise, causing the problem and a while to abate. The engine coolant temperature is different than the temperature causing the problem. Great the engine coolant is not overheating, this is a different problem.

The options are to continue w/ paying the mechanic for shotgun fixes at the problem w/ the hope that he wings that bird, or to follow normal diagnostic procedures and "slay that beast".

Start w/ getting the correct factory service manual for the engine. The FSM has invaluable information and is a required tool. Also a fuel pressure gauge is required for resolving problems w/ fuel injected engines.

  1. Thanks for informing about Narragansett Bay operation, 75°F water / 86°F air temperatures, great location. Those are warm but not not hot. This is a great boating location!

  2. Operation w/ the engine cover removed is required to determine if that insulated box is containing too much heat, preventing proper operation. If so, that invaluable information is then used to identify why there is too much heat, not to always operate w/o the box.

  3. Operation w/ the bilge blowers continuously on is a quick / easy step to see if it resolves the problem. It probably won't, because the heat is up high and the blowers are required to suck from low, to get possible heavier-than-air gasoline fumes. It could help in an all-systems-normal, but just over the threshold for proper operation, situation.

  4. A thermocouple thermometer is a very handy tool, in this case to dangle near the top of the engine in the closed box to measure engine compartment air temperature. At some point that "thermos" temperature rises high enough to cause the problem.

  5. GET a fuel pressure gauge! It is invaluable. The engine cannot run properly w/o sufficient fuel pressure. The engine will act as-described w/ insufficient fuel pressure. LOTS of guessing w/o making that required measurement. It is unlikely that the problem will be resolved w/o using a fuel pressure gauge as-described here.

  6. Bring the engine up to temperature and verify that it is possible to hold your hand on all the exhaust parts while the engine is running. If that is impossible then that must be immediately resolved! Either there is insufficient raw water flow or the exhaust has deteriorated and must be replaced. Either condition requires immediate resolution!

  7. The boat is operated in salt water. Get some riser gaskets and unbolt the elbow annually. This annual inspection is required. The exhaust parts will last for 3-4 years, then must be discarded and replaced. Scrape off the old gasket material. Inspect the raw water passages for excessive corrosion. Inspect the cast iron for sufficient SOLID wall thickness between raw water and exhaust gas.

  8. It is great the engine looks great. Plan on annual plenum removal for internal cleaning, throttle body cleaning and flame arrestor cleaning. It is impossible to maintain proper operation w/o following these steps.
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Old 28-07-2019, 10:30   #10
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Re: Mercruiser 7.4L MPI slowing down

Some thing as simple as a weak fuel line , gets bit hot , weak under heat causing fuel restriction? , An object within fuel tank moves around getting sucked near too fuel pipe ??
Lots of responses and not re- reading possibly this was mentioned
Pumps external electric with high & low fuel cell pumps ,
While back on my Volvo Penta gas 4.3 engines they have electric carter fuel pumps cells High & Low , ( don't know if Mercruiser has similar ) .
Would work fine then after 1/2 hr use power would drop from 2500 rpms boat on plane , too 1800 rpms , Boat could not get out of its own way , Then would open up , Turned out too be bad pumps ,
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Old 28-07-2019, 10:42   #11
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Re: Mercruiser 7.4L MPI slowing down

Bad pumps or a restricted inlet would show up as poor fuel pressure during the problem, contrasted to normal fuel pressure while operating properly.

Again leading to the fuel pressure gauge being an essential tool for an injected engine.
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Old 28-07-2019, 11:29   #12
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Re: Mercruiser 7.4L MPI slowing down

Some cruisers have a check valve in line between the tank and motor. Often located near the tank. Some setups/regulations require it depending on tank/motor levels. When I worked at the boat shop it sometimes caused problems on certain boats. Often on boats with large OB motors and especially ETEC Evinrudes.
Just a wild thought here for you to think about.
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Old 29-07-2019, 13:01   #13
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Re: Mercruiser 7.4L MPI slowing down

Quote:
Originally Posted by wingless View Post
This FSM flow chart shows the available DDT functionality.
I met with the mechanics today and spoke to them for a while. First of all they told me the first things that they did was clean the fuel pickup and anti siphon valve (they did clear some gonk out of it) but it didn’t fix issue.

They did hook up a fuel pressure gauge and do a measurement when the issue manifested and found there was no fuel pressure change. They also said the computer recording did not throw any code and everything looked normal thats why they were having difficulty identifying. They said when they’re running it at 4000 RPM the issue did not occur (it’s only occurring at about 3000 to 3500 RPM which is the sweet spot for the boat). They did suggest (before I did) hooking up a fuel tank and they also have what they call a boat-in-a-box which lets them hookup to the boats wiring to see if there’s an electrical issue. 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.

I assume replacing the ECM would be costly too (if the part is even available). One thing I never thought to try was to turn engine off for a minute when it does occur. If it is related to the computer but its fixing itself every time I take the boat out perhaps powering it down and letting electricity dissipate out of it would help.

Thank you,

Scott
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Old 29-07-2019, 13:30   #14
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Re: Mercruiser 7.4L MPI slowing down

Yeah, I forgot those have computer control modules now. By pass it! Oh never mind.. I didn't say that.
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Old 29-07-2019, 13:34   #15
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Re: Mercruiser 7.4L MPI slowing down

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?
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