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Mercruiser 7.4L MPI slows down

S

Scottd21

Member
[FONT=&quot][FONT=&quot]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.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]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.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]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.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]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 spe******t 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.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]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.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]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.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]The fact that it only occurs after a 1/2 hour to an hour sounds to me like something is heating up (even though engine temp is fine) and it sounded too me like a sensor issue but the map sensor was replaced and it’s still occurring.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]Any opinions or suggestions are appreciated.[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]Thanks,[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]Scott[/FONT][/FONT]
 
That marina is getting rich over you!

I'm thinking fuel issues. I suggest monitoring the fuel pressure as you run the boat and see if it changes when the problem develops. There's a Schrader valve fitting in the system where you can attach a pressure gage.

Jeff

PS: That marina should have done that BEFORE replacing half your parts!
 
fastjeff said:
That marina is getting rich over you!

I'm thinking fuel issues. I suggest monitoring the fuel pressure as you run the boat and see if it changes when the problem develops. There's a Schrader valve fitting in the system where you can attach a pressure gage.

Jeff

PS: That marina should have done that BEFORE replacing half your parts!
Click to expand...

Hey Jeff,

Thanks for your reply. I know they hooked up a fuel pressure gauge but the only thing that I am aware of that they used it for was to tweak the fuel pressure on the engine. If they had it hooked up I would think they would have checked it when problem was occurring but I will check with them on it. That would tell them if it’s an issue within the fuel system is that correct? But if they replaced the spark plugs, distributor cap, and rotor like I mentioned it sounds like they don’t know.

They also told me that issue does not occur at 4000 rpm only when running it in 3000s after time. That’s the part that I don’t understand if it were the fuel system wouldn’t it occur all of the time. It occurred when my gas tank was at various levels as well.

Thats the frustrating thing - it’s already up to $1700 with no results and not even knowing where problem is not to mention 2 weeks+ in the summer being gone. At some point wouldn’t it be cheaper to replace all the components of the fuel system? I even found the same engine with 150 hours online for $4,000.
It sounds like it could get to that point the way that it’s going!

Thanks,

Scott
 
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Even tho the tank vent test was negative they replaced the vent anyway? (red flag number one)
Logic says that there's nothing wrong with ignition system so why are they swapping ignition parts? (red flag number two)
If it's running like it's fuel starved then what are they looking at to remedy this?

You don't need another (whole new set of problems) you need a tech that knows which end of the horse gets the bridle.
 
o2batsea said:
Even tho the tank vent test was negative they replaced the vent anyway? (red flag number one)
Logic says that there's nothing wrong with ignition system so why are they swapping ignition parts? (red flag number two)
If it's running like it's fuel starved then what are they looking at to remedy this?

You don't need another (whole new set of problems) you need a tech that knows which end of the horse gets the bridle.
Click to expand...

The first thing they did was they told me to do the tank vent test and it seemed to have helped so they told me that it’s clogged. I dropped the boat off and they replaced the tank vent and ran the boat. They told me one of the mechanics didn’t like that it was hesitating so they asked me if I had the fuel filter changed. It’s a new used boat and where I had it commissioned they only change the fuel filter in the winter so we thought that might be the cause so they replaced that. I believe this is when they ran the fuel pressure monitor and replaced the fuel filter (they also said that they tweaked the engine fuel pressure) and tested the boat and the other mechanic did not notice it hesitating and told me you’re all set. I picked up the boat and it ran great for an hour and then it slowed down again so I dropped it off again. This is when they ran it again with the computer connected and took a recording. They looked at the recording which led them to believe there was an issue with the map sensor and said they confirmed that with a Mercruiser tech. They were both wrong because they replaced it and $250 later it’s still not working. It was after all that that they swapped the spark plugs.

I am going to meet with them tomorrow and get a full understanding as to what they know and what they I are going to try to do to resolve it besides guessing. I would think booking up a fuel pressure gauge and diagnostic tool would narrow it down wouldn’t it?
 
I'd also put an eyeball on the stepper motor. If it is malfunctioning you get all sorts of bad juju. You can buy a Mercruiser one for $140 or get one for a chevy truck for $45. Both are the saame Delphi unit.

It is also known as the Idle Air Controller which controls the air entering the engine upon deceleration and at idle when the throttle plate is closed.
 
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