Mercruiser 7.4L MPI slowing down

[Scottd21]

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 Forum: General Q & APost New Thread
Logged in as Scottd21

 

[Bt Doctur]

what is the fuel pressure at the rail when this happens?

 

[Scottd21]

The mechanics told me that they hooked a fuel pressure gauge to it. The only thing that they told me about it is that they tweaked the fuel pressure on the engine with that information. I would think they would have that information, wouldn’t that tell us if the problem is within the fuel tank or vent if the fuel pressure is changing then? I will have to check back with them on that. Thanks

 

[tpenfield]

You might want to learn how to diagnose and fix things yourself. The mechanic shop is playing a guessing game and you are paying the price of entry. Ask them to give you a fixed price on the resolution, you are paying for their lack of knowledge.

got a serial number for the engine?

will they give you the data file from the recording? Perhaps some of us can run it on our computer and give an opinion . . .

Mercruiser lists the MAP sensor at over $250, but the GM equivalent part is under $100. I always try to find the GM part number off of the part(s) and source them that way whenever possible. I think I replaced my MAP sensor for $15

 

[wingless]

Welcome to the forum.

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?

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.

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.

 

[Scottd21]

You might want to learn how to diagnose and fix things yourself. The mechanic shop is playing a guessing game and you are paying the price of entry. Ask them to give you a fixed price on the resolution, you are paying for their lack of knowledge.

got a serial number for the engine?

will they give you the data file from the recording? Perhaps some of us can run it on our computer and give an opinion . . .

Mercruiser lists the MAP sensor at over $250, but the GM equivalent part is under $100. I always try to find the GM part number off of the part(s) and source them that way whenever possible. I think I replaced my MAP sensor for $15

I agree I was planning on doing that but wasn’t planning on needing to know how to this soon (first year with boat). I also recently purchased “Boatowners Practical and Mechanical Manual” but there’s a lot to learn. I’ve been around boats my whole life but this is my first boat that I have solely owned and with this type of engine. Seems like things have changed a lot going from carburetor to fuel injected and all of this computer data, they are more sophisticated whereas a problem like this in the past could probably have been fixed pretty quickly and easily. Asking for a fixed price sounds like a good idea and they did tell me that they will figure it out and not one has beat them yet so if they are confident that they can fix it they should agree on that. I can find out about the data file when I see them on Monday.

I was able to locate an engine serial number: 0M010279 and this is a link to Mercruiser manuals for this serial number:
https://www.perfprotech.com/mercury-...g/umodel/22376

Thanks of your advice!