Mercruiser 7.4L MPI slowing down
07-26-2019, 10:46 PM
#1
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 seemed 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
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 seemed 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
Last edited by Scottd21; 07-26-2019 at 10:57 PM.
07-27-2019, 12:31 AM
#2
Charter Member # 55
Charter Member
My bet is your fuel pickup in your tank has a crack and is sucking air or there is a chunk of debris that moves and blocks the pick up tube.
You need to put a fuel pressure guage on it and see what happens when the symptoms repeat.
You need to put a fuel pressure guage on it and see what happens when the symptoms repeat.
07-27-2019, 01:11 AM
#3
Hey Griff,
The mechanic did tell me that they hooked up a fuel pressure gauge but I believe they used that to tweak the fuel pressure of the engine. So if they put a fuel pressure gauge on it and monitor it when it happens that would narrow it down to one of those things that you mentioned?
The only thing that I don’t understand is if its a physical issue in the fuel system why wouldn’t it happen all of the time when running at 3000 rpm, why after an hour and then happens for rest of trip until the next trip again.
Thanks,
Scott
The mechanic did tell me that they hooked up a fuel pressure gauge but I believe they used that to tweak the fuel pressure of the engine. So if they put a fuel pressure gauge on it and monitor it when it happens that would narrow it down to one of those things that you mentioned?
The only thing that I don’t understand is if its a physical issue in the fuel system why wouldn’t it happen all of the time when running at 3000 rpm, why after an hour and then happens for rest of trip until the next trip again.
Thanks,
Scott
07-27-2019, 01:32 AM
#4
Charter Member # 55
Charter Member
How full was the tank each time??? If it was full and the crack was below the tank level, it wouldn't suck air till the fuel level dropped to certain point.
If a chunk of debris is moving around it would get sucked toward the pick up.
It could also be fuel pump on its way out. I believe you have a mechanical pump and an electric pump.
Could also be fuel line collapsing when it gets warm.
The boat needs to be ran in the water and under load with a fuel pressure guage attached. It needs to be scanned at the same time.
Doing this should help narrow down the issue.
If a chunk of debris is moving around it would get sucked toward the pick up.
It could also be fuel pump on its way out. I believe you have a mechanical pump and an electric pump.
Could also be fuel line collapsing when it gets warm.
The boat needs to be ran in the water and under load with a fuel pressure guage attached. It needs to be scanned at the same time.
Doing this should help narrow down the issue.
07-27-2019, 01:46 AM
#5
Well it’s been occurring for a while and because my fuel tank holds 75 gallons and I generally travel within the bay, I will usually add 30-40 gallons at a time and it’s generally between 1/2 and 1/4 full. I will ask the mechanic to monitor it when this issue is happening if he hasn’t already. So if there a loss of fuel pressure when the speed drops then they should look further into the fuel system and what if fuel pressure is okay then? What would you recommend then? I appreciate your suggestions.
Scott
Scott
07-27-2019, 06:20 AM
#6
Registered
You are separating the systems to speed diagnoses. If fuel pressure holds when problem occurs then you can move on to engine controls. ( I suppose could still be injectors but doesn't sound like injector problem to me) Like Griff said have fuel gauge and scan tool on when problem occurs then you hopefully know which system to chase.. By the way I think you have 2 temp sending units, one for the gauge and one for the ECM. Gauge could show OK but ECM think its overtempt and go into derate.
07-27-2019, 06:38 AM
#7
Registered
My opinion, you should be paying for repairs that the marina is GUESSING. They replace a part and if it doesn't fix the issue then remove it, and U should only pay for the labor... or maybe need to find a more knowledgeable shop? I like the idea of fuel pickup/tank issue... don't most tanks have a way to remove the pickup and check the screen which maybe there? I'm by no means an expert so just tossing out ideas and opinions.
07-27-2019, 11:13 AM
#8
You are separating the systems to speed diagnoses. If fuel pressure holds when problem occurs then you can move on to engine controls. ( I suppose could still be injectors but doesn't sound like injector problem to me) Like Griff said have fuel gauge and scan tool on when problem occurs then you hopefully know which system to chase.. By the way I think you have 2 temp sending units, one for the gauge and one for the ECM. Gauge could show OK but ECM think its overtempt and go into derate.
07-27-2019, 11:15 AM
#9
My opinion, you should be paying for repairs that the marina is GUESSING. They replace a part and if it doesn't fix the issue then remove it, and U should only pay for the labor... or maybe need to find a more knowledgeable shop? I like the idea of fuel pickup/tank issue... don't most tanks have a way to remove the pickup and check the screen which maybe there? I'm by no means an expert so just tossing out ideas and opinions.
Ive done some research reading various posts and info online and the pickup and fuel vent screen were mentioned also so maybe that’s what they should check. They did replace the fuel tank vent so not sure if they replaced that in process but I will find out.
07-27-2019, 11:47 AM
#10
Charter Member # 55
Charter Member
Well it’s been occurring for a while and because my fuel tank holds 75 gallons and I generally travel within the bay, I will usually add 30-40 gallons at a time and it’s generally between 1/2 and 1/4 full. I will ask the mechanic to monitor it when this issue is happening if he hasn’t already. So if there a loss of fuel pressure when the speed drops then they should look further into the fuel system and what if fuel pressure is okay then? What would you recommend then? I appreciate your suggestions.
Scott
Scott
You are separating the systems to speed diagnoses. If fuel pressure holds when problem occurs then you can move on to engine controls. ( I suppose could still be injectors but doesn't sound like injector problem to me) Like Griff said have fuel gauge and scan tool on when problem occurs then you hopefully know which system to chase.. By the way I think you have 2 temp sending units, one for the gauge and one for the ECM. Gauge could show OK but ECM think its overtempt and go into derate.
