Loss of power / won't reach WOT

Tin Diesel

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The last couple of times I've been out on our boat, I haven't been able to reach WOT of about 4800 rpm. I've not been able to get over 2300 rpm.

It was suggested it might be a spun hub, but that wasn't it. Although I did have some cracks in the hub - replacing it didn't change anything. That makes sense because a symptom of a spun hub is that as you increase power, the engine will start to rev, but the boat won't go faster (because the hub is spinning). My engine wouldn't rev.

In the photo below of my instrument cluster, I was never able to get the engine over about 2300 with the throttle fully down. GPS speed was about 23.5 mph. I played with engine trim in increments, and could see the speed change in tenths of mph as i feathered up and down - but 23.5 mph was max. This is a Mercruiser 5.0 I/O on a 2007 2575 RL tritoon.

Two years ago, on Lake Travis, with two adults and a half-tank of gas, we hit 42 mph on the instrument speedometer (probably 39-40 on GPS). So I think my prop is the right one (Mirage Plus 15x17p). It was definitely fast enough to make me uncomfortable!

Note: currently the toons are clean, we had 2/3 tank of gas (40 gallons), two adults, two kids, bimini up, fairly calm lake. Engine is smooth running up to half-throttle, which gets me 2000 rpm and 18-19 mph. After that, the more I increase throttle, the more there is a weird kind of sluggishness - engine sounds a bit more labored but nothing happens.

I did some research online, and I found that this problem is common enough that there are multiple threads. One had a good list of possible reasons (see below).

My next two steps are going to be:
  1. Replace the fuel filter. I realize I haven't done that since I bought the boat from my sister-in-law 2.5 years ago. Also, she said they always ran normal (ethanol) fuel in it, and it had sat unused for 3 years after my brother passed (since I bought it, it has only had ethanol-free)
  2. Clean the flame arrestor. This is a $190 part, so I'm hoping I can clean it and not replace it.
If these don't help, I'm going to take it to my mechanic and have the fuel pump checked for adequate pressure (a neighbor with same engine told me the pumps can go bad pretty easily). Will probably do a tune-up at that point, and ask him to check the compression on each cylinder, and see if there is any ethanol-related damage to the fuel system.

Any other comments would be welcome. I thought this might become a good thread for us all in the future.

The checklist below is from another website:
Engine Won't Reach Operating RPM. Check
1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive (clean toons)
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.
Loss of power.jpg
 
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SEMPERFI8387

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Absolutely sure throttle cable is advancing completely when at full throttle. Stupid question, but sometimes the easiest is most overlooked.
 

Renegade34

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From past experience the first think I’d check is to see if you have water in your pontoons. It doesn’t take much water to really slow you down. If possible, put it on a trailer and rock the boat side to side. You’ll hear it sloshing around in there if you have a leak.
 

kaydano

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Or go forward about 3 mph and slam on the brakes. You'll hear sloshing from the drivers seat. Don't ask how I know.
 

SEMPERFI8387

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From past experience the first think I’d check is to see if you have water in your pontoons. It doesn’t take much water to really slow you down. If possible, put it on a trailer and rock the boat side to side. You’ll hear it sloshing around in there if you have a leak.
How would this associate to the motor only hitting 2300 RPM?
 

rodsfields

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The last couple of times I've been out on our boat, I haven't been able to reach WOT of about 4800 rpm. I've not been able to get over 2300 rpm.

It was suggested it might be a spun hub, but that wasn't it. Although I did have some cracks in the hub - replacing it didn't change anything. That makes sense because of symptom of a spun hub is that as you increase power, the engine will start to rev, but the boat won't go faster (because the hub is spinning). My engine wouldn't rev.

In the photo below of my instrument cluster, I was never able to get the engine over about 2300 with the throttle fully down. GPS speed was about 23.5 mph. I played with engine trim in increments, and could see the speed change in tenths of mph as i feathered up and down - but 23.5 mph was max. This is a Mercruiser 5.0 I/O on a 2007 2575 RL tritoon.

Two years ago, on Lake Travis, with two adults and a half-tank of gas, we hit 42 mph on the instrument speedometer (probably 39-40 on GPS). So I think my prop is the right one (Mirage Plus 15x17p). It was definitely fast enough to make me uncomfortable!

Note: currently the toons are clean, we had 2/3 tank of gas (40 gallons), two adults, two kids, bimini up, fairly calm lake. Engine is smooth running up to half-throttle, which gets me 2000 rpm and 18-19 mph. After that, the more I increase throttle, the more there is a weird kind of sluggishness - engine sounds a bit more labored but nothing happens.

I did some research online, and I found that this problem is common enough that there are multiple threads. One had a good list of possible reasons (see below).

My next two steps are going to be:
  1. Replace the fuel filter. I realize I haven't done that since I bought the boat from my sister-in-law 2.5 years ago. Also, she said they always ran normal (ethanol) fuel in it, and it had sat unused for 3 years after my brother passed (since I bought it, it has only had ethanol-free)
  2. Clean the flame arrestor. This is a $190 part, so I'm hoping I can clean it and not replace it.
If these don't help, I'm going to take it to my mechanic and have the fuel pump checked for adequate pressure (a neighbor with same engine told me the pumps can go bad pretty easily). Will probably do a tune-up at that point, and ask him to check the compression on each cylinder, and see if there is any ethanol-related damage to the fuel system.

Any other comments would be welcome. I thought this might become a good thread for us all in the future.

The checklist below is from another website:


View attachment 26830
If there are no alarms from the engine I’d say it was a fuel line issue. If you look on the rear deck you should have a small access hole and it might contain a fuel bulb (mine does anyway) and you can check to see if it collapses when you throttle up. It would mean the fuel system is not venting properly.
 

Tin Diesel

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Good thoughts but she is on a trailer all the time, and I've checked for water.
As Semperfi said, I doubt that would limit the RPM. I'm pretty sure I have a mechanical problem. Just trying to isolate everything I can fix before taking it to the shop.

Thanks Rodsfields, I'll look for a fuel bulb.

Fortunately my wife is perfectly happy if we never go faster than 20, so the frustration is all mine!
 

rbtnt

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Make sure all your spark plug wires are plugged in. Seen same issue caused by fuel filter on boat that was not used very much. As someone said earlier, check the throttle cable to see that it is advancing the throttle all the way.
 
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BigKahuna

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Hey Tin talked to my mechanic about your issue. He worked on my I/O for 5 years. I asked him what he would check first then on down the line. First would be bad gas/water in the gas. Second would be the fuel filters. You have 3 on that engine. Fuel/Water seperator, 1 inline coming from the gas tank, and one coming from the fuel pump. Could be clogged or really dirty. He said he wouldn't be surprised if you have an issue with the throttle position sensor. But if that was the case you would hear 3 beeps when you hit the key and it will leave a code. He ruled out bad fuel pump, blown head gasket, or bad cylinder. If any of those were bad you wouldn't be able to get to 2300 rpms. He asked if it was running rough or missing??? If it is it could be bad plug/s, wires, Cap. If it's idling okay and running smooth its not any of those........ Keep us updated .....
 

Tin Diesel

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Runs really smooth up to when it won't get more RPMs. Even then, it's smooth.

I doubt it's bad gas but I'll try to figure that out.

The only new thing I have noticed lately is that if I come down off of speed quickly, or sometimes when I'm shifting into reverse while maneuvering, the engine will die.

BigKahuna, I really appreciate the info you just posted. I will look for the other fuel filters and make sure that I change them also.

I'm hoping that's the answer. It would make sense, since it sat for so long unused and my brother had always used ethanol fuel in it.
 

BigKahuna

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Runs really smooth up to when it won't get more RPMs. Even then, it's smooth.

I doubt it's bad gas but I'll try to figure that out.

The only new thing I have noticed lately is that if I come down off of speed quickly, or sometimes when I'm shifting into reverse while maneuvering, the engine will die.

BigKahuna, I really appreciate the info you just posted. I will look for the other fuel filters and make sure that I change them also.

I'm hoping that's the answer. It would make sense, since it sat for so long unused and my brother had always used ethanol fuel in it.
Good luck to you. You'll figure it out!!!
 

Remediation

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We have been having a miss fire issue with our 2008 merc 350 for years and it almost got us stuck on the water. Have discovered that the cool fuel cell that houses high and low pressure pumps was painted on the interior of the cell. That paint will flake off and started clogging the filters and pressure regulator. If you google the problem there are several threads about the paint causing problems. Ours is in the shop getting new high pressure pump and regulator and screens and rest of paint removed from cell interior.
Last year it got so bad that we limped into the dock and pulled the boat. Mercury is helping with the problem. They sent injectors and lots of parts to the dealer. It has to be a Mercury certified shop and I had to pay the labor. Labor was about 50% of the bill. They had to pull the motor.
I just posted my adventures from yesterday after the repairs. The engine is running well.
 

Tin Diesel

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Thanks Remediation. How can I tell if I have the Cool Fuel Cell? I have a 5.0L 260 HP MPI from 2007.

I was out at the storage unit yesterday looking for the fuel filters mentioned by BigKahuna. I definitely do not have an inline filter from the tank to the fuel/water separator/filter. There is one line going into this filter on the forward side (from the tank).
Coming out of it are two fuel lines on the aft side... well, one line has an 'out' arrow, and the other line has an 'in' arrow. Both are black steel-mesh lines, and both disappear under the engine. I could not find out where they came out. And believe me, I tried!!!

I could not see anything resembling a fuel pump (from my shade-tree mechanic days) so I could not trace from it to the fuel rail to see if there was a filter after the pump.

However, my engine runs really smoothly. It just won't rev beyond 2300 rpm. So, I'm not sure if the Cool Fuel cell is an issue (at least now - but what I've read online is not encouraging!)
 

BigKahuna

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Thanks Remediation. How can I tell if I have the Cool Fuel Cell? I have a 5.0L 260 HP MPI from 2007.

I was out at the storage unit yesterday looking for the fuel filters mentioned by BigKahuna. I definitely do not have an inline filter from the tank to the fuel/water separator/filter. There is one line going into this filter on the forward side (from the tank).
Coming out of it are two fuel lines on the aft side... well, one line has an 'out' arrow, and the other line has an 'in' arrow. Both are black steel-mesh lines, and both disappear under the engine. I could not find out where they came out. And believe me, I tried!!!

I could not see anything resembling a fuel pump (from my shade-tree mechanic days) so I could not trace from it to the fuel rail to see if there was a filter after the pump.

However, my engine runs really smoothly. It just won't rev beyond 2300 rpm. So, I'm not sure if the Cool Fuel cell is an issue (at least now - but what I've read online is not encouraging!)
The fuel pump is under the engine. Had it replaced a couple years ago. They had to pull the entire engine out.....
 

Tin Diesel

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The fuel pump is under the engine. Had it replaced a couple years ago. They had to pull the entire engine out.....
We'll, that figures!
:oops:
 

lakeliving

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Engineers.....
 

Remediation

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If you have a coolcell the high and low pressure pumps are in the coolcell. Talk to a service department at a Mercury certified repair shop. They should be able to tell what system you have.
The web has several threads on it but for any help from Mercury it has to be through a certified shop. I had talked to Mercury direct. They were very polite but bottom line was they only deal with the shops.
 

Remediation

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Engineers.....
Lets make a cell that we PAINT that we will fill with ethanol fuel which is a cleaner and then inject the fuel through 5 micron injectors. What could go wrong?
 

Tin Diesel

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Well, I finally got the boat back out on the water after replacing the fuel filter. Same problem. Will not rev above 2300 RPM.

The fuel filter wasn't that bad. I cut it open and looked at it and there is no evidence of anything that would restrict flow. (I was hoping for a water/ethanol goo like I saw in my generator carburetor several years ago)

The throttle travels clean from start to finish. I don't think it could go any further so I don't think I have a cable problem.

I haven't cleaned the flame arrestor, although it does not look dirty at all. I may try that but I don't think it's going to make a difference.

As I said before, the engine runs smoothly but when it hits 2300 RPM, it feels like a governor is kicking in.

I'll add one other thing. This is something it seems to have happened all at once, although we don't take the boat out very often I'm sad to say. It does not seem to be getting worse, and and I don't think it's something that gradually came to this point, although we usually don't drive much faster than 25 mph.
 
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