Not sure about this one... Too much fuel pressure???

[cjustinstockton]

Hi, all. Long time reader - first time poster.

I have a Mercruiser 5.0 MCM carbureted sterndrive that I just had the carb rebuilt on. I had the local repair shop do the rebuild. When I put it in the water for the first time after the rebuild, it started right up and was running just fine. The idle seemed smooth and it was running okay. That is, until I got into the upper RPM's. When I tried to run it at WOT, it hit about 4,000 - 4,500 RPM's (wasn't really that wide of a range, I just didn't look at the exact number) it would run there for a second and then it sounded like it was starved for fuel and cut back about 500 RPM's. After a few seconds, it would surge back to the higher RPM's. The higher I put the throttle, the more frequent the intervals of this happening. I finally found a sweet spot of about 3000 RPM's where it wouldn't cut back but every so often.

I didn't want to just assume it was the carb that was just worked on even thought it didn't show any of these symptoms before. So, I did a few things before calling the shop. I replaced the fuel water separator, emptied the tank and filled with 93 octane and put some dry gas into the tank.

Long story short, that didn't work.

I took it back to the shop and after they got a chance to look at it, they told me that my fuel pump's internal regulator had failed and it was reading at 12 PSI of fuel pressure. They said when I'm hitting higher RPM's the engine is flooding. This just doesn't make sense to me given the symptoms. If it's blowing the needle and seat, shouldn't the symptoms be WAY more noticeable at lower RPM's?

My first thought was that the floats were not adjusted correctly. Because the fuel pump is internally regulated, it's a $300 part with another $200 in labor (even thought from the looks of it, it's about a 20 minute project). I don't want to spend this much for them to NOT fix the problem.

Thoughts?

 

[cjustinstockton]

Just a little supplemental information -
When speaking to the guy at the repair shop, he said that the engine was flooding and misfiring at lower RPM's. I DID notice it was running a little rougher than normal. He said as the RPM's increased the engine was then inundated with fuel and that's what caused the RPM's to kick back. I politely questioned his logic and he seemed to get a little grumpy about that. I told him that if he was absolutely sure about the diagnosis go ahead and fix it. If this isn't the problem, should I still have to pay for it? I'll take moral AND legal opinions on this .

I also asked him if it was possible just to add an inline fuel regulator instead of replacing the entire pump - he said that wasn't an option because that's "not the way it was designed". Can anyone speak to this?

 

[tpenfield]

Sounds counter intuitive, but see how the engine runs after the fuel pressure issue is resolved. You will certainly be charged for it.

​The mechanic is not going to rig anything up on the fuel system that is not according to the original design. They will just fix what is broken and get things back to working the way they should be.


​If the fuel pressure fix does not solve the problem, then I would look for a different shop to work the issue. Possibly, they did not rebuild the carb correctly and you could hold them to making good on their work. Other than that I'd move on.

 

[cjustinstockton]

Sounds counter intuitive, but see how the engine runs after the fuel pressure issue is resolved. You will certainly be charged for it.

​The mechanic is not going to rig anything up on the fuel system that is not according to the original design. They will just fix what is broken and get things back to working the way they should be.


​If the fuel pressure fix does not solve the problem, then I would look for a different shop to work the issue. Possibly, they did not rebuild the carb correctly and you could hold them to making good on their work. Other than that I'd move on.

Thanks, Ted - They should be done with it today. So, we'll see how it works out. Hopefully I'm wrong and it fixes the issue.

 

[kenny nunez]

I have never seen a standard marine carburetor that can hold 12 lbs. of fuel pressure without flooding over . If anything it appears just the opposit with fuel starvation.

 

[Maclin]

You should have experienced issues at every point in the rpm band, especially at idle, and going WOT should have eased them some. Coming back from high RPM down to 3k rpms should have drowned it in fuel I would think.. Would also be a strong smell of raw fuel, and more than likely seeing fuel coming out of the carb everywhere. Just sayin', we'll see I guess.

 

[cjustinstockton]

Update - They just called and told me it was done. $550 for a new fuel pump and labor... Seems a bit steep to me. They also said they heard a noise coming from the gimbal bearing. I just had the bearing replaced two months ago... On to the next saga :facepalm:

 

[Bondo]

Ayuh,.... Welcome Aboard,.... You need to find another marina or buy some wrenches,....

Yer symptoms are exactly opposite of their diagnosis,....

 

[tpenfield]

Run, Justin, run . . .

 

[cjustinstockton]

Update from my phone. They finished up. I took it out as they were closing. Currently taking advantage of my BoatUS membership to get back in....

 

[Grub54891]

Update from my phone. They finished up. I took it out as they were closing. Currently taking advantage of my BoatUS membership to get back in....

​Time to get a new mechanic. But try to get them to reimburse you for at least the labor.

 

[biggjimm]

I stopped at a very well known & I thought reputable Marina in Indianapolis the other day to acquire about a boat I've seen in the back lot the last couple of times I've been down that way.

After talking to the tech for a little while he commented on the fact that most boat owners are rich & stupid & how easy it was to "make a little extra beer money." I couldn't believe he was telling me this. I mean for all he knew I could have been a potential customer. I told him I was neither rich or stupid & they would not get any of my money.

I called back down to speak with the manager after returning home & he all but called me a liar.

It's unfortunate that some places are that way but they are.

Good luck & definitely find another shop to service your boat.
Jim

 

[achris]

Hi, all. Long time reader - first time poster.

I have a Mercruiser 5.0 MCM carbureted sterndrive that I just had the carb rebuilt on. I had the local repair shop do the rebuild. When I put it in the water for the first time after the rebuild, it started right up and was running just fine. The idle seemed smooth and it was running okay. That is, until I got into the upper RPM's. When I tried to run it at WOT, it hit about 4,000 - 4,500 RPM's (wasn't really that wide of a range, I just didn't look at the exact number) it would run there for a second and then it sounded like it was starved for fuel and cut back about 500 RPM's. After a few seconds, it would surge back to the higher RPM's. The higher I put the throttle, the more frequent the intervals of this happening. I finally found a sweet spot of about 3000 RPM's where it wouldn't cut back but every so often.

I didn't want to just assume it was the carb that was just worked on even thought it didn't show any of these symptoms before. So, I did a few things before calling the shop. I replaced the fuel water separator, emptied the tank and filled with 93 octane and put some dry gas into the tank.

Long story short, that didn't work.

I took it back to the shop and after they got a chance to look at it, they told me that my fuel pump's internal regulator had failed and it was reading at 12 PSI of fuel pressure. They said when I'm hitting higher RPM's the engine is flooding. This just doesn't make sense to me given the symptoms. If it's blowing the needle and seat, shouldn't the symptoms be WAY more noticeable at lower RPM's?

My first thought was that the floats were not adjusted correctly. Because the fuel pump is internally regulated, it's a $300 part with another $200 in labor (even thought from the looks of it, it's about a 20 minute project). I don't want to spend this much for them to NOT fix the problem.

Thoughts?

So many things are wrong here. Not sure if you have a mechanical pump or electric (please post your engine serial number ).. Either way it doesn't matter. There is no 'internal regulator', on either pump. With the mechanical pump it's a function of the spring, and the only way to increase the prssure is to replace the diaphragm spring. With the electric pump it's a function of the impeller and it's IMPOSSIBLE to over pressure (I have had one completely apart, I know what's inside.) OK. The electric fuel pump is a $300 item, but it's a 20 minute job to change out, 1 electric plug, the fuel in and out lines, then pull it off the mounting bracket.

As already stated, your symptoms are counter to the alleged cause. If the engine problems are now sorted, I suspect 'skullduggery'.... One more thing. Were you handed back the old pump? If not, go back to the shop and demand you get it. You may have bought a new one and had them fit it, but the old one is still your property, and should have been returned to you when you paid for the new one.

Chris...........

 

[Maclin]

I think cjustinstockton got stranded out on the lake first time out right after picking the boat up from the Marina

Hope not, but that last post from their phone did not look good...How 'bout an update there CJ ?

 

[cjustinstockton]

Hi, all. Sorry. It’s been a while. I finally figured this whole thing out. Turns out whoever had the boat before me had the original engine which was a mercruiser 5.0 EFI. They had some sort of failure with the port injection system. So, instead of fixing or replacing it, the ripped it off and bolted a carb on. EFI’s, as I have learned, take significantly higher fuel pressure. They never bothered changing out the fuel pump thus leaving me with significantly higher fuel pressure than I needed for the carb. This is why the engine was flooding. As counter intuitive as it may have seemed, they were right on. Running on the old gummed up carb was actually helping regulate the higher PSI. When they rebuilt it, there was nothing holding that pressure back anymore.

 

[achris]

This is a good example of why we ask for engine serial numbers. Had a serial number been listed with the first post (or even subsequently) a quick check would have it come up as an MPI engine... That would have lead to questions.... And would have revealed the problem, possible a lot quicker than 12 months...

Thanks for the update...

Chris.......

 

[Lou C]

For a carb engine you don’t want more than 6/7 lbs!