Engine won’t exceed 2000 RPM’s under load

Hoplite808

Well-Known Member
Jan 11, 2022
507
Honolulu, HI
Boat Info
1996 Sea Ray 270 Sundancer
Engines
Mercruiser 7.4 LX MPI EFI (Gen. V)
Been working on getting my boat to run right since January and still no joy. But I think I’m close.

Work done so far:
Rebuilt the top half of the 7.4. Rebuilt heads and new near about everything else. Compression reads within acceptable range on all 8 cylinders.
All new tune up parts. I checked the timing by inserting a jump wire in the test plug to connect A and B leads and then set idle to 1800 RPM’s with a digital tach. Then adjusted the timing to 8° BTDC.
Outdrive was just entirely replaced with a fully rebuilt one.

I took the boat out today and in neutral it revs all the way up to 4500 rpm’s. However under load it will hit a wall at 2000 rpm’s and no matter what the boat wouldn’t go faster than 9 knots.

Thought it might be old stale fuel but I ran it down to 1/8th tank and then added 25 gallons and no change. Also no signs of water in the gas when I checked the contents of the fuel separator.

I’m at a loss here. No idea what else could be wrong with it. Anyone have any ideas?
 
Been working on getting my boat to run right since January and still no joy. But I think I’m close.

Work done so far:
Rebuilt the top half of the 7.4. Rebuilt heads and new near about everything else. Compression reads within acceptable range on all 8 cylinders.
All new tune up parts. I checked the timing by inserting a jump wire in the test plug to connect A and B leads and then set idle to 1800 RPM’s with a digital tach. Then adjusted the timing to 8° BTDC.
Outdrive was just entirely replaced with a fully rebuilt one.

I took the boat out today and in neutral it revs all the way up to 4500 rpm’s. However under load it will hit a wall at 2000 rpm’s and no matter what the boat wouldn’t go faster than 9 knots.

Thought it might be old stale fuel but I ran it down to 1/8th tank and then added 25 gallons and no change. Also no signs of water in the gas when I checked the contents of the fuel separator.

I’m at a loss here. No idea what else could be wrong with it. Anyone have any ideas?
I suppose start with the obvious stuff. Is the bottom clean? Do you have the right prop and is it to spec?
 
Only thing I haven’t done is test the fuel pressure. Gotta find a schrader valve adapter to do that.
 
Figured it out. The fuel booster pump inside the vapor separator tank had begun to fail. Only had 20 psi of fuel rather than the 37 that is needed. Found a replacement pump with o ring set and rebuilt it. Engine restored to good working order now it appears.
 
It appears I was wrong about the pump being the last problem. Wasn't able to take the boat out that day but did this weekend. It still wont rev past 2000 rpms under load giving my 270 an apparent top speed of 9 mph. Could my injectors have gone bad? I don't know what else to look into here and the mechanics say they either wont work on gas engines here or are too busy to add me to their schedules.
 
It appears I was wrong about the pump being the last problem. Wasn't able to take the boat out that day but did this weekend. It still wont rev past 2000 rpms under load giving my 270 an apparent top speed of 9 mph. Could my injectors have gone bad? I don't know what else to look into here and the mechanics say they either wont work on gas engines here or are too busy to add me to their schedules.

I had a 2800 Maxum with the 7.4 mpi and Biii drive. Following rebuild, the thing would not go over 3000 rpm. We tried everything. One night after running it up and down the channel, it occurred to me to pull the air cleaner off. Damn thing ran right up to 4500 rpm.

Unlikely to be an exhaust blockage, but I'd check the intake.

The other thing I did was to buy this guy's MEFI scan software (http://mefiburn.com/), log the data at idle and under load, and sent it to him. He went through it and told me that the engine didn't think it was getting enough air. We checked the MAP sensor but it was right at atmospheric pressure at key on. Anyway, that was our first clue that it was something with the intake.

2000 is a pretty low rpm. Maybe something wrong with the rebuilt drive?
 
vapor separator tank. Gen 4's had them not sure if the 5's did.
Ahhh, I just rebuilt that. Fuel pressure tested low at 25psi. Changed the mechanical pump and no change so I rebuilt the vapor tank and that restored the fuel pressure to 37.

When I had the whole engine apart to rebuild the heads the injectors looked old but ok. I didn’t have any tools to otherwise check their functionality. I’m guessing at this point that the issue I’m having is a fuel starvation issue. The injectors are the only thing I have yet to replace in the fuel system at this point.

The only other thing I can think it might be is maybe it’s an outdrive issue. But I just had a rebuilt one installed by a mechanic that I trust. I would suspect more noise and vibration if that were the issue though.
 
Have you tried some simple things like running with the gas cap off to check for a venting clog? How about hooking up a portable gas tank directly to the fuel filter block? Doing the portable tank completely eliminates anything in the boat-side of the fuel system as a variable... including old, internally failing fuel lines acting as a check valve.

FYI, throttling up in neutral is a pretty useless test.

If it was the outdrive, you'd know it.

It certainly can't hurt, though, to pull the injectors and have them professionally cleaned/rebuilt (typically about $20/injector). Chalk it up to "it's a good idea to do that, anyways".
 
Gas tank is venting fine. Fuel lines are also all fine. The throttle up in neutral was letting me know it was capable of higher RPM’s while no odd noises from the block.
 
Gas tank is venting fine. Fuel lines are also all fine. The throttle up in neutral was letting me know it was capable of higher RPM’s while no odd noises from the block.
If you've checked the venting system and physically checked (or recently replaced) the fuel line - then good enough. But don't assume (couldn't tell from your post).

Also, try removing the AS valve.

EDIT: Monitor fuel pressure while underway.
 
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If this started at the same time as the outdrive was rebuilt, could your mechanic possibly have installed the wrong gear ratio? They make a very "tall" set for diesel usage, so it'd be like trying to get on plane while you're in overdrive.
 
If this started at the same time as the outdrive was rebuilt, could your mechanic possibly have installed the wrong gear ratio? They make a very "tall" set for diesel usage, so it'd be like trying to get on plane while you're in overdrive.

It has a Mercruiser 7.4 engine and we confirmed the gear ratio via the serial number as 2.0 prior to the rebuild and also found the gimbal bearing completely shot too. The mechanic rebuilt another entire outdrive at his shop and we swapped it completely. I was also having engine problems prior to the swap. There are no odd noises from the outdrive to suggest to me that it is defective. It’s just under load in relatively calm water it won’t get up on plane and the engine won’t rev past 2000 rpm’s. When I push the throttle for more speed once at 2000 the engine begins to struggle slightly and even misfires a little bit. Which I thought was a symptom of lean condition due to the fuel pump in the vapor tank since the fuel pressure tested at 25 psi before I replaced it. It’s at 37 now and no change. This is why I’m thinking now it’s perhaps the MPI injectors maybe were damaged by running with bad pressure or maybe just age. I just don’t know. I ordered replacements but they won’t get here for another week. If that doesn’t fix it. I just don’t know what the issue is.
 
In addition to getting the injectors cleaned and flow-matched as a set, have you checked all the other basics like spark plugs/gaps and plug wires in the correct order? Timing ok?

When you put the injectors back in, are they all seated properly with the o-rings? And electrical connections good to each one?

Like Dennis said, older fuel lines can definitely be a problem you could rule out by hooking up a portable tank as close as possible to the fuel rail.

Did you say you're seeing good fuel pressure even when you hit the wall at 2000 under load?
 
@Phil S the fuel lines are fine, they have been inspected and fuel moves through them freely.

The injectors are seated properly and the electrical contacts are good as far as I can tell with my multimeter.

All tune up parts replaced and firing orders and timing triple checked.

I have not checked the fuel pressure under load as I’m working by myself and I can’t handle the helm while also in the engine bay. But I did check it in neutral at 3000 rpm’s and it stayed steady at 37 psi after I rebuilt the vapor tank and replaced the pump in it.
 
mechanic said I needed a new fuel pump-replaced the water separator fuel filter -back to normal- wasn't getting more than 2000 rpm at full throttle
 

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