1990 Mercruiser V8 WOT 2800 rpm under load....

[shift6]

Hopefully someone can lead me down the right path....First off, I'm a novice to this whole boat ownership thing so bear with me! This spring I bought a 1990 Doral Cavalier with a Mercruiser V8 (OC775400) with an Alpha One outdrive (OC852452). It is equipped with Bennett Trim Tabs, and a 13 3/4 x 17 pitch aluminum prop. Motor just had new wires, cap and rotor installed prior to going in the water this year. Bellows and gimbal bearing were replaced last year.

The problem: in neutral, the engine will rev over 4500 rpm (I didn't push it to rev limiter). With the drive engaged, at WOT, the motor won't rev higher than 2800 rpm. It won't plane, and it feels absolutely sluggish. The only thing I've noticed is that the gear lube reservoir has been leaking at the bottom and is pretty low. Not sure if that would have any impact...

Anyone run into this load problem before?

Thanks in advance!

 

[Don S]

Re: 1990 Mercruiser V8 WOT 2800 rpm under load....

The problem: in neutral, the engine will rev over 4500 rpm (I didn't push it to rev limiter). With the drive engaged, at WOT, the motor won't rev higher than 2800 rpm.

First of all, revving in neutal is a non test. You could remove half the spark plugs and rev 10,000 rpm and blow the engine. And low rpm under load is probably the most common question asked.

Here is a list of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

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
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.

 

[shift6]

Re: 1990 Mercruiser V8 WOT 2800 rpm under load....

Thanks Don! I did see that post earlier when I was crawling around the forum. I was told by a local reputable marine mechanic that it could be the driveline coupler. I hope it's going to be something simpler that doesn't involve pulling the engine! Looks like I'll have to get it in for a once over.

In no particular order

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel Fresh fuel this year
2. Propeller pitch or diameter, damaged blades Both props are the ones used by previous owner
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 collapseOil level is good
5. Marine growth on hull and outdrive No growth, anti-fouling coat. Problem from first dip in the water
6. Wrong gear ratio in outdrive Original outdrive, should be the one that works
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. Boat is pretty empty
13. Engine Overheating No overheating problem
14. Engine timing and ignition system operation New wires, cap, and rotor
15. Remote control cables and linkage for proper travel to open throttle plates fully. Good

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