350 EFI low WOT

[Ferrellb07]

I have been using this forum for years for information, but have never posted anything. The information here has solved a ton of issues for me. I have searched and searched on my current issue and found a lot of people describing low WOT RPM issues, but none of them helped me. I am now asking for help as I feel that I have tried everything.

I have a 22ft Bowrider with a 2001 Merc 350 EFI(serial 0M007346). I can only reach 4000 RPM with only myself on the boat. I think I should be seeing 4600-5000 rpm. I have found Don's list for low WOT and I have been through everything, or so I think. See below for what I have tried already:

This is Don's list for possible causes of low WOT, and I will add in bold what I have tried.
1. Fuel condition. Type and Octane possibly old fuel, I have connected a separate red fuel can with known good fuel just before the fuel pump
2. Propeller pitch or diameter, damaged blades I have the same prop it's always had (SS Mercury 48-13702 21P) It does not appear to be damaged, I've even taped a stiff wire to the outdrive and it contacted the prop in the same place on all blades.
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test I performed this fuel system test with vacuum gauge and clear tubing right before the fuel filter, no bubbles.
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse.I did find where the previous owner had WAY too much oil in the crankcase. Twice the distance from add-full above the full mark on dipstick. I pumped the excess oil out and it is now at the correct level.
5. Marine growth on hull and outdrive. I pulled the boat out of the water and cleaned everything
6. Wrong gear ratio in outdrive no changes to the outdrive
7. Restricted carburetor air intake (clogged flame arrestor) I ran it with the flame arrestor off, no change
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive, I pulled the outdrive and looked up the exhaust belows and looked into the exhaust cavity in the lower unit, I did not see anything out of the ordinary.
9. Poor cylinder compression Compression Test Compression test checked out OK
10. Carburetor defective, or wrong type.EFI
11. Fuel pump pressure and vacuum I am reading 35 PSI at the fuel rail under load, this is what it should be for my engine. I thought it should be 43psi for some time, purchased a new fuel pump and installed. I pulled the vacuum line at the throttle body at idle and the fuel pressure jumped up to 35 psi. Still didn't help
12. Boat overloaded, improperly loaded, or improperly trimmed. I am the only person on the boat. I moved the trim up and down and could only reach 4000 rpm with the trim most of the way up
13. Engine Overheating Engine is not overheating
14. Engine timing and ignition system operation new distributor cap and rotor. New coil. New wires and plugs. Plugs are the merc recommended plugs for my engine and are gapped correctly. Checked and double checked to make sure plug wires are connected correctly. New magnetic pickup inside distributor.
15. Remote control cables and linkage for proper travel to open throttle plates fully No change to linkage. The throttle body opens fully. New throttle position sensor.


As you can see I am very stumped. I do not know what I should try next. Any suggestion is appreciated. Thanks

 

[QBhoy]

Has it always been like this or did it used to run higher rpm with the same set up...prop etc ?
Have you had it plugged in to see any faults ?
Is 5000 rpm Defo the max rpm range for the efi ?

 

[Ferrellb07]

I bought the boat in July and it ran fine, performed much better than it does now. I could get it up to 55 mph or so, but the tach didn't work. Unfortunately this problem started happening before I replaced the tach. I have now replaced the tach and can see only 4000 rpm and can reach only 48 mph. I do not know exactly what RPM it could turn before. I did purchase the little red light code reader and it does not show any fault codes.

 

[QBhoy]

Is it kept in the water ? Is the hull dirty ? I know that my boat speed drops to under 60 towards the end of season due to growth on the hull.

 

[Ferrellb07]

The hull is clean. I took the boat out of the water and pressure washed the hull. It did have some growth, nothing extreme. I put it back in the water and no change.

 

[Scott Danforth]

pull the exhaust elbows and make sure the shutters did not drop down and plug the exhaust.

you can build a diagnostic code flasher http://forums.iboats.com/forum/engi.../10266808-2000-merc-5-0-v8-efi-no-start/page2

I believe your serial number puts the motor at a 30psi system, however i am going off of memory.

if the regulator is pink, you have a 30 psi system. if the regulator is blue/white, you have a 43psi system

 

[alldodge]

Timing 8 BTDC
Plugs MR43LTS gap .045
WOT 4400-4800 RPM
Compression 150 PSI range
Fuel pressure 30 PSI at 1800 RPM

Has timing been set using base timing mode?

There should be a knock sensor on the starboard side of the block. Remove the wire on the knock sensor and see if it will reach WOT. If it does reconnect and need to find out this issue

 

[Ferrellb07]

Thanks guys.

I did put MR43LTS plugs in and gapped them to .045
The compression was spot on 150 PSI on all 8 cylinders
The fuel pressure regulator had the pink mark.

I have the code flasher with the red LED light that blinks. Its not showing any codes.

I have not messed with the timing, but I will look into that now.

I'll dump it back into the water next weekend and trying removing the knock sensor wire. This is something I haven't tried.

Thanks again. I REALLY want to figure this out before I have to winterize.

 

[Fun Times]

Are you jugging by the boat speedometer or a more accurate way such as GPS of some sort....Sometimes the boat speed o meter is considered not the best for continuous accuracy as possibly knowing the boat sits in the water, maybe the speedo water passage hole starting on/at the lower stern drive unit gear housing has become partially blocked with debris.

Being it's a new tachometer, would you happen to have another tool handy that could help verify it's accuracy at all RPM's as maybe an adjustment is needed on the new gauge....Maybe.

Without judging by the new RPM gauge and speedometer, does the engine sound and physically seem to perform differently/oddly then the one time out that when it hit 55 on the gauge?

^ 'Was the boat low on fuel, gear weight, water tanks? people weights , etc. between hitting 55 and 48 MPH?...More fuel, etc. generally lowers top end speed/RPM.

Ambient air temp, altitude, water conditions has some effect on overall engine/boat performance too....Any major changes in those categories at all by chance?

You've probably already did this little test but just in case; 'Next time when you're at top RPM, try carefully opening up the engine hatch compartment a little bit to see if allowing more air to the engine seems to help any.

AllDodge's knock sensor test is a good one plus you could try taking along an Advance Timing Light and watch the timing advances between the harmonic balancer and front cover timing tab to see if the timing fluctuates with RPM changes. Ignition Advance Curves and Point Setting

 

[Ferrellb07]

I am using GPS speedometer as well as the one on the boat. They are both very close to the same speed.

I have been out multiple times over the past 2 months as I have been troubleshooting. The weather and water conditions different each time, still nothing over 4000 rpm.

The fuel level has been close to the same each time.

The engine does sound different. It sounded like it was rev'ing higher before, but I had no tach. The engine doesn't sound wide open now, it sounds like it has more rev's to go.

I will look into timing and knock, this is something I haven't dug into yet.

 

[QBhoy]

Carefully....does it rev past 4000rpm in neutral ?

 

[Ferrellb07]

Yes, it will rev up in neutral to somewhere near 4800 if not higher.

 

[Scott Danforth]

the motor is starving for fuel when under load. since it only takes a few drops of fuel to rev it in neutral, that does not do anything.....except if you are on muffs.....then it burns your impeller from lack of water flow

your 350 mag is trying to burn 19.4 gallons per hour or 0.33 gallons per minute when under load at WOT. failure to provide the motor with that amount of fuel or the air required to mix with that amount of fuel will cause low WOT numbers

 

[ghunttwo]

Leaving a out drive boat in the water is really hard on it. Fresh or salt water? If salt water, only a hand full of barnacles will effect speed/ rpm's
Has the boat gained weight, meaning water soaked foam in the hull from a improperly bedded something below the water line. Transducer, truhull for a bait well or head, out drive to transom. I've seen it many times with large and small boats.
If the boat is on a trailer have it weighed then weigh the trailer alone. You should be able to google that boat and find new boat weight.
Just a thought, good luck

 

[QBhoy]

Above post is sure true. My boat looses at least 7/8mph gps with a dirty hull. Amazing the drag caused...especially over certain speed barriers. Every tiny detail counts at around 50mph and over.