Low RPM at WOT

N

newlyn

Member
Joined
Aug 15, 2005
RO Number
18724
Messages
59
I have been having this problem since July, and it is driving me crazy. Engine is a 2011 350 Mag MPI. (Boat is a 2001 310). Starts right up and runs fine, but won't go over about 3100-3200 RPM under load. Port engine runs fine all the way up to 4400rpm.

So far we have done a tune up, replaced entire cool fuel module and all injectors (had some paint chips), and checked anti-siphon. I have not had a chance to run it off a separate gas can yet, but that will be next. I noticed a small warp on the prop edge, but I doubt that would result in such a big WOT loss.

Any ideas are appreciated.
 
It might be the prop. The wrong or damaged prop will bring RPM down..
Also, that damaged prop will do damage to your prop shaft/outdrive because it's out of balance
 
Do you own yet or have access to a marine scan tool?

This can tell you if your throttle position sensor is going all the way to 100% when the dash control lever is at WOT. It will also indicate and tripped codes, whether the knock sensor is giving the ECM/ECC a message to retard timing , total timing at all rpm, all sensor data you can compare to the good engine, and compare fuel GPH flow.

It should also allow you to perform a power balance test electronically. shutting down either ignition pulse or fuel injection to each successive cylinder to see if a bad-combustion cylinder fails to lower rpm.
I have a pair of MPI engines and can't imagine not having a scan tool ( I have 2 kinds by Rinda, h/h & notebook SW.) But then, there are no authorized Crusader techs here.
 
Forgot to mention, suggest you triple check correct firing order at both ends of cables too. And do get the prop(s?) scanned and repaired.
Do these engines have distributor-less ignition?
I/B or I/O?
 
It is the original prop. I did see a slight warp on one edge. I would be surprised if that made a 1400 rpm difference, but perhaps. Also, no significant vibration.

I checked throttle linkage and it goes all the way.

Mechanic found no codes, but he is not a Merc certified mechanic so perhaps he doesn't have the right reader?

Engines are inboards.
 
You dont mention fuel filter change?? Assuming the problem existed before you did the tune up IMO you need to have the prop fixed. Ruling out known problems is always the place to start. What looks like a minor ding can twist a blade more than immediately obvious. On an IO I would suggest switching props but that is not realistic for an inboard.
 
Did you have this problem, then do the tune-up and still have the problem?

I was surprised how well our 5.7 EFI ran with a plug wire off.

How did the plugs look?

Could be time for a compression test.

Don't rule out a new part being bad.

You should be able to watch each injector pulse with a Noid Light, but I'd hope the tests you've done so far would have shown one not getting a pulse.
 
Yes, mechanic did the tune up to try to resolve the problem. I didn't see the plugs, but he did not say anything about them.

He also scanned and no codes.

Haven't done compression test but engine has less than 150 hours.
 
Not sure if MPI is different, but the distributor weights on my crusaders were so rusty they wouldn't advance beyond a certain rpm, replaced old distributor with DUI. Solved that problem.
 
Just an update: I am now getting 4000 rpm, but I am still missing about 700 rpm. Really not sure why I am getting higher rpm now. After replacing the fuel cool module and injectors I had a brief stint at 4000 rpm, but then it dropped down again to 3400. Last weekend we ran it hooked up to a laptop and fuel pressure guage. It ran at 4000 rpm and everything looked fine (no codes, timing ok, correct fuel pressure), but I am still missing about 700 rpm. Port engine runs at 4700 WOT. Planning to look at prop next, although I would expect any prop damage that causes a 700 rpm loss to result in noticeable vibration, no?
 
Props can make a big difference. Did it run WOT ever? I am a fan of Propscan I have seen the results more than once. If it were my boat I'd only be using a Mercruiser mechanic at this point. If you're satisfied with your mechanic that's ok but he needs to start from basics and eliminate everything one at a time. Did he do a compression check? Did he change plug wires? Are cap,rotor & plugs new? If all this is good then check the prop. A side note when my Chevy Tahoe had a bad injector the only way I could tell something was wrong was a slight loss of power but otherwise it ran fine.
Rob
 
Yes, it ran at 4600 rpm a year ago. Hauled the boat and the props look fine.

Haven't done a compression check yet. Also, plugs and cap are new, but not wires.
 
Check the specs for yours, but I believe your Vortec MPI engines should be propped to run about 4800(bare minimum)-5200 RPM at WOT,with 5000 or a little above as optimum with normal to full boat loading.

Your engines may not have taken kindly to being over-loaded.

If the boat is still hauled, you might do well to have the props scanned now( free scan and report at PropScan) to see what you really have for both props, and adjust from there.That's not to say there isn't something else going on with the low engine now.
 
If it ran 4600 rpm last year what is causing the poor performance? You may have to insist the mechanic do a compression test and go from there. The visual inspection of the props won't tell you what the pitch is or how balanced they are. You have to rule out mechanical(compression) electrical and fuel problems first then look at the props. Sandy is right you should be WOT 4800-5200 rpms so at 4600 rpms you are over propped.
 
WOT range for my Mercury 350 Mag Horizon is 4600-5000. The 4600 number on the starboard side last year was on the tach, with some growth on the bottom, so within spec. Port side is getting 4700 currently.
 
4700 will likely go up when the starboard side is repaired. Any progress with diagnostics on starboard engine? If possible figure this out before you put the boat away, believe me I feel your frustration having been thru similar issues with my own boat. Good Luck and keep us posted.
 
My plan is to pull the props and have them scanned, but they look fine to the naked eye. Boat is out of water now (had too many commitments in October) so limited in what I can do, but I figured I would also swap out the spark plug wires since that is cheap/easy enough.
 
It may sound stupid, but make sure the builder supplied tachometer is reading correctly if that is what you are using. My tach's do stick occasionally and read wrong. I added a synchronizer that always reads more accurately.
 
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