1998 350 Mariner Mercruise 350 Magnum 2800 RPM WOT issue

[grizer]

Ok I have a strange one which will require a real GURU to give some new hints. I have read lots of posts about RPM limits under WOT but none of them seem to be my issue. At dock and at idle, WOT will go up to 4500RPM+. Under load (in FWD), both engines are limited to about 3000RPM w WOT. Both engines behave exactly the same. If I try 1 engine at a time under power, no change, meaning each engine still limited to about 3000RPM. We pulled the boat, inspected the shafts, props, bottom, trim tabs and everything seems to check out. Drew some fuel into a glass jar and looks great. Replaced fuel filters and checks fuel lines even cleaned out the fuel vents. Still same issue. For giggles we did a compression test and both engines seem fine. We put 2 different diagnostic tools on and no codes are being thrown. Both engines purr like a kitten at idle and also run strong - just limited to the 3000RMP thing. We looked at the fuel pumps (CoolFuel2) and the input voltage is correct. According to Merc, with WOT we should see about 40psi+ but only get about low 20s. That seems to be wrong of course but the question is "is something causing it to be limited to 20psi or is it truly the pumps"..it.seems strange that both would behave exactly the same. Other than the input voltage to the fuel pumps, what are the other inputs and conditions that it needs to see in order for it to supply the full 40+psi? Furthermore, is their something common to both engines with regards to those other inputs/conditions needed for the fuel pump? Any suggestions welcome please as I am completely STUMPED!

[tomschauer]

You should see over 4000 rpm at WOT with both engines running under load, without load is meaningless, and with just one motor under load the load is too great for it to achieve rated rpms. I believe the fuel pressure should be 35-40 PSI if you have MPI mercury's. If its only 20 at idle, its probably less under load and you will never get to full PRM. First and easiest / cheapest is to verify there are no restrictions in the fuel supply to the pumps. You said you changed the filters, I believe these also have a filter in the cool fuel pump / regulator assembly that may be the issue, if not,its a good chance its the pumps and/or regulators.

[bud37]

Just to add, be careful revving it up in neutral / no load....to those numbers, Doesnt prove anything and there is a possibility you could damage things.

[grizer]

Thank you both of you, however no hints in either of those replies. If anyone else has more suggestions I could really use the insight.

[bud37]

Source of the post Thank you both of you, however no hints in either of those replies. If anyone else has more suggestions I could really use the insight.

A couple questions that may help,....to be clear you are saying both engines act exactly the same with fuel pressures etc.
Is this something that has just happened abruptly to both sides ( were there any alarms, codes in memory etc )?
Are these engines MPI or TBI ?
Any work been done lately in the engine compartment ?
Have you changed fuel source lately ?
Thats all I can think of , but this may help the guys a bit.....good luck man..

[tomschauer]

OK, let me try again. If you have mercury MPI motors and only 20psi at the outlet of the fuel pumps, you don't have enough fuel to get to full RPM under load.

Hint, its either a restriction in the fuel supply to the pumps or the pumps / regulators themselves.


Good luck.

[grizer]

Thanks to all of you. @bud37, yes both engines act exactly the same, there was no "event", no maintenance has been done that may have led to this, no alarms, no codes, no changes to fuel supply. @tomschauer, I understand that at 20psi it is not enough fuel pressure but my question is "could it be something else?" - I don't think I have a restriction. What I am trying to find is a working diagram or explanation of what the fuel pump needs to get as input to tell it how much fuel to send, in other words you have Voltage In and throttle position and fuel coming it, is that it? I guess I am looking for a Cool Fuel2 guru to say "try this, watch out for that, the throttle cable should be doing this..."

[buster53]

Ok, is this a fairly new issue or has the boat been that way since you bought it?

While the low fuel pressure is definitely worth looking at, is there any chance the boat is way overpropped?

[tomschauer]

you should have at least 35 psi fuel pressure, at idle, at half throttle at full throttle, in other words, whenever the engine is running, no matter what the throttle or shift cable is doing, The pump runs at one speed, it will pump 70 psi+ depending on load, the regulator drops it to 35-40.

[bud37]

Grizer.....the problem here as I see it is you are saying both engines are the same, which to me seems highly unlikely, unless you have extremely bad luck........so is this boat new to you ?......has it always exhibited this problem you are presenting ? Are the batteries feeding these engines up to snuff, low voltage will drive ecm's nuts, and the other question, are these engines Multi port injected or throttle body as it makes a difference as to how the regulators work.

This is important info the guys need to help you........