Engine choking. Can't go over 2000 RPMs.

Do you have an engine serial number ,Is the backfiring from the intake or exaust.
Have you checked the dist cap for carbon tracks, and has the sparkplug firing order been checked.
Does this motor use the "crab " dist cap. the flat one? There known to go bad .
 
Last edited:
I'm using that boat every weekend so I think plug wires are ok and they are new. I'll recheck again.
Engine serial number: 0F602266
Correct me if I'm wrong but if sparkplugs firing order is incorrect the engine would be shaking the whole boat even on low rpms. It could be easy detected.
I really don't know where backfiring occurs.
I don't know what are you asking about "carb dist cap the flat"
 
Yes it is as far as I remember. Are you thinking of a wire marked as no.17?
 
No #24


double check the firing order, 1,8,4,3,6,5,7,2 in a clockwise direction 1,3,5,7 on your right facing the engine 2,4,6,8 on your left
 
Ok thanks. Is that a wire that I can buy in autoparts store or marine store ?
 
auto parts should carry it. in a pinch you can use a sparkpug wire with a short wire inserted into the core(paperclip)
 
So I've changed that wire and still no changes. Mechanic came with computer and reset all codes but TPS and MAP sensors are coming back as faulty.
Next thing I'm gonna pull injectors off and have them checked.
Any other ideas?
 
Last edited:
Sounds like the MEFI unit is going in to failsafe mode. When in failsafe mode, the engine operates normally up to about 2000 RPM. Above that RPM limit, fuel delivery is limited to half the fuel injectors until the engine drops below 1200 RPM. Once engine speed drops below 1200, the system operates normally until 2000 is exceeded again.

So... What puts MEFI into failsafe?

Firstly check the condition of all wiring and connectors. As you have to plug and unplug the connectors to ensure the contacts are clean and dry, disconnect the battery to avoid surges. Check in particular system grounds. A bad or intermittent ground will cause all kinds of headaches!.

Secondly check all the sensors. MEFI can monitor and go into failsafe for low oil level, low oil pressure, high transmission temperature (inboards) low gear oil level (sterndrives), high coolant temperature, and maybe something else that I can't remember. :) Check all with a DVM.

Sterndrives also have may have the shift interrupt input. Ensure that's not intermittently signaling.

Then it would be helpful to run the engine past 2000 RPM while monitoring the data from the various sensors including TPS and MAP with the diagnostic tool. The values should be smoothly increasing as you smoothly and slowly add power. Watch for jumps in signal levels from not only TPS and MAP but also the knock sensors. ECT, and IAT should show normal values.

Finally, if you see everything else normal, but strange values from TPS or MAP, connect a DVM with at least a 10 megohm impedance to the MAP or TPS signal wire and read voltage while running the engine slowly up to 2000 RPM. You're looking for discontinuity, jump, or sudden lack of signal. These are three wire sensors with the black wire being ground, typically a grey wire for +5v reference voltage, and the other color wire, blue or orange I think? being the signal. Whatever is not black or grey is the signal wire.

If all that's normal, then you have to check the signaling between the DI module on the distributor and the MEFI module. Never did that so I forget how that's done. I know that the DI module only passes information to the MEFI until once the engine is running. It's only in control of timing and ignition during starting. I remember there's a voltage signal between the modules and a digital pulse signal. The voltage is easy to check but the digital signal requires a frequency counter and possibly a 'scope, but problems like that are rare. Usually it either works or doesn't but rarely would it send the MEFI into failsafe. And it does sound like you're in failsafe.

Oh... and what ARE the actual fault codes?
 
Last edited:
This sounds way above my ability. I think I'll take my boat to an authorized dealer to have them checked. Thank you
 
codes.jpg
 
Based on those codes and symptoms I'm guessing
1) bad or intermittent ground;
2) faulty wiring harness or plug, especially wiring harness ground splice;
There are two or three more possibilities, but the above are more likely. If not one of those, then I'd need to know the actual voltages on the sense wires, dark blue and green as the throttle is slowly advanced to 2000 RPM. (as related previously.)

Best regards,
Frank
 
So I finally started working on that engine again. It took me a while because I didn't want to take the engine apart due to few events on the water that I wanted to attend.
I took all injectors out and had them tested. They're all good.
I also did a compression test and one cylinder shows something different than others.
All are at 130 psi except one which is at 90psi.
Also 3 spark plugs were totally covered with this black stuff/gunk. A lot of it. To the point of covering the space where spark should ignite.
 
Last edited:
A loss of 40PSI is a big deal. You need to do a leak down test.
 
So the loss of preasure in that cylinder was caused by lack of shims under the rocker arms which pressed open valves. Now all cylinders read the same compression. Although we fixed that engine still won't go over 2000rpm.
I was really hoping it will solve the problem but it didn't.
Back to square one
 
Last edited:
Yareck said:
So the loss of preasure in that cylinder was caused by lack of shims under the rocker arms which pressed open valves. Now all cylinders read the same compression. Although we fixed that engine still won't go over 2000rpm.
I was really hoping it will solve the problem but it didn't.
Back to square one
Click to expand...
Curious what fixed your issue?
I have similar issue on Mercury 7.4 MPI
 
Backfiring out of the intake indicates a lean burn due to fuel starvation or timing. Backfiring out of the exhaust indicate rich fuel or incomplete fuel burn or timing. Generally speaking.
 
You must log in or register to reply here.

Forum statistics

Threads
113,143
Messages
1,427,148
Members
61,056
Latest member
Rafael Saraiva
Back
Top