99 Mercruiser 43L EFI

"I purchased this boat last ye

"I purchased this boat last year. The first time I took it to the lake (6000')Elevation. The boat idles great, gets up on plane fine, then when I start cruising my Tack hits 3800 RPM's and my speed is only 28MPH then my RPM's start to jump up and become irratic and the engine starts to back fire and cut out, but the engine volts show steady at 13. Its been back to the dealer 2x and they can't seem to find the problem. I have changed the cap, rotor, wires, plugs, fuel filter, oil and gas. Still does it. Any ideas what might be causing this? I have two props 14.5/19 3 blade and a 14.25 x 20 4 blade, doesn't matter which one I use same problem."

"Any codes from the PCM? At 6

"Any codes from the PCM? At 6000 ft the MAP sensor and TPS sensor will likely not agree on the air/fuel ratio. This is based on my auto experience, not this motor. I have yet to poke around an EFI Merc, but have lots of experience with autos. A coil breaking down can cause this too, as can a defective PCM. I'd hook up the laptop and check the fuel mapping and timing advance as well.

Hope this helps a bit"

"Bill, thanks for the response

"Bill, thanks for the response. I'm not sure what a PCM is, I took it back into the shop for a 3rd time and they replaced the coil and said they got a few codes but said they fixed it. That seemed to help some but now what I'm noticing is when I throttle up, my throttle handle seems to go forward past level and down some. When that happens is now when it starts. So I wonder if maybe that isn't somehow effecting the TPS or if maybe the TPS is going bad. I now can run 37MPH at 4400 RPM's."

"Your motor is a GM 4.3L EFI,

"Your motor is a GM 4.3L EFI, just like in a GM truck. The PCM, also called an ECM (Electronic Control Module) is mounted on a bracket on the motor. If the throttle goes farther than it did before, there was something restricting it and they fixed it the last time it was in. The TPS won't keep the throttle from going to its limit, that's done with the limit on the throttle body. Ask what codes they got and if they cleared them.

The TPS could be causing the erratic response if it's glitchy but your repair shop should have been able to find that and fix (replace) it. The only really reliable way to find a bad TPS if it has a glitch is with a digital storage oscilloscope, which they have, hopefully.

37 MPH @ 4400 RPM sounds about right for that motor at that altitude, depending on the size and weight of your boat. There's a power loss of about 3% for every 1000' of altitude increase.

The fuel table in the ECM is fine for that altitude and there are plenty of boats running at 6000'. Above 2% throttle, the MAP sensor is what the ECM uses for input as far as load and unless there's significant increase or decrease in throttle position in a short time, there's no major change in fuel delivery, either rich or lean.

One thing I would look into, however, is the location of the MAP sensor and ECM. If the MAP sensor is below the throttle body, the tube going to it can become filled with gas and will lead to erroneous readings by the ECM. If the gas gets into the sensor itself, it can be damaged but I would clear it out and try it before replacing it. Since the boat is a '99, I would make sure the battery cable clamps and battery terminals are clean and tight and that the point on the block where the rest of the grounds are attached in clean and tight, with the wires and terminals intact. I know some people like to place the clamps on the posts, so they can remove them easier when they want to but that's just asking for trouble.

The ECM on this motor is probably a Delphi MEFI 2 or CEFI 3 if it's more square than rectangular. There's a possibility that you have one of the earlier Motorola controllers, but I thought Mercruiser changed to those in 2000. Either way, it works the same and uses the same sensors and codes. Get the codes form the dealer and post them here."

"JIMN, Thank you for all the i

"JIMN, Thank you for all the information I will contact the dealer and ask for the codes when I get back in town. I will let ya know."