please reply if you have any clue: Engine rev + stall at mid to high RPM problem

[cinar]

Hi All,

I had started a thread but after some more check up, here is the situation:
1999 5.0 L Merc with Alpha 1.
In gear, I rev up normal throttle to 2400 and as the boat starts planing, Tachometer needle acts weird: jumps from 2400 to 3K, to 4K to 5K and stalls, then I back down the throttle.
When not in gear, I rev up; no problem at all.
When WOT, feels ok, does the above problem time to time.

Checked all sockets, put dielectric silicone.
Removed the tank air inlet hose from the fuel cap, just to make sure I don't have vapor lock.
oil pressure ok
water temp around 172F
I keep the engine fire inlet cap open to get the heat out.
I keep the blower on all the time to make sure it is getting the hot air out.
I cleaned the whole fuel tank, got fresh gas (95 octane)
I was suspecting fuel pump, so changed the fuel pump.
Got new fuel water separator filter
Did not change the fuel regulator

Still same problem.
I had new acoustics insulation inside the engine bay cover, I had bought from West Marine.
So, I think I have multiple problems:
1. engine was getting hot b/c of the thick insulation material, one of the sensor started to act funny when heat was on
2. vapor lock

I am thinking to change all sensors, but it will cost me. Any ideas?

Thanks

 

[Don S]

Re: please reply if you have any clue: Engine rev + stall at mid to high RPM problem

EFI or carbed engine?

Could be cracked coil tower, and the tower is arching to the negative terminal of the coil which will go directly to the tack and the ignition system. Could also be a a defective tachometer. Just disconnect the gray wire for the tach from the coil (not the gray wire going to the module) then run it.

 

[Fun Times]

Re: please reply if you have any clue: Engine rev + stall at mid to high RPM problem

Since you said fuel regulator, Is this a carbureted engine or fuel injected engine?

Do you have the engine serial number?

If the tachometer is bouncing around that could be a sign of an electrical or ignition problem vs a fuel problem.

You may want to try unhooking the gray wire off the back of the tachometer or coil to see if the engine runs better.

 

[cinar]

Re: please reply if you have any clue: Engine rev + stall at mid to high RPM problem

Since you said fuel regulator, Is this a carbureted engine or fuel injected engine?

Do you have the engine serial number?

If the tachometer is bouncing around that could be a sign of an electrical or ignition problem vs a fuel problem.

You may want to try unhooking the gray wire off the back of the tachometer or coil to see if the engine runs better.

5.0L EFI.
Tomorrow morning I will do your recommendations and check out the plugs.
I will also get the serial number. Thank you very much. I will let you guys know.
I also suspected coil, but why would it run ok when not in gear? Or if a plug issue, it shouldn't even idle.
What do you think about TPS sending wrong voltage when in gear (or load), I can check that out too with a needle, croc and voltmeter on the cable (used to do on my mustang)