150 V6 shutsdown at 4000 RPM in 2 minutes

[Clint.Chapman]

I have a 150 V6 Blackmax 1992 it was just recently in shop. While in shop they modified the carburetors to run hotter (about 165? @ 3000 RPM).

I changed the water pump impeller right after that because they said the pressure was weak. The pressure guage registers about 3-5psi at idle and then about 10-12 psi at 3000-4000 rpm (it did not seem to change much).

When running for about 2-3 minutes at 4000 RPM the motor shuts down, just like you turn off the key. It can be restarted right after that.

It sounds to me like the tuning of the carburetors was not done right or that the thermostats are not opening all the way.

Any thoughts would be appreciated.

- Clint

 

[CharlieB]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

2 -3 minutes of operation could be either a restricted fuel supply OR an ignition component heating and failing.

I'd bet on the fuel system, a weak pump or air leaking into the fuel line somewhere before the pump.

In any case, you need to test for spark immediately when it dies to be sure you are looking into the correct problem.

 

[Clint.Chapman]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

Thanks for the reply.

So you do no think it is a temp problem... Hmmm. Another thread (different web site) said the same thing.

Another clue(?): Last year I put a full tank of regular 87 octane gas in. No stabilizer, but have tried to crank the motor and run it in the drive way each month. Could the fuel be bad? Would that cause this?

It seems to me if that were the case, then the problem would happen at all RPMs and it would not restart as easily as it does.

 

[hkeiner]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

It is easy to over think what the problem might be based upon how the motor is running. As previously suggested, it is better to systematically troubleshoot the problem. Check if the spark is failing when the motor stalls and report back. If it does fail, you can troubleshoot the ignition system further and you will get lots of advice on how to do that. If the spark is not failing, then test the fuel system. Report back and you will get lots of advice on that too. Troubleshooting using items such as spark testers, DVA meters, fuel vacuum gauges, multimeters, portable fuel tank, spray bottle of fuel, etc. will help you find the problem quickly without the risk of fixing/replacing good parts unnecessarily based only upon a hunch or guess or clue from how the motor runs. Just my thoughts...

 

[Clint.Chapman]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

More info...

While trying to reproduce the stalling, it was extremely difficult to maintain a constant RPM above about 2000 RPM. For example, I would force the motor to about 4000 then it would stay there 3-5 secs. then decrease to 2200 RPM, then increase to 3500 RPM, decrease, speed up etc. After about 5 minutes of this, the motor quite, and there was no way to tell if the spark was present as I had to be right on the throttle to keep the speed up above 3000 RPM.

It appears it must be under load to cause the stalling. While running under load (the last time I had boat on the water), it also did not appear to have this increasing / decreasing RPM problem.

One other thing, the battery seems to be weak. I had to connect a starter battery to get it running, and when I disconnected it while running (with the old battery still in place), it did not run long. Coincidence?

 

[CharlieB]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

Changing RPM under a load as you describe is often a symptom of either a weak fuel pum failing to supply an adequate amount of fuel for that load

OR air leaking into the fuel system, preventing the fuel pump from supplying enough fuel for the load.

EITHER way, the engine is probably running LEAN while under load and you may be damaging/melting piston(s) if you continue to run it!

 

[hkeiner]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

The troubleshooting steps described in the below linked thread may help you determine if you have a fuel starvation problem or not. Doing the suggested tests and reporting back on what you found will likely lead to the solution more quickly than what you may get based solely upon how you describe the way the motor is running. Just my opinion...

http://forums.iboats.com/showthread.php?t=305659

 

[Clint.Chapman]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

Thanks for the help y'all, I very much appreciate your time and help.

CharlieB, It does not appear to happen under load. It does stall at 4000 RPM under load after several minutes (see above). (diagnosis the same?)

 

[Clint.Chapman]

Re: 150 V6 shutsdown at 4000 RPM in 2 minutes

There were a few bubbles in the line, so I replaced the fuel supply line from fuel/water separator - no more bubbles.

Engine is STILL all over the place. It will not hold RPM above 3500.

On vacuum test, it pulled about 7 in. Hg with fuel supply clamped and held steady.

With increasing RPM, there was little difference in the supply pressure so it looks like it is not upstream of the motor.

- Clint