Merc '95 150 warning horn question

[pablo1804]

'95 Merc 150 carb type, black, oil injected.

I'm having an intermittent horn warning together with a loss of RPM after running at about 4200 RPM for about 10 minutes. When the loss of RPM occurs, the horn makes the beeping sound, but it may be interrupted or disappear after a throttle reduction or reducing to idle. The RPM will not reset after cycling the ignition switch. When I run the engine the following day, it will come up to full power, but the same problem will occur again after about 10 minutes running at 4000 to 4500 RPM. I have changed the pump shaft sensor and reservoir level is correct.

Telltale water flow looks good, but I suspect it is a temp problem and not oil related. What I plan to do is hook a premixed tank to the engine (just in case), disconnect the reservoir and shaft sensor leads to the warning module and run it again. If the warning recurs, I think I will have eliminated an oil related source.

I have inspected the thermostats and found that they had been gutted by the previous owner and are just there as gaskets. Can anyone please suggest something else I might check before the next run, such as how to check cylinder temp sensors.

Any info would be greatly appreciated, as I do not have a manual yet.

Thanks,

Paul

[optimax135]

An intermittant beep is definately an oil problem. You may want to look at your warning module. It may be reacting toward a wrong signal or weaker signal from the sensor. But go ahead and try the spare tank to make sure your actually not losing oil at that RPM. Be careful with how you solve your problem. It may cost you a new power head. When in doubt mix oil in your gas until you find the problem. Good Luck! Dale

[sosmerc]

You may be losing ignition voltage to the cylinder that supplies the oil warning module with a voltage pulse. Check your output voltage to coils with a DVA meter at 4000 RPM. You may have a bad stator or a bad switchbox. (the sugguestion to run 50:1 in your fuel tank during testing is a good one).
You should also visually inspect the nylon oil pump drive gear...look for signs of shredding plastic and very thinned and sharp gear edges.
I don't think a malfunctioning oil warning system would cause a drop in rpm, unless somehow the module is grounding the green wire that goes to the outer switchbox.