1992 90 HP cuts back to 2500 rpm intermittently

[tthrone]

Hi,
I have a 1992 Evinrude 90HP outboard that developed a problem last year. It had been running normally, when at full throttle it suddenly cut back to about 2500rpm even while control was at full throttle. It intermittently throttled up and down by itself with control at full. No signal horns or other clues. I thought it might be a clogged fuel filter so I replaced but that changed nothing, Also, I replaced the thermostat, installed a fuel pump kit, rebuilt the carbs and replaced the fuel line from tank to engine but no change in symptoms except that now, when hot, it tend to stall at low idle. That could be carb adjustments.

I read that the engine could limit to 2500 rpm if overheating. The cooling water impeller is new too, but the symptoms remain. The engine usually starts and runs well for a while, maybe 20-30 minutes before trouble. I have had it cut down and then ramp up to full in as little as a few seconds or minutes of running.

I thought it might be the VRO fuel pump unit, but they are $500 to replace so before I do that, I want to try less expensive repairs. Any ideas?

[Joe Reeves]

NO Warning?? Do this.... Key in ON position (engine NOT running)... find the overheat sensor wire protruding out of the starboard cylinder head and ground it to the powerhead.

That should make the warning horn sound off constant and steady. Does it? If not, find out why as that horn is the only warning you'll receive as follows:

********************
(VRO Horn Warnings)
(J. Reeves)

NOTE: I retired around 1991/92. Possibly some of the later V4 engines and others may also incorporate a fuel vacuum switch that would enable a fuel restriction warning to sound as mentioned below, an unknown factor to myself.

1 - A steady constant beep = Overheating - The V/6 engines, possibly some others, have a fuel restriction warning which is also a steady constant beep.

2 - A beep every 20 or 40 seconds = oil level has dropped to 1/4 tank. (Late model engine = Every 40 seconds)

3 - A beep every other second = VRO failure, air leak in oil line, oil restriction, (anything that would result in a lack of oil being supplied to the engine).

NOTE - If the warning horn is the black plastic (overpriced) three wire type horn, the warning horn should beep once when the ignition key is turned to the ON position. If it does not, it is either faulty or someone has disconnected it (a stupid move!). At any rate, if it does not beep which indicates that the horn is non functional, find out why and do not run the engine until the problem is corrected.
********************

The drop in rpms to 2500 rpm is a built in "SLOW" warning along with the warning horn to prevent extreme overheating that would destroy the engine.

[tthrone]

Test results

Thank You Joe Reeves. I tested the temperature and the signal horn. I get a single horn when I turn the ignition key to "on", and when I grounded the temperature sensor wire to the block I got a constant horn signal, as you indicated. The horn sound is a bit weak but distinguishable.
So what next? Does this test just verify the horn is working? As I mentioned before, the impeller for the water intake is new as is the cup it rides in, and the thermostat unit is new. When I run the engine, there is water flowing out of the upper port which as I understand it, indicates water is flowing up and through the engine block. So it it possible the temp sensor is failing and sending an invalid signal to reduce fuel? Is there a way to test that? And what about the fuel restrictor. Is that an electronic unit that could be failing too?

Thanks again fro your reply and help.

[Joe Reeves]

You asked: "And what about the fuel restrictor. Is that an electronic unit that could be failing too?"

It is an electrical unit that could fail BUT I don't think your engine is equipped with it.

The dropping of rpms to exactly 2500 rpm indicates the engagement of the "SLOW" system due to a overheating problem.

Temporarily disconnect the two wires leading out of the PORT cylinder head
(overheat and SLOW sensor) just to see if that cures the problem "One Time", that is don't continue to run the engine for any length of time if that DOES correct the problem... you just want to see if that sensor in the cylinder head is bad or not.

If that cures the problem, replace that sensor.

This is a$$uming that the engine is not actually overheating. If you discover that the engine is in reality overheating, inspect the water pump first, then the water deflectors.

[tthrone]

Update on this problem. I replaced thermostats and I also saw a tip about the water deflectors, so I removed both cylinder heads and cleaned them out real good. There were some sandy deposits under the cyl head cover that I cleaned out. The deflectors were worn and the bottom one on the port side was clearly displaced. I replace all four deflectors and I replaced the temp sensors on both heads. New gaskets, of course and tested the temp sensors by grounding to the block. Constant signal on the horn as expected.

I had a water leak from under the exhaust manifold, so I removed that and replaced all the hardware (four bolt heads snapped on removal) cleaned and tapped the bolt holes, new gaskets and new hoses on the thermostats and the exhaust manifold. Put everything together - tight and dry! Ran just on muffs and did well. Telltale had good water flow. Put a temp sensor on both heads after 30 mins or running between 2500 and 3500 RPM and temp only rose to about 130 -140 deg.

Put it in the water today. Started and idled no problem, but as soon as I put under load, it had only half power (abt 2500RPM) Put In neutral, then hit throttle again and it rose up like normal for about 30 secs then cut back to 2500 RPM again. No warning horn and too short a running time to be overheating.

Previously replaced plug wires and plugs, rebuilt carbs and rebuilt fuel pump (diaphram and gaskets), replaced gas hose from tank to engine including new bulb, and new fuel filter. So what's next?
BTW - I replaced the water pump last summer and it has practically no use use since installed.

[tthrone]

One more thing - the water pump is 1 yr old with practically no hours on it.

[tcnoithat8]

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