1996 Mercury 115 Dies at Slow Speed

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
houndman
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1996 Mercury 115 Dies at Slow Speed

Postby houndman » Sat Apr 22, 2023 4:23 pm

My 1996 MERCURY 115-HP engine [later clarified to be a four-cylinder in-line engine in which only two of the four cylinders operate at low speeds, a design known as the 2+2 engine] suddenly shuts off at less than 2000 rpm. It doesn't spit and sputter. It dies suddenly as if it was switched off. Most times it will restart and will run for several seconds to a few minutes before it dies again. Sometimes it will not restart no matter what I do. I have to wait an hour or so before it will start up again.

If I successfully get the engine running and the boat on plane, the engine runs fine at speed.

On April 21, 2023 the engine ran for many miles at WOT and it never missed a beat. As soon as I got near the landing and brought the boat off plane and the rpms got below 2000 it died again.

I've owed the boat for ten years. I've never had any major problems. Just regular maintenance items. It's always run like a sewing machine.

Q1: what is a likely cause of the problem with the engine stalling at low speeds?

jimh
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Re: 1996 Mercury 115 Dies at Slow Speed

Postby jimh » Sun Apr 23, 2023 2:10 pm

When an engine stops immediately without sputtering or decelerating, the cause is like due to an electrical problem, and most like due to loss of spark.

Obtain an in-line spark gap test tool. The next time the engine refuses to start, insert the in-line spark gap test tool into the spark lead for the cylinder number one. You should be able to observe the arc jumping the spark gap in the tester if there is good spark voltage being generated.

In the 1990's Mercury engines were somewhat notorious for failures in their spark generation circuitry.

Also, mention more details about the engine.

Q2: Is this a two-stroke-power-cycle engine with a power head made by Mercury that has four in-line cylinders?

houndman
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Joined: Sat Apr 22, 2023 4:06 pm

Re: 1996 Mercury 115 Dies at Slow Speed

Postby houndman » Sun Apr 23, 2023 8:49 pm

[Clarifies that the engine that is the topic of this thread is a] 1996 Mercury 115-HP two-stroke-power-cycle engine with an in-line four-cylinder block in which only the top two cylinders operated below about 2,000-RPM. This engine is known as the 2+2 design.

jimh
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Re: 1996 Mercury 115 Dies at Slow Speed

Postby jimh » Mon Apr 24, 2023 10:21 am

Thanks for the clarification that the engine under discussion here is the infamous 2+2 engine. The 115-HP 2+2 engine is known for exhibiting running problems related to the unusual (and never repeated) design of the (more or less passive) system to disengage two of the four cylinders at low speeds by not having low speed jets in the carburetors for the two cylinders that won't be active at low speeds

Because the problem you describe is related to low speed stalling, another reasonable inference is the problem is due to lack of fuel flow from the low speed jets of the two carburetors associated with the two cylinders that are supposed to remain active at low speeds.

If you perform the recommended spark check test I described in my initial reply, and if that test shows that the two cylinders that are supposed to be operating are getting good spark, then you should look into the carburetors. Check for debris in the fuel system.

If the recommended spark check test I described earlier in my initial reply shows that there is a loss of spark, then the cause may be due to a fault in the spark generation system. In engines of that epoch, Mercury often used an unusual method for generation of spark. Two different spark generation systems may be used in your engine. One system provides spark at low engine speeds, and a second system supplies spark at higher engine speeds. Because the engine seems to stall without any stumbling or run-on, this behavior suggests an immediate loss of spark.

If you want to make a fundamental diagnostic test on the engine yourself, you will need a method to test for spark. An in-line spark gap tester is the simplest method to perform a spark test.

A good in-line spark gap tester is made by Lisle and is available from SUMMIT RACING. See

https://www.summitracing.com/parts/lil-20610

I have the Lisle tester and have used it many times as a method to determine if there is spark being generated by inserting the tester in the spark plug leads for each cylinder, and observing for the presence of spark when the engine is operating abnormally. Seeing the spark in the spark gap tester is made more easy if the tester is not position in direct sunlight.

If you want to perform service on your 27-year-old Mercury outboard engine, you should get the factory service manual from a Mercury dealer. Trying to work on an engine without any sort of factory service manual is often not a productive method to make repairs. Also, due to the very unusual design of this 2+2 engine, its age, and the relative obscurity of this method, finding an authorized Mercury dealer with technicians that are familiar with this engine may be difficult. Performing your own service may be your only resort to getting a remedy.

Perform the spark test and reply with the results observed.

houndman
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Joined: Sat Apr 22, 2023 4:06 pm

Re: 1996 Mercury 115 Dies at Slow Speed

Postby houndman » Sun May 28, 2023 8:51 am

UPDATE on loss of spark in the 1996 Mercury115 2+2 engine:

I purchased a spark checker, a remote starter switch and a DVA for troubleshooting purposes. After watching many videos on how to check the stator and trigger resistances and voltages, I checked mine and everything checked fine but [the 1996 Mercury 115-HP 2+2 engngine] still had no spark on any plug during cranking after the engine was warm. What little spark I did occasionally get was very weak. Everything pointed toward the switch box being bad.

Before buying an expensive [new switch box], I downloaded the trouble shooting guide from the CDI website. The second entry on the troubleshooting guide is to unplug the two yellow stator wires that go the the voltage regulator. I figured there was no way the voltage regulator would cause a no spark condition since it is an isolated circuit that only charges the battery. As soon as I unplugged the yellow wires [the engine's spark ignition generating circuit] starting throwing lighting bolts across the spark plugs.

I have twice run the boat and engine on a lake with the yellow wires unplugged, and the engine ran perfectly. I have not yet replaced the voltage regulator. I wanted to be certain the voltage regulator is the problem before ordering [a new voltage regulator] part.

I'm puzzled as to how the voltage regulator will cause this condition. Perhaps a certain failure mode causes the voltage regulator to emit high frequency [signals] which [interfer] with the switch box. I've ordered the Factory Service Manual to try to deduce how the voltage regulator can cause this condition.

I'll report back with results after I replace the voltage regulator.

houndman
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Re: 1996 Mercury 115 Dies at Slow Speed

Postby houndman » Fri Jun 02, 2023 5:01 pm

I have good news and bad news.

Good News: when I attempted to install the new voltage regulator yesterday I had to take off the oil tank to get to the connections behind the tank, however, the oil tank was stuck and wouldn't come loose.

The cause of the difficulty to remove the oil tank was a bullet connector on one of the yellow stator wires had melted into the plastic oil tank. Also, on each side of the bullet connector for about an inch of [the wire insulation] had melted, exposing the bare wires.

I cut off the bullet connector, and I soldered the two wires together. Then I ran the engine using a hose adaptor for a long time. The engine read fine'

Before the problem [with lack of spark] started, I had also noticed a lot of interference the SONAR screen, and the engine tachometer gauge had become flaky. Neither of those symptoms were apparent today.

I assume that the bad bullet connector connection was arcing and creating a lot of high frequency noise that was interfering with the switch box.

Bad News: I have a brand new, unused CDI Voltage regulator that I don't need and can't send back

jimh
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Re: 1996 Mercury 115 Dies at Slow Speed

Postby jimh » Sat Jun 03, 2023 6:26 am

Congratulations of getting the problem of the engine dying at slow speeds resolved.

Using an in-line spark checking tool, as I suggested, apparently helped you to identify the cause of the engine stalling at slow speeds as being related to loss of spark.

Your close visual inspection of the wiring associated with the yellow insulated conductors from the engine alternator coil winding led to the ultimate remedy of the problem. Although electrical current flow is invisible to the eye when occurring in a conductor, problems with electrical current flow often leave visual evidence. A careful visual inspection of all electrical circuit components can often lead to discovery of a problem, as you have demonstrated in your investigation into solving the problem of no-spark and ultimate stalling of the engine at slow speeds by finding the bad electrical connection in the circuit at the bullet conductor. The melting of the plastic oil tank adjacent to the conductors was evidence of generation of considerable heat, as was the melting of the wire insulation.

I also suggest to check the voltage on the engine cranking battery when the engine is running at speeds above 1,500-RPM to be certain the existing regulator is properly limiting the charging voltage output to 14.4-Volts or less. If you find that the existing voltage regulator is not maintaining the charging voltage output to a reasonable limit, you may need that new voltage regulator assembly you bought.

houndman
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Joined: Sat Apr 22, 2023 4:06 pm

Re: 1996 Mercury 115 Dies at Slow Speed

Postby houndman » Sat Jun 03, 2023 8:08 am

I checked the output voltage while it was running on ear muffs in the yard and the old voltage regulator is working fine. It outputs a steady 14.0 volts at idle and 14.2 volts at 3000 rpms.

I now know more about an outboard ignition system than I ever wanted to know., but knowledge is a Good Thing.

I'm just happy [the problem with the Mercury 115-HP engines is] fixed. Yesterday I took my 85-year-old pappy angling for Crappie, and we had a blast.