Trimming Causing Misfire? 3.0L Alpha 1

[Grumman59]

I have an '86 Alpha 1 with a Mercruiser GM 3.0L. It has an Oildyne-Eaton hydraulic pump and the trim control on the throttle lever.

I just completed a comprehensive list of maintenance work on the out-drive -new bellows, hoses, lower shift cable, gimbal bearing, new U/Js, water pump, resealed the upper leg, and new trim sender/limit switches.

Before doing this work, there was a problem whenever I would use the trim causing the engine to misfire and stutter. I put this down to the fact that the wires were shorting in the bell housing where the insulation had fallen off. I assumed the new switches would cure that.

Now I just took the boat to the lake to test everything out and for about 20 minutes, everything was fine. Go, stop, reverse, WOT for a few miles, no problems at all. Getting a good smooth 40mph easily. The trim worked fine and the gauge worked too.

Then, I began to get the same symptoms as before when trimming and I also noticed that the voltmeter would jog back to less than 12v each time I pressed the lever. After that, the motor would misfire and run rough all the time whether I used the trim or not. Then, after mooring for a cool-off period, it ran perfectly well again for a bit but the trouble soon returned.

So I disconnected both the trim switches and tried the boat again using the trailer button for "UP" but it was still the same with the motor running rough all the time.

Sorry this is so long but any help or ideas would be appreciated.

 

[bruceb58]

Re: Trimming Causing Misfire? 3.0L Alpha 1

You have a voltage drop somewhere because of a resistive connection. Start with the connection at the starter. There will be the one cable that goes to your battery and the other one that is a smaller gauge that feeds the rest of your boat. Check that one.

 

[180shabah]

Re: Trimming Causing Misfire? 3.0L Alpha 1

While your at it, you should clean ALL the power connections.

 

[Grumman59]

Re: Trimming Causing Misfire? 3.0L Alpha 1

You have a voltage drop somewhere because of a resistive connection. Start with the connection at the starter. There will be the one cable that goes to your battery and the other one that is a smaller gauge that feeds the rest of your boat. Check that one.

So how would using the trim affect the engine performance? By affecting the coil voltage maybe? Unfortunately, I didn't have a multimeter with me when I tested the boat. I didn't think to feel the coil to see if it was unduly hot.

The voltage at the coil is 12v with the ignition switched on but the engine stopped although it's a resistor type coil.

 

[180shabah]

Re: Trimming Causing Misfire? 3.0L Alpha 1

Whats the voltage at the coil when the trim is active? bet it is less than 10v...

 

[Grumman59]

Re: Trimming Causing Misfire? 3.0L Alpha 1

No, it's still 12V. (I thought with the ballast resistor, it ought to be 9v or so except during starting).

It just dips from 12.5 to 12V when I operate the trim. But this is without the engine running and the problem may also be intermittent. The trim works and sound fine. No strange noises.

 

[bruceb58]

Re: Trimming Causing Misfire? 3.0L Alpha 1

It just dips from 12.5 to 12V when I operate the trim.

You need to measure with the engine running or the points closed. If the points are open, there will be no current going through the coil and hence no voltage drop across the ballast resistor.

 

[Grumman59]

Re: Trimming Causing Misfire? 3.0L Alpha 1

You need to measure with the engine running or the points closed. If the points are open, there will be no current going through the coil and hence no voltage drop across the ballast resistor.

Thanks, I'll fire it up on the muffs later and try that.

What I don't understand is why the trim would affect the engine running anyway. The trim pump has its own heavy duty wiring direct from the battery complete with fuses etc and the pump sounds good. The battery is new and all the wiring looks clean.

Before I renewed them, I thought the problem was the sender/limit switch wires shorting in the bell housing where the insulation had come off.

But I still got the misfire problem with the new switches whenever I would use the trim. At first, I thought I'd got the new wires pinched but the misfire was still there when I pressed the trim lever, even after I disconnected the wires from the motor.

 

[bruceb58]

Re: Trimming Causing Misfire? 3.0L Alpha 1

I had the exact same problem. I had a voltage drop due to a bad connection. In my case, operating the trim actually killed the engine.

 

[6meter]

Re: Trimming Causing Misfire? 3.0L Alpha 1

Your trim motor takes away the juice you need for the engine. Bad wire connections somewhere or alternator. Should be putting out 14.2 volts when running even with the trim motor running.

 

[Grumman59]

Re: Trimming Causing Misfire? 3.0L Alpha 1

Your trim motor takes away the juice you need for the engine. Bad wire connections somewhere or alternator. Should be putting out 14.2 volts when running even with the trim motor running.

That's interesting because I did check that on the water.

The alternator is putting out almost 15v (14.7 specified in the manual) but when I operated the trim and experienced the misfire, it would drop to around 12V. Never less.

 

[bruceb58]

Re: Trimming Causing Misfire? 3.0L Alpha 1

Where were you making your measurements?

Bottom line, go through all your connections, including grounds and make sure they are all clean. Use some sand paper on all the cable ends and make them shiny bare metal.

 

[Grumman59]

Re: Trimming Causing Misfire? 3.0L Alpha 1

Where were you making your measurements?

Bottom line, go through all your connections, including grounds and make sure they are all clean. Use some sand paper on all the cable ends and make them shiny bare metal.

From the voltmeter on the panel, not super accurate, but a guide at least. I'll take a multimeter with me next time, for sure.

 

[bruceb58]

Re: Trimming Causing Misfire? 3.0L Alpha 1

That is on the same circuit as your ignition basically so it proves you are getting a drop.

So...the path to your ignition is basically a thick wire from your starter to a circuit breaker. From there it goes through the engine harness up to your dash and to the ignition switch. From the ignition switch, it goes back through the engine harness to your coil or ballast resistor. Your voltmeter at your dash is basically meausuring what you would have going to your coil or ballast resistor if you have one. The drop proves that its back toward the engine and the common spot is probably at the starter itself.

You really need to use a multimeter to measure the voltages.

 

[bruceb58]

Re: Trimming Causing Misfire? 3.0L Alpha 1

One more thing. Make sure your dwell is set properly. If the dwell angle is too low, the coil is not going to get as high of a charge in it. This will of course be magnified with a lower voltage to the coil. I still think its a bad connection...just giving you plan B.