Engine Accelerates/Decelerates on its own

[Tom Kazakoff]

The engine is a Mercruiser 357 Alpha 4v. While running, the engine will start to speed up. If I back off on the throttle, it will speed up again after maybe a minute. This continues. After trolling for a couple of hours, the engine will do the opposite. If I am running at 18 mph, soon I will be at 16...until it gets to about 12 mph. If I back off the throttle, than accelerate, all is good. This pattern continues.

Prior to this behavior, after trolling, the engine would run fine for about 10 minutes, then it would decelerate to around 5 mph.At this point I could not accelerate at all. The engine would backfire, and miss violently. I changed the sensor that is inside the distributor. This caused the engine to go from the backfire, missing to run as described above. Prior to the new sensor, the engine still did (does) the "accelerate on its own" behavior

Salmon season is coming soon and it would be great not to get stuck 20 miles from the harbor again..

Thanks,
Tom Kazakoff

 

[Scott Danforth]

welcome aboard

you have dirt in the carb or another fuel system issue causing it to run lean

check the contents of your fuel filter
get a carb rebuild kit
check your fuel pressure

 

[Bondo]

Ayuh,...... Welcome Aboard,...... I agree with Scott, I'd be lookin' at fuel delivery,......

That motor has the Merc/ Weber AFB carb, Right,..??

 

[QBhoy]

You almost certainly have water in your fuel or your fuel is way past it’s use.

 

[Tom Kazakoff]

I am not sure of the carb type. It is what ever comes with a complete re manufactured engine.
The fuel pump has been changed and fuel pressure checked. The engine has also had a new distributor cap,
rotor,coil, new wires and plugs. I intend to pull the plugs and check the color. There was a flaky wire from the ignition that would cause the engine to cut out when wiggled, but that has been replaced.

We have also pulled the fuel intake probe from the tank. It was clean with no obstructions.

I have been doing some research, and a few new possibilities came up. One is the Map sensor. I am not sure if my engine has one, but a bad one does describe some of the symptoms I have. The others sensor is the Block Pressure Sensor. If anyone has any experience or thoughts on changing these, please advise.

The other going theory is the electronic modue. This is expensive, and it would be great to know it will solve the problem before buying one.

I have not done anything to the carb. A rebuild kit sounds like good advice. I was also told to check the inline filter that is right before the carb.

Thanks for the input, and hopefully with some persistence I can have a reliable boat again.

Tom Kazakoff

 

[Bondo]

I have been doing some research, and a few new possibilities came up. One is the Map sensor. I am not sure if my engine has one, but a bad one does describe some of the symptoms I have. The others sensor is the Block Pressure Sensor. If anyone has any experience or thoughts on changing these, please advise.

The other going theory is the electronic modue. This is expensive, and it would be great to know it will solve the problem before buying one.

Ayuh,..... Those sensors are used on fuel injected motors, not carbed motors,.......

The T-Bolt ignition usually works,.......... or it don't,........ as in no spark, won't run at all,.....

 

[Scott Danforth]

I have been doing some research, and a few new possibilities came up. One is the Map sensor. I am not sure if my engine has one, but a bad one does describe some of the symptoms I have. The others sensor is the Block Pressure Sensor. If anyone has any experience or thoughts on changing these, please advise.

you dont have an ECM or a MAP sensor on a carbureted motor.

I am not sure of the carb type. It is what ever comes with a complete re manufactured engine.


I have not done anything to the carb. A rebuild kit sounds like good advice. I was also told to check the inline filter that is right before the carb.

you need to know what carb you have so you can properly troubleshoot it as well as get the correct rebuild kit.

have you emptied the contents of your fuel filter into a clear container and looked? simple test to see if you have water or crud in your fuel (you will need a new fuel filter)

what is your fuel pressure?

 

[Rick Stephens]

Enough with diagnosing by buying random new parts. Seems like the only thing determining what parts you are buying is which is the next least expansive. Diagnostics, not parts swapping, is what will fix this. Start with fuel system, not ignition.

 

[Lou C]

Start with fuel filter, fuel pump, fuel vent and anti siphon valve. If these check out ok them move to carburation.
problems with electric fuel pumps and pump relays are fairly common.

 

[Lou C]

PS does this engine have a conventional dist cap or the crab style cap? The crab style ones apparently can cause trouble often.