Strange RPM problem

Don Lucas · Apr 10, 2015

But I've had problems with the tachs on both the boats I've had. The first problem was just corrosion on the sensor wire. My current one seems to come on go and I'm pretty sure it is the tach itself and it seems to be dependent a lot on the temperature.

The problem you seem to have sounds more like a wire problem. Check and clean the connector at the flywheel. Check the wire harness at the plug at the engine (on my last boat it corroded and I had to jumper it), and on the back of the tach.

I've kind of lost interested in fixing my tach to get the reading right. But it seems to have moisture in it that keeps me from reading the engine hours and that is important to me.

Capt jgw · Apr 10, 2015

Alternator kicking in when a heavy load, like the fridge, comes on? I once had a bad voltage regulator that would cycle full field on and off. RPMs would drop and rise about 150. I knew it wasn't the tach, I could hear the change in speed.

Stu Jackson · Apr 10, 2015

Jim, do you have an external fuel ("lift") pump?

Glad you had a great trip - you deserved it!

Stu

woodster · Apr 10, 2015

Jim Seamans said:
Reference

Tack - no, it really is dropping the RPM's

Alternator - no, this is happening well after the the batteries have gone into float mode

Fuel lift pump - dont' think so it is a standard 3GM30F

SCRATCH SCRATCH

is your vent tube good and clear on the tank ...also check and make sure nothing is mashing the fuel lines

31seahorse · Apr 10, 2015

(1) A clog in the fuel system could be the cause. There might be buildup in the small screen that is often in the pick-up tube in the fuel tank. (2) A mechanic told me that fuel lines can deteriorate on the inside and bits can block the flow intermittently.

I have experience with the first problem.......clogged screen. Easy fix. Welcome back to the Chesapeake Bay! Phil

Jake N Eggs · Apr 10, 2015

In general, the governor controls the fuel delivery rate. The governor is usually weights that are thrown outwards due to centrifugal force. There is a spring mechanism that tries to move the throttle control to idle. When the throttle is opened or increased it puts greater tension on the spring that is trying to move the mechanism to idle.

The governor is always moving with the spring tension as a function of its operation. That is why a governor holds engine rpm at a given speed plus or minus a standard percentage. It could be that while running for some time at a given rpm, when the engine comes under a little more load the governor is not reacting to the decrease in speed until the speed drops to a critical point. Once the governor reacts it comes back to the speed set by the spring tension of the throttle control. This could be caused by wear on the governor shaft causing the spring to catch or it could be the hysteresis of the warm governor spring.

Usually if it is a fuel supply problem, where the pipes are blocked or the filter is clogged, the rpm will not recover without a reduction in fuel demand. If you ran for many hours after the rpm recovered I would suspect the governor issue before the fuel supply issue.

Good luck.

Johnb · Apr 10, 2015

I had a similar problem on my 3qm30 that went away after replacing the camshaft driven primary fuel pump. Intermittent strange loss of power and then recovery. One way or another it could be fuel starvation.

It was a cheap item and easy to get at.

chuckwayne · Apr 10, 2015

HI Jim, Welcome back! we had another 3" of snow this week - the cover may not come off Escape until July!
All good suggestions...... I've also had seaweed on the prop cause that kind od sudden drop - then if the weed clears the prop it comes back up again.
I've also experienced the screen clog doing the same thing on my 29.5 - but I asked about the 356 and Hunter said it doesn't have a screen on the pickup

Granfalloon · Apr 12, 2015

I had problems with my Tach on a 2GM20F. I bought a device called Tiny Tach for Diesels. You connect a sensor to one of the fuel injector lines. It as a digital readout of RPM and engine hours. It has countdown timers and flashes when it is reached. For example 50 hours and you change your oil. Not as expensive as a new Tach. More accurate and easy to install.

grandpasteve · Apr 20, 2015

No sense message sent from phone, sorry. Delete if possible.

ramblinrod · May 26, 2015

Could be plugged fuel line. Just pull it off and blow into it, back into the tank. If solved, you may want to think about cleaning out the tank.

Could be water. Ensure the water separator is emptied.

Could be an air leak into the fuel system. Check the gaskets on fuel filters and water separators.

Ramblin' Rod
www.sheenmarine.com

hvbaker · Jun 4, 2015

I just had a very similar problem. 2500 rpm dropped down to about 1000 for several seconds then back up. At first after running for 6 hr then again after about 1/2 hr then as rapidly as once every 5 minutes. Pulled over tied up replaced 2ed fuel filter problem went away. Continued trip for another 4 hr no problems.