# Engine rpms fluctuating



## c40eb (Sep 12, 2002)

Today, I started my 1988 Westerbeke 20B Two for the first time this season. I had replaced the filter elements in the Racor fuel/water separator, and the primary fuel filter earlier.

After bleeding the system using the manual fuel pump lever, the engine started just fine.

However, at any speed over idle, the rpms are fluctuating (over 1000 rpm range). Along with that, the voltmeter is fluctuating as well. 

So, both gauges are going up and down continuously at any engine speed over idle. I can hear the engine noise pitch change as it fluctuates. Also noticed the temp gauge didn't seem to be registering at all, though the engine was warm (had an overheat issue due to coolant air lock, which I think I've fixed).

The weird thing is, the actual engine speed doesn't seem to be changing that much...not 1000 rpms worth. Seems just to be a little speed change. But, the gauges are jumping wildly.

Any ideas what might be causing this? I thought maybe air still in the fuel lines causing erratic running, but the fluctuation is constant (i.e. a steady up and down of rpms).

Thanks for any help!


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## poopdeckpappy (Jul 25, 2006)

my guess is a lift pump problem, I have a Perk 4-108 that did the same thing


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## scottyt (Jul 19, 2008)

it sounds to me like an alt problem, maybe just connections. if the tach comes of the alt like some do that can make the tach do funny things. then as the load on the motor from the alt is drastically changing it can pull the rpms down. on my boat switching to charge a dead battery can pull the rpm down 500. ( 8 hp yanmar 55 amp alt )

first thing i would check is the engine ground, but if it started okat thats probably fine

edit you can test this by pulling the alt belt, if it does the water pump too just pull the wires of the alt, should just be 3 or 4, no need to pull the big one to the battery thou, but you should anyway for safety


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## zeehag (Nov 16, 2008)

only common thing on the boat is the ground....check those.....


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## svHyLyte (Nov 13, 2008)

If your batteries are fully charged, your voltage regulator may be kicking the alternator on and off which loads up the engine for a moment and, with the tach driven by the altenator, causes the RPM indicator needle to fluctuate more widely than the actual variation in engine RPM. Start the engine and try applying some loads to your electrical system. Lights, electronics, etc. Or, just leave a few lights on over night. Odd's are the variation you're seeing will disappear.

FWIW...


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## c40eb (Sep 12, 2002)

svHyLyte said:


> If your batteries are fully charged, your voltage regulator may be kicking the alternator on and off which loads up the engine for a moment and, with the tach driven by the altenator, causes the RPM indicator needle to fluctuate more widely than the actual variation in engine RPM. Start the engine and try applying some loads to your electrical system. Lights, electronics, etc. Or, just leave a few lights on over night. Odd's are the variation you're seeing will disappear.
> 
> FWIW...


Interesting...thanks. The batts are fully charged. I'll double check all wiring (engine ground & alt wiring), then apply loads.

Thanks for all the replies...!


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## c40eb (Sep 12, 2002)

*Update to fluctuating RPMs*

Ok, checked wiring, and from what I can tell, all are good and connected. Ground is good...engine starts fine...fires right up.

I did find a disconnected wire, but it was the temp sensor wire. Also, disconnected and reconnected the R, L and P wires.

Now, the wild bouncing is less. Just a little...very little discernible rpm change from the engine.

But, the voltmeter is WAY to high. Reading over 15.6v. Batteries are getting too much charge. I can hear the electrolyte bubbling away (when engine gets shut off). It was doing this from the start of this problem. Checked with the boats vmeter and my digimeter. Checked voltage at alternator...all agree. Too much into batts.

What I've read indicates overcharging is caused either by a broken/disconnected sensing wire (still not sure which one this is, or where it goes) or faulty regulator.

Guess I'll bring the alternator in to a shop for testing & rebuilding (if nec).


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## TSOJOURNER (Dec 16, 1999)

c40eb said:


> Guess I'll bring the alternator in to a shop for testing & rebuilding (if nec).


Sometimes the regulator is part of the alternator, but usually - at least in boats - it is not. It's a separate thing.

Here's one, so you know what you are looking for:









You already know the alternator is working. After all, if it was not working, it would not put out 15 volts, right?
So don't take the alternator in. Alternators only put out what they are asked to, so the culprit is the regulator one way or the other.
And the sensing wire is almost always built right into the wiring harness, so the probability of a loose sensing wire is just a theory. Probably.

Your regulator is acting just like it would if it could not see the batteries at all. 
*WHAT? NO VOLTAGE??!! FULL SPEED AHEAD!!!*

I'd replace the regulator


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## delan (May 2, 2009)

This is a diesel.
nothing electrical will affect the rpm's except load, and that no more than 50rpm or so
It's an air leak, on the fuel intake side, re-check the seals on the racor, you replaced them all right?


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## c40eb (Sep 12, 2002)

This Mitsubishi 50A alt has an internal regulator. Have to take apart the alternator to get at it. Fun! 

All three wire plus the battery cable were connected correctly and well. However, that doesn't mean the sensing wire is actually working!

I agree the alternator is acting like it is not sensing the batteries' charge and blasting the volts to them. So...either the voltage sensing wire is shot, or the regulator is kaput.

I did replace all seals in both filters. Double checked them and can't find any leaks. I'll triple check the fuel lines to ensure they're not pulling air.

I was see-sawing between electrical and air leak in fuel line. When this first started (see OP), the RPMs were bouncing wildly (as was the voltmeter)...over 1000 rpms on the tach. Though the engine did not sound like it was racing that high. So...that made me think electrical...that and the voltmeter bounce, and high output. 

I'll bring the alt in to get tested...then will know one way or the other.

Thanks for the replies...I really appreciate the input.


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## delan (May 2, 2009)

You may also check the ground to the guages, if it's corroded or unattached, the voltmeter will find ground via the engine sensors, intermittently.
Check all the grounds.


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## c40eb (Sep 12, 2002)

*Update #2*

Well, brought the alternator into Ron's Auto Electric in Windsor, ME. Ron stuck it on the big testing machine and confirmed there is something up with the regulator. It's "bouncing"...meaning, the output is fluctuating.

He's gonna pull it apart and check out the regulator.

So...seems I'm on the right track.

Thanks again to all for the input.


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## c40eb (Sep 12, 2002)

*Update #3 (FINAL!)*

Well, picked up the alternator today. Ron said someone previously had decided to paint various parts of the alt. Parts they shouldn't have. Interior surfaces that should be bare to make a good ground were painted.

He cleaned it, tested it and all is good.

I just reinstalled it, cranked up the engine, and all looks great! Gauges steady and volts where they should be. The regulator is regulating correctly.

So...now just a matter of finishing the cleaning & waxing and getting in the water. Hope by end of next week.

Thanks to all who replied! Much appreciated.


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## zeehag (Nov 16, 2008)

goood to hear ya got the problem fixed----


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## russread (Apr 1, 2019)

Sounds like 2 problems. On the overcharging, there was another possibility, besides sensing wire/regulator... Check your battery bank. One dud battery will demand way too high voltage from the regulator, as it tries to recharge an apparently flat battery bank.


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