RPM fluctuation

[gaffer45]

Hi, Anybody have any idea why one 5.9 6BTA which runs great will get to cruise at 2500 rpms then have the rpms drop to 2000 and then return 2500, and repeat. New fuel filters. The other engine runs fine. 150 hours on recons.

Topic

[Brian]

Racors and last-chance filters changed. Topped off fuel, sea trialed last night, problem solved! Wish all could be so easy.

David, gaskets are fine at fuel fill. Thanks.

[David Shaffer]

Have you checked the O-Rings on your fuel fill? Water can get into your fuel that way and create your problem.

[Brian]

Rob:

Thanks. Yeah, I found last night easy enough. Takes a while to prime with those but got it done. Racors were absolutely loaded with algae, especially starboard side. I never saw it this bad and need to keep a closer eye on them. Put 30 micron’s on knowing I’ll be changing again in a week or so but I want to clear this up. Also a pair of vacuum gauges so I can monitor a bit easier. I’m going to start using Biobor JF religiously. I never really had a problem with my fuel based on where I had formerly purchased. I switched to a new location a few years back and this problem has become more prevalent, despite their being a large volume seller. Seeing this there is no question my RPM’s were impacted. I’m surprised the motors ran as well as they did. Going to top off tanks tonight and do a quick sea-trial. Looking forward to seeing the RPM’s back to normal. Fingers crossed!

Thanks again for your help.

Brian

[Rob Schepis]

480CE Lift Pump

Brian wrote:

The lift pump – little black button right close to filter? I think that’s the color. I noted location last time I was down there.

See attached (and don’t let the missing last chance filter fool you, it’s remote mounted to the opposite side on this particular engine)

[Brian]

Hi Rob:

I think they are struggling to make WOT RPM’s but fall off before reaching. Then the cycle starts up again a short time later. The more I think about it the more I feel that’s all it is. I’m pretty certain it’s the filters. Changing tonight with hopes it’s as easy as that.

The lift pump – little black button right close to filter? I think that’s the color. I noted location last time I was down there.

[Rob Schepis]

I would expect that if the fuel filters were an issue the engines would still struggle and could make WOT but briefly and then rpm’s fall off. What you are explaining sounds more like you are hitting a finite rpm number without any fluctuation at all, right?

Installing the on engine filters – install them dry and pump the lift pump with the key on.

[Brian]

Hi Rob.

Thanks. 10-4 on the “480 CE'” name. Good point.

I think we’ve crossed paths in the past. You sent me a photo of my boat in fact. Small world.

The first thing I thought about was getting ECM’s flashed/updated but getting Cummin’s marine out takes an act of God and they are not cheap. My engine sync seemed to be acting a little odd and I may have screwed things up through adjusting the RPM using the bridge +/- RPM switch. I was also out in some snotty weather last week and fuel was less than 100 gallons before I topped off. (I never run that low) Things may have sloshed around in the tanks and perhaps its as simple as changing the Racors but tell me, could that decrease my WOT speed then? Not that I run WOT but the fact that it is now seemingly restricted bothers me and I need to address. My guess is that possibly, but I’ve never experienced this before. I also wonder if the system I have with the engine sync has memory which perhaps I can clear buy removing all DC power for a period and then reconnect. Is essence, a reboot to resolve my messing with the RPM control and perhaps clear the sync allowing WOT. Coincidentally, my programmed engine cruise control speed is 2400 RPM and that is about my ceiling right now. A bit less than the 2600/2650 I formerly saw. (This was calibrated by Cummins several years ago when I had them out to flash the ECM.)

On another note, racors are easy but when changing the fuel filters on the engines, would I need to bleed the system? I keep spares but have always had a “real” mechanic throw those on. Thanks.

Brian

[Rob Schepis]

Hey Brian. I passed by your boat a few times on the dinghy a couple weeks ago. We spent a few nights at the Brewers Yard after being on the hook at Napatree.

You should always refer to your engines as 480CE’s, not 6CTA’s, as the quasi-electronics on your engine cannot be ignored, especially when troubleshooting.

Never hurts to change out all filters to take them out of the equation.

You can try having ECM’s updated.

Since you have twin engines you can either measure fuel pressure at the last chance filter or you can swap the lift pump/check valve between the engines and see if the issue stays or follows.

[Brian]

Similar issue

I’ve got 6CTA 8.3’s on my boat and started experiencing similar issue. Seems to only affect starboard side motor. I cruise as 2400 RPM. It will drop to 2200 or so and then recover. This occurred yesterday on way back from long trip. Both engines also seem to max out at around 2450-2480 RPM. Just a few weeks ago I would see 2600 +/-. Have not had a chance to look at or change racors yet but think maybe the fuel I purchased could be the cause. (I hope) Wondering if I sound also replace the filters on the engine for piece of mind. ???

[Rob Schepis]

gaffer45 wrote:

Something new, It has a lot of water in the fuel water separator , they were just replaced . Bad fuel?

If you think you may have taken on a bad load of fuel — Do you always fill at the same fuel dock or have you recently been somewhere new? Any dock neighbors having a WIF issue too?

Tony mentioned pics of your fuel fills. How about the 0-rings on them, are they good and lubed? While I am sitting at the fuel dock taking on fuel I always clean and lube the cap o-rings and any dock attendant whom I haven’t met before always says, “I’ve never seen anyone do that before”…………

[Tony Athens]

Send me a picture of the off-engine water separator filter.

When was the last time you checked it?

When was the last time you used the boat?

When was the last time you fueled?

Where are your full fills? Pictures please

Where are you fuel tank vents? Picture please.

Take off the on-engine filter, dump it into a clean glass bowl — Look closely–Pictures please

ADDRESS each item above.

Water in the fuel does not mean you have bad fuel, it means you have “WATER IN YOUR FUEL TANKS”..

You need to sort a few things out BEFORE to use the vessel again.

Tony

[gaffer45]

Something new, It has a lot of water in the fuel water separator , they were just replaced . Bad fuel?

[Tony Athens]

RPM dropping

Do you see the RPM on the tach change, or is it really dropping 500 and then recovering..Throtte linkage and lever is 100%, right?

If it is really dropping, then you need to install a 0-50-ish psi liquid filled pressure gauge on the on-engine fuel filter and watch it while you are running.

Always done this or something new?

Simple things first..

Tony

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gaffer45

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