Engine RPM

[furball]

What could cause engine idle rpm to monmentarily drop from 720 to 600 when I use the bow thruster? 2 second bow thruster shot and idle drops for a couple seconds then comes back. Had another idle issue and hard starting 2 weeks ago that I'm suspect is related. Changed all fuel filters, checked vent lines. No smoke, no other apparent problems. Hoping this new piece to the puzzle helps narrow down the problem.
Thanks,
John
2006 28TE with Yanmar 6LP

[DougSea]

My very quick reaction is that you have a battery issue and it's hitting the alternator hard, and putting load on the engine, as the voltage drops.

[furball]

Thanks Doug,
New batteries and new ACR start of this season. I took a video on my IPhone because it was very repeatable. Watched the volt meter drop to 12.3 or so under thruster load and immediately return to 14 on release. 1-2 seconds later, rpm drops for about 5 seconds then comes back. I've done this at the dock with 7 year old batteries and never saw an rpm drop. Couple of weeks ago I had a sudden issue at idle, unsteady rpms and then constant 850 at idle. Then hard start. Changed filters and ran good this weekend but I don't think it's filters or fuel quality. I think it's fuel pump related. Is the fuel pump electric? Voltage doesn't vary from the alternator. Both analog dash gauge and Link20 monitor show 14 volts. What electric draw could effect rpm. What mechanical issue could put load on the engine? I need to understand Diesel engines better. Need to revisit taking that Mack Boring diesel class.
Thanks for the help.

[RobS]

Fuel pump is a standard mechanical pump. Why did you replace batteries? What is ACR? Back in my days on two wheels that would be automatic compression release

[DougSea]

Is the ACR (BlueSea Systems Automatic Charging Relay) a new item on the boat? Is it the only change between the old battery system and the new one? If so I might temporarily remove it and test. Is it installed correctly?

The voltage from the alternator is only part of the story, can you measure the amp draw? Because that's where the load on the engine would show itself.

There are only two ways I can see a big electric load like a thruster impacting a Diesel engine. Load, as I suggested above, or a fuel pump as you also ask. Do you have an electric lift pump? Shouldn't be too hard to track down.

Let us know how the mystery unfolds!

[jleonard]

I instyalled an ACR 2 years ago and had trouble with it. Bottom line the Blue Seas tech guy said its because my battery banks were too different in size and when the main one is low it drops voltage too low to transfer to the other.
It is now bypassed.

[furball]

I had 2 4D batteries from the original build, 2006 to 2013. Got 7 years out of them. End of the season last year, they had given their all and were replaced by 1 4D and 1 Group 31, both AGM. Shortly after battery replacement the charger died at the very end of the season. March brought a new charger and I also replaced the ACR (Automatic Charge Reply). I replaced it because since I bought the boat in 2007, the electronics, mainly the depth finder, would drop out during re-start, regardless of battery switch position. The ACR and charger were install at the end of March this year. I've been on the boat every weekend since then with no battery issues, no more electronic drop outs on restart and no issues with the thruster. I will bypass the ACR this weekend to check it but I think it's a fuel pump issue. I can measure amp draw for the thruster with the link20 system.
I really interested in how the fuel pumps work on a 6LP. There is obviously a vacuum pump that pulls from the tank through the filters to the engine and then a separate pressure pump to the injectors. Are either of these electric or are they mechanical? With the idle issue I had it's seems the fuel pump is slow to respond or not resetting to the correct idle position internally. Because this never happened, even with 7 year old nearly dead batteries in the past, and it's a week after this sudden idle issue, I'm just trying to confirm a fuel pump issue before I pay to have it rebuilt or replaced.
So if I bypass the ACR and it still occurs, pump problem? Nothing else could effect idle rpm, alternator load or fuel pump.
Thanks everyone, sometimes it's nice to be able to think out loud so to speak. Thanks for the feedback.

[jleonard]

120 rpm is a big drop due to alternator load. My Cummins 6BTA would drop maybe 50 to 75 when the grid heaters (intake air heaters) cycled.
I doubt you have an electric lift pump. Most likely a diaphram type or piston type mechanic pump. Usually when the lift pump starts to go south you have trouble at higher rpm, not at idle.
But it's a boat.

[RobS]

John, I noted above it is a mechanical pump. It's a shaft driven vane pump built into the injector pump. 100% mechanical. You may have more than one issue going on here that coincidentally occurred in a timely manner.

[jleonard]

John, I noted above it is a mechanical pump. It's a shaft driven vane pump built into the injector pump. 100% mechanical. You may have more than one issue going on here that coincidentally occurred in a timely manner.

Vane pumps are very robust. How many engine hours on that Yanmar?

[RobS]

T-R was not equipped with air heaters but I was on another 6LP 28TE that was, they surely sent the volt meter diving down but I recall no noticeable effect on the tach, however it was not my boat so I can't say there was none, but would have been minimal if at all...

Long shot, but have you checked linkage and idle adjustment on the inj pump to be sure it has not come loose? Are all other parameters up to snuff still, ie speed and WOT ? Etc..

[Bidgood]

Just an observation on my boat. When I use the bow thruster the rpm meter drops. I checked it and the engine does not actually slow. It is the current draw from the thruster that does it to the meter. I have and analog rpm meter.

The good new is that the thruster works just fine.

Tom

[furball]

Engine just hit 900 hrs. I have checked the linkage and idle adjustment when the initial idle problem happened two weeks ago and everything looked ok. Idle was good, steady all day until I backed into the slip. Hit the thruster to get a little closer for a bow line and the idle dropped. Both the analog and digital tach drop and I can hear the engine change rpm and get very rough for a few seconds. I took a video of the gauges. Wonder if I can post it here? Everything else seems fine. Hit 3820 and still climbing under load, hit 4200 no load in the slip. No white or black smoke. Fuel burn according to Floscan is right where it always is, right on Yanmar's fuel curve. That would seem to rule out some type of restriction in the fuel line.

[jleonard]

Possible that the thruster is drawing too much amperage? Meaning on it's way out? Maybe needs brushes? Bearing binding up?
Is there a way to check amp draw on it?

[furball]

Good idea, I can read amp draw on the Link20 battery monitor. It sounded like there may be some growth in the tunnel or on the prop the other day. Planning to do a bottom check next weekend.
How does the engine maintain correct rpm under load at idle? Is it computer controlled air fuel mix? Is it mechanical? If the alternator suddenly loads the engine do to a draw, how is the system supposed to work? Obviously you need more fuel under load to maintain idle rpm, what controls that? I think it's mechanical within the fuel pump or is it electronic? Someone else suggested ground to the engine.
Thanks,
John