RPM Drop

You guessed it. Post your technical questions or solutions about your boat's engine and transmission here.

Moderator: KenKrawford

Post Reply
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

RPM Drop

Post by zman18 »

While motoring today we experienced an odd issue in that on occasion we would get a drop in the engine RPM. It would go from 2300 to about 1500 for maybe 5 - 6 seconds than go back to normal. Otherwise the engine is running great. It seemed that this would happen if we got a large wake for a motor boat. Fuel filters have been recently replaced. Appreciate any thoughts as to what might be the cause. Thanks
John Zoda
Compass Rose
Hull# 484
wolfe10
Posts: 587
Joined: Mon Dec 11, 2006 2:58 pm
Location: Ft Myers Florida

Re: RPM Drop

Post by wolfe10 »

How full is the fuel tank?

If showing less than 1/2 tank (actual level will be less because tank is NOT rectangular), a large wake MAY expose fuel pickup to AIR.

If in a situation that elicits this behavior, be down in the aft cabin and listen/feel the fuel pump. If it races, it is moving AIR, not fuel.
Brett Wolfe
C350 #180
"Vindaloo"
Ft Myers FL
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: RPM Drop

Post by zman18 »

Yes tank was just about full. We are heading home after a cruise and ran the engine about 8 hours today (into the wind as usual). Early today we experienced the same drop in RPM a couple of times, without being hit with a wake. However after that the boat ran fine with no other RPM issues. Very odd appreciate any thoughts.
John Zoda
Compass Rose
Hull# 484
wolfe10
Posts: 587
Joined: Mon Dec 11, 2006 2:58 pm
Location: Ft Myers Florida

Re: RPM Drop

Post by wolfe10 »

Any electrical symptoms-- change in gauges like drop in voltage?

Is the electric fuel pump still working? Easy to detect by putting your hand on it. If not, use a voltmeter and/or test light to verify that there is 12+ VDC going to it.
Brett Wolfe
C350 #180
"Vindaloo"
Ft Myers FL
User avatar
scott.monroe
Posts: 33
Joined: Fri Mar 15, 2019 5:13 am

Re: RPM Drop

Post by scott.monroe »

Problem certainly sounds like a fuel flow issue, engine becomes fuel starved as demand is high.
Might suggest starting at the source and move up the chain. If fuel tank is full, then you go to your fuel pickup in the tank. If anything is loose in tank, whether manufacturing left overs (i.e. gasket material, aluminum shards, dropped screw), biological contamination, sludge etc. and that gets caught in the pickup that would lead to restricted fuel flow. If the tank gets a good jostle that stirs everything up, could explain a lot. Good tank cleaning would take care of that (use a portable fuel polisher with a long pickup would take care of sludge and biologics, a yard should have one and can be done on the full tank. If other debris the tank would need to be emptied and checked visually).
Next area to look is the fuel tank fittings, elbow and shut off valve, they can get clogged. But this would most likely not be an intermittent problem. You'd just experience lower than normal RPM at load, but normal idle.
The same would be true with clogged primary and secondary filters. Normal idle but lower than normal RPM's at load.
The electric fuel lift pump could be the source of an intermittent problem if there is a loose wire, short, or failing pump. As a previous comment mentioned, you can hear/feel it running. If you can check it during one of your intermittent problems to see if the pump is working. If not sure where it is page 3 in the service manual shows schematic of the engine.
Scott Monroe
C350 #409 / Southern Yankee
North Kingstown, RI
wolfe10
Posts: 587
Joined: Mon Dec 11, 2006 2:58 pm
Location: Ft Myers Florida

Re: RPM Drop

Post by wolfe10 »

The electric fuel pump is easily access from the aft cabin. Remove the cover lid for the shelf over the engine. The square (at least on ours) electric fuel pump is right there. In fact, when changing fuel filters, I disconnect the positive lead (quick connect from Catalina) and touch it to the battery side of the glow plug relay to run the pump to prime the system.

Just turn on the key (don't start it) and you will easily identify it there.
Brett Wolfe
C350 #180
"Vindaloo"
Ft Myers FL
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: RPM Drop

Post by zman18 »

Thanks for the suggestions. It will give me a couple of items to troubleshoot
John Zoda
Compass Rose
Hull# 484
vineyardsailor
Posts: 14
Joined: Sun Jun 07, 2009 3:07 pm

Re: RPM Drop

Post by vineyardsailor »

Had almost the same problem - it was the fuel lift pump
Triumph
Posts: 256
Joined: Thu Nov 03, 2005 7:34 am
Location: Tampa, Fl

Re: RPM Drop

Post by Triumph »

You mentioned that the fuel filters were replaced recently, I'd start with the gaskets on those and make sure you don't have a minor air leak.

Bill Cullen
TRIUMPH
pjr
Posts: 12
Joined: Sun May 14, 2006 9:33 pm
Location: Georgian Bay, Wiarton, Ontario

Re: RPM Drop

Post by pjr »

In our case, we suffered occasional rpm drops with our nearly new Imagine (brief, referred to them as the engine 'stumbling') for some time. It even stopped a couple of times. While changing the fuel filters, I brushed against the fuel pump and it started. I realized I had not heard that in a while; unwrapped the miles of electrical tape around the wiring there and found one arm of 'Y' quick-connect/splitter had broken and was only making occasional contact. Replaced the Y connector (and some of the tape) and all has been fine since.
(I should add that the little inline filter on the inlet to the fuel pump is tricky to replace - confined space, my elbow bends the wrong way etc)
Peter Richards
IMAGINE
Peter Richards
Catalina 350, # 402, 'Imagine'
User avatar
TBOT422
Posts: 424
Joined: Sun Jun 26, 2011 5:36 pm
Location: Clearwater, FL

Re: RPM Drop

Post by TBOT422 »

Keep in mind the fuel lift pump is powered through the pre-heat solenoid when the engine is not running and the key switch is in the 'pre-heat' position. However, it is powered by the oil pressure switch when the engine is running. So in addition to the fuel lift pump itself possibly being the issue, it could also be the oil pressure switch, or any of the wiring connections between them. Since it is intermittent, it is most likely a wiring connection.
Gary & Janet
The Best of Times (Hull #422)
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: RPM Drop

Post by zman18 »

Appreciate everyone's thoughts and suggestions. Since this problem is so intermittent I think I will start by cleaning all my terminals on the fuel lift pump and oil pressure switch. While also checking for any loose hose clamps in the fuel lines that might allow for a small amount of air to get in the line.
John Zoda
Compass Rose
Hull# 484
User avatar
russp
Posts: 189
Joined: Fri Apr 01, 2011 5:31 am
Location: Paynesville, Australia

Re: RPM Drop

Post by russp »

Just to add to the mix, my experience is that once fully primed the engine will run without the assistance of the fuel lift pump, the fuel pump itself generates enough lift to pull fuel up from the tank to the filter. Might be wrong, but seem to recall that my engine performed fine for quite a while after the fuel lift pump connection broke.
Russ Peel
Avalon #150
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: RPM Drop

Post by zman18 »

While troubleshooting this problem today I ran a test on the fuel lift pump. My understanding is that when the key is in the glow plug position I should hear continuous clicking from the pump. When we did the test I was only getting a single click each time we turned the key. Not sure if this is indicating a problem with the pump?
John Zoda
Compass Rose
Hull# 484
wolfe10
Posts: 587
Joined: Mon Dec 11, 2006 2:58 pm
Location: Ft Myers Florida

Re: RPM Drop

Post by wolfe10 »

John,

Check voltage to the pump with ignition ON. That will tell you if voltage issue or pump issue.

As we say in Texas "That Ain't Rite".
Brett Wolfe
C350 #180
"Vindaloo"
Ft Myers FL
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: RPM Drop

Post by zman18 »

IMG_2457.JPG
IMG_2457.JPG (116.96 KiB) Viewed 6125 times
Ran a jumper between the solenoid and the red wire on the fuel lift pump and got the clicking sound indicating that the pump works. Went I tested the double blue wires that connect to the red wire for the fuel lift pump I found that there was 12 volts there with the key on. So my confusion is that if I run 12 volts directly to the pump it works but does not work with the normal connection even though it appears that there is 12 volts at the connection point. Any thoughts as to what I'm missing? PS I do hate electric problems :(
John Zoda
Compass Rose
Hull# 484
wolfe10
Posts: 587
Joined: Mon Dec 11, 2006 2:58 pm
Location: Ft Myers Florida

Re: RPM Drop

Post by wolfe10 »

John,

Just thinking out loud here.....

So, the question: "What would cause you to read 12 VDC positive to the blue wire, but when connecting it to the pump, the (proven good) pump doesn't work?"

Could be resistance between battery and blue to red connection. With no load, could read 12, but with the amp draw of the pump-- insufficient voltage to run the pump. SO, check voltage between the blue/red connection and good ground with ignition on. If voltage goes from 12 VDC with red wire disconnected to below, say below 10 VDC with red wire connected you may have the answer-- look "upstream" for a corroded or loose connection or compromised wire.

Also remember that the electric fuel pump gets its power from the pre-heat solenoid when starting and through the oil pressure while the engine is running. Be sure in which "mode" you are checking for power to the fuel pump from which source.
Brett Wolfe
C350 #180
"Vindaloo"
Ft Myers FL
Post Reply