Diesel Troubleshooting Help Needed

Bill Cochrane · Post by **Bill Cochrane** » May 28th, '07, 05:06

CD36, Perkins 4.108 "new" model. Cruising at 2000 RPM, at intervals of from 15-45 mins, a brief surge of +200 to 300 RPM lasting from one to two seconds, then settling back to the 2000 point...until next time.

Any thoughts on possible causes/places to look/things to fix?

Thanks, Bill

Al Levesque · Post by **Al Levesque** » May 28th, '07, 06:36

No message.

Russell · Post by **Russell** » May 28th, '07, 06:43

Do you have a smart regulator for your altinator? Is it perhaps lowering the altinator output then raising it again? That could cause this, check your amperage output when this is happening. Big altinators have very noticable effects on engine loads. When my 150amp altinator goes from bulk mode into float mode the engine revs up considerably, it was disconcerting at first and I thought there was a problem.

David van den Burgh · Post by **David van den Burgh** » May 28th, '07, 07:17

In our experience, the surging has always been indicative of restricted fuel supply, originally at the fuel pickup tube - before we removed the screen - then at the fuel filters.

If that's what it is, it likely won't be long before you're stuck without an engine - until you clear the crud or change the fuel filter(s). Our surging was within the 200-300 RPM range as well.

Stan W. · Post by **Stan W.** » May 28th, '07, 08:19

Something is blocking delivery of fuel. Probably gunk on the screen on the end of the fuel tank dip-tube. It won't be long before the surge is followed by the engine quitting. Remove the screen, throw it away and just keep an eye on your primary fuel filter.

Bill Cochrane · Post by **Bill Cochrane** » May 28th, '07, 12:25

I assume the fuel supply fitting on the tank unscrews, and the dip tube is attached to it? I have the aluminum 53 gal tank in the bilge adjacent to the galley sink.

Bill Cochrane · Post by **Bill Cochrane** » May 28th, '07, 12:26

Russell, good thought, but the alt/reg combo is a couple years old with no changes this season. I do know that the alternator can suck several hp out of the engine, and it makes sense that a sudden drop in that requirement would cause a brief blip until the governor caught up.

John Vigor · Post by **John Vigor** » May 28th, '07, 16:40

I note with puzzlement that the concensus so far seems to be fuel starvation. Perhaps I'm not the brightest bulb on the tree, but why would less fuel make the engine go faster?

Doesn't the engine need more fuel to go faster than it was before?

The alternator doesn't seem like a possible cause either, because the increase in engine speed lasts only a few seconds at a time.

My guess would be a rare governor/throttle fault, or something in a locker rolling against the throttle cable and then rolling back. I ocassionally give myself a fright by accidentally advancing the cockpit throttle with my knee or a loop of line, but that doesn't seem to be the case this time.

What if a dollop of extra fuel were to be ingested from the lubrication oil now and then? Is the oil pan overfilled? Could some overflow be coming up the breather into the air intake as the boat heels? Some diesels have been known to run away like that.

Could the fault be air starvation? Something rolling around and blocking off the air now and then? If the fuel mixture suddenly became richer because of that wouldn't the engine try to run faster, and wouldn't there be a little lag while the governor got things under control again?

I love engine mysteries, specially when they have happy endings.
Where is Sherlock Perkins when we need him?

John Vigor

iropicc · Post by **iropicc** » May 29th, '07, 10:43

mr. V. Try this simple test to prove stravation of fuel makes an engine run faster. 1. Shut the fuel petcock on your lawnmower. 2. Start the lawnmower. 3. Before the engine dies from lack of fuel, you will notice a discernable increase in rpm. YES I know most real sailors don't own lawnmowers. Just color me the unreal sailor

Oswego John · Post by **Oswego John** » May 29th, '07, 10:54

Same thing with my chain saw.

But I don't know why.

O J

John Vigor · Post by **John Vigor** » May 29th, '07, 11:03

Oh come on, guys, surely you know the difference between diesel and gasoline engines.

Sure a gasoline engine will run faster and hotter when the ratio of air to gas increases. But a diesel atomises a precise amount of fuel for each power stroke at a set governor speed. How do you think that could be "leaned out," except by blowing more air into the combustion chamber?

The mystery for me is how can the engine speed increase if the fuel is reduced, thus making the mixture richer, not leaner?

John V.

Didereaux · Post by **Didereaux** » May 29th, '07, 11:25

John Vigor wrote:Oh come on, guys, surely you know the difference between diesel and gasoline engines.
.......The mystery for me is how can the engine speed increase if the fuel is reduced, thus making the mixture richer, not leaner?

John V.

The terms 'richer' and 'leaner' are based upon the amount of FUEL in the mixture, not the amount of air. Further an engine lugs when too much fuel is added because then all the fuel is not burnt in the chamber, thus reducing the total energy released(heat) of the 'explosion'. Ideally every last molecule of fuel is burned in the chamber before the exhaust valve opens thereby relasing the maximum energy and heat.

Run your engine too 'lean' and you burn valves and piston tops. Burn a too 'rich' mixture and you carbon up the valves and such, the overage of fuel is not completely burned and deposits as carbon.

But surely you knew that already!

miguel mascaros · Post by **miguel mascaros** » May 29th, '07, 14:44

I did experienced similar behavior in my MD7A it was increasing RPM's for a short period (say 5 seconds)prior to stalling...It puzzle me to no end....I went up and down thru the fuel system and finally the problem was solve after changing the fuel line from tank to first fuel filter.......
This is my experience in this issue....
Hope you work it out and post the results....

Matt Cawthorne · Post by **Matt Cawthorne** » May 29th, '07, 16:54

Several years ago I decided to draw the tank down so that I could clean it. Late in November the fuel was down to what read 1/8 of a tank. I took the boat out on a blustery day and had a great sail. I started the motor and pounded into the chop for a little while to get home before dark. Just as I entered the creek the engine started to run faster, then returned to normal. I knew something was wrong and got the anchor ready. It started to surge again one or two times and then died. I got a tow back to the slip and parked the boat. Shure enough, the tank was empty of all but about a cup of fuel. There is something about the introduction of air or the operation of the governor that must temporarily increase the fuel flow.

Matt

Bill Cochrane · Post by **Bill Cochrane** » May 29th, '07, 18:05

Changed the primary and secondary filters today after talking to a guy in the marina who also has a 4.108 in his Tayana...same symptoms, problem was clogged filters.

The primary had a good coat of crud (technical term), which was surprising because the vacuum was only about 4-5" (7 is the point at which you're supposed to begin thinking about changing the filter.

Unfortunately, in the process of bleeding the engine I managed to damage a fuel fitting and am now waiting for a professional repair.

At any rate, thanks for all the comments...the consensus here and on Boatdiesel archives, as well as my friend with the Tayana, seems to point to fuel starvation and/or air in the lines. Will see what happens when I get the engine running again.

The Cape Dory Board

Diesel Troubleshooting Help Needed

Diesel Troubleshooting Help Needed

Transmission slippage? no msg

Fuel Supply?

David is right on.

Screen on dip tube -- access and removal

Alternator/Regulator

Less fuel but more revs?

"lean out not hike out "

RPM Increase

Surely you know the difference

lean vs rich .>Re: Surely you know the difference

Could be drawing in air.

Further info