Uneven Engine RPM - Volvo Penta MD2030

[onlinelee]

Hi all,

I wonder if I could get some suggestions regarding a problem I am having with my 2003 Volvo Penta MD2030.

Whilst on a 6 hour motor today I noticed having set the throttle at 2250 rpm every few minutes the rpm would drop to 2000 rpm then within a few seconds recover to 2250. The fluctuation was a bit more dramatic when I opened and set the throttle to 2500 rpm.
When set at 2000 rpm the fluctuation was minimal. When I returned to the dock the engine idled normally and in natural I took the rpm to 3000 and held for a moment, again without problem, so my feeling is that this problem only occurs under load.

The engine makes no smoke and apart from this issue appears totally normal. I did notice a little vibration on approach but this went when I nudged the throttle a little.
Both filters were replaced 60 hours ago back in May this year.

Any suggestions would be greatly appreciated.

 

[odaydokay]

What mean "both filters?" How about the one inside the fuel pump itself? If that is partially clogged, you could get some surging. You can also tie the boat up real well including spring lines if you are at a dock and run it up in gear.

 

[Capt jgw]

Or plugged tank vent or screen at the end of the tank pickup tube. If there is a screen there, remove it, you don't need it.

 

[Benny17441]

90 % of engine problems are fuel related. It does sound like a fuel flow restriction that becomes more evident a higher RPM. My first instinct would be to check the primary filter for excessive water collection or tank debri. Sometimes when we go out into large swells particles that have been resting at the bottom of the fuel tank get dislodged and can clog the primary filter rather quickly. As others have mentioned a blocked vent or screen in tank can have a similar effect. The fact that it becomes more evident under load is consistent with having restricted fuel flow. The governor demands more fuel to maintain certain RPM but the required fuel cannot get there in time. The other possibility is a restricted exhaust flow. If replacing the fuel filters do not correct the situation then have the exhaust mixing elbow inpescted. Pues hombre a comer jamon y a tomar vino!

 

[onlinelee]

Hi guys,
thank you all very much for your comments & suggestions. I had guessed it was probably a fuel supply issue and clearly your replies confirm this.
1: I did not know there was a filter in the fuel pump!
2: I don't know what a tank vent / screen looks like, will investigate that.
3: I will remove the primary filter and inspect that as Benny suggests and replace if in doubt.

I will post the results of my finding shortly, meanwhile, muchas gracias amigos

 

[onlinelee]

Update... Thank you all again for you input. Benny, spot on, I could not see anything in the filter bowl so I replaced the primary filter cartridge and then tested it from the dock as suggested by odaydokay, all seems fine now.

 

[Benny17441]

1: I did not know there was a filter in the fuel pump!
2: I don't know what a tank vent / screen looks like, will investigate that.
3: I will remove the primary filter and inspect that as Benny suggests and replace if in doubt.

1) Me neither, some have a little screen but seldom gets fouled.
2) When you put fuel in the tank it displaces air out of it and when you use fuel from the tank air comes in to take its place. The vent is usually composed of a hose and a thru-hull opening fixture for air to come in and out. This opening may have a screen to keep bugs out but it is not uncommon for bugs to make nests in it and block the flow of air. If no air can come in no fuel can get out. The tank also has a pick-up tube to syphon fuel from near the bottom of the tank and this pickup tube has a screen to filter out large contaminants. It can get clogged disrupting fuel flow. In an emergency disconect the fuel line and blow back until you hear bubbles.
3) I have not learned yet how to accurately inspect a fuel filter, I would suggest to just replace it.

I have been in a position to change and clog three primary filters in one run. I can tell you from experience that in an emergency these diesel engines can run without a filter element inside the primary. They can burn fuel with a healthy amount of water in it and you can rely on the secondary filter to keep unwanted particles out of the injection system. if you want to test to see if a primary filter change is necessary just remove the element and see if there is a change in how the engine runs.

 

[onlinelee]

Hi Benny,
I really appreciate you taking the time to explain that, its a steep learning curve for me having been an office boy all my working life!! As per my earlier update, I decided not to try and clean out the old filter, just replace it, testing on the dock seems to have sorted it. Your comments about the filter are interesting, my primary system requires the actual filter as its case forms part of the part - if that makes sense!!!
1: I checked out Volvo parts list, I can't find any mention of a fuel pump or injection pump filter.
2: Please see attached picture of my fuel tank, I can see the pick up and what I guess is a return. I can't see the breather, maybe its further back, thats another job! Regards Lee

 

[odaydokay]

The breather will come off the top of the fuel tank and in may cases exit high on the transom or side just under the rail. The screen can indeed get clogged.

Also, I have all assumed that an electric lift pump pulls fuel from your tank. When you turn on the ignition (but not the glow plug and/or starter button) you should hear it humming or ticking. Many have a small sieve or filter, so that needs servicing annually by removing and cleaning. Another thing happened to me a couple of times was that an electrical connector at the pump (in this case the ground cable) got loose and there would be a quick cut out of the pump, a drop in rpm and then a resumption of full power. Finally, on the same trip it just died and I found the problem and fixed. I think this is probably NOT what is going on in your case, but in the interest of being thorough, best to check the connections.

 

[odaydokay]

What mean "both filters?" How about the one inside the fuel pump itself? If that is partially clogged, you could get some surging. You can also tie the boat up real well including spring lines if you are at a dock and run it up in gear.

I have now done some research. You should have this lift pump in that engine, but I cannot see that it has a sieve, screen or filter.