Inconsistent Idle

[theonecalledtom]

Hi peeps, you may remember me from such times as "how come my engine wont start" and similar fun.... this time the engine [Universal 5432] is running. In fact it's running pretty well all told, start's, moves the boat, hasn't really worried me in the last, umm, well since last time it worried me....

Anyway, this last week the idle started getting a bit inconsistent. By this I mean that it will run at what we set it too, say 1000 rpm, then, for a period of several seconds drop to a lower rpm, perhaps 100 rpm lower in this case.

Shortly thereafter it'll return to it's normal setting and run there for perhaps another 30 seconds or a minute, before repeating.

Initially this seemed to happen right after startup then stop but when returning to our slip last night it was happening while cooling down after running at 2300 for about an hour.

So far guesses include dirty injectors or some other flow problem in the fuel system, probably nearer the engine than earlier in the fuel system.

What do you guys think?

[Boracay]

Have you checked the fuel lines, filters, fuel tank breathers?

[daddle]

That whole fuel governor thing can do that if it has gotten some gunk or a spot of rust on it. It's on the engine oil/crankcase side of things, not the fuel. No easy cure.

100RPM could be the alternator/regulator adjusting the load.

[theonecalledtom]

Daddle and Nigel Calder are in the same club. In Marine Diesel Engines the useful trouble shooting diagram has hunting, which would be a better description than poor idle I think, down as being caused by the Governer sticking / Loose Linkage.

From the Wiki we see this description: Diesels

Fuel delivery
A vital component of all diesel engines is a mechanical or electronic governor which regulates the idling speed and maximum speed of the engine by controlling the rate of fuel delivery. Unlike Otto-cycle engines, incoming air is not throttled and a diesel engine without a governor cannot have a stable idling speed and can easily overspeed, resulting in its destruction. Mechanically governed fuel injection systems are driven by the engine's gear train.

Nigel also goes on to say that there could be several other reasons for the rpm hunting, including misfiring cylinders, load variation and poor injection.

Prior to this issue occurring we spent a night on the hook in heavy chop (fun!) so its possible that sludge in the tank was stirred up.

Haven't had a real chance to look at the engine since this started happening, guess I'll start with the filters to see if there is visible dirt in there and see if I can get a visual on the governor linkage.

Good news is that while looking into this I found a nice modification to the air filter and confirmation that the engine is probably fitted with the wrong thermostat, explaining why it runs too cool. Gotta love google.

[SkiprJohn]

Diesels hunting is usually more frequent than 10 to 30 seconds in the difference of RPM. I'd say some gunk in the fuel would be more likely.
regards,

[theonecalledtom]

Originally Posted by daddle

100RPM could be the alternator/regulator adjusting the load.

Okay - this seems closer to the truth. I spent some time looking at the idle today. I found that with the ignition off the variation didn't happen. Then I checked the charge voltage and found that the batteries weren't charging (they had been while connected to shore power).

After a few cycles on the ignition switch the problem went away and normal charge voltage was resumed....

The problem will be back sometime!

[UKEngineer]

Tom,

The batteries will not charge with the ignition off as the circuit required to charge them from the alternator is not made. Hunting is often to do with the centrifugal govenor, it can be worn for example if the engine has been been regularly used at a certain RPM. It maybe that you need a new shaft.

It seems strange that the ignition should have any effect on whether the hunting happens or not as it a fuel pressure problem rather than an electrical one. Maybe you just happened to get lucky with the ignition off, or was the engine not under load?