A racing engine. Cause?

[AndrewB]

My Perkins 4108 has recently developed the habit of suddenly racing for a few seconds before returning to normal speed. This happens a couple of times hourly in use. There is nothing wrong with the throttle linkage. What could be causing this?

 

[tritonofnor]

Extra revs has to equal extra fuel, so take a close look at the high pressure pump, and the injectors.

 

[Lakesailor]

You don't have a slight gas leak on board do you? Diesel engines ingesting LPG or similar race uncontrolably.
Just a thought

 

[sailorman]

the perky 4108 has a damper devise on the injector pump it is an anti-stall devise (similar to modern ford diesel engines) you need to check the user hand book to get the settings, its quite easy to do ( well i managed it /forums/images/graemlins/cool.gif)

 

[jimbaerselman]

Check that your pre-heat switch isn't sticking in the pre-heat position, or that there isn't a short in the pre-heat circuit.

The preheater works by heating a coil in a plug in the inlet manifold, and when this coil is hot, it opens a valve and lets fuel into the intake. This will cause your revs to increase arbitrarily from 800 to 3,000 or so, especially when you're in idle with no load on the engine.

The temporary solution is to disconnect the wire going to the preheat plug.

 

[capt_courageous]

Hi
On many diesels the throttle control cable does not go direct to the fuel pump rack which controls the amount of fuel. It goes to what is in effect a governor. This then controls the rack. Check to see how your engine is actually controlled - could be the governor itself is acting up.

 

[Gymnopedie]

Just check the sump oil level; any significant excess could over-pressurise the sump blow past piston rings and effectively provide an additional combustion fuel supply; although I would expect this to cause more continuous racing (succeeded by a lot of exhaust smoke and a big bang!).

 

[oldharry]

More usually with blowby causing excess sump pressure, the oil is forced out of the breather tube which is connected to the inlet manifold. once enough oil has been dumped in the manifold the engine runs away on it until it has used it all.

Last time I saw it happen was a Turbo Perkins Prima which revved wildly to around 7000 rpm for around 15 seconds producing a vast cloud of smoke then stopped. It survived and is still running quite happily with a new turbo, as the old one had caused the sump pressure.

Another Perkins i knew was badly worn, and regularly pumped oil into the inlet manifold if it was run above 4000 rpm. Would go mad for 15 - 20 seconds, then continue to run quite happily. Owner was eventually persuaded to have it rebored, and life became less exciting for him! To stop it happening he lead the breather hose into an empty oil tin, with another pipe to the inlet manifold which sucked most of the fumes in but left the oil collecting in the tin. He would stop every hour or so and put it back in the engine to stop the level getting too low..... /forums/images/graemlins/smile.gif. After all, as he said, the engine was shot anyway.

 

[jsl]

Have you changed the fuel filter? If the filter is a bit blocked, the pump will struggle to get the fuel through it, occasionally getting a bit more than it has been 'expecting'. Changing the filter is by far your simplest first step, assuming your sump is not over-full. But of course you have probably done that already.

 

[dovekie]

Andrew. We have a 4108 and we had intermittent racing of the engine. To cut a long story short, it was a leak from a corroded fuel pipe between the lift pump and the high pressure pump. Copper pipe from the coolant circuit was just touching the steel fuel pipe, electrolytic corrosion. Probably tiny amounts of air leaked in at first, causing the racing. Gradually got worse - eventually the pump failed (? grit from the corroded pipe). Big cost for a new pump. So it might be worth checking those fuel lines if other replies haven't given you an explanation.

 

[jimbaerselman]

You've had an absolutely fascinating list of possible causes. After ten years living with my 4.108, I'm obviously lucky to only have seen one cause, but I'm dying to hear what you find out!

 

[Billjratt]

Agree with Dovekie, jst had the same thing - air being drawn into the fuel system prior to the last filter. I could bleed it from there by loosening the union on top of the filter and pumping the lift pump. ( did it without stopping a few times!)
Symptoms were revs rising by 100 - 150- revs for about a second, getting more and more frequent until the engine stops if you don't bleed... I replaced and painted lots of pipe, and made sure it was properly clamped so as not to rattle loose again. Job done.

 

[AndrewB]

Many thanks guys ...

... for all these suggestions. I've tested the simpler ideas, but beginning to suspect its something to do with the high pressure pump. I'll be checking all over for air leaks - that sounds possible.