Stopping the urge to surge
QI have a narrowboat fitted with a Vetus P4.19 engine (a marinised Peugeot/Citroen XUD9 1.9 normally aspirated lump). The engine has now clocked up nearly 7,000 trouble-free hours.
However, recently the engine speed ‘surges’. It does not ‘run-away’ as such, as you can close the throttle and it falls back to idle speed, but sometimes you set the engine revs to say 1200 revs and it will, quite by itself, increase revs to 2000 or so.
My questions are: Is it dirty fuel/ water in fuel or similar. Or (as the engine is quite highish hours) is this the first sign of unburned fuel getting into the crankcase and beginning to develop “diesel engine runaway”
I usually put diesel conditioner in the tank twice a year (certainly before winter) and I usually have the tank full (certainly before winter), but I have never had the tank cleaned.
The engine has started to smoke recently, although not badly. It hardly burns any oil and starts easily, so I believe the compression is good, although I’ve never had it tested. KEITH HAMSHIRE
ATONY REPLIES: After a bit of Googling I found that engine either has a Bosch or Rotodiesel rotary injection pump. The Bosch pump is mechanically governed so unless dirty fuel is, in effect, causing a weak, faster burning mixture, which is very unlikely in a diesel, the problem could be a sticky governor inside the pump. I am not familiar with the Rotodiesel pump but from what I can see it looks like a CAV DPA clone. The illustrations I found show a mechanically governed version but CAV also made hydraulically governed versions that exhibit your exact symptoms when suffering fuel starvation. The mechanical version might also do it if the governor was sticking a bit.
Could any gas or other volatile combustible material be leaking into the engine bay?
I can set your mind at rest about a potential runaway. For this to happen you would need a lot of diesel in the oil. This causes the sump lever to rise, the oil to thin and smell of diesel. Your words do not suggest this is happening.
With high hours, any smoking is more likely to be caused by worn/ dirty injectors but injectors do not normally cause the engine to rev itself up. Burning lots of oil may cause it to rev up but for that to happen you would also have clouds of exhaust smoke and heavy oil consumption.
You make no mention of how regularly you change the fuel filter(s) and clean any water traps. Likewise you say nothing about regularly pumping, siphoning or in some other way removing whatever is lurking in the bottom of the fuel tank. Tank draining needs doing annually.
I think the first thing you need to do is to see what is in the bottom of the tank and remove and dispose of anything that is not clear diesel. If you find black, brown, or grey slime you probably have diesel bug. If you find a lot of pinkish emulsion it’s a reaction between an emulsifying fuel additive and water. It all needs getting rid of but if the containers are allowed to stand for several days you may well find emulsified fuel separates and as long as there is no sign of bug the fuel part can go back into the tank. Then change your fuel filters and drain/clean any water traps. Take a great interest in what is trapped inside them. Again you are looking for water and signs of bug.
Check for diesel bug