Knock Knock

[No 6]

Not a joke - but the noise coming from an AEC Routemaster engine on an example that is stored here.

Starts up and runs and ticks over nicely, but let the engine get warm and work up the revs and a sharp knocks starts and won't let go.

Sounds in the middle of the block and very hard metallic - valve slap? little end???

Any ideas??

[billyboy]

sounds like it could be a little end on the way out

[G-CPTN]

Need more information whether the noise is constant or whether it varies with speed and / or load.
Could be piston slap (worn bores).
Does it smoke (oil smoke) after over-run?
A deep-down rumble under load could be mains, or (different conditions) big ends.

Get a broom handle and place one end against the block and the other against your ear (take care not to get caught in any rotating machinery of course). You should be able to locate the precise location (ie which cylinder(s) and how far up or down).

It might be detonation from a dribbling injector . . .

[billyboy]

When last was the fuel pump recalibrated?...maybe its a fuel knok as it only happens when up to running temperature. Never had much to do with AEC engines (Leyland/DAF man me)

[No 6]

Like I said - I know nothing about the vehicle - it is merely stored here, but I will run it up again, and check the detail

On start up there is no knock anywhere - you'd buy it!

After a few minutes I ran it up and the knock kicked in, engine went out of beat and tried to jump off its mounts - never looked at the exhaust as we were shutting it down.

The knock was sharp, metallic, not heavy enough for a big end, but heavy enough to make you sit up and take notice - and once in play did not stop on returning to tick over

On shut down a nasty curl of smoke came out of the breather

Will have another play!

[G-CPTN]

engine went out of beat and tried to jump off its mounts

Indicates a fuel (injection) fault IMO.
Imagine taking a plug-lead off a petrol engine (and worse, having a high compression charge that doesn't fire).

Could be a sticking valve I suppose . . .

[billyboy]

needs the head off to check the piston is not hitting the valve. Check the fuel injection system first though

[G-CPTN]

Why should a piston start to hit a valve?
I suppose if it started to stick after the engine had been running.
(The indication seems to be that on startup there is no misfire or knock)

Had a mysterious misfire on a Detroit Diesel V engine.
All static checks OK. On stripdown it transpired that a fuel leak from a injector pipe (within the engine casing) had diluted the lubricating oil and subsequently caused wear of a roller-follower that operated the injector. Thus the injector-timing was wrong as the injector was late (due to the increased clearance caused by the flat on the roller-follower) causing the engine to misfire. In addition, the engine sump oil level continued to rise . . .
Checking the injector clearance statically gave the correct value as the roller-follower rotated onto the high point.

I'm not suggesting that the Routemaster has the same fault BTW.

Is it in fact an AEC engine, or is it a Leyland engine?
Or is it perhaps an Iveco or Cummins engine?

Originally Posted by billyboy

needs the head off to check the piston is not hitting the valve. Check the fuel injection system first though

Originally Posted by billyboy

needs the head off to check the piston is not hitting the valve. Check the fuel injection system first though

[billyboy]

Well, had another think about it. Try changing the two middle injectors for the two end ones and see if the noise moves to either end of the block. We only had the one AEC engine. very reliable, sweet on tick over but noisy at revs. sounded like all six were detonating.
I much prefered the Leylands, fords,Gardeners and DAF's.The facdt its ok cold and only does it when hot makes me think its a fuel detonation problem rather a bearing or a little end

[Mr Scammell]

When the knock starts, crack off the injector pipes one at a time until knock goes or is less prominent. Change injector for the one furthest from noisy cylinder and try again. If knock moves its the injector, if not its a small end that is probably worn and it is possible when engine hot and end worn piston could hit head.