What causes engine knock from a bad injector?

[LCA078]

I know bad injectors can cause a knock but I'm lost on physically what is going on to cause the knock. I've searched the forum and internet but haven't find a good answer.

My understanding is engine knock is caused from pre-detonation of fuel that causes an abrupt increase in cylinder pressures, usually in advance of when it's supposed to burn. The rapid increase in pressure is the actual knock that we hear. I understand this is easy to cause with gasoline engines where the fuel is already introduced during the compression portion and the fuel/air mix can detonate from hot spots, bad fuel, etc. but I don't understand pre-detonation on diesel engines where the fuel is injected 'always at the right time'. I think this is especially true on the NHC 250 where the mechanical system that fires the injector (cam/rocker) isn't real easy to advance. I believe a failing mechanical injector will only inject less fuel or later in the stroke so I'm lost on how detonation can occur.

I'm sure there's a simple explanation...so feel free to edge-u-ma-cate me.

 

[winfred]

premature opening (popping at a low pressure) of a failing injector would effectively advance the timing in that cylinder changing the noise, one thats dumping too much fuel would imbalance the power output across the set. normal diesel operation is controlled detonation

 

[Jon0249]

It's possible you have a stuck lifter. Run a full bottle of Seafoam in your crankcase at operating temp for 20-30 min, then change oil & filter. Worked for me, got rid of a very loud knocking/tapping. Even if it's not a stuck lifter, this is a good procedure anyway.

 

[simp5782]

If its a red river truck more than likely a snap ring was left out of a wrist pin or a rod cap is loose

 

[frank8003]

If its a red river truck more than likely a snap ring was left out of a wrist pin or a rod cap is loose

bad bad bad

 

[Floridianson]

Does the knock happen when running through all the gears or just after you hit 4th high and 5th high?

 

[LCA078]

I don't have a knock in my M923. I'm only trying to understand how a bad injector can cause knocking as it's prevalent in a lot of posts in this forum. I just can't understand the mechanics of it so it led me to ask the question.

Now, my non-running M925 will be a different story about knock. I bought it as parts truck mainly for the winch and hardtop because the previous owner said it ran fine but then developed a knock he couldn't figure out so he stopped driving it a couple years ago. I just got the truck delivered and haven't even barred the engine over yet. But I have a feeling the knock in this truck is not because of a bad injector...

 

[WillWagner]

I don't have a knock in my M923. I'm only trying to understand how a bad injector can cause knocking as it's prevalent in a lot of posts in this forum. I just can't understand the mechanics of it so it led me to ask the question.

Now, my non-running M925 will be a different story about knock. I bought it as parts truck mainly for the winch and hardtop because the previous owner said it ran fine but then developed a knock he couldn't figure out so he stopped driving it a couple years ago. I just got the truck delivered and haven't even barred the engine over yet. But I have a feeling the knock in this truck is not because of a bad injector...

View attachment 765149

That it typically what happens when something gets dropped down the push tube cavity, lands on the cam follower and doesn't get fished out.

What winfred posted is it in a nutshell on a fuel knock. Usually accompanied by acrid smelling black smoke.

 

[LCA078]

Thanks Will. I've read most of your posts about adjusting the valves and injectors and appreciate the info. It really helps me comprehend the mechanics of what's happening when the pieces and parts are whirling around together turning fuel into noise and motion.

I also remember one of your posts about NOT using the socket-style screwdriver bits to torque the top section. Hopefully you're right and I find a bit lodged in the cam follower when I pull it out. Of course it has to be the cam follower behind the injector pump...

 

[WillWagner]

If the cam isn't toast...IF...just measure the gasket stack you took off and you can put another cam box on from another engine, new gaskets of course, without having to time it. Sometimes the broken pieces parts will hit the piston skirt. If that happens, the piston and possibly the liner will need replacing. If it got hit don't re use it.

 

[Keith_J]

Leaking injectors will cause far advanced injection timing..actually there will be fuel in the combustion chamber as the piston is approaching TDC.