diesel knock

[doyle]

hi guys

been on ebay looking at repairable vans and cars, ive noticed that some of the vans have diesel knock.

as I understand, knock is caused by fuel igniting as the piston is compressing the air half way up the cylinder instead of tdc.

what would be the causes for diesel knock and how would be the typical fix for it


ps murse thanks for your advise that helped me on my exams cheers
thanks
terry

 

[Belle427]

Leave them well alone would be my advice, most people know what is wrong with them thats why they sell!
Incorrect pump timing and dodgy injectors can influence it and it can be very expensive to put right.

 

[Burnerman]

I have to agree with that one....just check out how many issues there has been with Vivaros for example.
Old type indirect injection vehicles maybe, but there's few of them left.
John

 

[Peter.N.]

I agree with the above, it depends on the age of the vehicle and type of engine, older IDI engines give very few problems but the newer the engine the more problems you are likely to encounter. The best of the common rail engines are the early Peugeot/Citroen 8 valve Hdi's, I have two vehicles with them that have both done around 200,000 miles and are still mostly original.

Peter

 

[Mursal]

No worries, obviously everything went well !!

Very hard to get a bargain, especially when they actually tell you it has a knock. It may not be related to injector timing at all, just a convenient way to tell you it has a problem noise.

Yes, you are correct diesel knock is related to injection timing in relation to the position of the piston in the cylinder. In an ideal world, top dead centre (TDC) would be the spot to inject the diesel in on top of the piston. Which will self ignite, due to the pressure in the cylinder, forcing the piston back down the cylinder on the power stroke.
http://auto.howstuffworks.com/engine1.htm (click the button to see the 4 stroke cycle)
But, it takes a little time for the fuel/air mixture to ignite, so we have to inject before TDC. 5-25 degrees, depending on engine design and engine speed. It is this point of injection, that is the must likely cause of diesel knock. Modern high speed direct injection engines (diesel injected directly in on top of the piston), now have multiple injection points. With only a few hundred milliseconds separating them, so they must use high speed injectors, which are expensive to replace and do give trouble.

http://www.howstuffworks.com/diesel.htm

 

[Peter.N.]

No worries, obviously everything went well !!

Very hard to get a bargain, especially when they actually tell you it has a knock. It may not be related to injector timing at all, just a convenient way to tell you it has a problem noise.

Yes, you are correct diesel knock is related to injection timing in relation to the position of the piston in the cylinder. In an ideal world, top dead centre (TDC) would be the spot to inject the diesel in on top of the piston. Which will self ignite, due to the pressure in the cylinder, forcing the piston back down the cylinder on the power stroke.
http://auto.howstuffworks.com/engine1.htm (click the button to see the 4 stroke cycle)
But, it takes a little time for the fuel/air mixture to ignite, so we have to inject before TDC. 5-25 degrees, depending on engine design and engine speed. It is this point of injection, that is the must likely cause of diesel knock. Modern high speed direct injection engines (diesel injected directly in on top of the piston), now have multiple injection points. With only a few hundred milliseconds separating them, so they must use high speed injectors, which are expensive to replace and do give trouble.

http://www.howstuffworks.com/diesel.htm[/QUOTE]

Except in the 2.0. 8 valve Hdi.