300 tdi knocking

harryb · Jan 12, 2019

Hi all, been a long time since I posted on here but I have a persistent knocking on my 300tdi discovery that I can't get to the bottom of.

It probably started 2 or 3 years ago as a slight knock that I put down to injectors however it has got a lot worse in the last 6 months, I had the injectors tested which came back as being fine, it's not the vacuum pump or lift pump, with a stephoscope it sounded like it was coming from the top of the block from one of the middle pistons so I then stripped the engine which had:
-New big ends
-New main bearings
-New piston rings
-New piston that had picked up a bit on the skirts
There was no noticeable wear on the small ends, a very slight bit of movement but nothing obvious as being worn, the pistons and bores were all within the wear limits, the pushrods were all straight and the cam and followers all looked in good condition.
It all went back together and still knocks! I've since tried a set of known good injectors and that made no difference and tried retarding the timing (which had been checked) which also made no difference.
The knocking is now at a point where it is very obvious under load, on over run it quietens right down but as soon as you put even the slightest movement on the throttle pedal it comes back and once warm becomes more of a tinking than a knocking if that makes sense.
So I'm now at a bit of a loss of where to look next, any ideas?

Al2O3 · Jan 13, 2019

What's the compression like? While you had it apart, did you check the piston upstand, valve face position from cylinder head face, cylinder head gasket thickness and condition of all valve stem caps?

jamesmartin · Jan 13, 2019

harryb said:
Hi all, been a long time since I posted on here but I have a persistent knocking on my 300tdi discovery that I can't get to the bottom of.

It probably started 2 or 3 years ago as a slight knock that I put down to injectors however it has got a lot worse in the last 6 months, I had the injectors tested which came back as being fine, it's not the vacuum pump or lift pump, with a stephoscope it sounded like it was coming from the top of the block from one of the middle pistons so I then stripped the engine which had:
-New big ends
-New main bearings
-New piston rings
-New piston that had picked up a bit on the skirts
There was no noticeable wear on the small ends, a very slight bit of movement but nothing obvious as being worn, the pistons and bores were all within the wear limits, the pushrods were all straight and the cam and followers all looked in good condition.
It all went back together and still knocks! I've since tried a set of known good injectors and that made no difference and tried retarding the timing (which had been checked) which also made no difference.
The knocking is now at a point where it is very obvious under load, on over run it quietens right down but as soon as you put even the slightest movement on the throttle pedal it comes back and once warm becomes more of a tinking than a knocking if that makes sense.
So I'm now at a bit of a loss of where to look next, any ideas?

there should be no movement at all in small ends beside sliding,any play its worn out

harryb · Jan 13, 2019

Al2O3 said:
What's the compression like? While you had it apart, did you check the piston upstand, valve face position from cylinder head face, cylinder head gasket thickness and condition of all valve stem caps?

I can't say I checked compression and regrettably didn't check valve face position or piston upstand since I ordered the parts in advance to try and get it back together as soon as possible, I just used a 1.6mm gasket since that's what was already on there. Valve stem caps are all still there though

jamesmartin said:
there should be no movement at all in small ends beside sliding,any play its worn out

I couldn't notice any play in the thrust direction but could just about notice a bit of movement when rocking it along the pin if you get what I mean, they were all similar I think though, would that amount be enough to make it knock? When fitting the new piston you could feel the difference between the old and new gudgeon pins in terms of the snugness of their fit. Any ways of confirming its small ends without stripping it? Got to the point where I don't really want to throw even more money at it without being fairly certain that I'm going to sort the problem.
Cheers for your help

jamesmartin · Jan 13, 2019

harryb said:
I can't say I checked compression and regrettably didn't check valve face position or piston upstand since I ordered the parts in advance to try and get it back together as soon as possible, I just used a 1.6mm gasket since that's what was already on there. Valve stem caps are all still there though

I couldn't notice any play in the thrust direction but could just about notice a bit of movement when rocking it along the pin if you get what I mean, they were all similar I think though, would that amount be enough to make it knock? When fitting the new piston you could feel the difference between the old and new gudgeon pins in terms of the snugness of their fit. Any ways of confirming its small ends without stripping it? Got to the point where I don't really want to throw even more money at it without being fairly certain that I'm going to sort the problem.
Cheers for your help

im not suggesting its little ends, youd expect it worse on overrun, how do you know its not vac pump or lift pump,you really should check valve stand down and piston height

harryb · Jan 13, 2019

jamesmartin said:
im not suggesting its little ends, youd expect it worse on overrun, how do you know its not vac pump or lift pump,you really should check valve stand down and piston height

Sorry I thought you were saying it might be little ends. I've had the vac pump off and that made no difference and I've fitted a new lift pump.
I realise I ought to have checked the stand down and protrusion but with a 1.6 gasket and no sign of any contact having been made I can't imagine that would be the cause of the knocking would it?

Al2O3 · Jan 13, 2019

harryb said:
Sorry I thought you were saying it might be little ends. I've had the vac pump off and that made no difference and I've fitted a new lift pump.
I realise I ought to have checked the stand down and protrusion but with a 1.6 gasket and no sign of any contact having been made I can't imagine that would be the cause of the knocking would it?

Unlikely if you didn't see any signs of contact on the piston tops. The tolerances are very fine in there though and so it's good to know where you stand. I was thinking along the lines of combustion 'diesel knock' being affected, but I was just thinking out loud. Are you pretty sure it's a mechanical knocking?

harryb · Jan 13, 2019

Al2O3 said:
Unlikely if you didn't see any signs of contact on the piston tops. The tolerances are very fine in there though and so it's good to know where you stand. I was thinking along the lines of combustion 'diesel knock' being affected, but I was just thinking out loud. Are you pretty sure it's a mechanical knocking?

I know and it was something I regretted not checking once it was back together... Well I was thinking it was diesel knock for a while but I'm now certain it's not injectors and I've also retarded the timing till it was smoking away and the knock was still there so the only thing I could think of was an issue with the injection pump but then on more thinking I wasn't sure that from no throttle on overrun to a tiny bit of throttle could make the noise come and go so much if it was diesel knock?
Nothing wrong with thinking out loud and any thoughts are appreciated because I'm at a bit of a loss now really as to what it could be!

300 tdi knocking

harryb

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Al2O3

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jamesmartin

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harryb

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Al2O3

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