Kubota L4740 Engine "knock" Help?

[jshwhite]

Recently sold the L3710 and got a low-hours L4740. I love it - that extra 10 hp makes such a difference. It had 365 hrs on it when I bought it and I've put about 75 more on it.

It's making a sound that has me a bit scared. When I first got it, I thought it sounded like it had a knock. The guy that had it before me didn't use it much and it just sat a lot - outside. So my first thought was maybe bad fuel, sticky injector, or something like that. I flushed the fuel system, changed filter, and worked it hard. Still sounds the same. So...I need to do something about it.

Here's a video. Kubota L4740 knock - YouTube The sound doesn't come through perfectly, but hopefully those of you with trained ears will hear it.

Thoughts? Bad injector? Bad valve lash? Spun/worn rod bearing? (I sure hope not!! ) I have a lot of mechanical experience, but not much with internal diesel engine work. Thanks.

 

[TMGT]

That doesn't sound too good, one thing you might check first is for an exhaust leak or a bad muffler.
I have a reel mower with a 4cyl kubota that I thought had a rod knock, finally figured out it was the muffler causing the sound. No leaks, just real tinny.

 

[nybirdman]

I agree with the exhaust leak;let's hope that what it is.Definately sounds different than my L4240.

 

[dstig1]

I was also going to suggest exhaust/muffler leak/hole/etc. Kinda rusty in there so maybe the muffler perforated? Most likely at the bottom of muffler & pipes, I'd guess.

 

[rScotty]

OK. You mean the knock, right? Yes I hear it plainly. First of all I'd say it is from one cylinder... not the other three. Darn, I was hoping it would be something simple.
In fact, I was hoping it would be typical rocker/tappet noise, but no..... and it does not sound like any kind of exhaust leak to me. That's a metallic noise I'm hearing. You described it as a knock and I agree. It isn't a heavy knock, but it is one.

. Nor does it sound like a stuck EGR valve. Those sometimes have a very loud rattling knock, but but it's a lightweight higher pitched knock and more rapid.

That noise I'm hearing is definitely from the engine - probably the rear - but it doesn't sound like a knock you'd get from small end bushings (no double-knock sound) - and it is too light weight to be big end rod. Plus it didn't change much under load. So I don't think it is big end.

It is way too lightweight to be crankshaft.

And while that engine runs good at idle and lower RPM, the the whole engine sounds to me just a bit light and ratty when revved up....

I'm going to say what I'm hearing is diesel piston slap in one cylinder due to a faulty injector. It would be like having one weak cylinder. Diesel pistons make a knock quite unlike gassers which have a lighter ringing sound to their piston slap.

But I'm going to say that this is not a clearance or wear induced slap, it is slap brought on by that cylinder's injector. Maybe I'm all wet, but I think that cylinder's injector has either lost it's proper pop-off specification or is putting out a less efficient injector spray. Maybe both. Either or both of those will change a diesel's normal compression ignition timing and make just exactly that kind of knock.

Keep in mind it's just a guess, but it fits what I hear.

If yours is a common rail diesel (that engine changed to Tier IV interm about 2008) I'd first check all the electrical connectors to the injectors. And if you don't find anything wrong there, I'd pull that injector - and the one next to it - and get them tested.

New injectors can be expensive, but don't go there. If it works as well as it does in the video there is no need. Find a good old school local diesel shop to test and rebuild them. It doesn't have to be Kubota to do that work, it can be any brand. Here I'm assuming that the Kubota injectors are rebuildable although I don't actually know that. At any rate, lots of shops will check them while you wait. Do that first.
good luck,
rScotty

 

[TMGT]

OK. You mean the knock, right? Yes I hear it plainly. First of all I'd say it is from one cylinder... not the other three. Darn, I was hoping it would be something simple.
In fact, I was hoping it would be typical rocker/tappet noise, but no..... and it does not sound like any kind of exhaust leak to me. That's a metallic noise I'm hearing. You described it as a knock and I agree. It isn't a heavy knock, but it is one.

. Nor does it sound like a stuck EGR valve. Those sometimes have a very loud rattling knock, but but it's a lightweight higher pitched knock and more rapid.

That noise I'm hearing is definitely from the engine - probably the rear - but it doesn't sound like a knock you'd get from small end bushings (no double-knock sound) - and it is too light weight to be big end rod. Plus it didn't change much under load. So I don't think it is big end.

It is way too lightweight to be crankshaft.

And while that engine runs good at idle and lower RPM, the the whole engine sounds to me just a bit light and ratty when revved up....

I'm going to say what I'm hearing is diesel piston slap in one cylinder due to a faulty injector. It would be like having one weak cylinder. Diesel pistons make a knock quite unlike gassers which have a lighter ringing sound to their piston slap.

But I'm going to say that this is not a clearance or wear induced slap, it is slap brought on by that cylinder's injector. Maybe I'm all wet, but I think that cylinder's injector has either lost it's proper pop-off specification or is putting out a less efficient injector spray. Maybe both. Either or both of those will change a diesel's normal compression ignition timing and make just exactly that kind of knock.

Keep in mind it's just a guess, but it fits what I hear.

If yours is a common rail diesel (that engine changed to Tier IV interm about 2008) I'd first check all the electrical connectors to the injectors. And if you don't find anything wrong there, I'd pull that injector - and the one next to it - and get them tested.

New injectors can be expensive, but don't go there. If it works as well as it does in the video there is no need. Find a good old school local diesel shop to test and rebuild them. It doesn't have to be Kubota to do that work, it can be any brand. Here I'm assuming that the Kubota injectors are rebuildable although I don't actually know that. At any rate, lots of shops will check them while you wait. Do that first.
good luck,
rScotty

You might be surprised at what noises a muffler can make, i spent some time tracking down a knock on a V1305 in a mower, sounded like someone was tapping the side of the block with a ball peen hammer!
Finally narrowed it down to the muffler, no leak, muffler seemed to be intact (I didn't cut it open), so possible a baffle finally rotted through.

I'm not saying that's the OP's case, but it's a simpler and less expensive avenue to explore first.

 

[mcfarmall]

I'd take a walk down the exhaust leak path before getting too worried.

 

[jshwhite]

Wow, I go sleep and come back this morning to find lots of helpful ideas. Thanks everyone.

Re the exaust leak idea, when loading the engine I do hear the classic "ticking" sound of an exhaust leak. So it very well could have one. I'll give it a good looking over.

TMGT, interesting you mention a bad muffler. I talked to a buddy of mine and he said his D3 dozer was making a "knock", which turned out to be a bad muffler. Question is, how to test that? Take it off and run the engine? Seems that would create another sound altogether

rScotty, thanks for the idea/explanation. Hey, a bad injector would be better than a spun rod bearing! And actually, I kinda thought the knock was a separate sound from the exaust leak, so maybe I actually have 2 problems.

Too many projects going at the moment, but I'll try to get to this in a few days and report back...

 

[TMGT]

Wow, I go sleep and come back this morning to find lots of helpful ideas. Thanks everyone.

Re the exaust leak idea, when loading the engine I do hear the classic "ticking" sound of an exhaust leak. So it very well could have one. I'll give it a good looking over.

TMGT, interesting you mention a bad muffler. I talked to a buddy of mine and he said his D3 dozer was making a "knock", which turned out to be a bad muffler. Question is, how to test that? Take it off and run the engine? Seems that would create another sound altogether

rScotty, thanks for the idea/explanation. Hey, a bad injector would be better than a spun rod bearing! And actually, I kinda thought the knock was a separate sound from the exaust leak, so maybe I actually have 2 problems.

Too many projects going at the moment, but I'll try to get to this in a few days and report back...

I have access to electronic devices designed to isolate sounds on a chassis, however taking the muffler off will work, it'll be louder but the noise should dissappear if its the muffler.

 

[dodge man]

I thought it disappeared or got better a higher rpms.