View Full Version : What does knock sound like?

04-04-2009, 01:25 PM
I started starting the 70D a while back. It ran for a few minutes and then died. I double checked everything and found it was almost out of oil. I put in about 6 gallons worth and turned it over again. It runs without shutting down and (to me) the engine sounds really good.

Supposedly, this engine has a knock in it, but I don't hear anything that sounds like a knock. What does knock sound like? What should I be looking for?

04-04-2009, 02:32 PM
Big End Bearings, White Metal Melted Out and therfore not enough Compression etc etc, or Just Check the Fuel Tank Level and hope you had a close Escape of a costly engine repair, I have had a close encounter in the past.

The Big End KNOCK:- is the Knocking from the Big End Bearing, I have throw a rod in the past the worse case I guess, From Experience the Clearance Dia for the Big end Bearing and the Dia of the Crank Dia get too great. you have to remember the Crankshaft is doing "X revs per min" and the Big End is being fored up and down at 5 Times or even more the Perifferanal Speed so with melting the big end bearings by heat generated from no oil, Oil is also a cooling factor in the engine, the big end of the con rod, becomes like a slide hammer, and breaks the bearing bolts, and breaks the crankcase, cam shaft and block anything in his way at high revs etc.

Or you have had a Knocking Sound for some time, and you still have a couple of Pints in the lub system, which as saved the main mechanical fuction of the engine,, you may have a:-

Diesel Knock from a worn diesel injector, or damaged injector tip. Or you might just be a little Sensive to the Normal running of the Engine, there is a Knock in the Clylinders at the Ignition cycle, and there can also be a PINKING in the Ignition cycle. Pre ignition and Post Ignition.

It all depends on the type Of KNOCK, Different friends knock on my door in different ways, I hope your engine problem is a Friendly Knock


04-04-2009, 02:48 PM
There are two things right off hand to check and with it idling take a 3/4" open end wrench and open each fuel line one at a time till fuel comes out and see if one of them takes the knock away when it,s open and if it goes away sounds like a bad nozzle. You could swap that nozzle with another one in the engine and see if the knock follows and if so pretty much pins it down to a bad nozzle. If the knock is still there take the oil filters out and see if they have some aluminum grindings or chips caught in them and if so sounds like you got trouble there, scored bearing or bearings.

04-04-2009, 05:33 PM
Sorry, I didn't explain myself well. I am asking what knock sounds like. I don't hear anything. The engine sounds normal to me - just like the other motor grader I have that runs fine. I was only told that the engine has knock when I bought it, but I don't know that for myself.

04-04-2009, 05:58 PM
There are many faint knocks and clatters from a engine that's running, and the noise is worse when they are cold. "Diesel knock" is the explosive combustion of diesel fuel under high compression that is readily identifiable with any diesel.

However, audible knocks can be caused by excessive clearances or faulty fuel injection. Clearances larger than normal in (mostly) big end bearings will come across, as a louder-than-normal regular deep knock. The knock is audible above the regular "diesel knock" and generally increases at higher speed, and when the engine RPM is varied slightly but rapidly (the vroom-vroom of rapid throttle increase and decrease).

Another source of knock is excessive wrist pin clearance. This is generally louder but lighter in pitch than a big end bearing, and is regular through the rev range and throttle application. A mechanics stethoscope is an ideal tool to try and pinpoint knocks.
Have someone operate the throttle, varying the RPM, while you place the stethoscope on the lower parts and upper parts of the engine block, to try and pinpoint the sound source area. Check at idle, and check at higher RPM.

Injection knock is a very loud knock that is alarming, and easily heard. It is caused when air gets into the fuel, or the injectors are faulty, and not atomising the fuel properly. Injection knock can be found easily by utilising the process described above by SJ.

Oil Slick
04-04-2009, 06:19 PM
Raklet, click on the links below. I got piston and rod knocking video's. Then a picture of whats next :eek:

#6 Piston Knock,

#2 Rod Knock,

I think the camera is shaking because I'm crying LoL

04-04-2009, 07:19 PM
I thought that I was going to have to make a post like this this yesterday.

I went out to the ranch for the first time in a long while after working on a hard deadline on a big history changing R&D project at work for the last few weeks.

When I fired up my old D4 6u I noticed a hollow sounding knock. Shut it down, luckily there was a Cat mechanic working on one of my relatives challengers. Asked how long the cat was sitting in the back of the barn, told him a few months, he grabbed a 3/4" wrench started it up and started bleeding injectors and after #4 was bleed it went away, and I was out making washboards.

I guess I was lucky on my tractor but with little or no oil in the motor grader I would be cutting into filters and "looking for gold" as some mechanics would say.

04-04-2009, 08:39 PM
Raklet, click on the links below. I got piston and rod knocking video's. Then a picture of whats next :eek:

:eek: That rod knock video is terrible! I don't have anything that sounds like that.

04-04-2009, 09:39 PM
Mike, got a little over rev going on there to cause that or or pulling too hard with the 8 on.

Oil Slick
04-05-2009, 02:14 PM
A- I bought the truck with a tired motor (#6) and gave it a complete in frame rebuild. About 200 miles after the rebuild it spun #2 rod bearing. Called the engine rebuilder and they sent a tow truck and put a brand new crank in for free. The suspected cause was an obstruction in the oil passage. Not my driving :cool:

04-06-2009, 01:44 PM
Ok, opinion time.

I found a local old gent that has 40+ plus years working on CATS. He came out and listened to the engine with me. He couldn't hear anything right off and had it run for several minutes to get good and warm, still with nothing apparent showing up. He told me to get the tires fixed so I could take it out and put a load on it.

We were about to shut it down when he messed with the throttle linkage. Finally, we could hear some noise. It occurs when the throttle is rapidly moved from high rpms to low rpms. The noise is not audible going from low rpms to high rpms. We cracked each of the fuel lines, but no change there. The noise can still be heard, but not at any steady rpm level. Only when the engine is retarded from high rpm to low.

The old gent suggested I remove the inspection covers and check to make sure the rods are tight and inspect the bearings. I guess that will be my next step. Also I am going to change the oil filters and look for metal shavings. Any other opinions, suggestions?


Old Magnet
04-06-2009, 02:54 PM
I have found that a rod or main bearing knock is usually louder under load. It's the piston pin that will knock when engine is unloaded from high rpm to low.

04-06-2009, 05:29 PM
Is the piston pin the same as the wrist pin that was referenced earlier?

I have tried to do some reading about piston pin knock. Does the following apply to these diesel engines?

A piston (wrist) pin knock. Loose piston pins will generally knock louder when the engine is not operating under a load. A piston pin knock is sometimes mistaken for a connecting rod knock although the pin knock is not as loud. If it is possible to adjust the exhaust valve so that it stays open all the time, the pin knock will double up so it makes twice the number of sounds.

I am assuming that to change it, the rod has to be disconnected, head removed, and the piston pulled out the top. Is that right?

How can I find which one? Or, while I am into it, am I better off pulling all of the pistons, inspecting them, doing general engine inspection, etc?

Old Magnet
04-06-2009, 07:04 PM
Piston pin...wrist pin...same thing.
Can be a little tough to locate when knock is slight.....I'm a big believer in the screwdriver or bar to the ear when probing for source of sound. You could try the valve thing you describe as that would unload the piston but be sure your not raising the valve to the point where it may hit the top of the piston. Noise would be loudest when piston is floating with minimum load so that it tends to take up the maximum movement up and down at the wrist pin when changing direction. Also the sound comes from much higher on the block. The knock is a higher pitch...sorta like taping on a solid heavy piece of iron with a ball peen hammer as apposed the the heavy low frequency rod or main knock which sounds more like some one inside the block trying to bust out (some do) using a heavy hammer.

Yes, on process of piston removal.

How far in to it you want to go is up to you but the usual approach is while your in there, check it all.

04-08-2009, 12:19 PM
Sounds about right :) I Purchased a cat from a farmer, who had mentioned the cat use to run :D but the engine needs some work :D :eek:

After the Knocking sound of a big end, you end up with a big BANG lol.

I was driving a Series 3 landrover with a 2.5 landrover 90 diesel fitted, I was going down a big hill on the motorway with my foot to the floor, I heard a knocking sound, which I guessed was BAD, I started to slow down and stop, when I noticed Steam out of the hood, and the windscreen was getting cover in oil, I decided to Keep Driving, I had to get of the Motorway at least, I managed to drive 6 miles throu a town centre, and Pulled up at my Friends GARAGE :D , I explained I was in the Shxt, I had a breakdown job in the back of my motor. I kept the engine at high revs as to not let the thing stall, He asked me to try and start the engine again, I replied NO POINT, just look under the hood, I asked if I could use his shop, and he even throw me the keys to an Imported GMC, I ordered a re-con block from my local engine shop, I can remember the guy telling me, its xxxxxx and you cannot exchange that block, because theres a hole in it. I did try to Explain it was only one hole :D . I didnt get an early Warning Sound :confused:

pics of cat 30, conrod for No.1 cylinder has no main bearing, and 1/2 the cam-shaft is missing, There is also some Engine Block Minor Damage :D

04-08-2009, 12:35 PM
lol - just some minor damage in those photos :). I hope the inside of my old girl doesn't look like that. ;)

Old Magnet
04-08-2009, 01:36 PM
Was running when parked.....:) :)

04-08-2009, 04:24 PM
Good one OM!! Love it!!