Mystery knock...what is it?

[Classicfordguy]

Since I bought my truck just under two years ago it has had a knocking noise, seems worse when the engine is cold and the timing retard or advance or whatever is on till it warms up.

I just went on a 2000mi trip to indiana pulling a trailer and the noise has become much worse. When sitting at idle the noise will come and go, on and off every 30 sec or so. I tried cracking the injector lines and none seemed to make it go away completely, it just changed a bit since the engine would slow as I cracked each line.

It does seem to go away a bit when I accelerate hard but not completely.

I have noticed recently that it is much easier to make black smoke. I used to shift at 1600 rpm going around town but now I have to shift higher to avoid black smoke.

The truck has 123K miles on it, its a 7.3L factory turbodiesel in a 94 F250, to my konwledge the injectors and pump are original, I just don't want to spend the money replacing them if the realy problem is something bad like a wrist pin.

You guys have any idea what it could be or things I could do to figure it out?

Thanks in advance, one day I'll buy a helm manual, untill then your guy's help is awsome.

-Rob

 

[Diesel JD]

Hey Rob...the knock may very well be the injectors... there's a diesel shop in Altamonte Springs that seems like a good place to buy these type of parts. I would pull the injectors and take them there or to another good diesel shopa nd have the injectors pop tested and checked for spray pattern. A real funky spray pattern or low pop pressure will cause all kinds of bad noises. Also the 7.3 s often dropped valves so hopefully that is not your problem. How is your oil pressure and can you tell where the noise is coming from?

 

[towcat]

silly question...
how's the condition of the flexplate?

 

[suv7734]

+1 on the injectors. Many have had a 'bad wrist pin' that turned out to be a leaking injector. I would look into this soon before you put on too many miles (if you're still driving the truck) because of your other post about the oil leak. As DieselJD said there are know issues with valves on these engines. If you have, for instance, a badly worn guide that is leaking straight through it can create a huge amount of blowby and more specifically pressurize the crankcase. This could be why you are seeing so much oil around that grommet in the valley pan. Hopefully it is just a bad injector and a leaking seal but you want to find out before it potentially drops a valve on you. I've seen both scenarios and the end result of one is not pretty.

 

[krawlr]

Since cracking the lines didn't change anything and he's switched to a ZF5, my guess is lift pump or valvetrain.

Will a DMF make noise under load?

You can pinch off the inlet hose on the lift pump and see if it changes the sound. (if it's the old mechanical one)

A chunk of garden hose held to your ear can help dial in on the location.

 

[towcat]

Will a DMF make noise under load?

If the DMF is seriously worn out. you bet. BUT.....accelerated wear happens when the IP is dying a slow death.

 

[h2odrx]

If the DMF is seriously worn out. you bet. BUT.....accelerated wear happens when the IP is dying a slow death.

Can you explain this???

 

[Classicfordguy]

Im sure hoping it's the injectors but cracking the lines and hearing no difference scares me.

towcat, what flexplate do you mean? Arent flexplates only in automatics?

I can't for the life of me pinpoint the location of the knock, I realy don't think it's the daul mass, its comming from inside the engine somewhere. I used a 2ft screwdriver as a stethascope but couldn't narow it down.

Krawler, what will pinching off the lift pump inlet do, don't understand what im looking for there?

The noise seems far too big to be valve train in my opinion uless its a lifter not returning properly then smacking into the low part of a cam lobe.

Thanks for the responses, keep em comming

-Rob

 

[krawlr]

Krawler, what will pinching off the lift pump inlet do, don't understand what im looking for there?

It should change the sound of it if it's failing.

 

[84TD]

I have a mystery knock in my truck but after hearing the exact same noise in another 6.9 I ruled it out as normal, I can only hear it in the cab, it sounds like a light off beat knock that seems a little random compared to the diesel clatter. It gets louder near the pedals. I cant hear it at all from out side the truck but if I put a stethoscope on the fuel filter bracket, near the alternator I can hear it pretty good. I think its prob the lift pump but I am not sure. Does yours sound like that at all?

Anyone else heard this on theirs?

 

[Classicfordguy]

I'll give the lift pump a shot, my noise is easily heard outside the truck, kinda embarassing when its at its loud points.

-Rob