Me and a buddy just got done rebuilding his 7.3 idi for his 93 f-350. Runs great but it’s got an awful knock. We are both scared it’s something on the bottom end. We know the knock is coming from the rear passenger side cylinder because when we loosen up that injection line the knock gets really quiet. We pulled the intake and valve cover and discovered nothing. The rocker arm bolts where tight and no collapsed lifters or bent pushrods. It has a brand new set or R&D injectors, and even after swapping them around the noise is still present. At this point, I’m out of excuses that would not merit another rebuild, so as a last resort, I decided to turn to this forum for any last opinions. I’m considering running it until it blows, but maybe it’s an easy fix (probably not). It sounds very very similar to this video I found on YouTube: https://m.youtube.com/watch?v=Zc_rUh23HcU&feature=youtu.be However, I was unable to find any verification that the knock in the video was actually a rod knock. That’s just what the title says. The creator of the video has not posted any follow up videos, nor has he responded to comments. Let me know what you think.

 

Could be rod knock, or piston slap. Dumb experiment, but have you tried retarding the timing or changing the offending cylinder's injector? If you have a weak injector, I'll fire the fuel in prematurely and cause a knock.

 

At this point, I really don’t think it’s the injectors. They are brand new, and even after swapping the injector from the bad cylinder to another, the same problem. No difference. I’ll try and adjust the timing on the pump a little and see if that makes a difference, but I don’t understand how that could only effect one cylinder. If it was out of time, it would effect all cylinders. The one thing that makes me think it isn’t the bottom end is the fact that when the injection line is disconnected, there is no more knock. If it was bottom end, wouldn’t there continue to be a knock even if the line was disconnected?

 

Yah. I missed the part where you said you moved around the injectors. I listened to the yt video and that sounds pretty bad. It definitely sounds like it's in the lower end. Maybe a bent rod. Cracking the injector removes the pressure spike in the cylinder, which is why it gets quiet. If you had a bad piston or a bad rod, it would quiet it down since it's just along for the ride while the line is loose. I'm guessing it's a non turbo with the factory air cleaner. Those have a problem of leaking water into the air cleaner after a good rain storm. Since the engine is tilted back, the water runs to the back cylinders and hydro locks them.

 

That is certainly possible. After we rebuilt the motor, we pulled it out of our shop to have it out of the way, and didn’t mind putting the hood back on. Instead, we put a tarp over the engine bay. But It is possible some water may have made it into the intake because there where a few holes in it and it did rain. I did put a thick rag over the intake just in case, but who knows. One other possibility I have thought about is maybe that the wrist pin bushing in the rod may be warn out. When we pulled the old pistons out, one of them had the side of the piston broken off, and thinking back, this may have been a result of the worn out bushing causing piston slap and breaking off the side of the old piston. We didn’t replace those, so maybe the knock could be the play in the bushing. It didn’t knock before the rebuild though. Is it possible this could be the cause?

 

Typically, when piston slap gets that loud, it already broke a section of the piston off..it's pretty common in jeep 4.0. But I would bet you have a bent con rod from water.

 

Double post Sorry

 

any update on this problem?

 

Six months later and the engine is rebuilt once again and running great. It was piston slap due to a warped cylinder.