I did a search and never really uncovered anything conclusive.
'93 IDI NA E4OD 207K

Every now and then when I crank it up for the first time of the day I will get a very loud thumping sound that seems to be isolated to a single cylinder. It is very loud but is not a mechanical knock like a rod knock. It sounds more like a "THOCK,THOCK,THOCK" from the driverside firewall/floorboard. I sounds almost hollow like it is coming from the intake. Duration can be between 1 thump up to 5-6 seconds of thumping on cold idle.

New injectors and IP several months before it started.
No hard time cranking or starting.
New and functional GP's.
Thump never returns after 1st start of day, if it even thumps then. Maybe 1 in 20 starts it will thump. Sometimes 2-3 days in a row.

My only guess is maybe the beginning of cavitation problems? I have the SCA's in it but can't speak for the first 195,000mi. Seems like maybe if the piston happens to stop at a low point in the cylinder, coolant can collect and cause a thump or knock on startup until it clears. My thoughts are that possibly the rod has bent enough to allow a certain volume of water w/o bending further or breaking?

Maybe some kind of valvetrain problem?

No loss of coolant, no oil/residue in coolant system.
Temp always stable.
Oil pressure is where it has always been. Changes are between 4-5K and I use Delvac or Delo400 with Wix 51734 PSD filter.
Truck runs strong as ever, no smoke, nothing.

Has anyone else experienced anything like this?

 

I'd throw a bunch of diesel additive in your diesel, and see if the problem goes away.
I suspect you've got a sticky/gummed/failing injector, or at least that's where I'd look.
Injectors(especially rebuilt ones) will sometimes randomly fail, or stick after a while. It probably worked fine on the test bench, but over time it just wore what little was actually keeping it working right.

I'm suspecting sticky because it clears up. It might just be a matter of it getting gummed up a bit somehow, and once it warms up, it works fine.

 

Maybe you have a small air leak and one of your engine/transmission mounts has failed.

 

The injectors are new Stanadyne E codes. I know that doesn't grant immunity but this doesn't sound like a dead cylinder. It sounds almost like a very loud backfire in the intake. I will try to video the truck each time I start it to see if I can catch it in the act.

 

First guess is a failing injector, second is a failing lifter. Any chuff out of the intake or exhaust when it happens?

 

It's hard to tell because it is so loud. I would say that there is some noise from the intake when the air cleaner is removed. I noticed it when I was starting it after changing the injection pump. I'm sure it was doing it before, but I never had it running w/o the air cleaner in place.

What are the signs of a bad lifter?

It didn't do it this morning, so no video.

 

What your describing is exactly what my truck did this evening when I started it...If you ask me it sounded more like a valve train issue like an intake valve sticking or something. As stated above maybe a lifter failure?? Didnt really sound like an injector to me...

 

Okay, so maybe one of the valves sticking open initially at start up. That sounds logical to me. Maybe the rocker is lifting off the valve but the valve is hung for a few seconds and compression/ combustion is occurring in intake or exhaust due to heat from GP?

I guess my intermittent problem will have to become a constant one before I do anything about it.

I have a 12V Cummins I could swap in but this 7.3 runs good enough that I'm not in any particular hurry.

 

This also sounds like a collapsed lifter to me. My '85 F-350 would do this occasionally on start up as well. Very loud with the air cleaner removed. Removal of the valve covers and observing the rocker operation narrowed it down. It decided to act right eventually. My guess is an oil passage in the lifter was blocked.

 

Lifter for sure. First try NOT using wix and go with motorcraft oil filter. They don't tend to drain over night, which usually leads to noisy lifters on a cold startup.

 

Hmmm I'm not as convinced as some of these others that it's a lifter but it' a darn good theory. But it certainly seems oil related, that it goes away as soon as whatever it is is lubed. To that end I'd make two changes, switch to the Rotella T6 5W40 and ensure the filter you're using has an oil drain back valve. The 5W over the 15W makes for a thinner oil when cold and thus it gets there faster. This matters even more on a lifter issue as the passages in the lifter are rather small and it's the last place to get pressurized oil. The drain back valve is exactly what it says. Just like air intrusion issues in the IP, air will eventually fill the oil passages and pump if allowed to drain back to the pan.

If those two changes make it better then that alone says a lot for diagnostics.

Pic of drain back valve, you can see it via the holes in your filter, the orange thing.