Connecting rod broke below the piston. Looking for ideas on cause.

[Godell50]

I've had a 7.3 idi in my truck for about a year now. All stock except for the "bowl" cut out of the intake and the fuel screw turned up two flats. I have a pyro on it and it never peaks above 900 degrees. 1975 F250 "high-boy". It usually stays at dads farm and gets used off road with the occasional trip to town. Mom hit a deer a few weeks ago so I gave her my car til hers is fixed and took the truck to school about 120 miles away. Headed back home Wednesday and 5 miles into the trip it threw a rod through the block at 60 mph cruising down hill. Tore it down and it's broken in the middle (looks like below the piston is where it started). Didn't come off the crank or wrist pin, both sides spin freely on the journal/wrist pin, valve train looks healthy, pulled the head and the piston face looks fine, screen is in place on the intake and had a filter on so nothing could've gotten down there anyway, water separator was in place, checked it and there was just diesel in it. Rings are intact and piston moved freely in the cylinder. About 2 to 3 inches of the rod are just gone (probably out the pan, looks like it took a 3 rd burst from a ma-deuce). I have another 7.3 to go in but after seeing that the rods were cast and experiencing this, I put it back in the corner and pulled out a 390 today to start freshening up. I hate to go back to gas with it but I dont want to put another time bomb in the truck. I have a lot to learn so I asked a guy who used to be a diesel mechanic, and hasn't steered me wrong yet, and he said his best guess is a casting flaw in the rod. I beat the hell out of this engine around the farm yanking logs out to cut and dragging trailers out of muddy fields, I have a hard time believing it just let loose cruising down a hill if its had a weakness there since day one. Looking for others thoughts on a possible cause. Thanks in advance.

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[Ironman03R]

My experience with rod failure has been that it was stressed at some point then fails later when you least expect it.
I have not had an IDI rod failure, but GM small blocks and 1.9 Saturn's.
My years at the Saturn dealership I saw a lot of what you describe, where the customer says "it just let go on the way to work" .That might have been true, but what they don't tell you is 2 weeks ago they put a fartcan muffler on and was reving the **** outta it to hear how bad ass it sounds
I wouldn't worry about it and just put another IDI in it, these engines are old and metal fatigue s over time, chalk it up to a freak event and roll on with it.

 

[Thewespaul]

That’s pretty typical for the na rod faliures I’ve seen, which is why I really prefer to build the turbo blocks. The na rods seem to be strong but seem to fail at very random power levels, instead of as soon as you pass a certain threshold them breaking. This tells me the quality of the rods are not very consistent, some seem to be very brittle and snap at low power levels for no obvious reason

 

[icanfixall]

Sorry you lost an engine but no need to feel another idi is going to fail like this one did. There are thousands of these engines doing all kinds of hard work and not very often does this forum read about failures like this. You yourself posted you trashed this engine on the farm hauling logs and "stuff" everywhere. My thoughts are this rod had a flaw in it. Finally it let go.

 

[IDIoit]

they break very randomly.
i developed a tick in mine, tried to chase it down by going through the rockers, injectors, and all of the other BS for diagnosing.
after all of the work, the tick was still there..
turned the engine off, and it broke.
had been running the engine for several years, and while i may drive it pretty hard, i dont tow with it....
all IDIT blocks from here on out.

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[bbjordan]

Change the rod & pan and keep running it.

 

[Macrobb]

I have a '88 7.3 IDI block that did the same thing... about a year after I had it rebuilt. I'd been running it at much higher than stock power levels(250RWHP) for quite a while, no issues... then started it up one morning cold and it ran on 7. Revved it up a little, "clunk" and oil is draining through a bunch of new holes in the pan.

Rod broke exactly like that - just after the small end. In my case, the big end and rid stayed attached, bent/twisted a bit as it was banging around, but... same random failure.

Sadly in my case, it bade some cracks in the upper end of the block and tweaked the lifter galleries a bit, or I'd have just JB-welded the minor holes where it met the oil pan and ran it again...

 

[chris142]

Usually that's caused by water in a cylinder. It can't compressed so the rod bends. Any chance water ran down the air filter bolt hole?

 

[Godell50]

Thanks for the replies guys! I think I'm still going to switch to the gasser and save this 7.3 (non turbo) for another project that won't see such hard offroad use (my uncle recently gave me a 64 f250 in need of some tlc). The 390 I'm going through has 428 scj rods so they should hold up. It's been sitting so I'm going to re-ring it but otherwise it looks good.

Chris142 I wondered about that but I dont see how. It still has the rubber grommet on it and it wasn't raining or anything of the like.

Bbjordan a chunk of the block came off with the pan so that's not really an option. I'm currently tearing it apart and saving what can be reused, scrapping the rest.

The only thing I cant explain is all the other cylinders have the soot brushed away in the shape of a valve. That kinda scared me when I saw it but they're all like that. Except the one that failed. I'm not sure what that's all about. I'll go take a picture of both of them and post it.

 

[Godell50]

The black one is a good cylinder. All 7 look like this one. The random scuffs are from the work bench. The round one is from the valve.

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[Thewespaul]

You’ve been burning coolant in that cylinder for awhile now, the engine hydralocked

 

[Godell50]

Thewespaul how do you tell? The color? Forgive my ignorance. I don't have too much experience with diagnosing failure. The head gasket was in good shape, I'm fairly confident it wasn't leaking.

 

[Godell50]

Also, how do I diagnose that in the future. Engine seemed to run fine. Smoked a little on cold start but once warm the exhaust looked normal. You could only see a puff of black at loaded full throttle on a hot day so it wasn't being masked. I just want to be able to notice this and prevent catastrophic failure should it happen in the future.

 

[Godell50]

Thanks for sharing your observations btw.

 

[Thewespaul]

Yep, the lack of carbon deposits are from the steam cleaning effect, the white color is from the coolant burning off. Just like if you leave a pot of water on the stove too long and it evaporates, it will leave a similar white coating. Your headgasket may be fine, but the head could be cracked or the block cavitated.