Complete Misfire on Three Cylinders

[Captain Obvious]

So I'm out driving around today, just minding my own business... All of a sudden, out of the blue, it starts running exceptionally poorly. Somehow (still not sure how), I managed to limp it about five miles home. Get home and start the troubleshooting. It's completely missing on the rear three cylinders and front firing out the intake like the timing is way off.

So, what can cause a complete miss on the rear three cylinders and run like turds on the others?

ECU?  Nope. That's not it.
Dropping resistor? Nope, not it either.
Bunch of lash pads fly out? Nope.

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth."

https://youtu.be/4nE4QhK4LMY

Woof.

[Reptoid Overlords]

It's gotta be the AFM

[Captain Obvious]

Uhhhhh.... Did you watch the video?      

[Zed Head]

9 minutes ago, Captain Obvious said:

So I'm out driving around today, just minding my own business... All of a sudden, out of the blue, it starts running exceptionally poorly. Somehow (still not sure how), I managed to limp it about five miles home. Get home and start the troubleshooting. It's completely missing on the rear three cylinders and front firing out the intake like the timing is way off.

So, what can cause a complete miss on the rear three cylinders and run like turds on the others?

ECU?  Nope. That's not it.
Dropping resistor? Nope, not it either.
Bunch of lash pads fly out? Nope.

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth."

https://youtu.be/4nE4QhK4LMY

Woof.

Is that really your engine in the video, or were you just telling a story to set up the puzzle?  Pretty cool.  Not sure why it ran like turd on three, seems like three should have been okay, unless this is a case where the timing chain actually did jump a tooth or two.

[Captain Obvious]

Sorry guys. I was trying to be cute and I guess the issue doesn't come across so obviously in the video. That's what I get for holding my antique crappy  camera in one hand and reaching in with my other hand to crank the engine. If you look at the camshaft while I'm cranking the engine, you'll notice that only the front half turns. The back half doesn't move at all.

My crankshaft completely cracked into two pieces.

This is just behind the center support bearing:

[Reptoid Overlords]

I saw it, I was only joking about the AFM comment.  My dry sense of humor. 

[Zed Head]

Camshaft.

I saw that.  That's what I meant about the front three should have run fine unless cam timing was off.  I think I read "front fire" as a problem with the front three cylinders though, so I guess you were running on half power, cam timing is probably fine for the front three.

Front firing is typical of a flattened cam shaft lobe.  Really common on small block Chevys.  So that does make sense since the back six valves were stuck closed or open, or in between.

Any thoughts on why it broke?  Have the towers ever been moved?

[S30Driver]

Wow. must have been weakened from those 10,000 rpm shift points of your spirited driving .

Only thing I can think of is a bearing froze after the break or a flaw in the metal that decided that was the time to let go.

 

 

[heyitsrama]

? was it a stock cam? Is there a possibility for the valve's to slap the pistons in this case?

Edited May 16, 2019 by heyitsrama

[Av8ferg]

Holy Cow!!! [emoji15]

 

Sorry this happened to you CO. Glad you didn’t have to leave your car in a less than affluent area. At least they wouldn’t have been able to drive it either. Well...the silver lining may be that this is what brakes the inertia and gets you starting that F54/P90 engine rebuilt and in your car? [emoji2369]

 

If you gave me 100 guesses without that video on what caused that I would have never said.... “oh, easy a cracked cam”

 

 

 

 

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[Av8ferg]

When I was reading about the full engine rebuild I read about doing either MPI or DPI inspections for cracks in the Crank and Cam and Connecting Rods. I thought that maybe it was overkill but now I think your incident shows that it might be worthwhile after all.



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[Dave WM]

were the cam towers ever removed and replaced?