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Captain Obvious

Complete Misfire on Three Cylinders

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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.

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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.

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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:
P1140729.JPG

P1140728.JPG

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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?

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

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.

 

 

  • Haha 1

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

Edited by heyitsrama

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Holy Cow!!!

 

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?

 

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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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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I bought this car used a couple years ago, and I don't know the history of the engine. I did get some pics from the PO including some shots with the head off the engine, so I know it's been apart at least that far. I don't know if he ever pulled the cam off the head or took the towers off the head. It's stock parts (cam is Nissan "A" grind stock), but I don't know if it's original to this car. I suspect that it is, however.

I've been driving this car on that engine since I bought it, and have never had a hint that I had trouble like that brewing. No idea what happened. I was not at high RPM's when it let go, and I don't think I've ever hit redline with that engine. I take it to maybe 5K often, but that's about it. Anyway,  no idea why it finally let go, but it's gone now!

So I haven't dug into it at all and don't know if that rear cam section is seized in the journals. I kinda doubt it. I suspect it spins just fine. I'll find out when I start taking stuff apart.

And about a path forward... I could either put a new cam in that engine, or...

15 hours ago, Av8ferg said:

may be that this is what brakes the inertia and gets you starting that F54/P90 engine rebuilt and in your car?

Exactly what I was thinking as well. I kept telling you I was going to do it this summer, right? The only problem is that it's Z driving season right now and it would be faster to toss another cam into my existing engine.

15 hours ago, Av8ferg said:

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

Yeah, me either. That's the "whatever remains, however improbable, must be the truth."  part!   LOL

Edited by Captain Obvious
My cam is an "A" grind

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Well, for sure put a new cam in it. That’s the fast way to go and you’d likely keep this engine as a spare I presume anyway.

So, what intrigues me is what was the mechanism for the failure. Is it possible something got in the head and jammed the cam for a moment. Why did it break where it did? Either the cam experienced some sort of trauma or it was faulty and had a small stress riser that finally gave way.
I took a 4 week course on aircraft accidents and we had a structures class that focused on metal fatigue and a lot can be determined by looking at the fracture. You can tell If it failed slowly or immediately. I’d think the area most prone to cam fracture would be where to sprocket connects. This is where the load is being applied.

f9f60712b7eb286d6a72370ec2396b67.jpg


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I am sure CO is going to do a post mortem with detailed looks at the fracture, its his nature.

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so now I have to get a spare camshaft for my spare head for my spare engine....Paranoia is a difficult thing to overcome.

better still, Camshaft phobia..

Edited by Dave WM
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Might as well get a spare Z while your at it. An hour from my house I found a 76 the yard will deliver to my house for $800. Minimal rust and tighter 100%. Tempting but my wife would for sure stab me with a dull spoon if it did it.


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you will get no help here in support for NOT getting it. Make a deal she can get a new couch if you get the car. I use that one a lot. Some how the couch deal never happens though.

ROFL

Edited by Dave WM

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CO, do you have a spare camshaft that you can install? just want to know if there is ANY binding that you can detect.

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If it was mine, I'd go get the head or engine rebuilt.  Some of those valves may be bent.

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I agree a replacement head might be a good move.
I’m pulling that N42 head next time I’m away for work. There are 2 N47’s in that yard. Meet me there and we’ll pull them both. $70


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