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Old 07-09-2020, 01:37 PM   #1
CF12Valve

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96 12 Valve Dead Cylinders #3 & #6

I recently bought a 1996 12 valve sight unseen with 300k miles. I've been trying to restore it and was told that it had fueling problems. I have replaced all of the fuel lines to the tank, the return overflow valve, the in tank fuel module, and did a filter change. I also changed the oil and replaced a few other things that I seen were messed up.

So the problem I am having now is that I'm getting fuel to all of the injectors, the truck runs, but runs rough. I originally thought that the #3 and #6 injectors were bad because when I cracked the lines, the truck stayed running the same. I had all of the injectors tested and cleaned and they were good so I put them back in while also switching the suspected bad injectors into different holes. The point I'm at now, I know the injectors are good and I'm getting good pressure to all of the cylinders, but there is no combustion going on. I was thinking either the valves were bad or the rings are bad so I'm going to check them. Before I take the head off I will be doing a compression test whenever my compression tools get here. I was thinking that since cylinders 3 and 6 are on the same stroke, could the timing be off? I noticed that whoever had the truck before had cranked the star wheel all of the way, the preboost screw all of the way, and had a #0 fuel plate in the truck, which made me think that they may have tried to adjust the timing. Could the timing be the cause of this? If so I'll order a timing kit and check that as well. Any help or input would be greatly appreciated.
 
Old 07-09-2020, 01:42 PM   #2
jasonc

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Timing wouldn't cause a dead skip on the same two cylinders.

How much blow by does it show out the oil fill?
 
Old 07-09-2020, 02:05 PM   #3
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Probably not a bad idea to go ahead and pull that head.

Have you pulled the valve covers and given it a look while it’s spinning?
 
Old 07-09-2020, 02:06 PM   #4
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Quote:
Originally Posted by jasonc View Post
Timing wouldn't cause a dead skip on the same two cylinders.

How much blow by does it show out the oil fill?
Right on. If it has dead cylinders it would have high crank case pressure.
It may be timing, or, bad B&P in the injection pump.
 
Old 07-10-2020, 08:38 AM   #5
CF12Valve

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It's got a good amount of blow by but not an extreme amount. I haven't checked it out with the valve covers off yet, but did have to adjust valve lash on most of the valves because they were off so much. I'll get the compression checked today and then take off the head from there.
 
Old 07-11-2020, 04:54 PM   #6
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Is it using oil?
 
Old 07-11-2020, 06:23 PM   #7
CF12Valve

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Well after getting 3 readings of 120 psi on cylinder one and 2 readings of 20 psi on cylinder two I decided to take the head off without testing the other cylinders. I think I found the problem. Every single piston was melted and completely messed up the cylinder walls. I don't know how this motor was running at all.
 
Old 07-11-2020, 06:56 PM   #8
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Dang. Hate to hear that.
 
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