Hello I have a 96 l31 that I'm planning to install into my k2500 today, but I have some concerns. Last night I was putting on the finishing touches before I take the thing of the engine stand, and while trying to find TDC to install my distributor I noticed something that worries me. Everywhere I could find said to put a finger over the #1 spark plug hole to feel for compression pushing against it as you rotate the crank. The problem is I don't feel anything. I can feel a slight bit of vaccume when the piston starts going back down but no audible hissing sounds like everyone is describing. I verified TDC by removing a valve cover but my question is with all new piston rings and the cylinder bores being freshly honed by a machinist should I be concerned with the fact the thing isn't blowing my finger off of the hole while spinning the crank with a ratchet? Any input is greatly appreciated!
Cylinder Compression
- Thread starter katwood16
- Start date
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Greetings katwood16,
When you installed the timing gears & chain, did you line up the timing marks per the manual?
We need accurate cam/crank phasing in order to generate proper compression.
And if that is known-good, we then need to verify how you adjusted your valvetrain. (Lifter preload?)
It's possible that if the valves are set 'too tight' that the held open valves will bleed off the compression
resulting in no joy.
At the same time, your observation about the 'vacuum when the piston starts going back down'
is a bit puzzling at this point. To me this makes cam phasing 51% probability vs too-tight valves
at 49%, but no matter what I'd want a brand new engine to push my thumb off the spark plug hole
with any speed on the ratchet. (Better test/easier to spin is all plugs out, instead of 7 plugs in and
turning over slowly. Much easier this way to 'feel' the compression stroke vs the other 3 cycles.)
Hope this helps. Let us know what you discover.
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If you have it on an engine stand you can pull the #1 spark plug and see the piston through the spark plug hole.
If you're 180* out you won't feel compression. Turn it over once more and see if you have compression.
If you're 180* out you won't feel compression. Turn it over once more and see if you have compression.
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Have a good feeling that the valves are the issue. I tried to take a short cut and do the valves the ol vice grip garage way (spin pushrods till you feel resistance while tightening the valve then when you have went through four engine cycles you turn all the valves 3/4 of a turn) I have yet to check to verify but that sounds very plausible
Worst case, pull the valve cover off, watch the rockers and observe piston. Recheck lifter settings, but stab distributor first, the recheck settings. That's what I'd do. With the lack of compression. Presuming you have mechanical timing correct.
Update: Took the valve covers off and backed off all of the valves and took out all the plugs and just tested with my finger over the sparkplug hole. I am getting good compression on all except 1 and 8. I put in all the plugs except those two and put my ear next to the hole to feel if any air comes out while rotating and it does but it's not enough to push my finger off. The valves we just done by a machinist as well as the piston rings and the timing by myself. You guys were saying it could be a possible timing issue but wouldn't there still compression just at the wrong time? Which leads me to believe that either the machinist botched the valves or I botched the piston rings somehow. I am ******** actual bricks and really need some advice. I have the thing damn near fully assembled and really want to be done and not have to take steps backward.
Do you have more than one thread going on this problem.?
What does installing the plugs have to do with low pressure on cylinders on opposite ends of the block?
It's either an assembly issue, block; or the valves are leaking from poor seat fit or valve adjustment.
Or worst case it's full of cracks.
What does installing the plugs have to do with low pressure on cylinders on opposite ends of the block?
It's either an assembly issue, block; or the valves are leaking from poor seat fit or valve adjustment.
Or worst case it's full of cracks.
You will get a compression stroke every other rotation of the crankshaft. On the off stroke, the exhaust valve is open and it won't build compression. Did you turn it over more than once?
Image from MotoIQ's website. https://motoiq.com/suck-squish-bang-blow-part-1/
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Image from MotoIQ's website. https://motoiq.com/suck-squish-bang-blow-part-1/
Get a sparkplug-air chuck adapter and see if you can find out where the air is escaping. Maybe do a leak down. Did you oil the cylinders?
This is what makes me believe it's not timing after spinning it around like 10 full rotation the best I get out of 1 and 8 is like someone softly blowing on your ear. I am getting my compression tester today and a leak down kit and will update when I have actual numbers. Sorry for making two different threads after a couple hours on this one I was unsure anyone was gonna answer me.
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