low compression

bholmes

Registered User
OK, I have gone thru several diagnostics, initially thinking I wasn't getting fire (but I was) fuel pressure is good, plugs damp with gas but not starting. I checked my compression tonight on one cylinder and I was registering 45psi!! Tomorrow I will check all of the cylinders but I was curious if anyone might have some insight. It has fresh rings and good pistons. Obviously i could have messed up the timing and bent the valves but I am pretty sure that wasn't the case. I have timed several engines and never had a problem. The camshaft is an aftermarket comp cam and I am running stock rockers and pushrods. Do I need adjustable pushrods and/or rockers for this cam? Is the base circle that different? Typically I thought a hyd lifter would compensate if its not too radical of a cam. Is it possible to over torque the rockers and hold the valves open that way? I'm used to small block chevy rockers!
 
Wow don't even know where to start on this one. First adjustable push rods are only to be used with light cheaker springs. Once you have good geometry between the rocker and lave tip u remove the adj. push rod cheaker measure it and order the correct length push rods. Plenty of write ups on this even 3.8 specific.
 
Lol! I miss typed. I reply on my phone so somtimes I goof. I know how to do adj pushrods and measure. I guess I was asking if anyone knows if i need custom length pushrods or adjustable rockers (which I assume they make). Or if on a moderate cam the hyd lifter should take up the difference. I wouldn't see much need if the has circle is the same personally but I'm not well versed in the 3.8
 
Well you should post the specs on your cam that will help sounds like maybe you need longer rods. Story of my life!!!!
 
Lol! I miss typed. I reply on my phone so somtimes I goof. I know how to do adj pushrods and measure. I guess I was asking if anyone knows if i need custom length pushrods or adjustable rockers (which I assume they make). Or if on a moderate cam the hyd lifter should take up the difference. I wouldn't see much need if the has circle is the same personally but I'm not well versed in the 3.8

With pedestal mounted rockers, it should be possible to hit the torque setting within 1/4 to 1 full turn of the wrench, after the pushrod tip touches the rocker cup. If the amount of tightening after first contact was more than 1 turn, the valves are probably hanging open. If that is the case, then you should use a pedestal shim kit to raise up whichever rockers are too low.

Note that this happens when your pushrods are too long, because of head milling, a base circle change, or whatever.

In most cases, a new camshaft is a regrind, which would want a longer pushrod. In that case, I don't see how the valves would be hanging open. But it's worth checking that the pushrod length is okay in any case, so this is a good thing to do.
 
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