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3 cylinders 0 compression

OK this is my first time forgive me, I have a 1976 f100 custom single cab 2wd 302 c4. It has 3 cylinders with 0 compression the other 5 cylinders have 120 psi , I pulled the heads and checked the pistons, no holes in any of them cylinders were clean with no ring groove, I had the heads rebuilt and installed new lifters and push rods no change still no compression in 3 cylinders it has 40 pounds of oil pressure at idle when its warm starts easy every time pretty much just bump the key it doesn't smoke or burn oil either could it be a flat cam, if not how would I know if the rings are bad, the engine was rebuilt when I bought truck it is bored .040 thanks for any info
 

Fellro

Moderator
Staff member
Did you make sure the heads were flat checked when they were rebuilt? Also, which 3 are dead? If it is all side by side, it would almost have to be head, but if it is the back three on both sides, make sure the valves are moving, you may have a broken cam. For 0 psi, that seems more likely. However, as I mention, it has to be the back 3 cylinders. If it were bad rings, or even the head warped for that matter, it will pump up some pressure.
 

blacksnapon

Moderator
Staff member
Using air fittings for changing valve seals, get the suspect cylinders to top dead center. Pressurize the cylinder. If the air noise comes out of the tailpipe, bad exhaust valve. If it comes out of the intake, bad intake valve. If it comes out of the oil fill tube, bad rings.
 

Fellro

Moderator
Staff member
Definitely make sure the valves are moving.
 

BuzzGun79

Nov.TOTM 2012 / 2012 TOTY
2,388
55
1rst step remove the valve cover..see if the valve train is working properly.You can also determine ring wear by conducting a "wet compression" test take some automatic transmission fluid and squirt some in the failing cylinders..if the readings come up the rings are worn.
 

Fellro

Moderator
Staff member
Normally, that would be effective for checking rings, but it has 0 compression. Even bad rings won't show up so low.
 

1970Custom

They call me Spuds
14,107
447
Middleton, ID
Just a thought, have you checked the firing order? Had my 351 running on four holes because somebody installed a 302 cam. Also had no compression because the valves were open on the compression stroke...
 

Fellro

Moderator
Staff member
Head bolts back on these are re-useable. If it was simple crack, the heads would still allow for some compression, this one is blowing a 0.

I could have sworn he had come back and responded the pistons were bad...
 
OK this is my first time forgive me, I have a 1976 f100 custom single cab 2wd 302 c4. It has 3 cylinders with 0 compression the other 5 cylinders have 120 psi , I pulled the heads and checked the pistons, no holes in any of them cylinders were clean with no ring groove, I had the heads rebuilt and installed new lifters and push rods no change still no compression in 3 cylinders it has 40 pounds of oil pressure at idle when its warm starts easy every time pretty much just bump the key it doesn't smoke or burn oil either could it be a flat cam, if not how would I know if the rings are bad, the engine was rebuilt when I bought truck it is bored .040 thanks for any info

Hello and good evening.

Pull the plugs on the 3 cylinders, add some oil and then retest just to confirm, nothing more. Even though other members are on track this was an old trick to determine if it is the rings(compression will elevate) or bad valves(no change in compression).

Something to do I suppose.

Boon
 

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