Random misfire at idle 454 vortec

[Schurkey]

I just performed a leak down cylinder 1 tdc and i believe the worst has been found. Cylinder 1 100 percent loss instantly too crankcase. Bad rings

100% leakage, and not via valves or into the cooling system?

Maybe if the rings are broken and with pieces missing. I'd be suspecting a hole in the piston.

 

[elevatorman]

100% leakage, and not via valves or into the cooling system?

Maybe if the rings are broken and with pieces missing. I'd be suspecting a hole in the piston.

Not sure made 150 on compression test

 

[elevatorman]

Not sure made 150 on compression test

And no nothing too cooling system or anywhere else i can hear

 

[Schurkey]

I'd be really suspicious of having 150 psi cranking compression pressure and 100% leakage.

Sure the leakdown tester was hooked up properly? Were you at TDC Exhaust (valve overlap) and not listening at the throttle body (with throttle partly open) or tailpipe?

The engine needs to be at TDC-Compression (both valves closed) and then you still need to open the throttle and listen, and put your ear to the tailpipe to assure there's no leakage past the valves/seats.

There's always SOME leakage past the rings...but 100% is HUGE. Having said that, I saw more than one "Pontiac" 151/2.5L Iron Duke with 60--80 percent leakage. They drove into the shop, but they were truly gutless/lazy.

 

[elevatorman]

I'd be really suspicious of having 150 psi cranking compression pressure and 100% leakage.

Sure the leakdown tester was hooked up properly? Were you at TDC Exhaust (valve overlap) and not listening at the throttle body (with throttle partly open) or tailpipe?

I was performing test alone so i used a whistle too get on compression stroke. Actually just cracked the air open at 50psi and it was coming out the oil fill tube fast as it was going in.
I am redoing test tomarrow with 2nd hand too verify

 

[Schurkey]

So you're not using an actual leakdown tester, and not listening at the throttle body/tailpipe?

 

[elevatorman]

So you're not using an actual leakdown tester, and not listening at the throttle body/tailpipe?

Yes i have an actual tester. I did go too the exhaust felt nothing and nothing at throttle body. I didnt really give it much thought tonight just closed the shop up and called it a night

 

[Road Trip]

100% leakage, and not via valves or into the cooling system?

Maybe if the rings are broken and with pieces missing. I'd be suspecting a hole in the piston.

I'm with Schurkey -- 150 psi compression test *and* 100% leakage into the crankcase
don't jive/hard to explain? On the other hand, if the 7 other cylinders give you the
expected results (verifying your test gear for you) ...then this last cylinder could be
visually checked for a hole in the piston with a cheap (free to rent) un-articulating
borescope.

And if you happen to discover a holed piston, taking a pic of the borescope display
and posting it here will net you much praise from the remote troubleshooters reading along. :0)

But good on you for trying to gather as much data on your ailing big block as possible.
As a fellow 454 owner, I'm rooting for a successful outcome.

 

[Schurkey]

Don't "FEEL" for leakage at the tailpipe or throttle body.

LISTEN for leakage at the tailpipe and throttle body, and open the throttle plates some when listening.

You will hear the leakage long before you feel it.

 

[elevatorman]

Ok so i was initially not performing the leakdown correctly i was under the impression that you apply x amount of air and watch it hold without a constant air source. So that being said i repeated the process correctly and at 100 psi inlet side cylinder pressures were all at 90 psi or 88. Air was observed slightly at crank exhaust and throttle body not sure what too make of it.