I performed a leakdown test on my 425 in my '66 Toronado, and all cylinders don't look too bad, but when I test cylinder 4 and am putting air pressure into the cylinder, the gauge that indicates leakdown percentage climbs very slowly until I get 80 lbs into the cylinder. Then the leakdown gauge all of a sudden jumps up and reads correctly. It's almost like something in the cylinder is not sealing until the air pressure gets past a certain value. What is the most likely cause of this? Can piston rings exhibit this behavior? I still need to do a compression test, so I don't know how this cylinder behaves while cranking. Thanks all!

 

Could be a sticky valve or ring. Are you chasing a problem or trying to create one? I'm of the opinion that leak down follows compression testing. If the leak down is good, send it.

 

^x2^
Generally, perform a leak down test when prior results suggest further diagnostics. (1) Perform both a dry and a wet compression test on each cylinder - compare results between dry & wet; (2) Perform a basic vacuum test noting readings suggested from chart below.

 

Possibly a weak valve spring or sticky top compression ring on that cylinder. Could try a ring soak on that cylinder, drive it and re-test.

 

I'm probably trying to create an issue, honestly. It was just out of the ordinary as no other cylinder did this. I started with leakdown testing because I was assuming the worst with a case of disappearing coolant. Then I pressure tested the radiator and found a loose hose clamp (go figure lol). I'm going through and replacing valve springs just because I don't know how old they are.

I'm leaning towards sticky rings still because if I remove the air pressure from the cylinder and then refill, it does the same thing. I would think you'd have to turn the motor over to recreate that "problem" if it really was a valve causing that.

 

Valve seals too? IIRC some CO
members have found positive lock seals that don't require machining. They're probably the same but, I'm not sure if they were for 425 heads.

 

the valve stem seals are still soft and pliable, so I didn't change them out. Intake side on all of them was some sort of weird seal, might be that positive lock type you're talking about, unless factory used different seals on intake and exhaust valves. The exhaust seals are just regular umbrella seals.

 

IDK "VGG" but AFAIK, MMO works IMHO...just 'sayin

 

Wrong thread perhaps? VGG is vice grip garage on YouTube. It's a great show.

 

Yup...my mistake, learned about Vice Grip Garage. Thanks I'll check it out.

 

Have to keep your eye on Jeff (Sugar Bear). 😊