Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Leakdown mystery

General Engine Building Discussion

Forum Posts



Tech Articles

Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.

= Resolved threads


Sorry post.

Alright, I just finished getting my rig all setup to get my v2p clearnances using the check-spring method. So far not a lot of fun. I decided to do a leakdown test while the cams were out since all the valves should be closed at the same time then. All the cylinders checked out fine except for cyl #1 which also happens to be the one I put the check springs in. Leakage was around 70% for cyl #1. There’s no good reason why a ring would be bad, it was rebuilt by a good machine shop 3k miles ago before I somehow got sucked into this hobby and frankly it drove really well a few days ago before I started this particular project.

I was supplying 70psi to cyl #1 and the tester was showing a sustained 20psi in the cyl.

I tested all the cylinders twice to make sure.

Moving on, no air escaping from the intake side, that I’m certain of. I used the checksprings to push the valve down with 20psi of air and when I do that then air comes out the intake manifold vaccum nipple. It was quite hard to push that valve open even with the check spring against that 20psi of air pressure.

No air escaping through the exhaust. I’m certain of that too as I did the same experiment of manually opening an exhaust valve against 20psi. Air only comes out the exhaust when I manually press the valve open.

No air escaping into the coolant.

No air coming out the oil dipstick. I put my ear over the dipstick heard nothing, put my finger over it and it changed nothing.

I swear it sounded like the air was coming out somewhere in the head. I tried to move my finger around in there and find it, thought maybe the oring on the leakdown tester wasnt seated right and it was coming up the spark plug hole, or maybe somehow around a valve stem because the check springs aren’t that strong despite it seeems to make a good seal on both the intake and exhaust.

So I put a capful of oil down the cylinder, waited a minute or 2, then tried again. Exact same result, leaking about 70% I swear the air sounds like it is coming up through the head somehow. No evidence of air at intake, exhaust, coolant or dipstick.

I thought maybe I wasn’t precisely enough at TDC but given cyl #4 tested fine and that’s #1’s buddy piston (they are both at TDC at the same time) then I think that rules out the not quite at TDC possibility.

Seeing as though there’s nothing I can do about this right now I decided to move on. And put the cams back in and figured at least I’d measure my v2p clearances as that was the mission anyway.

So after I put back in the cams, re-installed the timing belt, mounted the degree wheel, found true TDC I decided to test the cylinder again with the degree wheel at precisely TDC. Low and behold it tested out just fine this time. I think it was actually the best cylinder of the lot. I tested it with 70psi and it held almost 100% of it.

So either the cap of oil I put in an hour prior took that long to seal a ring gap, or I just had bad luck seating the air hose in the spark plug hole for several attempts earlier and now it was seating correctly, or it’s happenstance. I do think it tested better than the other cylinders (at this point) because of the cap of oil I put in there and this is a cold leakdown test, but I have a hard time believing that the cap of oil didn’t do anything until a long time after I put it in. I would think if it’s leaking 70% around the rings then it would have quickly pushed the oil into those gaps with 70psi of compressed air shortly after I put the capful in.

I don’t think anything I did so far in the project could have jeopordized the rings and they were fine beforehand. And just to say it again no air was coming out the dipstick. It could be that in the last test I was precisely at true TDC on my degree wheel and that made the difference? Though in the earlier tests when #1 was leaking #4 was not which is at top at the same time.

It’s also worth mentioning that when I was finding true TDC with the degree wheel and piston stop I could definitely hear the compression fighting the piston stop as my piston stop has an air hole in it to alleviate that pressure. The compression coming out the tiny hole in the piston stop sounded like it normally does and the difficult to press it out was the same as usual.

Any thoughts?

i think you have it pretty well covered you have done ever check i would have done.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?