Bad leakdown on rebuilt head
 

Help!!!

I just got my piston/cylinder & head put back together and decided to do a leakdown on my newly rebuilt heads.

Cylinder 2-6 got less than 10% leakdown. However, cylinder #1 showed massive leak of 40% through the intake!!!

I've tapped the valve spring couple of time to reseat the valve and it now has 15% leakdown.

Should I be getting this high of a leakdown value until the valve is seated? Or do I have a poorly rebuilt valve job?

thanks,
anthony

It seems you have done what you can to seat the valve, and with that amount of leakage you still have a problem. Is the engine back in the car?

The engine is a 993 3.6. I just finished the cam timing so I only had the #1 intake installed. I took the rocker out, but the leakdown stayed the same.

I would hate to take the head out, but the air escaping from the intake valve is definitely not normal right?

A higher than normal leakdown could be expected from an engine that hasn't been run. However What you have doesn't sound too good. It sounds like the valve may be slightly bent, or wasn't lapped properly. If it were mine I would be pulling the head. Did you use new valves during your rebuild? Were the valve seats replaced? How about the guides?
What exactly was done to the heads while you had them off the car?

I dont think running the engine is going to fix this problem, "unless it was blowing past the rings" I have seen valves that lapped and blued well but did not pass the pressure test, and lapping it will not fix it, it must be cut correctly, thats why I always pressure test any head I do, just for this reason, I would pull it apart..

The shop that did the head said they perform the standard rebuild job with new guides, recut the valves, etc. They said the valves were good.

I am surprised they did not check to see if it would hold pressure.

Sounds like I need to tear it down. Bummer. But at least I didn't find out about when the engine's in the car.

if it went from 40 to 15 just by bipping the spring with a rubber hammer, you would think maybe there was some foreign matter in there and maybe there still is some.

I put the rocker on and rotate the engine to open the valve. Didn't see debris or anything out of place last night.

I'll try it again and have a closer look.

But I want to confirm that valves don't need time to seat right? They should be good from the get go (no leak) after the rebuild.

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anthony

Valve seating is perfect from new.

I agree there must surely be some crap in there..nothing else could explainan improvement with a bashing!

Kind regards
David

depends on the way the seats were cut if they will get better with time. a serta cutting machine would most likely produce a seat with the smoothest and most accurate finish. a seat cut with a stone can vary between pretty darn smooth to having scratches all around. depends on the type of stone, how fresh the finish on the stone, (some stones leave a smoother finish after they have been used on several seats previously) and how much seat material gets picked up by the stone. intake seats are much softer than exhaust seats and easily clog the stone with spots of metal that in turn don't do much for getting a nice smooth seat. then there's the finish on the valve itself, which can show some wavyness if the valve is particularly hard, or if the grinder is getting old, or if the stem is not chucked up dead center due to grit in the chuck. then there's the pilot that the arbor spins on. could be worn out or bent even, causing the seat to be cut on an angle from the guide bore. that's the specific reason that i lap all my seats to the valve that will run in that hole. no blue paste though, because it gives a poor reading. just water based fine lapping compound and a suction cup lapping stick. the valve seat should be the same tone of gray all around when wiped clean. darker gray on one side and very light on the other shows there is poor contact. the valve will always show gray all around because all it takes is a high spot on the seat to do that. lots of variables. so, yes, it could get better in time, but it's still a crapshoot.

Wooohooo!!!!

I used air to blow out anything in the intake port, but there weren't any to begin with anyway since I sprayed the head with carb bleaners before putting the head on. Gotta be clean here.

Then I noticed some red paints? (guess the stuff they used when lapping the valves) on the valve seat. So I sprayed carb cleaner on a q-tip and cleaned it all off.

Guess what, the valve seated and the leakdown is less than 10% just like the other 5 cylinders.

Glad I don't have to tear it back down and waste the 4 days of work I had put into putting the head back on and timing the cams.

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anthony

That is certainly good news. Pays to be a little persistant and not react too quickly!
Good luck -