Need help with valves that leak boost...
 

I have a toyota tacoma, and its turbocharged. I also have some street cams. Anyway, I was getting a bunch of bad issues with tuning and idle, so I did a boost leak test. First I found that i had a leak from an injector, and found a half eaten injector o ring. I replaced it, but still had a slow leak in pressure. I found, finally, that my intake valves for cyl #3 were allowing air to pass into the cylinder ( i found this by pulling the spark plugs and feeling for air escaping when I pressurized the system). Anyway, I still have my stock head which I pulled off in perfect working condition a while ago. So i pulled the problem head, and began prepping the other (stock) head for installation. Before taking it to the machine shop, I decided to put my intake manifold on it and doing the same test, just in case. FYI, both ot these head were tested with the cams pulled, so the valves should be completely closed. What I found is what freaks me out.

My stock head ALSO LEAKS FOR THE INTAKE VALVES OF CYL #3!!!!!!!

What could this be? This is not an engine that was designed for FI, so could it be an unnoticed design flaw where intake pressure, with no cyl compression to counteract the push, gets by these particular valves? Or is it a crazy coincidence? Or could there be something wrong with my #3 chamber (piston, injector...) that could cause this problem regardless of what head is installed (the piston shows on signs of interference impact, BTW)?

Anybody have an clues or insight? I would hate to take the head to a machine shop and install it and in a week find that the problem persists...

Re: Need help with valves that leak boost...
 

Hmm, first off what kind of clearances is there between the piston's and the head? Your piston could be in backwards, causeing the intake valves to hit the piston because the exhuast valve reliefs are smaller.

In our motorcycle shop we have this kawasaki mojave 4 wheeler, the stock 4 stroke 250 cc motor was rebuilt before we got it at another shop, with a high compression wisco piston. The installed it backwards, one of the intake valves hit the piston and it broke the valve. They charged the guy $780 for the full rebuild, and it broke the next day. The wouldnt refund his money so he took it to us.

Thats about the only thing i can suggest from my experiance.



Sean

Re: Need help with valves that leak boost...
 

That is an interesting idea, but the pistons are still stock from factory, so i dont think it is that...
I am not quite sure how one could install a piston backwards, anyway.... ???

Re: Need help with valves that leak boost...
 

if it were backwards, it would be turned 180 degrees looking at the top of the piston. basically, the vavle relief's are sized according to the size valves. intake valves are allways larger, so turning the piston 180 degrees will make the exhuast relief's sit where the intakes should be, since the exhuast relief's are smaller, the intake valve will hit the piston. If its from the factory than i doubt thats your problem. Maybe its just a flaw and its made obvious under higher hp.

Re: Need help with valves that leak boost...
 

since you have 2 heads, take some valves that are good from one head and put them in place of the leaking ones on the other. you should take it to a machine shop to make sure the seats are cut to match the new valves