Larry

iTrader: (4)

I need to do a leak down test on my engine. How do you keep your piston at TDC when doing a leak down test? Particularly with an automatic transmission. As you apply air pressure your piston pushes down then the exhaust valve starts to open. With 100 psi test pressure your piston will see almost 1200 pounds of downward force, that would be pretty hard to hold with a wrench should the piston start downward.

Street Racer

iTrader: (65)

hmmm they way did mine was. I loosened all the rockers and pulled the pushrods as well to make sure they were not bent. Then just filled up the cylinder with air and waited to see what the gauge said. Needless to say I have a new motor in it <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

Larry

iTrader: (4)

I was hoping for a little easier way than having to pull the valve covers. That may be the only way though.

Bakerman

iTrader: (1)

This topic interests me. I just finished changing springs in my car, so I had the 'ol compressor hooked up to the sparkplug hole.

I was quite surprised when I hooked it up and heard a GOOD bit of air blowing past the piston. Apparently I have a ton of blowby.

This disturbed me for awhile, then I just chalked it up to being an aluminum motor. Every cylinder was nearly identical, and no valve leakage at all. Motor only has 20K on it, and it hasn't been abused.

Comments? Has everyone else experienced this? I don't remember this on the old steel blocks.

Darren

I Guess you guys don't do this for a living. Its normal to get some leakage. And for 1200 lbs that a slight miscalculation. You can set up a socket and ratchet on the crank bolt and it will hold it.

LS1derfull

All you need to do is put piston at true TDC to keep it from moving with air pressure in cylinder.This takes a certain feel but you can do it with pressure applied, get it close apply presssure thru leak tester and tweak crank adjustment until it stops fighting you. That is where test can be done.

Bakerman

iTrader: (1)

I held mine fine at 60lbs. Didn't try 100lbs, but I don't believe it would be too hard based on the feel of 60lbs.

I know some blowby is normal, this was just much more than I would have expected. Might explain some of that piston slap <img border="0" title="" alt="[Roll Eyes]" src="images/icons/rolleyes.gif" />

speedy_g

I just did a leak down test about a month ago to check for bent valves. I had no leaks in any cylinder but two. I took the valve covers off because the gaskets can be used two or three times without needing to be replaced because they are rubber, at least mine were. I also took the intake hose off that connect to the throttle body so the air would blow out if it needed to. I ended up also taking the rocker arms off and the pushrods out. I had three bent pushrods and two bent valves . Oh well, the price I pay to have fun.

Larry

iTrader: (4)

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Darren:
<strong> I Guess you guys don't do this for a living. Its normal to get some leakage. And for 1200 lbs that a slight miscalculation. You can set up a socket and ratchet on the crank bolt and it will hold it. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Don't think I miscalculated Darren. The area of a 3.90 diameter is 11.946 sq. inches. Multiply this by 100 psi and it calculates 1194.6 pounds. That's actual force applied to the top of the piston. This doesn't mean that much force is directed directly to the turning of the crank. That would be variable depending on stroke length and position of piston. I would assume at this force level if the piston is half way down you would not be able to hold the crank by hand although I don't know what torque level the crank would see.

I'll give LS1derfull's method a try. With the piston right at TDC there is no force on the crank. I just didn't know how to find that position without the heads off. Applying pressure slowly and rotating the crank should work fine. Looks like it's a two person job.

Thanks to all!

JS

iTrader: (4)

Take off ALL the rocker arms and fill each cyl. with 120psi,bring motor to TDC and spin each cyl. 4 to 5 times using a remote....

I just did mine and had 3 cyl. at 25% leakdown <img border="0" title="" alt="[Frown]" src="gr_sad.gif" />

JS

<small>[ April 10, 2003, 09:26 AM: Message edited by: JS ]</small>

WMSuperSport

iTrader: (23)

I used a "Jimmy Rig" method of finding TDC. I stuck a small rod into the spark plug hole and raised the piston until you feel it at the top. Just make sure you dont booger up the spark plug threads by pushing the rod too far into the cylinder and then putting force on the crank, in turn pushing the rod into the threads. Sounds funny on paper but it works for me.

Larry

iTrader: (4)

What orifice did you guys use to get a pressure drop when doing the leakdown. Seems like it would have to be a known flow value to determine percent drop. A known value that our engine is suppose to flow.

<small>[ April 10, 2003, 08:37 AM: Message edited by: Larry ]</small>

JS

iTrader: (4)

I have the tavia tool...
I checked every cyl. and there were 2 that were 3%,3 that were 14% and 3 that were 25%.I didnt know u needed a value with the rockers off and at TDC?I could understand if say all the cyl were off but mine were all over the place and varied from low to extemely high.

I could hear the air comming out of the shortblock and into the pan.

JS

Larry

iTrader: (4)

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by JS:
<strong> I have the tavia tool...
I checked every cyl. and there were 2 that were 3%,3 that were 14% and 3 that were 25%.I didnt know u needed a value with the rockers off and at TDC?I could understand if say all the cyl were off but mine were all over the place and varied from low to extemely high.

I could hear the air comming out of the shortblock and into the pan.

JS </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">What size orifice is in the Trivia tool? To get a pressure drop you must have an orifice between the regulator and the gauge that is reading the pressure to the cylinder.

Larry

iTrader: (4)

I just finished my leak down test using a .040 orifice. No problem keeping each piston at TDC. I removed all 8 spark plugs first. I then pressured up each cylinder with 20 psi and rotated the crank clockwise until it moved back and forth slightly without any resistance. You will know when it not on TDC as it is very hard to rotate until you get there even with only 20 psi. In fact 20 is about all I could even have in the cylinder and be able to rotate the engine. Once I reached TDC then rotation was easy again. I did not hold the crank bolt when I pressured up to 100 psi. The crank did not rotate. I had a range of 96-98% leakdown. My engine is in great shape. I did find my problem, all 8 plugs were very black. I'm way too rich.