Looking for TDC

[Rustee]

I feel stupid asking this and I watched video and I knew the answer 10 years ago but when finding TDC to do a leak down test, is it the second highest stroke that blows the air out? And is it before or after the air blows out that it's at TDC? I'm about to do the test (hopefully tomorrow) and just want to make sure I'm getting the piston to sit where it needs to be. Also am I supposed to be lining up the timing marks per each piston when doing this test?

 

[GregBelleville]

You want TDC on the exhaust stroke. So as you're cranking the motor you can feel with your finger over the spark plug hole for pressure when it's on the exhaust stroke.

 

[Rustee]

You want TDC on the exhaust stroke. So as you're cranking the motor you can feel with your finger over the spark plug hole for pressure when it's on the exhaust stroke.

So when I'm feeling the air come out take my finger off and put something in there to get it at it's highest point? Then put the gauge on it?

 

[GregBelleville]

So when I'm feeling the air come out take my finger off and put something in there to get it at it's highest point? Then put the gauge on it?

Yep

 

[tworley]

I used the longest part of a broken, plastic clothes hanger and dropped that down the hole. Once the plastic piece hit its highest point while turning the engine on the compression stroke (I had to double check a few times) I knew I was at TDC.

 

[Rustee]

I used the longest part of a broken, plastic clothes hanger and dropped that down the hole. Once the plastic piece hit its highest point while turning the engine on the compression stroke (I had to double check a few times) I knew I was at TDC.

Okay now I'm confused. Is it the compression stroke or the exhaust stroke that the piston needs to be TDC on? Or is the compression stroke and the exhaust stroke the same thing?

 

[tworley]

Okay now I'm confused. Is it the compression stroke or the exhaust stroke that the piston needs to be TDC on? Or is the compression stroke and the exhaust stroke the same thing?

Two different things, but happening in the same cylinder. Compression brings the piston up along with air/fuel for ignition, the exhaust stroke pushes out the gases after they've burned. Basically during the compression stroke, the valves are closed. On the exhaust stroke, the valves are open a small amount. So you want measure for TDC on the compression stroke.

 

[tworley]

Also, @Chris , this should probably be moved to General Discussion.

 

[Rustee]

Two different things, but happening in the same cylinder. Compression brings the piston up along with air/fuel for ignition, the exhaust stroke pushes out the gases after they've burned. Basically during the compression stroke, the valves are closed. On the exhaust stroke, the valves are open a small amount. So you want measure for TDC on the compression stroke.

So before it starts going back down?

 

[tworley]

So before it starts going back down?

Correct. Compression stroke, when the piston is at its highest point. Like I said earlier, I used the long portion of a broken plastic clothes hanger to help determine when the highest point was reached. Some people use dowels, corks, stoppers. Whatever works to help you see it.

 

[Rustee]

Correct. Compression stroke, when the piston is at its highest point. Like I said earlier, I used the long portion of a broken plastic clothes hanger to help determine when the highest point was reached. Some people use dowels, corks, stoppers. Whatever works to help you see it.

does that piston come up and down once or twice during each pass? It just comes up and then goes back down one time before going to the next cylinder right?

 

[tworley]

does that piston come up and down once or twice during each pass? It just comes up and then goes back down one time before going to the next cylinder right?

Piston comes up on one revolution of the crank, then back down on the next revolution.
If you're on tdc and the exhaust valve is open, then rotate the crank one more revolution to get on the compression stroke.

 

[Nashvillwill]

Since this topic is fresh, I’m curious if anyone has had trouble with specific LD kits. Specifically with threading the hose into the spark plug hole.

Mine had swivel air fittings. In other words, the threaded part would spin on the hose because of a slip fitting. With the spark plug holes so recessed, I couldn’t confidently thread it in without fear of it getting stuck. Did I get a crummy kit, or am I doing things wrong?

 

[Rustee]

Piston comes up on one revolution of the crank, then back down on the next revolution.
If you're on tdc and the exhaust valve is open, then rotate the crank one more revolution to get on the compression stroke.

So it comes up twice before moving to the next cylinder and it's the second time that I need to get TDC on. How I tell when the exhaust valves are open for that cylinder?

 

[BlueC]

So it comes up twice before moving to the next cylinder and it's the second time that I need to get TDC on. How I tell when the exhaust valves are open for that cylinder?

I think you're getting things confused, or at least I'm confused. Are you doing a leak-down or compression test? Leak-down is static, compression is dynamic. Exhaust valves will be hanging open during the exhaust stroke.

 

[Rustee]

Piston comes up on one revolution of the crank, then back down on the next revolution.
If you're on tdc and the exhaust valve is open, then rotate the crank one more revolution to get on the compression stroke.

So it comes up twice before moving to the next cylinder and it's the second time that I need to get TDC on. How I tell when the exhaust valves are open for that cylinder?

I think you're getting things confused, or at least I'm confused. Are you doing a leak-down or compression test? Leak-down is static, compression is dynamic. Exhaust valves will be hanging open during the exhaust stroke.

Well I tried 7 or 8 different times on cylinder 6 and 5. I put a piece of a wire hanger in there and waited for the air to blow out, then to the top and stopped. Gauged it and it would force air out the exhaust, intake manifold or other spark plug holes. I tried different ways many times but could never get it to hold pressure. Then I took a flash light and shined it into the spark plug holes and a couple of them had a little bit rusty water sitting on top of the pistons. One had a decent sized puddle in there. I even watched the valves go up and down via spark plug hole while trying to get them to close and hold air.

 

[JKP]

Leak down test is with both valves closed, piston at the top of bore on compression stroke.

If you have water on the pistons and it won't hold air when at TDC compression, sounds like a blown head gasket between cyls 5 and 6.

 

[Rustee]

Leak down test is with both valves closed, piston at the top of bore on compression stroke.

If you have water on the pistons and it won't hold air when at TDC compression, sounds like a blown head gasket between cyls 5 and 6.

I'm returning it. I found a 2006 for $500 from a Jeep dealer. I seen a video of it running and it sounded pretty strong at 170k miles. Do you guys know if a 2006 is a direct fit? It had a 6 speed manual tranny on it so should I keep my flex plate, fly wheel, and OPDA (or is it ODPA)? Anything else I should take from mine for the swap?