larpy

I was reading threadsrecently about leak down tests and I did a search to see how they are performed. I noticed many members said their mechanics gave their car a "clean bill of health" and did not recommend performing a leak down test during the PPI. This got me thinking, Are mechanics NOT doing leak down tests because they are difficult to do or time consuming? Is it like the mechanic that only replaces some of your wires and plugs because they can not get to all of them without a lot of work? Anyone ever watched the leak down test and know how long it takes?

Quadcammer

The basics are:

Bring cylinder to TDC on the compression stroke
Insert air line from shop compressor into spark plug hole (with fitting of course)
Pressurize the cylinder, typically to 100psi
See how much the pressure drops, and note whether the air is rushing out the intake, exhaust, or crankcase

The time consuming part is getting to the plugs, especially on the turbo. Aside from that, its not difficult.

Should take maybe 3 hours on an NA, about 5 on a turbo.

race911

3 hours? Remove mufflers, remove bottom plugs. Absent rusted fasteners, how long is that? Yeah, have to do it on a warm engine, so maybe muffler handling isn't so pleasant. But once you're there, 15-30 mins for actually doing the 6 cylinder TDC-and-pressurize sequence. (I get that a turbo is a whole different animal being a single plug engine, and you have to do it from the top.)

Quadcammer

I was being generous, but yeah you're right.

matt777

I watched mine being done. I don't recall the time but it was less than an hour. I have a leak down tester myself that records the pressure drop on a gauge. This shop had another type of leak down tester that I'm not familiar with. Its in a little box. I think it was just a fancier unit. Some shops won't do the leak down test if the car doesn't do well on the other inspections. It just saves you a little money. The shop I used also said they could measure the movement of the valve in the guides if the leak down was bad but this wasn't necessary.

cgfen

yes, leak down test (pressure decay test) is challenging on our cars due to spark plug location / access.

i would think ~ 4 hours on one of our cars, i did one in 35 minutes last weekend on a 340 'Cuda, but access is significantly better

larpy

How many have actually watched them do the test? I bet there are a lot of shops claiming they are doing the test but actually are not or not doing a complete test.

Slow Guy

How expensive is the leak down tool? (mid grade, not a cheap Walmart one) I don't mean the compressor as I've got a 25 gal. upright. I have a compression tester too.

TheOtherEric

While we're on this, can somebody explain how you determine TDC for each cylinder? I've never understood this.

matt777

I believe they range from $40 to $250+. IIRC mine was $85.

There should be a TDC mark on the crank puller (Z1) although you can be out 360 degrees. The distributor rotor position tells you this although there are other ways of determining this. You then turn the engine 120 degrees to get each cylinder at TDC (per the firing order). I believe there are 2 other marks that help with this. I'm glad our lifters are hydraulic!

http://www.pelicanparts.com/techarti...tic_timing.htm

avslash

How do you rotate the engine???

Breaker bar on crank pulley bolt??

yumyum1667

Race911,

Just curious why a leakdown on a warm engine? For my PPI my shop insisted on doing the leakdown on a cold engine.

MarkD

heat expands metal and a warm engine gives you the most accurate readings.

No idea why someone would insist upon a cold engine except maybe to avoid dealing with a hot engine, which, I can empathize with... it's not fun!

Slow Guy

Thanks.


Just for extra info for those new to auto mechanics...

A single cycle of a 4 stroke engine is 2 complete rotations of the crankshaft. Intake/compression/power/exhaust are the 4 strokes. There are 2 TDC (top dead center) on each cycle (in each cylinder), 1 at the end of the compression the other at the end of the exhaust stroke, you want the one at the end of the compression stroke hence that's where the spark plug would be firing and why you look for the position of the distributor rotor in relation to the dist. cap.

Also, at "true" TDC the piston is all the way at the top of the cyl. and both valves (intake and exhaust) are closed, this is so when the spark ignites all the exploding gas pressure is being used to push the piston down (the power stroke). At "false" TDC the exhaust valve is open because that's the end of the exhaust stroke.

If you tried to do a leakdown test at "false" TDC all the compressed air would flow right out the the exhaust valve and your leakdown would seem to be 100% (bad but a false reading) You want both valves closed so the test will tell you if there is any air leaking by the valves (or piston).

"True" and "false" TDC were strictly my terms for this discussion to differentiate the 2 TDC's.

The 3 marks 120 deg apart on the shaft pulley times 2 rotations of the crank equal 6 (compression) TDC's, one for each of the 6 cylinders on our cars. If they were 8 cylinder engines the pulley marks would be at 90 deg. on the crank, times 2 rotations would equal 8 TDC's (one for each cylinder)

- On the intake stroke the piston is going down which has the effect of sucking the air/fuel mixture into the expanding cylinder. (intake valve open)
- On the compression stroke the piston is going up and compressing the air/fuel mixture (both valves closed)
-- then the spark plug fires --
- The explosion from the firing of the plug on the compressed air/gas mixture causes the piston to go down which is the power stroke. (both valves closed)
- Next the piston goes back up (with the exhaust valve open) and pushes the combusted gas out the cylinder which is the exhaust stroke.