Leakdown questions

[MrJTP2001]

1968 911 Sportomatic with the 911 (no alpha ) engine. About 60K miles but only 200 in the last 20 years.

Before I send this engine in for evaluation/rebuild I'd like to do a leakdown on it to give myself a better idea of its condition before I listen to the mechanic. But, I want to do a high pressure leakdown at BDC. Typically that would require pulling the rockers but, if I backed off the valve adjustment completely would that be enough to insure valve closure at BDC? Oh, yeah, this will be a cold leakdown. Assuming I go through with it, what numbers should I look for with a cold test?

[RSTarga]

You mean TDC? I wouldn't bother loosening the rockers, just make sure they have correct clearance. I don't think a cold leakdown is very accurate due to expansion etc. At least you'll know it your valves are leaking.

[MrJTP2001]

Actually the test I'm planning is a high pressure (100psi) test at BDC. At TDC high pressure might cause problems which is why TDC leakdowns are done with low pressure (so the cylinder won't try to move).

[edmayo]

In my shop we do all the leakdowns at shop air pressure which is right at 90-100 psi. This is done with the PISTON at TDC (I think that's what you meant, the cylinder doesn't move) since the crank pulley is conveniently marked at 120 degree intervals you just rotate the crank untill the right mark lines up for the cylinder you're checking. I've never heard of the condition you mention of 100 psi causing problems at TDC. Can you elaborate?

[MrJTP2001]

Thanks Ed, however my engine is sitting on an engine stand with nothing to keep the cylinder from moving under pressure. Under that condition do you still recommend TDC? The instructions that came with the leakdown tester (pretty old) says to do the test at BDC (and 100psi) to prevent the engine from turning over. I've never done the test at high pressure or at BDC, I just thought I'd follow the instructions for once. Silly me.

[304065]

Never heard of a BDC leakdown. You shouldn't have to take the valvetrain apart for this test. Do it at TDC, plugs in the other five cylinders and lock the flywheel if it's on your P201, easy.

Do NOT connect the air source with anything like a wrench on the pulley. Henry Leland had a friend who died in the hospital after a friend of his took a starting crank in the jaw. This moved Leland to seek out Charles Kettering's electric starter and the rest is History. . . don't YOU become History. . .

These tests really work. . .

http://forums.pelicanparts.com/911-e...11-normal.html

[MrJTP2001]

It's accidents like that which were mentioned in the instructions as to why do it at BDC so the piston has no place to move. I'll have to think about locking the flywheel, tomorrow when I'm actually awake. This is a Sportomatic engine and the normal presentation when the engine/transmission are separated is the carrier plate, not the flywheel. The plugs will be in all other cylinders but that's marginal help since some of those cylinders will have open valves. I'm leaning toward just doing a low pressure test and see what I get for numbers. I really don't want to remove the carrier plate. I haven't looked at the valve timing chart but at BDC the exhaust valve might be starting to open. At least that's what I assumed when the note said to pull the rockers. I'll sleep on it.

[edmayo]

Were those instructions written with radial aircraft engines with props attached in mind? First, it doesn't matter if the engine is in the car or on your kitchen counter, if you have the pulley mark lined up it won't kick the piston down,,,,,and even if it did kick the piston down,,,,nothing happens, just start over. It is a good idea to take the wrench off the pulley that you were using to rotate the engine so it doesn't get tossed off if the engine were to kick over. That is all that can possibly happen. Once again sounds like a case of too many lawyers involved. Oh, and we never use flywheel locks for leakdown tests! And by the way David is correct in that a warm leakdown is prefereable to cold one, but you take that into account.

[SpawnyWhippet]

Strange one here with my cylinder number 3. I have my 1972 2.4T engine out the car and doing a clean up/reseal and check over. I did a leakdown and got the following results:

On cold engine, prior to valve adjustment:
1: 5%
2: 9%
3: 26%
4: 18%
5: 10%
6: 4%

Cold engine, after valve adjustment:
1: 10%
2: 9%
3: 92%
4: 12%
5: 6%
6: 6%

Somehow cylinder 3 has completely lost the ability to pressurize. I have rechecked the valve gaps several times and they seem OK. I followed the procedure as per the 101 Projects book by Wayne Dempsey.

Any ideas why the cylinder would no longer pressurize - exhaust valve stuck open or something? I poked a camera into the plug hole and it is pretty coked up. Also the plugs were very black after only about 100 miles of running.

[ESully]

Sometimes carbon prevents the valve from fully closing.