leak down test procedure question

Ive never performed a leak down test and was wonder if it really matter if the engine is at TDC or can it be 180 out? I guess if you have the valve cover off then its easy to determine this. But can you leave the covers on and still do this test?

thanks,
Aaron

 

It has to be done at TDC and at the end of the compression stroke for each cylinder (at other TDC and BDC positions, both valves won't be closed).

One fairly easy way IMO to do it is to remove the distributor cap(s), and make marks on the distributor housing rim corresponding to each contact postion inside the cap(s) -- when the rotor is pointing at the mark for cylinder X, that cylinder is near TDC and ready to fire.

 

Ah, right. The other TDC is right between exhaust valve closing and intake opening.

Will take the distributor cap off to determine positions. That would make this easier.

thanks for the info!
Aaron

 

This test should be done to a warm engine?

How much of a change would typically occur between a cold and warm engine?

thanks,
Aaron

 

The test is "most informative" with a warm engine, but still has "great value" even with a cold engine.

On a new (but broken in) engine, the difference is negligible.
On a used engine, the difference might sway you choice to rebuild now or later.

 

Ideally the piston should be exactly at TDC for each cylinder (no simple task on some engines) with the car in gear with the parking brake on to make sure the pressure doesn't push the piston down.

It's essential that the technique be identical from cylinder to cylinder. Small changes can make a big difference. This is one reason why there can be big differences in the results depending on who did the test.

 

Some people hold the gas pedal wide open, some don't
Some people remove the air filters, some don't
Most people remove all spark plugs before begin the test
Most people connect a battery charger to the car before and during the test

 

^you are refering to a compression test. The op was asking about leakdown, which the procedure doesn't make you turn the engine over to build pressure.