Can a leakdown test be made on 2 cylinders at the same time?

[MoPar~Man]

This pertains to the small block, if it matters.

If one cylinder is in TDC position on compression with valves closed, is there going to be another cylinder also with valves closed, maybe at the bottom of it's stroke?

 

[Toolmanmike]

Not unless you have 2 testers? What's your angle here?

 

[rags]

if you take the rocker shafts off all the valves will be closed. that's the only way i do the test.

 

[413]

The correct way to leak down test is to test with piston at the top and both valves closed. Piston at top is when it matters if the rings leak or not, and the most cylinder wear is at the top,

 

[MoPar~Man]

Based on what I just measured, on my 318, pistons come to TDC in pairs, and both their valves are closed at that point, so a leakdown test can be made on both (one after the other) without having to move the crank. Those pairs are 1/6, 8/5, 4/7, and 3/2. Now maybe that's a function of the cam shaft, and some cams don't behave that way. I don't know. But I did verify I can pressurize those pairs. (I'm not saying they're both on a compression stroke. Just saying they're at the top and their valves are all closed).

Yes, I guess unscrew the 4 rocker bolts and close all the valves at the same time. I don't have a torque wrench handy to screw them back in so I didn't do that. I also wanted to see if anything was going on with lifters and rods so ...

I only have a 5 gallon tank that I can pressurize with an air compressor. I charge it to 100 psi then connect it to the cylinder with an inline valve. Turn the valve and use a stop watch to time 10 seconds then turn it off, take note of tank's new pressure. I'm seeing 5 to 8 PSI drop in all cylinders except 7. I see 12 - 15 psi drop for 7.

Compression test (engine cold, mid 60's ambient temp, engine not run for a few days, throttle jambed open) gives 150 - 160 psi range for all cylinders. 7 comes in at 150. By the third crank they're all 140, even 7.

I don't think that's too bad for a cold engine, but I'm wondering if 7 is causing my rough idle, vacuum needle really bouncy.

 

[rags]

I don't have a torque wrench handy to screw them back in so I didn't do that.

i've never used a torque wrench to tighten these bolts. but a successful hack. two pistons will be at tdc at the same time. the other is on the exhaust stroke where overlap of exhaust and intake valves occurs. good way to check cam timing. lower compression in one cylinder can cause a rough idle. i've seen it be the cause of misfire codes set at idle. compression is something that everybody overlooks when diagnosing that issue. if the compression leak is caused by blow-by from a stuck ring, that would cause a high crank pressure. you'd actually see it puffing out the breather with the engine running. also the plug would be oil fouled.

 

[volksworld]

theoretically the other cylinders on tdc are at overlap where the exhaust valve is closing and the intake is starting to open...if you had a solid lifter cam they would definately be open and you couldnt do a leakdown test at that spot...your hydraulics have bled down enough to get away with it but you could potentially misdiagnose an open valve for a burnt one....as far as your method of timing how long it takes to leak a certain amount i can't interpret that as i've only used a real leakdown tester that tells what percentage of the air fed into the cylinder is escaping....

 

[rags]

a rough idle, fluctuating vacuum gauge and lower compression in cyl. 7. something that @CBODY67 would be familiar with, the scopes had a cylinder kill function. kill one cylinder at time and watch the rpm drop. something that could be easily replicated and would indicate how much your weak cylinder is contributing to the rough idle.

 

[MoPar~Man]

I don't have lower compression in 7. The compression tests fine in 7. It's the leakdown test where it bleeds more air than any other cylinder. 2 or 3 times as much. When I fire it up again, I'll know soon enough if a bad coil is the reason for the rough idle.

I did search for how you can have a good compression test but a questionable or not so good leakdown test, couldn't really find an explanation for that.

 

[rags]

The com ipression tests fine in 7.

i went back a reread it 150-160 and 7 is the lowest.

By the third crank they're all 140, even 7.

my mistake. been following this across 3? threads. sorry.

 

[MoPar~Man]

Yea, sorry, I'm throwing out lots of number here. I charged the battery before this last comp test, and had the throttle open. The engine was cold, but the comp numbers this time were higher. 5 cylinders were 150 - 152 after 4 cranks, 3 were 160. As for the leakdown tests, aside from cyl 7, the rest of them leaked 4 to 8 PSI out of my tank. Cyl 7 leaked 12 to 15 psi.