What cylinders are closed when #1 is at TDC and what ones are closed at BDC on a 389 Pontiac V8?

Question

Try to do a leak down test i know how to find top dead center just need to know what other cylinders are closed with the #1.

Answer 1

"No" other cylinders are closed when number one is on the TDC. They are all in some various state of the 4 cycle engine cycle awaiting their turn at cylinder up and both valves closed for combustion. Leakdown tests, which is not the same as a compression test are usually done on a test bench in a machine shop using a head gasket and flat surface with compressed air, etc. You can do this on the car as you are apparently attempting, but have to rotate the engine to each cylinder's turn in the cycle to do the test. The reason that the engine fires in the order already mentioned in another answer is that that is the order that each cylinder is on compression and at or near tdc for that particular cylinder to fire. You can do this by watching the distributor rotor and seeing which cylinder it is pointing to and then pumping that cylinder up with compressed air and then after removing the air source letting it sit to see how long it takes to "leakdown". That tells you the condition of valves, rings, etc. A much easier test is the regular compression test where you screw the guage into the cylinder and after pulling the coil wire so the engine won't crank allow the starter to roll it over for a few seconds. that reading depends on the type of engine but can range from around 120 up to 180 or so on some higher compression engines. What is most important in that is that all the cylinders be within 20 percent of each other in reading. If one or more is out of that range there there is some internal wear problem causing it and engine needs repair. Compression guages are cheap and available at all parts stores. get the ones that screw in and seal with an o-ring and that have a release button so that you can use them more accurately. What you might have been confusing is that when you have number one cylinder on the TDC of firing there are about half the valves in the engine that can be adjusted at that same time. Then for valve adjustments you would turn the crank 180 from tdc and do the remaining valves as listed in the repair manuals. We usually have adjusted valves by simply watching when they are closed by observing the rocker arms or camshaft lobes and then adjustting. Hope this helps. By the way, Library should have manuals that cover the old 389 engine and you will find it listed in detail in both the "Motor" brand manuals as well as "Chiltons which in those years cover a range of cars. For instance Chiltons has a 54-63 which should cover most of the 389's and there is a 64-71 as well. These are also all continually reprinted and cover all AMerican cars built in those years and can find new at bookstores and on internet. I keep all the ones from 54 up to 76 or so when both started losing some of their quality of info.

Answer 2

Just do a compression test, the leakdown deal is too much of a hastle. And if the compression test shows all cyls being over 120 psi, you're good to go.

Answer 3

The order should be; 18436572; but you don't need that for a leak down test. Just install your psi gauge in one hole at a time and crank the engine for a few revolutions. Then watch the gauge.

Answer 4

as said,pull number one plug,put finger over hole and turn engine over,you will feel compression blowing out ,when it stops that is top dead center on compression.
watch the timing mark or rotor position in distributor

Answer 5

The cylinders are not all closed you mean valves intake and exhaust ...you can ID what stroke it's on by holding your finger over the plug hole.