I just got my car home, and it runs fine but when I did a compression check I got a high of 129 and a low of 100. How can I tell if it is valves or rings? It doesn't smoke at all. It has oil leaks all over the place, is it easier to leave the motor in to replace gaskets, or does this sound to you like a rebuild is needed. I have one bad motor mount so I guess that I will have to pull the motor partially any way. this is a very helpful website, thank you
When you did the compression test, was the motor warm, and did you block open the throttle? Compression is considered satisfactory if the lowest reading cylinder is more than 75% of the highest reading cylinder. With a high of 129, 75% would be 97, so you are OK. If you inject a table spoon of oil into the cylinder, and the pressure comes up, it is rings. A more accurate test would be a leakdown test. Each cylinder is brought up to TDC, and compressed air is introduced through the plug hole. Then the % of leakdown is measured. How does the engine run, does it miss? If not, and it doesn't smoke, you are good to go. Reseal it and tune it for increased performance.
Hi Candice and congrats on your car. 69s are beautiful units. My 72 8.5:1 compression motor had a high of 150 and a low of 138 psi. I hate to differ w/ Larry but all manuals and mechanics I have investigated agree that a difference of 10% is maximum acceptable. I would think your High Compression 430 would have a high cyl of somewhere between 150 and 160 on a warmed, fast spin. I would probably pull her out and inspect. I am new to this site, too. It is like finding an Oasis in a desert if you are a Buick enthusiast. There are some really knowledgeable and helpful folks here. Tom.
Tom, I'm quoting from the Buick chassis manual. I think those readings are somewhat low, but if the compression test was done on a cold engine, and the throttle wasn't blocked open, that could account for that. And BTW, it's OK to disagree with me:laugh: I don't know everything, but I'd like to:grin:
My main source for that kind of data is Petersen's Big Book of Auto Repair. They used manufacturer reccomendations for cylinder pressure until the 70s then foot noted the 10% formulae after that. I just assumed it was the manufacturers statement. BTW, does anyone know if petersen published any more Big Books after 1986? Can't find anything on the net or at book stores. Tom.
Hey Candice! Congrats on the GS. I had a 68 GS long before I was grown up enough to know what I had. Wonderful car. Anyway, I'd run that comp test again with the throttle blocked open all the way and the coil wire removed at both ends. I'm not an expert in the class of Burek or Weiss on this board,... but... When I am looking for overall condition of an engine, I like to see the cylinders within 10% of each other. That's ten percent of the highest reading cylinder. As Larry said, the oil test is good for a quick way to find the rings/valves answer, and the leakdown test is the best way to find out just how much wear that bad boy may have. I pretty much always give way too much information on procedures so please don't feel like I am talking down to you. To expand a bit on Larry's description of a leakdown test, there is a special tool (isn't there always?) to do exactly this. The easiest to use is the dual guage unit. As with most things that go in the garage or tool box, buy a decent one to start with and try to stay away from the cheapies. It connects to a compressed air supply and applys that air pressure into the cylinders of your engine via the spark plug hole. You then read the guages to determine the amount of leakage for each cylinder. Again, the closer they are together on these measurements, the better off you are, within reason. If they all leak from 35 to 37%, that is obviously not good!! Keep in mind that there are many things the leakdown test can verify for you. If the reading on one cylinder is higher than all the others, then further investigation is warranted. If air can be heard escaping via the carburator, you either have a bad intake valve or an intake valve open. (Be sure both valves are closed and the particular cylinder you are checking is at TDC) If you hear it via the exhaust pipe, exhaust valve problem. If it is coming from the valve cover on either side, I'd suspect rings would be in order. So, how does this beast run? Idle smooth? Carb and ignition in good shape? Before you make the decision to yank that motor, do some checking on it's condition first. Go with a complete tune up, from the points if you still have them all the way through the plugs. If you are an experienced carb rebuilder, go through the Quadrajet, particularly checking out the throttle bushings and the well plugs underneath. Both are famous for leaks. Throttle bushings give you a very well hidden vacuum leak (don't ask how I learned this little bit of info) and the well plugs don't plug very well..... Even if they aren't currently leaking, mix up some appropriate epoxy and seal them for good. Let us know how you are doing with your new prize. Post some pics if you can?
