211 Posted February 3, 2007 Share Posted February 3, 2007 Borrowed a compression tester from a buddy. Changing the plugs this weekend so I figured I'd do a compression test while I was at it. What #'s should I see on average? Anything else I should know while I'm at it??? Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 3, 2007 Share Posted February 3, 2007 Number isn't important. You are looking for the cylinders to be within 5-10% of each other. Test with throttle wide open, all plugs removed, and fuel pump fuse removed or pump disconnected. Crank for 5 seconds per cylinder to avoid draining the battery too much. GD Link to comment Share on other sites More sharing options...
del Posted February 3, 2007 Share Posted February 3, 2007 the actual numbers are not the important part. what you are looking for is consistancy between cylinders. if the highest cylinder is over 10 to 15 percent more than the lowest, other tests are needed. if every cylinder is within 10 to 15 percent no worries. Link to comment Share on other sites More sharing options...
[HTi]Savage Posted February 3, 2007 Share Posted February 3, 2007 Number isn't important. You are looking for the cylinders to be within 5-10% of each other. Test with throttle wide open, all plugs removed, and fuel pump fuse removed or pump disconnected. Crank for 5 seconds per cylinder to avoid draining the battery too much. GD WEll yes ur looking for cylinders to be within 5-10% of eachother but number does matter. good cylinders should be upwards of 120 psi. lower than that is not normal. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 3, 2007 Share Posted February 3, 2007 Savage']WEll yes ur looking for cylinders to be within 5-10% of eachother but number does matter. good cylinders should be upwards of 120 psi. lower than that is not normal. Due to testing differences - both in methodology, and in equipment, and other factors it is best to NEVER compare numbers to anyone else. The test is purely a pass/fail as to if it *has* compression, and how close the cylinders are. GD Link to comment Share on other sites More sharing options...
[HTi]Savage Posted February 3, 2007 Share Posted February 3, 2007 True but, the number is helping u determine the condition or the motor. If u have the holes on one side at 90 and 105 and 120 and 130 on the other side ur going to have an unballanced motor and cause for ruff running. Link to comment Share on other sites More sharing options...
thealleyboy Posted February 3, 2007 Share Posted February 3, 2007 Agree with most of the commentary about the compression readings... Think of the motor as being "balanced" whether on the low- or high end. After all, we are talking about the motor {ideally} wearing out gradually, and evenly. If you can verify this, it'll tell you things are peachy keen. Readings of 70 psi across the board, at 250k, wouldn't alarm me in the least. Obviously, consistantly strong compression will indicate good power and efficiency. However, a worn motor can be quite relaible {indefinitely} if it does not deteriorate further. You can control this to some extent, by good maintenance. The compression test is most useful in weeding motors that are already past the point of no return. Keep that in mind, and don't get too fixated on big numbers.... good luck, John Link to comment Share on other sites More sharing options...
4x4_Welder Posted February 3, 2007 Share Posted February 3, 2007 70psi would have me pretty worried, since it's hard to idle an engine with less than 90psi. The factory spec is around 150psi, I have had these engines run from ~120 to 175. The important part is balance, though, if you have three at 150 and one at 120, that's cause for concern. If they are all at 110 or so, start looking into a rebuild or rebuilding your engine, since they are pretty well done by that point. Link to comment Share on other sites More sharing options...
[HTi]Savage Posted February 3, 2007 Share Posted February 3, 2007 Exactly. Yea sure ur comp can be 90, 110 or whatever but the fact of the matter is if ur under 120 the motor if obvously worn. This is the pourpose of a comp check, to see how worn a motor is. u will have far more power if ur within specs then under. This an a leak down test go hand and hand. Link to comment Share on other sites More sharing options...
thealleyboy Posted February 3, 2007 Share Posted February 3, 2007 70psi would have me pretty worried, since it's hard to idle an engine with less than 90psi. The factory spec is around 150psi, I have had these engines run from ~120 to 175. The important part is balance, though, if you have three at 150 and one at 120, that's cause for concern. If they are all at 110 or so, start looking into a rebuild or rebuilding your engine, since they are pretty well done by that point. Yeah, 70 psi is kinda on the low side although I've seen some old ones hang in there a long while with some pampering. I would probably give a worn, but consistant motor the benefit of a doubt over one with readings all accross the board. The point I was trying to make is that a motor that is wearing evenly is a good sign. Definitely a good core candidate for an overhaul. And yes, you probably should start thinking replacement when the compression drops in the double digits on one, or all, cylinders... good luck, John Link to comment Share on other sites More sharing options...
211 Posted February 3, 2007 Author Share Posted February 3, 2007 Thanks for the overwhelming support on this one guys. I forgot to mention; the motor is a '85 carbed ea82 w/ 157k. That being said, do I still need to disconnect the fuel pump? I'll update this in a day or so once I get some #'s BTW should I check each cylinder 2 or 3 times to get an average, or is that necissary? Link to comment Share on other sites More sharing options...
goobaroo Posted February 3, 2007 Share Posted February 3, 2007 yes u want to disconnect the fuel pump so that when u crank over the engine it doesnt fire up on the other 3 cylinders! that could be bad. u should also pull the plug wire off the coil so that u dont get spark and fire up the engine on whatever fuel it can suck up from the lines between the fuel pump and the carb, remember theres gas in the bowl of the carb too. Link to comment Share on other sites More sharing options...
mikeshoup Posted February 3, 2007 Share Posted February 3, 2007 Considering you do the test with all four spark plugs removed, I don't think you have too much worry of the engine starting. But yeah, disconnect the fuel source that way you minimize the amount of unburned fuel you're dumping into the cylinders. Your cat con will like you for it. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 3, 2007 Share Posted February 3, 2007 It's not just that - the fuel will wash the oil off the cylinder walls creating a bad sealing condition durring the test. But in a carb, with the plugs removed it shouldn't matter too much. It's just standard testing procedure, and it's good habit to follow for all vehicles. GD Link to comment Share on other sites More sharing options...
daeron Posted February 4, 2007 Share Posted February 4, 2007 It's not just that - the fuel will wash the oil off the cylinder walls creating a bad sealing condition durring the test. But in a carb, with the plugs removed it shouldn't matter too much. It's just standard testing procedure, and it's good habit to follow for all vehicles. GD Kind of like unplugging that MIG/TIG welder you are repairing BEFORE you stick your fingers inside.. Link to comment Share on other sites More sharing options...
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