98sub2500leg Posted May 14, 2009 Share Posted May 14, 2009 98 Sub 2.5L dohc;Manual trans. With all plugs out and hand turning the crank, 3 cylinders measure 58-62psi, the last one measures 75. Measured with a Sunpro (cheap generic) compression tester. Compression seems to measure low, but the engine runs good otherwise. Have the engine out, TB ready to remove, the hds come off for HG replacement. Not sure if I need a full overhaul while it is out or I just have a crappy tester. The tester is about 12-14 years old, but kept in the orig box. Not sure why 1 Cylinder measures higher than the other 3. The HG is bad I know(oil in coolant), which is why the engine is out. If the HG is bad in 1 small spot, which is what I suspect, then I would think I would measure low on 1 cylinder and higher on the 3. Link to comment Share on other sites More sharing options...
ericem Posted May 14, 2009 Share Posted May 14, 2009 I would guess there is coolant in that one head with higher compression which is giving you the high compression reading. Honestly I would get another guage. that is very very low compression. Did you try a wet test in the other cyclinders? Link to comment Share on other sites More sharing options...
OB99W Posted May 14, 2009 Share Posted May 14, 2009 With all plugs out and hand turning the crank, 3 cylinders measure 58-62psi, the last one measures 75. Measured with a Sunpro (cheap generic) compression tester. [...] How many revolutions did you manage ''hand turning the crank'' for each cylinder measured? Does the tester screw in, or does it have a hand-held rubber cone? Link to comment Share on other sites More sharing options...
CNY_Dave Posted May 14, 2009 Share Posted May 14, 2009 I thought for a compression test you normally would use the starter to spin the engine? Dave Link to comment Share on other sites More sharing options...
porcupine73 Posted May 14, 2009 Share Posted May 14, 2009 The soob numbers of like 190psig per cylinder are fully charged battery, cranking engine, and also throttle blocked wide open. Hand turning would almost be more like a cylinder leakdown test. Link to comment Share on other sites More sharing options...
nipper Posted May 14, 2009 Share Posted May 14, 2009 Yes handturning tells you nothing. nipper Link to comment Share on other sites More sharing options...
98sub2500leg Posted May 15, 2009 Author Share Posted May 15, 2009 I would guess there is coolant in that one head with higher compression which is giving you the high compression reading. Honestly I would get another guage. that is very very low compression. Did you try a wet test in the other cyclinders? I may borrow a gauge to recheck. I bought the car new 11 years ago. I have 108km on the engine. I kept it up pretty well the whole time and before I took it apart even with the intermittent overheating problem from the HG's, the engine still ran as well as when I bought it. I know I should have done the compression test while it was still running. I realize the hand crank wouldn't give full compression, but it still should give a relative indication from cylinder-cylinder. It is possible there is coolant in that one cylinder. The plugs all looked the same so no indication there. I was hoping not to have to do a full overhaul. Hopefully the head is not cracked, but I should find that out soon. Link to comment Share on other sites More sharing options...
98sub2500leg Posted May 15, 2009 Author Share Posted May 15, 2009 The soob numbers of like 190psig per cylinder are fully charged battery, cranking engine, and also throttle blocked wide open. Hand turning would almost be more like a cylinder leakdown test. True, definitely not the real deal, the numbers would never stack up to the specs. but it does hold compression at a low level, which should give indication for leaks and cylinder-cylinder comparison. Link to comment Share on other sites More sharing options...
98sub2500leg Posted May 15, 2009 Author Share Posted May 15, 2009 How many revolutions did you manage ''hand turning the crank'' for each cylinder measured? Does the tester screw in, or does it have a hand-held rubber cone? Yes, I checked it again and it is absolutely worthless to test it this way. The harder you physically crank it, the bigger number you get on the gauge, sometimes takes 2 full cranks to register. I knew I should have checked it before tearing it down. The engine runs so well that I let it slide. Mistake on my part. Link to comment Share on other sites More sharing options...
OB99W Posted May 15, 2009 Share Posted May 15, 2009 As you've already determined, manually cranking an engine doesn't lead to reliable cylinder compression measurement. Besides a reasonable (and consistent) speed, it requires about three compression strokes, which on a 4-stroke engine would be six crankshaft revolutions per cylinder. Consistently doing that by hand for each cylinder is impractical, if not impossible. If your main concern is the condition of the heads (cracks, warpage, valve sealing, sealing-surface corrosion, etc.) rather than that of the rings, a good machine shop can pressure test them when any other work is done. Link to comment Share on other sites More sharing options...
1-3-2-4 Posted May 24, 2009 Share Posted May 24, 2009 Consistently doing that by hand for each cylinder is impractical, if not impossible. . Yeah I've turned the engine over by hand with the crank pulley only unless you like sore hands and cuts not worth it.. Link to comment Share on other sites More sharing options...
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