mtcookson Posted February 1, 2006 Share Posted February 1, 2006 When doing a compression test, should the compression numbers always be in spec even if its not spinning very fast? For instance, I have a VG30E that I'm unsure of the condition. It supposedly burned oil however when taking it apart I found a good amount of oil in the intake manifold which makes me think possibly the PCV was bad. Anyways, I only had it sitting on the floor and did a compression check as it sat. Somewhat cold, maybe 50 degrees or so, intake off, one exhaust manifold off, no accessories on it nor even the crank pulley, really the only thing on it was the timing belt and the flywheel and starter. We used a battery charger to test it and it wasn't quite spinning as fast as it would when cranking the engine in the car... maybe 150-200 rpm if that. The readings were only around 120 or so, some at about 90 psi. Basically I'm trying to figure out if the engine is indeed bad or if the readings were quite off due to the circumstances. Thanks in advance for any help you guys can offer. Quote Link to comment Share on other sites More sharing options...
Austenp405 Posted February 1, 2006 Share Posted February 1, 2006 In theory, if the engine were good, then there shouldn't be a difference. But, if there is a compression leak, I think the slower speed would give the air more time to leak out, resulting in a lower reading. Also, everywhere that I've read always says to bring the engine up to normal operating temperature first.I don't know what effect that would have, though. Anyone more knowledgable than me want to chime in? Quote Link to comment Share on other sites More sharing options...
mtcookson Posted February 2, 2006 Author Share Posted February 2, 2006 I should add that this engine has been sitting in the garage for a while as well. It hasn't been cranked over in at least 6 months I believe. It does have oil in it and surprisingly the oil looks real good (previous owner must have got an oil change before pulling the engine I guess ) Quote Link to comment Share on other sites More sharing options...
george.bryant Posted February 2, 2006 Share Posted February 2, 2006 hook up the starter motor to power and get it to spin fast? Quote Link to comment Share on other sites More sharing options...
mtcookson Posted February 28, 2006 Author Share Posted February 28, 2006 Figured out the problem... apparently whomever worked on this engine last flipped the cam gears around (left on right, right on left). I guess they just looked at the engine from the front and thought the right side was the driver's side and the left side the passenger (right is passenger, left is driver). Luckily the gears are only half a tooth off from each other so there shouldn't be any valve damage. I'm putting it all back together now so should hopefully have a running Z31 by the weekend. Quote Link to comment Share on other sites More sharing options...
george.bryant Posted March 1, 2006 Share Posted March 1, 2006 that was pretty stupid of whomever to put the cam gears back in the wrong way round. lol. what motor is this on btw? Quote Link to comment Share on other sites More sharing options...
mtcookson Posted March 1, 2006 Author Share Posted March 1, 2006 lol, definitely. This is a 84-86 Maxima VG30E that I'm going to put into a 300ZX Turbo. The original engine wasn't taken care of very well either and its pretty much toast... would need new pistons and such. I'd prefer using the Maxima engine anyways for the extra compression (9:1 vs. 7.8:1) as its not too overly high and should make some awesome low end power. Quote Link to comment Share on other sites More sharing options...
rubin Posted March 1, 2006 Share Posted March 1, 2006 engine should be brought to operating temperture before doing a compression test so that the piston rings expand and the gaps are minimized Quote Link to comment Share on other sites More sharing options...
mtcookson Posted March 1, 2006 Author Share Posted March 1, 2006 yeah, i just had no way of doing it. i mean... i could have put the engine in but if it were bad that would have been a ton of wasted work. however, now that i know what the problem was i'm definitely putting the engine in now. Quote Link to comment Share on other sites More sharing options...
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