JSM Posted October 1, 2009 Share Posted October 1, 2009 This may be a dumb question, but I'm a little stumped. I did search. This may be my gauge as well. I picked up a 83 Turbo w/a P90A head. When I do a compression test I get a solid 30 across all 6 cyl. I recall when my hydraulic heads were done on my Ford Taurus we got no compression until the oil pressure built up. The car has been sitting for a little bit but I cranked the car enough to build some pressure up and I have oil in it. Thoughts? Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted October 1, 2009 Administrators Share Posted October 1, 2009 If anything, your cranking compression would be higher with hydraulic slack adjusters. When they collapse, the valves will close earlier, thereby trapping more air in the cylinder during cranking! (Intake valve closes after the pistons has started its travel UP on the compression stroke so the first portion of the compression stroke isn't compressing. When the intake valve closes earlier form collapsed lifter, that cylinder will trap more air and register a higher cranking compression). Regardless, only 30 PSI on all cylinders means something is VERY wrong! Cam timing is VERY off, (was the head removed and reinstalled?) Holes in pistons, broken rings, valves bent or hanging open for one reason or another, or a very blown head gasket to allow all 6 cylinders to only crank up to 30 PSI. If all 6 cylinders are exactly the same, I am more inclined to believe improper cam timing, bent valves from starting the engine with improper cam timing, improper valve train set up if the head was just rebuilt which would not allow the valves fully close. Quote Link to comment Share on other sites More sharing options...
JSM Posted October 1, 2009 Author Share Posted October 1, 2009 If anything, your cranking compression would be higher with hydraulic slack adjusters. When they collapse, the valves will close earlier, thereby trapping more air in the cylinder during cranking! (Intake valve closes after the pistons has started its travel UP on the compression stroke so the first portion of the compression stroke isn't compressing. When the intake valve closes earlier form collapsed lifter, that cylinder will trap more air and register a higher cranking compression). Regardless, only 30 PSI on all cylinders means something is VERY wrong! Cam timing is VERY off, (was the head removed and reinstalled?) Holes in pistons, broken rings, valves bent or hanging open for one reason or another, or a very blown head gasket to allow all 6 cylinders to only crank up to 30 PSI. If all 6 cylinders are exactly the same, I am more inclined to believe improper cam timing, bent valves from starting the engine with improper cam timing, improper valve train set up if the head was just rebuilt which would not allow the valves fully close. Thanks Paul. I'll need to ensure my guage is accurate first. Something just doesn't seem right with the 30 PSI. The consistancy really threw me. Head gasket maybe, but all 6 cyl? Not seen that one before. It would run on Starting fluid also. Previous owner hasn't a clue on what was done or not, so I really don't know myself. 130K on the motor and it appears untouched. Now i'm really curious. Quote Link to comment Share on other sites More sharing options...
sweetleaf Posted October 1, 2009 Share Posted October 1, 2009 I would double check that gauge. 30PSI across the board seem too even to be any kind of damage. Quote Link to comment Share on other sites More sharing options...
Noddle Posted October 1, 2009 Share Posted October 1, 2009 I know year ago, on a carbie engine, I got low readings, but after I opened the throttle (jammed it open), the readings were high. Nigel Quote Link to comment Share on other sites More sharing options...
Leon Posted October 1, 2009 Share Posted October 1, 2009 I know year ago, on a carbie engine, I got low readings, but after I opened the throttle (jammed it open), the readings were high. Nigel That is part of the compression test procedure! Make sure you're at WOT while cranking. I have my brother press down the pedal, if you don't have a helper use a heavy object on the pedal. Quote Link to comment Share on other sites More sharing options...
JSM Posted October 1, 2009 Author Share Posted October 1, 2009 That is part of the compression test procedure! Make sure you're at WOT while cranking. I have my brother press down the pedal, if you don't have a helper use a heavy object on the pedal. Really? I have NEVER done that before. Quote Link to comment Share on other sites More sharing options...
Tony D Posted October 1, 2009 Share Posted October 1, 2009 Really? I have NEVER done that before. Woah, Holy Terrible Test Batman! That is Auto 101, compression checks can be argued over cold or operational temperature, but anything but a WOT is never in question. Normally you will get good compression on the first few cylinders taken due to air in the plenum. If you are quick, then the numbers start to go down and by the last cylinder are crap giving terribly misleading results to the test. Quote Link to comment Share on other sites More sharing options...
JSM Posted October 1, 2009 Author Share Posted October 1, 2009 Woah, Holy Terrible Test Batman! That is Auto 101, compression checks can be argued over cold or operational temperature, but anything but a WOT is never in question. Normally you will get good compression on the first few cylinders taken due to air in the plenum. If you are quick, then the numbers start to go down and by the last cylinder are crap giving terribly misleading results to the test. You learn something new every day! I've probably done a 100 or more compression tests and have never done this! I know, I know... I only know what I've been taught and I guess dad taught me wrong! Time to eat some humble pie! Now I'm real curious! What is really amazing, is that I've never encountered this before. http://www.geocities.com/dsmgrrrl/FAQs/compression.htm Quote Link to comment Share on other sites More sharing options...
jacob80 Posted October 2, 2009 Share Posted October 2, 2009 Yeah I would definitly make sure that gauge is accurate, try it on a friends car or something and see what you can come up with. Quote Link to comment Share on other sites More sharing options...
Tony D Posted October 2, 2009 Share Posted October 2, 2009 Gauge Accuracy... Meh. The numbers are consistent, some would say even if your gauge is showing 30 across the board and the engine runs, then the functional test of the compression check passes. Expecially if you see no change doing the 'wet test'. The compression check is not in any way an end-all to engine diagnostics, it's only one step in a comprehensive evaluation. Cylinder Leakdown, evidence of smoke, oil consumption, sounds, smells, running smoothness.... it's a lot more comprehensive than most people would think. Sure getting an alarming number might get you closely looking at the engine---and in that context the current gauge you have did exactly what it should have done: get you looking at everything to see a bigger picture. I have some glycerine filled Ashcroft gauges I retrofitted to Harbor Freight compression sets. I like the setup, save they can' t be armored nicely so I have to wrap them in foam or be very careful when working with them. Same goes for the EFI pressure tester, etc. You can always get another gauge, once you have a decent setup you like working with. Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted October 2, 2009 Administrators Share Posted October 2, 2009 Gauge Accuracy...Meh. The numbers are consistent, some would say even if your gauge is showing 30 across the board and the engine runs, then the functional test of the compression check passes. Expecially if you see no change doing the 'wet test'. The compression check is not in any way an end-all to engine diagnostics, it's only one step in a comprehensive evaluation. Cylinder Leakdown, evidence of smoke, oil consumption, sounds, smells, running smoothness.... it's a lot more comprehensive than most people would think. Sure getting an alarming number might get you closely looking at the engine---and in that context the current gauge you have did exactly what it should have done: get you looking at everything to see a bigger picture. .... Spot on ole-chap. Quote Link to comment Share on other sites More sharing options...
JSM Posted October 2, 2009 Author Share Posted October 2, 2009 The WOT test added nothing. I'm going to check my other Zxt w/the same gauge and check the results. A little oil squirt also added no additional PSI. Quote Link to comment Share on other sites More sharing options...
woldson Posted October 3, 2009 Share Posted October 3, 2009 Regardless, only 30 PSI on all cylinders means something is VERY wrong! Cam timing is VERY off, (was the head removed and reinstalled?) Braap Check timing marks, also is the cam stock? Quote Link to comment Share on other sites More sharing options...
JSM Posted October 3, 2009 Author Share Posted October 3, 2009 Bone stock 280zxt. Quote Link to comment Share on other sites More sharing options...
JSM Posted October 4, 2009 Author Share Posted October 4, 2009 It runs! Never did get another gauge or check it on another car, but she runs! Tank was so rusty it was scary. Had to hook up another pump altogether. I new it would run! Yeah right! Need to change the plugs and check for vacuum leaks. The hoses were so dry rotted in some spots. Quote Link to comment Share on other sites More sharing options...
woldson Posted October 5, 2009 Share Posted October 5, 2009 Nice job! Guess that compression test will just have to be chalked up to Hummm for now. Then again, does it make any power,,,hummmm. Quote Link to comment Share on other sites More sharing options...
JSM Posted October 6, 2009 Author Share Posted October 6, 2009 Even better news! Compression test tonight yielded a solid 145 across the board. So the only conclussion I can determine is there wasn't enough oil pressure? Other thoughts? Quote Link to comment Share on other sites More sharing options...
Leon Posted October 7, 2009 Share Posted October 7, 2009 If it had been sitting for a while the rings might have gotten stuck and by running it you likely loosened them. Quote Link to comment Share on other sites More sharing options...
ozconnection Posted October 7, 2009 Share Posted October 7, 2009 If it had been sitting for a while the rings might have gotten stuck and by running it you likely loosened them. Yep, what he said! Quote Link to comment Share on other sites More sharing options...
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