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| Compression test results | |
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| Tweet Topic Started: Aug 21 2014, 05:18 PM (1,624 Views) | |
| M4tthewd | Aug 21 2014, 05:18 PM Post #1 |
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Matt
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I did a compression test on my 1996 1.3L, all the cylinders were basically all the same. ![]() I read my Haynes manual for my old 1991 and it says psi from 156-199, as well as the how to on this site. The thing is though, all my cylinders were at 130 psi. She runs amazing, starts up no matter how cold it gets, and believe me it gets cold where I am. Gas mileage is great. Nothing to complain about what so ever! I'm just curious as why it's suppose to be no less then 156, yet I'm at 130 and have absolutely no problems. I know it's a different engine then what's in my old 1991, does that change the standards for psi? |
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| Coche Blanco | Aug 21 2014, 05:43 PM Post #2 |
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Troll Certified
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Maybe. Did you hold throttle open? |
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| M4tthewd | Aug 21 2014, 06:24 PM Post #3 |
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Matt
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Yeah, foot to the floor for wot. My old mans a mechanic and says its perfectly healthy and not to worry. But I'm just curious as why my results are down. I don't suppose it could be the tester it's self? I'm not really concerned, just curious :p |
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| Coche Blanco | Aug 21 2014, 06:35 PM Post #4 |
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Troll Certified
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If it runs and the are even, he's right. Don't worry about it. |
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| chessir | Aug 21 2014, 07:26 PM Post #5 |
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I have been reporting the same phenomenon on my recently acquired 93 virt which I rebuilt. After rebuilt my compression numbers are 100 130 100. Yet the car seems to drive perfectly fine at all speeds.I even calibrated my compression tester to borrowed compression testers. There must be some explanation to this as most here report more normal compression values after rebuilt. |
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| t3ragtop | Aug 21 2014, 07:48 PM Post #6 |
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Turbo3 and Twincam Tweaker
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a compression of 130 across the board is in the range of requiring a rebuild, a new g10 will be at 200 psi. the litany of denial requires that you say "it runs fine." there is a notable difference between an engine that "runs fine" at 130 psi and an engine that "runs great" at 200 psi.
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| chessir | Aug 21 2014, 08:01 PM Post #7 |
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My rebuilt included cylinder honing,rings, head gasket, valves, and lifters. Head and block clearances were within speck. What could be the cause of such low compression values then. 100,130,100? |
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| t3ragtop | Aug 21 2014, 08:26 PM Post #8 |
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Turbo3 and Twincam Tweaker
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it's pretty typical to measure low compression on an engine that has just been rebuilt and not broken in. as the rings bed in, the compression goes up and so does fuel efficiency. the other answer is less flattering. maybe there was something not quite right with the rebuild. i've had engines that were running right at the edge of not running and when the temperature dropped in the late fall they just stopped starting. the biggest cause of low compression for me has been burnt exhaust valves but when an engine is at 130 across it would more likely be rings frozen up with carbon and or excessive bore taper. |
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| chessir | Aug 21 2014, 08:52 PM Post #9 |
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Hi t3ragtop, I believe you are right that something is amiss with the rebuilt. I just cannot put my finger on it and I fear hard or no -start problems when cold. The compression values are supposed to diagnose the engine health.Come to think of it I never did measure bore taper.Is bore taper diagnosis common for a 117m engine? |
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| myredvert | Aug 21 2014, 08:53 PM Post #10 |
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myredvert
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There is a big difference between the concepts of designed performance, specified minimum performance, and then below that level "usable" performance. In my opinion the denial t3 described usually takes the form of a failure to even acknowledge (let alone objectively consider) the differences between them. When someone says "my compression is pretty low, but it still runs, the wheels don't vibrate off at idle, it still burns less oil than gas, it makes it up the steepest hills I normally encounter, and my fuel economy isn't bankrupting me, so I think I'll live with it and keep driving it for a while longer" then you know that person understands the difference and is making a reasonably well informed decision they can live with and probably won't regret. When people ignore the differences and choose to keep their head
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| truckjohn | Aug 21 2014, 09:04 PM Post #11 |
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Did you pull the fuel pump relay so you wouldn't be pushing fuel into the cylinders? Failure to do this will drop the number a LOT... |
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| t3ragtop | Aug 21 2014, 10:00 PM Post #12 |
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Turbo3 and Twincam Tweaker
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bore taper and out of round condition is pretty common with the g10 engines. you can run a hone in the cylinders and install new rings but if the holes aren't round and the cylinder walls aren't straight you will end up with problems.
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| perfesser | Aug 21 2014, 10:35 PM Post #13 |
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Elite Member - Former Metro owner
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If the compression for all three cylinders is very close, the engine will run "smooth", as in little to no vibration. Each cylinder develops the same power, so you have a balanced power output. If the compression for all three cylinders is very close, and at the high end of the acceptable range, the engine will run "smooth" and strong!! Each cylinder develops the same power, and that is close to the amount designed into it. 130, 130, 130 is smooth, but not strong. The low readings are probably the result of uniform wear in the power cylinder. You can refine the surface of a cylinder by honing, but you won't make any significant difference in dimensions. If the cylinder is tapered or egg-shaped before, it will still be tapered or egg-shaped after honing. If the wear is out of specs, a new standard set of rings will not seat properly. Oh, sure, you can give yourself a warm, fuzzy feeling by spending money and doing things that won't really address your problem. Your warm fuzzy won't raise the compression one psi. The only way to be sure of the block's health is to measure it (if you have the skills and equipment) or have it measured by someone with the competence to do so. The block on the engine I rebuilt looked really good (visual), but I followed Geo Glenn's counsel and measured it, and found out it needed to be bored 0.020" oversize. I bought the new pistons, the machinist worked his magic, and I have a smooth and strong engine now. Only have about 1500 miles on the rebuild so far, so I haven't done a compression test yet, but several Metroheads who know much more than I about such things have pronounced it a good job. The old engine builder's motto applies here: "Do it right or do it over." |
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| thevipofdbz | Aug 22 2014, 07:43 PM Post #14 |
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Advanced Member
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when I got my 95 1.3 my comp was 115 or so on all. However when I lifters broke and messed up my cam shaft I test again and I was 180-170 on all. I rebuild another 1.3 honed and new rings and it was 150 on all. What I am trying to say is gauges are not always right. Mine is a Harbor freight. |
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| freegeo | Aug 23 2014, 09:30 AM Post #15 |
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That is so true. Like some have said on here, if the cylinders are close in psi to each other it will run smooth. If you are having the head done you should at the very least check out the size of the cylinders and see how they match up to the FSM specs. Do a compression test after the work is complete and see what difference there is. If new rings were put in do another one are a few thousand mile to see if it has changed any. |
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