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Old 12-22-2006, 05:41 PM   #1
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Default Compression #s got me confused...

I've been trying to pinpoint some oil consumption. 1-2quarts 500-750miles.

After boosted runs I smell the faint smell of burnt oil, which led me to fix a small leak at the oil return flange.

Been a week since and I still think I can smell burnt oil...haven't had a chance to measure the loss.

I went and took some compression numbers to see if the rings might be shot. The dry numbers looked good, but the wet got me confused:

#1 190 / 230
#2 185 / 280
#3 183 / 210
#4 205 / 250

I guess I should just do a leakdown test and really pinpoint it, and double-check for an CAS or front seal leak.

thoughts?
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Old 12-22-2006, 06:28 PM   #2
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The numbers would signify bad rings. Maybe valve seals too.

Did you use just a couple of drops of oil for the wet test?


TDC #2 & Blow air in it. See where the air goes.
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Old 12-22-2006, 06:39 PM   #3
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i mean i poured like a tsp/tblspn. im going to do it again this weekend...weird thing is there is no visable smoke from the exhaust...although I see the slightest amount from the oil filler if i take it off just after shutting down the engine. If anything I want it to be the valve seals.
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Old 12-22-2006, 06:47 PM   #4
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#1 190 / 230 21% ring
#2 185 / 280 retest
#3 183 / 210 15% borderline ring or valve seals
#4 205 / 250 22% ring

May as well TDC and blow air into each cyl, one at a time. Where the air goes will tell you what's wrong.
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Old 12-22-2006, 08:17 PM   #5
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WTF the specs on a healthy motor are more than 160 psi he is running 180+ on all cylinders f course it is gonna go up when he ads oil becouse the rings are working i would look to the oil drain and cas. I doubt blowby is your problem, you could try VTA. :gay:
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Old 12-22-2006, 08:31 PM   #6
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WTF the specs on a healthy motor are more than 160 psi he is running 180+ on all cylinders f course it is gonna go up when he ads oil becouse the rings are working
Uhm NO. 5% more wet than dry indicates worn valve seals. 20% more indicates bad rings. I'd also like to see more conisistent dry numbers from hole to hole, but Mazdas are kinda sloppy in that regard.

Low compression is a different failure--like head gasket or open valve. That's obviously not the problem. Oil consumption is.

Like I said before, he can shoot compressed air into a tdc cylinder to pinpoint the issue. But 20%+ increase in wet compression means rings.
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Old 12-22-2006, 09:58 PM   #7
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So if 150 on a cylinder when all the others are around 190 doesn't increase at all when you add oil, it would most likely be valve seals or a stuck open valve?
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Old 12-22-2006, 10:22 PM   #8
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The high dry numbers might be lots of carbon build up. You need to be fairly precise adding oil on the wet cylces- too much and you raise the compression with the extra oil.

That's a lot to run by a valve- so it would have to be some big damage. Of course the other culprit could be the head gasket. Crank and watch the rad for bubbles. Blown (leaking) headgasket can also pressurize the crankcase causing oil consumption and/or excessive oil in the vent system.
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Old 12-22-2006, 10:30 PM   #9
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All i know is my #2 cylinder had 95 psi compression while the rest had 170, and adding oil got me 230 psi. When i got it apart the ring lands on #2 where gone rings where still in one peice but the piston had failed. I doubt that with compression numbers like that he has a ring/valve/headgasket problem sounds more like a normal leak to me.
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Old 12-22-2006, 11:35 PM   #10
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Quote:
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So if 150 on a cylinder when all the others are around 190 doesn't increase at all when you add oil, it would most likely be valve seals or a stuck open valve?
Yes, could be seals, but I'd bet on a valve. You can do the pressurized air trick to figure out if it's an intake or exhaust valve. An open exhust valve really won't hurt the motor.
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Old 12-22-2006, 11:47 PM   #11
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I'm going to retest...i think i let some crank more than others...and I was very imprecise with the oil....just the dry numbers seem high to me.

if you add oil to the low cylinder and the compression's the same, it's the valves.
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Old 12-22-2006, 11:59 PM   #12
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Bring the car to operating temp. Be very precise with the oil. Watch the gauge, stop spinning the motor the 4th time the needle twitches. I'm hoping that you dropped too much oil in #2.

But at this point, it's obvious that there's a problem. The air trick will point to the culprit.

I don't think it's the head gasket becuase the compression is high and fairly even.
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Old 12-23-2006, 10:25 AM   #13
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I've seen a "blown" head gaskets show 160# with the other good cylinders showing 180#. Just a small leak to an oil return galley. 20# was enough to cause oil in the cc vent system and minimal blow by (oil consumption) at the rings. Car would have probably driven forever like that, just messy and requiring oil added between changes.
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Old 12-24-2006, 03:40 PM   #14
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Redid the test yesterday and made sure everything was constant.

#1 180 190 5%
#2 175 180 2%
#3 180 185 2%
#4 187 195 4%

looks good to me. I'll just drive it and make sure my consumption wasn't fixed by fixing the oil leak and see if all is good. If not, leakdown test will prove more conclusive.
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Old 12-24-2006, 07:23 PM   #15
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get a leak down test. I've seen comp test look good and a leak down show that a motor was toast
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Old 12-24-2006, 08:16 PM   #16
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well, i had a pretty bad oil leak from the return...so I can figure that's where the oil was going. I did the check cause I was over-cautious. If oil consumption resumes, then I'll do a leak-down test. Otherwise, prepping for the dyno next week.
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Old 12-25-2006, 03:27 AM   #17
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230rwhp come on you know you need to see it.
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Old 12-25-2006, 12:07 PM   #18
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If the motor is toast I would just crank the boost and see how much power you can make w/o blowing it up.
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Old 12-25-2006, 12:57 PM   #19
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haha, yeah I think I should be around 220rwhp....if my BEGi FMU was functioning I'd crank it up a tad and see a little bit more....however the smallest FMU disc I have is a 6:1 so I'm already at 120psi of fuel.
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Old 12-25-2006, 01:02 PM   #20
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megasquirt FTW!!!!
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