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Low compression or bad testing setup!!???!! Yikes!

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Low compression or bad testing setup!!???!! Yikes!

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Old 12-05-2018, 04:50 PM
crdouger
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Low compression or bad testing setup!!???!! Yikes!

I have an '05 R53 with 160,000 miles on it. I'm pretty sure I need to put a new timing chain on it, But before I go through all that work I thought I'd do a compression test on it to make sure I wasn't wasting money on a shot engine. I borrowed a compression tester from Napa and my readings were alarmingly low.
#1 - 85psi
#2 - 80psi
#3 - 75psi
#4 - 72psi
Notes: I did it with a battery charger on to keep it charged. I left the other three spark plugs in. And I cranked it until the readings wouldn't go any higher which was about 20 seconds of cranking.

QUESTION: If that's as bad as seems, I'm a little surprised that they are as even across the board as they are. I read elsewhere on this forum that the length of the compression hose might effect the readings. One guy had a reading of 90psi and then shortened the hose and got a reading of 160 on the same cylinder. I can't really shorten the hose because the tester is a loaner from NAPA.

Is my engine really junk or is my testing bad. Before the timing chain problem it ran great, started fine, and had lots of power. It did consume a fair amount of oil, but a lot of that is leaking from the valve cover or somewhere else because the bottom is an oily mess. I don't want to throw the car away, but I also don't want to waste my time on replacing the timing chain if the motor is really junk. Any solid advice is appreciated.

Thanks!!
 
  #2  
Old 12-05-2018, 04:53 PM
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You should take all of the spark plugs out when performing the compression test, The starter will spin faster, which is what you want.
 
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Old 12-05-2018, 04:55 PM
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Will that give me more accurate readings or will it just get to the same reading faster? I can do the test again with all of them out if that's what I should do...
 
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Old 12-05-2018, 05:11 PM
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I tried with all of the spark plugs out and it cranked noticeably SLOWER (which I had read somewhere) but my readings were about 15psi higher with the other plugs out. I'm guessing the loaner compression tester from NAPA is an inexpensive one. The rubber hose feels soft and flexible to the touch. I wonder how much that is affecting my readings.
 
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Old 12-05-2018, 08:06 PM
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Compression testing

crdouger
See if the attached file helps......
Others may have more to add.
 
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Compression testing.pdf (54.7 KB, 20 views)
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Old 12-06-2018, 05:17 AM
crdouger
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Mike, That's helpful. It looks like there is a little more testing that I can do and see what comes out of it. Do you know where those testing instructions came from? Did you write them? - Doug.
 
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Old 12-06-2018, 05:51 AM
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I hope you disconnected the fuel pump fuse, as you'd be fuel washing the cylinders! Also, you need to have the throttle WOT so that you can get the highest numbers. Usually the actual numbers aren't as important as the difference in PSI between cylinders. You can throw a oil cap of oil down each cylinder to see if the numbers go up greatly as well. Definitely check the voltage of the battery, as that could influence the numbers you get as well.
 
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Old 12-06-2018, 06:18 AM
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Originally Posted by crdouger View Post
Mike, That's helpful. It looks like there is a little more testing that I can do and see what comes out of it. Do you know where those testing instructions came from? Did you write them? - Doug.
Well, yes I wrote them! But they are based on experience gained over the years. The point is that they are cheap to do, don't need any fancy equipment, don't take long to do & can at least point you in the right direction.
If you get inconsistent values on all tests, it may indicate a worn valve timing chain, affecting all cylinders, but I'm not certain how modern variable-valve-timing mechanisms influence this. Others may know.
 
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Old 12-06-2018, 06:43 AM
crdouger
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I did not disconnect the pump fuse. I also did not do it WOT. So...two strikes against me. I will re-do the test this weekend and see what happens. Thanks for the insight. It makes sense to turn off the gas so you don't wash the cylinder walls. But how does the WOT make a difference?
 
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Old 12-06-2018, 08:44 AM
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Neither gas nor WOT will make a massive difference to the results - but all the readings will be lower so it's harder to tell where the differences are. It's just the 'best' way to do it.
Don't jam the throttle open at the engine end - it's a drive-by-wire system & the synchronisation will get messed up - put a brick on the gas pedal.
For test 2, inject the oil one cylinder at a time - put a rag over the cylinders after you test them - the oil may spray out of those.
I've revised the attachment a little bit.
 
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Old 12-06-2018, 01:57 PM
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Ok. Thanks for the advice. I'm going to replace the timing chain and then re-test the compression and see how it looks. How badly do the timing chain sprockets wear. I see kits that just replace the chain and guides and use the original sprockets. Is that ok to do? That bottom sprocket seems like it can be a nightmare to get off and on again. I'd love to not have to replace it.
 
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Old 12-06-2018, 02:46 PM
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I think it's OK to replace just the chain & guides. I'm sure others will chime in with their opinions.
(I should probably do mine soon so I'll be interested to see).
 
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Old 12-06-2018, 10:02 PM
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did you depress gas pedal while crank?
 
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Old 01-02-2019, 05:26 PM
crdouger
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quick update in case you are wondering. I replaced the timing chain and tensioner, but not the sprockets. The lower cast aluminum flange right below the bottom sprocket, which seems like it could be intended to hold the chain on the sprocket or at least prevent it from skipping teeth, was broken off. So I changed the oil and cleaned everything back up. It started up nicely and runs nicely. So I checked the compression again with a tester from Napa with the same dismal results of 70-90psi...very disappointing! Then I decided to try a tester from Orielly's to eliminate that I happened to have a broken tester...and my results were between 150-160 across all four cylinders!!! Yay.

Tomorrow I"m going return the tester to Napa and let them know it's a broken piece of junk. :(
 
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Old 01-02-2019, 06:00 PM
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Hate to be the bearer of bad news but when chains are replaced sprockets should be replaced too. At least this was the SOP for when I worked on engines with chain cam drives and even before that with motorcycle chain drives.
 
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Old 01-02-2019, 07:55 PM
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Although I replaced the gears when I did mine I think the replacement chain will be worn out again before the old gears will give out. Just my two cents. You will be fine, just be sure everything is torqued to the newer proper spec's
 
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