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  #1  
Old 06-10-2012, 06:24 PM
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Compression test today - 420SEL 1896

Sadly, the results were very bad... I might have to sell my project car as a parts car. This has also been my daily driver for several years but recently started idling roughly.

PSI right bank: 90, 55, 82, 90
PSI left bank: 65, 55, 75, 70

I was wondering why the O2 sensor light was coming on.

:-(

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  #2  
Old 06-11-2012, 08:18 AM
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Was that with a hot or cold engine?
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  #3  
Old 06-12-2012, 09:17 PM
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Cold

Cold but then I did the test again at operating temp.
Marginal improvement - the 90's went to 105. The 55's went to 67.
Still a long way from optimum 123...
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2000 Porsche 911 Millenium Edition Carrera 4
2010 Lexus RX 350
2002 Kia Rio
2002 Hyundai Accent
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  #4  
Old 06-12-2012, 11:13 PM
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engines wont even start under 90lbs, i think your compression tester is not reading correctly,does it use a hose screwed in or rubber seal pressed into hole? double check readings throttle open, warm engine , fully charged battery?
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  #5  
Old 06-13-2012, 04:37 PM
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123psi is optimum? For what?

My near 300000 mile M103 still was in the 160-170psi range.

Try a new tester. Are you sure you are operating it correctly... plugs out, throttle fully open, etc
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  #6  
Old 06-13-2012, 05:41 PM
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Agreed, thats way too low, you'd have way worse than a rough idle.....new tester time!

How many miles on the motor? My dad's 420 has almost 300k and runs great....the bottom ends of these engines are extremely durable.
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  #7  
Old 06-14-2012, 11:33 AM
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OK, you've convinced me...

I'll get a new tester immediately.

The rough idle paired with the O2 sensor light coming on (nearly every trip), suggested to me that perhaps it was a burnt exhaust valve. I think I eliminated electrical by changing the O2 sensor, all the spark plug wiring, the distributor rotor and the ignition coil - all new - still running roughly.

Initially I thought vacuum. So I have been all over the vacuum system too. I have changed nearly every rubber connector - no change in the rough idle.
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  #8  
Old 06-14-2012, 06:39 PM
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Worn injectors can cause rough idle, as well as low fuel pressure(failing pumps), clogged fuel filter, bad pressure regulator, bad spark plugs (did you change these also?) Distributor cap can cause problems too.

Does your economy gauge work properly or seem to behave normally? I had a rough idle one day and it was because the line to the econo gauge fell off the engine.
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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 88k - mine - (OC-91,200)
'17 Metris(VITO!) - 16k - wifes (OC-17k)
'01 E320 Wagon - 159k - mine (OC-160,000)
'01 E320 - 179k - dad's (OC-182,500)
'07 E350 Wagon - 133k - dad's (OC-132,500)
'01 SL500 - 52k - dad's (OC-52,000)
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  #9  
Old 06-14-2012, 07:14 PM
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Quote:
Originally Posted by pawoSD View Post
Worn injectors can cause rough idle, as well as low fuel pressure(failing pumps), clogged fuel filter, bad pressure regulator, bad spark plugs (did you change these also?) Distributor cap can cause problems too.

Does your economy gauge work properly or seem to behave normally? I had a rough idle one day and it was because the line to the econo gauge fell off the engine.
Yes I changed the plugs, and the fuel filter. I will check the economy gauge but it appears to be working normally.

I have thought about injectors - maybe that's another avenue to pursue, along with a new compression tester!
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2002 Kia Rio
2002 Hyundai Accent
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  #10  
Old 06-14-2012, 07:31 PM
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Quote:
Originally Posted by Hit Man X View Post
123psi is optimum? For what?

My near 300000 mile M103 still was in the 160-170psi range.

Try a new tester. Are you sure you are operating it correctly... plugs out, throttle fully open, etc
Yes I'm sure; I did it how they taught me at school.

AllData stated in the specifications section (for my engine) that the normal compression is 8.5 bar (@~14.5 psi/bar = ~123 psi). Perhaps I miscalculated the conversion from 8.5 bar to psi...

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2010 Lexus RX 350
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2002 Hyundai Accent
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