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Old 03-09-2002, 09:37 PM
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Join Date: Jun 1999
Location: Surrey, Beautiful British Columbia, Canada
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Engine Compression Test GAUGE

sorry for this elementary question. finally got a
chance to purchase gauge. do i just screw it
in like a spark plug? manual says remove
all spark plugs and crank engine. thanks for any help.

what should be a good compression on 89 300E?

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Old 03-09-2002, 09:45 PM
Join Date: Nov 2001
Location: Cape Cod Massachusetts
Posts: 1,427
Commpression test ?

Usually you would do them one at at ime so you don't get any wires mixed up but you MUST also disconnect the high voltage lead at the coil!

If you pull all the plugs at the same time you would be safe disconnecting the high voltage lead at the coil also. The engine should turn a couple of revolutions for accuracy, note the reading, release the pressure, and move on to the next cylinder. A warm engine would be most accurate.
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Old 03-09-2002, 10:08 PM
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thanks billybob.
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Old 03-10-2002, 01:11 AM
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The throttle will also need to be kept open in order to allow air to be drawn into the engine; otherwise, you'll obtain low readings.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC

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Old 03-10-2002, 11:47 AM
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I had never heard that the throttle should be kept open. Has anyone tried both methods and seen a difference?
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
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Old 03-10-2002, 12:25 PM
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If the throttle is closed, you are pulling a vacuum.
You want the cylinder at atmosphiric pressure before the compression stroke. If it is not, you will get a low reading.
Opening the throttle insures the piston is full of air Before the compression stroke.
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Old 03-11-2002, 03:36 PM
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On K-Jet engines, should you prop the air metering valve open? Makes little sense to open the throttle if the metering valve upstream is closed.

91 300SE
81 300SD
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Old 03-12-2002, 02:24 AM
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The position of the throttle valve is not important on any engine. If this were the case then a cold engine would always be difficult to start due to low compression. My service CD tells me to remove all the plugs, disable the fuel and ignition and then take a reading after 2 or 3 turns on the starter ( M117 V8, a normal compression being 8.5 bars and not below 7.5 bars with no more than 1.5 bars between any 2 cylinders ). It's always worked fine for me on any of the cars I have looked after over the years . . .
1993 190E 2.0L (Euro.) sold
1991. 500 SEL (Euro.)
1991. 300 CE (Euro.) sold
1993. 500 E (U.S. spec.)
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Old 03-12-2002, 10:18 AM
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A compression test can certainly be done without opening
the intake air, but in a test you want the HIGHEST possible reading to test the components [ rings,valves, etc.]
So restricting the intake air passage on any gas engine
may give an inaccurate reading.
At cranking speed, there May be enough intake air to get a good reading, but open intake [ throttle/choke/etc.] assures there is.

Common tech pratice .
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