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TECHNICAL INFORMATION How-to and detailed information on specific technical subjects

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Old 14th April 2011, 06:21 AM
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Default 6.9 Compression Test

Most of us know the procedure involved in carrying out an engine compression test. Right ? Engine hot, accelerator pedal fully depressed, and turning over engine for about six or seven revolutions. Normally, to carry out the test, it needs two people, namely one to activate the turning of the engine, and the other that holds the compression gauge in the spark plug hole.

Now, there MAY WELL BE another method of carrying out the test. Some gauges are supplied with a screw type fitting which, of course, can be screwed into the spark plug hole. The English LUCAS compression tester features this type of fitting. I had one of those, and actually decided to run the engine with the tester plugged in, checking ONE cylinder at a time. The readings were as follows at the time, back in February 2005.

1 - 88 lbs. 5 - 95 lbs.
2 -117 lbs. 6 -100 lbs.
3 - 80 lbs. 7 -100 lbs.
4 -100 lbs. 8 - 80 lbs.


From those readings, one could be forgiven for thinking that the engine was completely worn out. Yet, there were no real issues for about three years until the head gasket developed a leak at the back of No.4 cylinder. That problem, of course, was rectified in 2009 by a valve grind and de-coke. The engine has run well since then - no oil problems, no smoke on the over-run etc. I am interested what others are thinking about the method of checking the compression with engine running. Regards Styria.

Last edited by Styria; 15th April 2011 at 08:03 AM.
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Old 14th April 2011, 09:31 PM
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Default Re: 6.9 Compression Test

The "old type" of compression testers i.e. with just the rubber bung, have been around since Adam was a cowboy and the "later type" with the screw thread have also been around since the Ark. Where have you been !!! Why would you test it with the motor running ? Apart from being at risk of getting a kick off the spark plug lead and burnt fingers from the spark plugs, what advantage is there ?

Back to your compression readings, as a rule of thumb, any variation beyond 10% is cause for concern and investigation, however those readings appear to be very low to me. I would have expected a minimum of 140.

Last edited by CraigS; 14th April 2011 at 09:36 PM.
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Old 15th April 2011, 08:44 AM
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Default Re: 6.9 Compression Test

I wouldn't like to follow this method just because I don't like the idea of running on 7 cylinders. To get an accurate figure I'd have thought it'd be necessary to run the engine the same amount of time for each cylinder, which would be difficult. BTW a note on the threaded compression testers; only buy ones that have a detachable threaded end. If it cannot be detached from the tool & it's necessary to turn the entire tester around to screw it into the spark plug hole, it's VERY easy to cross the thread & ruin the head.....
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Old 15th April 2011, 08:51 AM
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Default Re: 6.9 Compression Test

Hi Craig, as you'd know I have been around since the days of Noah's Ark - why, I was actually offered a seat on a crowded bus this morning. Lovely Chinese guy (almost my age) - what was he thinking ? I am of course aware as to the correct procedure of testing the cylinder compressions, but I was looking for opinions whether, in fact, testing the compression with the engine running was in fact:

1. A more accurate way of obtaining readings compared with the usual method.

2. I was being stupid, or perhaps even neglectful, in using the 'engine running' method to obtain the readings.

3. Could it also be that carrying out the test with the engine running could have given me lower readings ?

These are the questions that I am posing and seeking an answer for to if, of course, there is an answer. Luckily enough, I did not burn myself nor did I get any shocks from any of the eight plugs that I removed in turn. Please correct me if I'm wrong, but the information in the Mercedes Technical Data manual indicates that the minimum compression reading should be no less than 125 lbs. with a maximum variation of 25% being allowed between the highest and lowest readings. Regards Styria

Last edited by Styria; 15th April 2011 at 08:53 AM.
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