Over the past year, my brick's compression in each cylinder (1/2/3/4) has slowly increased on 7/27/2002 of 145/145/146/153 psi, on 6/30/03 of 150/157/156/160 psi, and 166/171/172/174 psi today. The differences between cylinders is proportional, so I doubt that my technique is off. Besides, I'm using the same compressor tool with its braided tubing. This increase in compression is counterintuitive, unless I am taking better care of the engine. My flame trap is clean, but I'm a little concerned that an increase in the compression could be a bad thing. Your thoughts?

Did you just buy thr car before you started checking the compression? I've had the same increase over time up to a steady level after buying a car. I always assumed that I was changing the oil more often than the PO and the fresh oil gradually loosened the rings to give better sealing. In any case that compression is in the normal range.

Checking compression= remove one plug at a time with the engine at operating temps.
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john,,1985, 245:Ti, 246k mi..so.california

John, a flag went up when I saw your method: "Checking compression= remove one plug at a time with the engine at operating temps."

I've never seen that method recommended before. Does it have an advantage over the "traditional" procedure — (Pulling ALL plugs and keeping the throttle wide open) ?


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Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67

That much of a change in the 'good' direction does seem strange, but engine block temperature and ambient temperature can vary the ring-cylinder fit somewhat. Cranking speed (which also varies with engine temp and battery condition) may be responsible for some of the difference. Maybe you've been using better oil and filters? Or maybe some oil is getting past the guides, and the head is starting to get some carbon deposits?

Nothing to worry about, though - you won't blow the head through the hood. If anything, you're getting some slight increase thermodynamic efficiency (Carnot cycle) with an increased compression ratio.

I tried to clean the combustion chamber and valves with carburetor cleaner sprayed through the vacuum hose (per brickboard postings). I did this one week ago.

If the valves did have a little carbon on them, this may help that to come off and allow the valves to seal a little better against the valve seats. Also some fluid surely went into the cylinders- may have cleaned up the rings a little bit for better sealing. If a LOT went in there, and sat for a while, its advisable to change your oil.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K

Just a thought - could it be the gauge? I would however think that if failing slowly, it would do so with a lower and lower reading, not higher. I had one that sat stuck at 30 when released, and gave me a reading of 110. I finally got a Snap On and read 155. A new cheapo unit read 170. I've read elsewhere that calibration between units is often all over the place, and to pay more attention to balance between cylinders.

throttle valve open?