Hilton

Cool bit of kit

GregBBRD

I generally check cold engines.....after all, when I'm doing a compression check, I'm generally looking for any potential problems.....and a cold compression check will show more problems than a warm one.

If I get some really wacky readings cold, I'll go warm the engine up and re-check it.

dr bob

Interesting note in the 'analysis' section of the manual -- readings may be accurate plus or minus 1 bar. My kind of gauge!

Compression test readings vary from gauge to gauge for several reasons. Beyond impact/drop damage and the treatment most gauges receive in the toolbox, the most common is differences in the volume of the gauge itself and the connecting plumbing. The gauge works with a piece of coiled tubing inside, where added pressure causes the tube to "uncoil". That uncoiling movement is translated mechanically to the gauge pointer. Longer tubes provide more linear gauge needle response, but the actual volume in the tube and attaching plumbing can become a significant part of the total cylinder volume. Better gauges use stainless steel tubes, lesser ones may use brass or copper, which work-harden with repeated cycling but more importantly become temperature sensitive.

These and other reasons related to warranty by engine manufacturers cause the makers to spec differences between cylinder pressures as percent, rather than some absolute number to tell you what it should be.

The Fixer

We drove all the way to VT from Philly once to look at and hopefully buy a car,
I check compression and couldn't believe the thing even started, it did run bad. I checked a few more times to rule out me and found consistent/same bad readings and passed on it.

Later when checking my old Jag 4.2L (That i new was good) with the same gauge i found terrible readings again so i decided to buy a new gauge and realized the gauge went bad.

It went bad simply from age as it was kept out of harms way in a soft case
but outdoors.

That 4.2L I6 in my '72 Jag had 177K miles on the bottom end but had a fresh valve job and head gasket, factory "high compression head" : 9:1 compression, average was IIRC) around 165 lbs cold

But that old motor had readings that were within 2-4 lbs. btwn
each cylinder COLD. I was shocked. It did run well too.

Miss that one.

EDIT: I probably warmed it up a little but not much.