Hi folks, I just did a compression test on my 67 D100 and ended up with more questions than answers. Trying to figure out if the gauge is goofy or I have more problems than I expected. When turning the motor over for testing, the gauge will pump up to about 120-130 psi, then immediately drop down to about 40-50psi (this was consistent across all cylinders). I've looked all over for air leaks on the gauge, finally managed to hook it up to my compressor and it read a steady 95psi (which is correct). So it seems the gauge is good, but the dropping reading is confusing. Is this telling me I've got a rebuild in my future, or does this seem odd to y'all too?

The truck doesn't seem drastically underpowered, though after 17 years it's possible I just didn't notice the change over time and it's actually pretty crummy. It certainly takes its time getting up hills. If it is actually 40psi, would it even move?

I'm hoping to do a leakdown test, once I can finagle a connector to my air system. That will likely be awhile, as I'm redoing the brake system first.

I appreciate any input - thanks - Jason

Compression gauges have a one way check valve in them. When a motor builds compression then stops, it will bleed down the cylinder. The one way check valve is necessary so the gauge holds its reading when this happens.

If your gauge rises to a value then suddenly drops, that check valve is broke. It reads steady with your air compressor because the pressure is constant.

Yep, the schrader valve is failing.

Hey thanks for the reassurance folks!

... can I borrow a compression tester from you....?

The Schrader valve for the compression tester is a special low spring version.

The normal valve cores you get take about 20-25 psi to open (gauge will read 20-25 low)

So you need to find the ones for compression gauges..............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Perhaps the gauge is calibrated with the offset?

Unless it's an expensive tool truck brand gauge or you have easily available valves, it may just be best to buy a new gauge. If your ok with borrow and return then check parts stores, pretty sure they were part of the loaner programs.

Yeah, this was an Amazon special, think it cost $25. I think I can return it, instead of messing around with it more (I've already disassembled and put Teflon tape on parts of it).

Thanks again, if there's any exciting news I'll post it soon.

I would just buy new standard schrader valves and replace it. You can expect the readings to drop roughly 20 psi. The new school way to do a compression test is to hook up a oscilloscope to the starter and references off plug wire number one. Essentially we are inspecting wave form peaks to ensure you have balanced compression across the board. We measure via current draw. The reason I say all of that is I would be looking for balanced numbers. Of course a max pressure reading of 40 psi is bad across the board but you can hear that.

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Thanks mechanized, the word "oscilloscope" just sent me into a spiral of despair! I think I'll stick with the old-school method, and leave the oscilloscopes to the smarter guys like yourself. Maybe eventually I'll get that precise, for now I'm just trying to get on top of it!

Cheers - Jason