Checking compression with a multimeter

[Skydancer2992]

Scotty Kilmer has a trick for checking cylinder compression and fuel pump operation using a graphical multimeter.

Should work on an airplane motor and pump. Might be worth carrying in a plane kit.

The one he is using: https://www.ebay.com/itm/353314765737


 

 

[N201MKTurbo]

9 minutes ago, Skydancer2992 said:

Scotty Kilmer has a trick for checking cylinder compression and fuel pump operation using a graphical multimeter.

Should work on an airplane motor and pump. Might be worth carrying in a plane kit.

The one he is using: https://www.ebay.com/itm/353314765737


 

 

You know you can turn the propeller by hand and feel the compression. It is pretty easy to tell if you have a dead cylinder. We already have a fuel pressure gauge that tells us if our fuel pump is working.

Still a good trick for quickly evaluating an old beater car. 

[EricJ]

A buddy of mine makes his living troubleshooting cars at professional garages.   Basically, the shop calls him when they can't figure something out.   He also gets called by shops for anything related to reprogramming or reflashing smart boxes, and, surprisingly, does a lot of this work for dealerships.

My 20-year-old Ford Ranger with a 4.0L V6 got a lopey, shakey low speed shudder and was generally sad.  I did the usual stuff of changing injectors, smoke testing the intake for leaks, etc., etc., and couldn't fix it.    He stopped up to look at it and his Ford diagnostic tools have this function built in, so the first thing he does is disconnect the coil pack and connect his diagnostic tool and turn it over.   It just took a few seconds to see that I had a dead cylinder by monitoring the system voltage during start...he knew it without even looking at the scope, though, by just listening to it turn over.   He does this stuff enough that he was like, yeah, you can hear it.   

It had a burned valve seat on that cylinder and replacing the head fixed it.   Derp.

The automotive world has a lot of cool tools like that.   He did say that for the most part only the Ford diagnostic tools have it nicely automated, that most of the other manufacturers you have to do a bit more work to sort it out, but it's a well-known trick.   Things like OBD ports make it a lot easier, and most GA airplanes don't have anything like that.   The airlines have had stuff like that built-in for decades.

[Raymond J1]

It is very well known, it is called "compressiometer", it was very used on diesel engine, compression ignition so, we put a small cardboard on the device that will draw the graph and you have the recording. The downside is that you can have good compression even with worn segmentation... Just a good oil film in the cylinders for example. The measurement of the leakage rate as it is done aims to trap all possible ways of having a leak : valve that does not close, but also broken segments, cracked piston, cracked cylinder... What a compressiometer does not see in the brief moment of its measurement.