Compression test

[Rawze]

(01-16-2018 )Hammerhead Wrote:  That's a really good idea, attach a manometer on the crank vent and use Insite to run the engine on one cyl at a time and record the results.
Not only simpler, but far less labour intensive and probably less expensive tooling...[i]and, can be done at much closer to, if not at operating temp for best results.

Simply running the engine on one cylinder at a time will reveal any significant difference in compression. A manometer is not even required actually. Any measurable amount of compression loss differences will reveal itself quickly when firing the engine by only one cylinder at a time. A manometer will not reveal any valve or other head related seepages or air flow issues that cause compression losses. I have seen several engines of the past where compression is lower on one cylinder that is enough to measure. For a CM871 that has suffered years of soot problems, it is usually number 2 or number 3 cylinder. Firing the engine on one cylinder at a time AFTER ensuring you have done an injector leakage test (the bubbles in the glass of water video I made) so that you don't get false results will reveal a mechanically weakened cylinder.

Find a cylinder that does not want to run well?- Swap injectors around to ensure it is NOT an injector issue. If it stays with the cylinder then it is a legit compression problem.

the reason WHY is it usually number 2 or number 3 cylinder is that soot likes to get cached and harden into carbon chunks right behind the ECM and build up this volcano right at the intake valves in the head, then starve the cylinder of intake air, weakening it and lowering compression. It eventually takes its toll on the cylinder(s) and makes the other cylinders have to work harder to make up for it.

The diag machine that measures current and the cam position was mentioned earlier works the best on forced induction engines and is the most accurate way of comparison.

In fact, you could make your own with a coil and tiny magnet on the overhead cam in a safe place and measuring the voltage at the positive starter post, feeding both signals into a cheap usb oscilloscope for a laptop. record the screen (or a real o-scope and a video recorder), then play it back after cranking for a bit. You would simply set the trigger of the o-scope to the overhead cam signal so you got a stable screen, and place the magnet so that it passed the coil at TDC of cylinder number one. Reading the screen from left to right would be cylinders 1,5,3,6,2,4 in that order. This would be far less effort than taking things apart and sticking gauges into cylinder bores.