Shorting plug wires for testing

[Truckedup]

Some people think I know something about modern engines.. Hey whatever....Friend suspects a weak cylinder in his 2004 Suburban...He has no compression gauge so I said just insert a pin under the boot on the 8 coils and short out one cylinder at a time and check rpm drop...He said it'll burn out the electronics and instead pull off the coil signal wire harness.......Well, will shorting the wires harm anything?

[rebelrouser]

I have shorted plug wires for many years. On coil on plug engines I normally unplug the connector on the coils. But for any engine newer than 1996, federal OBD II regulations require that the computer be able to detect misfires and record them. So I seldom take the time to short plug wires, I just look on the scan tool, for misfire codes, and all vehicles have some misfire testing built into the scan tool, usually they kill the injectors not the ignition. It will set either random misfire codes = all cylinders missing some, or a specific cylinder code. Then you can look on mode 6 and it will record all the data for the engine at the time it set the code. If you don't have a scan tool, not as easy as it used to be. Lots of decent used scan tools on ebay, especially snap-on tools. I recently bought a use Modis, with four channel scope, good to 2012 model year for $1.500.00 good price just for the scope.

[RCJ]

You could knock out a HEI by shorting the plug wires.

[Truckedup]

You could knock out a HEI by shorting the plug wires.

So a fouled spark plug will also knock it out?.... Pulling plug wires on a running engine works but shorting them is safer ... I pulled a plug wire on a car years ago ,got shocked and dropped the wire on the battery which blew up all over me...No permanent damage other than Raggedy Ann clothes..
My question is about late model stuff, like the coil near plug LS....

[peejay]

I'd throw an amp-clamp on the positive battery cable and connect it up to a scope and crank the engine. A weak cylinder will show up plain as day and no need for a compression test. Good luck even doing a regular compression test on a lot of newer cars.

[user-23911]

With modern engines you use the OBD2 tool to disable injectors one by one.

[Olds455]

With modern engines you use the OBD2 tool to disable injectors one by one.

Cylinder power balance test? Fairly easy with the correct tool, as long as you know what you're doing. Turn off each injector 1 by 1 and note the rpm drop of each cylinder.

[MadBill]

I got fooled doing that once. Did an RPM drop test at ~ 2,000 RPM on a TPI Camaro, which showed a couple of bad cylinders. Many hours work later, I figured out it was showing maldistribution of EGR gas.