Ask the Mechanic

Steve Thompson, Contributing Columnist

READER: We have owned and worked on many John Deere engines using the "pencil" fuel injector. As the engines aged, and we began working on them, we never found a way of checking the compression with a pressure gauge. These engines should run around 320 pounds of compression, and we cannot seal a rubber fitting on a gauge at this high of compression. All of our fittings in our compression set must be threaded into the head. Can you tell me how I can check the compression on these engines?

STEVE: Checking the compression on an engine using the "pencil injector" requires a way to seal the compression for checking. With no threaded port in the head, the tool must seal the compression with sealing pressure. The good news is that you probably have a tool that will check the compression hanging around your shop on the shelf -- an old pencil injector. We simply sawed off the injector at the spray tip and return line tip of an old injector, and sealed the high-pressure fitting on it. Then, we welded an air quick-connect fitting that works on your existing pressure gauge kit on the return line end. The modified "pencil" injector will seal on the head using the same hardware as the original injector.

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-- Email Steve Thompson care of askthemechaniccolumn@gmail.com, and be sure to include your contact information and phone number.

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