Diesel Injector Cutout Test

[TedG]

I know that on electronically-controlled engines you can perform an injector cutout test, by disabling each injector in succession. Can I do the same with my '79 240D, by cracking the line fitting at each injector? I am getting a bad popping sound (like very loud spark knock) from one cylinder, and a very rough idle. This has now escalated into a noise I can hear in the passenger compartment while on the freeway even. It sounds like a rapid tapping. I suspect a clogged or bad injector. I would like to find the problem injector if possible. I had this same condition about 30,000 miles ago, at which time I replaced all four injectors with Bosch reman (about $40 each as I recall).
I plan to use Diesel purge this weekend to see if that alleviates the problem.
Any advice is appreciated.

Thanks.

Ted