You are right I don't understand the issue you bring up. I have never tested and injector or used my tester so Maybe that will help clue me in. My thoughts are that you may be seeing a loose or high fluctuating needle as acuaracy which it is not. If it over shoots 10 % then thats your unknown where as a liquid filled typically wont over shoot as much. It all goes back to accuracy of the gauge. Yes a cheap air filled guage will move around alot but what is it reading. Technically it is probably reading "jerk" or "momentum" which in this case doesnt tell you what you need to know. It boils down to money spent on gauge, in my case not very much. I'm cheap but I do like to learn so I guess I'll have to see. Thanks

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83 300D 227,xxx miles, sold
86 300 SDL 130,000 miles, sold