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1993 Isuzu pickup stalls when taking foot of accelerator

My 1993 Isuzu pickup with a 2.3L 4 cylinder has started to stall when I take my foot off the accelerator, it runs fine as long as I am giving it gas, but when I take my foot off the gas to stop at a light, it dies. It always starts right back up. Recently, it also stalls when I crank it unless I am giving it gas. It seemed like vacuum but the shop did a smoke test and could not find anything. Finally I cranked up the idle adjustment so that it idles around 2k RPM. So far so good, but obviously not the right answer.

Any thoughts?

We need to know if the engine carburated, throttle body injection, or multiport injection.

Tester