1991 Chevrolet Other engine

First check if there's any diagnostic fault codes. Here's how:

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Next, check the fresh air hose between the mass air flow sensor and throttle body. There can't be any leaks in that hose. The next step requires a scanner so you can view live data. That will allow you to see what the Engine Computer is seeing. and you may be able to identify a sensor reading that is incorrect but not bad enough to set a fault code.