If your Blazer is having idle problems, the IAC valve *might* be the culprit, but it's more likely a symptom of a larger issue. Trying to "adjust" it directly will likely damage it or be ineffective. Instead, you should troubleshoot the following:
1. Check the IAC Valve for Obstructions: The IAC valve can get dirty or clogged. You can carefully remove it (after disconnecting the battery's negative terminal!), clean it with a suitable cleaner (carb cleaner is often used), and inspect it for damage. Make sure the plunger moves freely. However, cleaning it might not solve underlying problems.
2. Check the Throttle Position Sensor (TPS): A faulty TPS can send incorrect signals to the ECM, leading to poor idle. A scan tool can check the TPS voltage, or a multimeter can be used to test its output. Replacement is often the solution if faulty.
3. Check the Mass Airflow Sensor (MAF): The MAF sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can also cause idle problems. Cleaning it (carefully, with MAF sensor cleaner) is often a first step, but replacement may be necessary.
4. Vacuum Leaks: Check all vacuum hoses and connections for leaks. Even a small leak can significantly affect the idle.
5. ECM Issues: A failing ECM can cause a myriad of problems, including idle issues. This is a less likely culprit but should be considered if other checks yield no results. Diagnosing this requires specialized equipment and knowledge.
6. Dirty Throttle Body: A dirty throttle body can hinder smooth operation. Clean it thoroughly with throttle body cleaner.
Instead of trying to manually adjust the IAC valve, diagnose the underlying problem. If cleaning the IAC valve and checking for vacuum leaks and sensor issues doesn't solve the problem, you'll likely need a scan tool to read diagnostic trouble codes (DTCs) from the ECM. These codes can pinpoint the source of the idle problem. A mechanic with a scan tool and experience with these systems is your best bet at this point. Trying to adjust the IAC without proper diagnostics could lead to more problems and potential damage.
