* Fuel pressure: A weak fuel pump or clogged fuel filter can cause these symptoms. You need a fuel pressure gauge to check this. The pressure should be within the specifications listed in your owner's manual or a repair manual for your 1991 Chevy Blazer 4.3L.
* Throttle Position Sensor (TPS): A faulty TPS won't tell the computer the correct throttle position, leading to poor fuel delivery and idle problems. This is a common culprit. It's relatively inexpensive to replace and can be tested with a multimeter.
* Mass Air Flow Sensor (MAF): This sensor measures the amount of air entering the engine. A dirty or faulty MAF will send incorrect information to the computer, affecting the fuel mixture and leading to poor running. Try cleaning it with MAF sensor cleaner (following the instructions carefully) before replacing it.
* Computer (ECM): While less likely given you've already replaced some components, a faulty ECM can cause a wide range of problems, including those you're experiencing. This is a more expensive and involved diagnosis.
* Vacuum Leaks: Check all vacuum lines for cracks or leaks. A small leak can significantly affect engine performance, especially the idle. Use a vacuum gauge or spray a soapy water solution on the lines while the engine runs to check for leaks.
* Ignition System (beyond cap and rotor): While you replaced the cap and rotor, other ignition components like the ignition coil, ignition module, or even wiring could be at fault. These require more specialized testing equipment.
* Crankshaft Position Sensor (CKP): A bad CKP sensor can prevent the engine from running smoothly or at all.
Troubleshooting Steps:
1. Fuel Pressure Test: This is the most important first step. A low fuel pressure will explain the symptoms you describe.
2. Vacuum Leak Check: Visually inspect all vacuum lines and use the soapy water method.
3. Clean the MAF Sensor: This is a quick and easy check that often yields positive results.
4. Check the TPS: A multimeter is needed here, but it's a relatively easy test to perform.
Recommendation: Get a repair manual specific to your 1991 Chevy Blazer 4.3L. It will have wiring diagrams, specifications, and troubleshooting procedures to help diagnose these issues. A code reader might also be helpful, although codes won't always pinpoint the exact problem with these older vehicles. If you are not comfortable working on your vehicle yourself, take it to a qualified mechanic.
