I. Battery and Charging System:
* Weak Battery: This is the most common culprit. Have the battery tested at an auto parts store (they usually do this for free). A load test will determine if it's holding a charge properly. Look for corrosion on the battery terminals and clean them if necessary.
* Alternator: A failing alternator won't recharge the battery. While the engine is running, have the voltage checked at the battery terminals. It should be around 13.5-14.5 volts. Low voltage indicates a problem with the alternator.
* Battery Cables: Check the cables for corrosion, loose connections, or damage. Clean or replace as needed.
II. Starting System:
* Starter Motor: A failing starter can intermittently refuse to engage. You might hear a clicking sound when you try to start the vehicle. This often requires professional diagnosis or replacement.
* Starter Solenoid: This is a part of the starter system and can also cause intermittent starting issues. Again, a clicking sound is a common symptom.
* Ignition Switch: A faulty ignition switch can prevent power from reaching the starter. This is less common but possible.
III. Fuel System:
* Fuel Pump: A weak or failing fuel pump might not deliver enough fuel to the engine. Listen carefully near the fuel tank when the ignition is turned on; you should hear a brief humming sound from the fuel pump. If not, or if it's weak, it may need replacement.
* Fuel Filter: A clogged fuel filter restricts fuel flow. Replacing it is relatively inexpensive and worth doing if it hasn't been changed recently.
* Fuel Injectors: Faulty injectors can prevent fuel from reaching the cylinders. This is more difficult to diagnose and usually requires a professional scan tool.
IV. Ignition System:
* Ignition Coil: A failing ignition coil can prevent sparks from reaching the spark plugs.
* Spark Plugs & Wires: Worn or fouled spark plugs or damaged wires can interrupt the ignition process.
* Distributor (if applicable): Older vehicles may have a distributor. Problems within the distributor can affect ignition timing and spark delivery. (Less likely in a 1999, but possible depending on engine)
* Crankshaft Position Sensor (CKP): This sensor tells the computer the engine's rotational position. A faulty CKP sensor can prevent the engine from starting. A scan tool is usually required to diagnose this.
V. Other Potential Issues:
* Security System: If the Blazer has an aftermarket or factory alarm system, a problem with it could be preventing the engine from starting.
* Neutral Safety Switch: This switch prevents the engine from starting unless the transmission is in park or neutral. A faulty switch can cause intermittent starting problems.
* Computer Issues: A problem with the engine control module (ECM) or related sensors is less likely but possible. A scan tool is essential for diagnosing computer-related issues.
Troubleshooting Steps:
1. Start with the simplest checks: Battery, terminals, cables.
2. Listen for sounds: Clicking (starter/solenoid), humming (fuel pump).
3. Check fluids: Ensure you have adequate engine oil.
4. Visual inspection: Look for any obvious damage or loose connections.
5. Obtain a scan tool: If you suspect a computer-related issue, use a scan tool to check for diagnostic trouble codes (DTCs). Auto parts stores may lend you one, or you can purchase a less-expensive OBDII scanner.
If you're not comfortable working on your vehicle yourself, take it to a qualified mechanic. Providing them with any observations you've made will help them diagnose the problem efficiently. Don't randomly start replacing parts; systematic troubleshooting will save you time and money.
