1. Verify Fuel Delivery:
* Fuel Pressure: Even with a new fuel pump, you need to verify it's delivering sufficient pressure. You'll need a fuel pressure gauge to test this. The pressure should be within the specifications listed in your owner's manual (or readily available online). Low pressure points to a problem beyond the pump itself, like a clogged fuel line or regulator issue.
* Fuel Injectors: Check if the fuel injectors are actually spraying fuel. This can be done with a noid light (testing for electrical pulse) or by listening closely for the characteristic clicking sound of the injectors while cranking. A faulty injector or wiring can prevent fuel from reaching the combustion chamber.
2. Ignition System:
* Distributor (if applicable): While less common on later models, some 1999 Tahoes might have a distributor. Check the cap, rotor, and ignition coil for wear or damage.
* Cranking Speed: Is the engine cranking over quickly and strongly, or is it slow and weak? A weak crank could be due to a bad battery connection (even with a new battery), a failing starter motor (despite a new one – perhaps the installation wasn't correct), or a low charge in the battery (despite being new, check the charging system).
* Spark Plugs and Wires: Check the spark plugs for wear, fouling, or damage. Inspect the spark plug wires for cracks or damage. A weak spark or no spark will prevent combustion.
* Ignition Control Module (ICM): This component can fail and prevent spark. It's best to test this with a professional scanner or multimeter.
3. Sensors:
* Crankshaft Position Sensor (CKP): This sensor tells the computer the position of the crankshaft, which is crucial for timing the spark. A faulty CKP sensor will prevent the engine from starting.
* Cam Position Sensor (CMP): Similar to the CKP, a faulty CMP will disrupt the timing and prevent starting.
* Mass Airflow Sensor (MAF): While less likely to prevent starting entirely, a faulty MAF can cause problems with fuel mixture and prevent a proper start.
4. Computer Issues:
* PCM (Powertrain Control Module): Although rare, a failing PCM can cause a variety of starting problems. This requires a professional diagnosis with a scan tool.
Troubleshooting Steps:
1. Listen carefully: Pay attention to any unusual sounds (clicking, grinding, etc.) during cranking.
2. Visual Inspection: Check all connections for corrosion, looseness, or damage.
3. Scan Tool: Using a scan tool (OBD-II reader) can retrieve diagnostic trouble codes (DTCs) that pinpoint the problem area. This is invaluable.
4. Professional Mechanic: If you've exhausted your own troubleshooting efforts, take your Tahoe to a qualified mechanic for diagnosis and repair.
Remember safety first! Work on your vehicle in a well-ventilated area. Disconnect the negative battery terminal before working on any electrical components. Avoid working on the fuel system unless you're familiar with fuel system safety procedures.
