Ignition System:
* Worn spark plugs or wires: This is the most common cause. Under load, the increased demand on the ignition system highlights weaknesses in worn parts. The spark may be too weak to ignite the fuel-air mixture consistently.
* Ignition coil(s): A failing coil may not produce sufficient voltage under high demand, leading to misfires. These often fail intermittently, making diagnosis tricky. Coil packs are common in this era of Tahoes.
* Distributor cap and rotor (if applicable): If your Tahoe still has a distributor, cracks or carbon buildup can prevent a strong spark. This is less likely on a 1998, but possible depending on the engine.
* Ignition control module (ICM): This part controls the ignition timing and could be failing intermittently under load.
Fuel System:
* Fuel pump: A weak fuel pump may struggle to deliver enough fuel under high demand, resulting in a lean mixture and misfires. This is more likely to show up under acceleration.
* Fuel injectors: Clogged or faulty injectors can deliver an inconsistent fuel spray, causing misfires.
* Fuel pressure regulator: A faulty regulator may not maintain the correct fuel pressure, leading to a lean mixture and misfires.
Other Potential Causes:
* Mass Air Flow (MAF) sensor: A faulty MAF sensor provides inaccurate readings to the engine computer, leading to improper fuel/air mixture. This can cause misfires under various conditions, including load.
* Oxygen (O2) sensor: Similar to the MAF sensor, a faulty O2 sensor can provide incorrect information to the engine computer, leading to poor fuel mixture and misfires.
* Catalytic converter: A clogged catalytic converter can restrict exhaust flow, causing backpressure that leads to misfires. This is less likely to be the sole cause, but it can exacerbate other issues.
* Vacuum leaks: Leaks in the intake manifold or vacuum lines can cause a lean condition, leading to misfires under load.
* Engine compression: Low compression in one or more cylinders can lead to misfires, especially under load.
Troubleshooting Steps:
1. Check the easy things first: Start with the spark plugs and wires. Inspect them visually for wear and tear. If they look questionable, replace them.
2. Read the trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the engine computer. This can often pinpoint the problem area.
3. Perform a compression test: This will check the health of your engine's cylinders.
4. Check fuel pressure: A gauge is needed for this test to verify that the fuel system is delivering sufficient pressure.
5. Inspect the ignition system components: Visually inspect the ignition coil(s), distributor cap and rotor (if equipped), and ignition control module for signs of damage or wear.
Important Note: Without further diagnostics, it's difficult to pinpoint the exact cause. Trying to diagnose this without proper tools and knowledge can be frustrating and potentially damaging to your vehicle. If you're not comfortable performing these checks yourself, take your Tahoe to a qualified mechanic for diagnosis and repair.
