Vacuum Leaks: This is a HIGHLY probable cause. The Northstar engine (if your Seville has one, it's common for this year) and other engines from that era are highly reliant on vacuum for various systems. A small leak can cause erratic idle and surging under load. Check all vacuum lines for cracks, loose connections, and proper routing. Pay close attention to the lines connected to:
* EGR Valve: A malfunctioning or leaking EGR valve can cause surging.
* MAP Sensor (Manifold Absolute Pressure Sensor): A vacuum leak near the MAP sensor will give inaccurate readings, leading to surging.
* Brake Booster: A leak here will impact braking and could contribute to idle issues.
* Intake Manifold: Cracks in the intake manifold are a common source of vacuum leaks.
Throttle Position Sensor (TPS): A faulty TPS provides incorrect throttle position information to the engine computer (ECM). This leads to inconsistent fuel delivery, causing surging. A simple test involves checking the TPS voltage with a multimeter while slowly moving the throttle. Look for a smooth, consistent voltage change.
Mass Airflow Sensor (MAF Sensor): This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can give inaccurate readings, causing the ECM to deliver the wrong amount of fuel. Clean it with MAF sensor cleaner (following instructions carefully), or replace it if cleaning doesn't solve the problem.
Idle Air Control (IAC) Valve: This valve controls the air entering the engine at idle. A dirty or malfunctioning IAC valve can cause erratic idling and surging. Cleaning it is often the first step, but replacement may be needed.
Engine Coolant Temperature (ECT) Sensor: An inaccurate ECT sensor can cause the ECM to provide incorrect fuel mixtures, leading to surging.
Fuel Pressure Regulator: A failing fuel pressure regulator can cause inconsistent fuel pressure, leading to surging.
Dirty Fuel Injectors: Clogged fuel injectors can prevent consistent fuel delivery.
Transmission Issues (Automatic): In some cases, problems within the automatic transmission can manifest as engine surging, particularly during shifts or acceleration. A thorough transmission check might be needed if other causes are ruled out.
Computer (ECM) Problems: While less likely, a faulty ECM can cause a wide range of problems, including surging. This is usually diagnosed last after other possibilities are eliminated.
Troubleshooting Steps:
1. Start with the simple things: Check all vacuum lines meticulously.
2. Clean the MAF and IAC valves: This is often a low-cost, high-reward step.
3. Check fuel pressure: Make sure fuel pressure is within specification.
4. Check the TPS voltage: Ensure it's within the manufacturer's specifications.
5. Consult a repair manual: This will provide detailed diagnostic procedures and specifications for your specific engine.
6. Use a OBD-II scanner: A scanner will retrieve diagnostic trouble codes (DTCs) that can pinpoint the problem area.
If you're not comfortable working on your car yourself, take it to a qualified mechanic. Trying to diagnose this without the proper tools and knowledge can lead to wasted time and money. Explain the symptoms clearly to the mechanic. The surging is a significant problem and shouldn't be ignored.
