Fuel System:
* Fuel injectors: Worn or dirty injectors can cause a poor fuel spray pattern, leading to rough running, especially when cold. The cold fuel can exacerbate the issue. Injectors often benefit from cleaning or replacement.
* Fuel pressure regulator: A faulty regulator might not maintain the correct fuel pressure, leading to inconsistent fuel delivery, particularly when the engine is cold and demands more fuel for starting.
* Fuel pump: A weak fuel pump might not deliver enough fuel at low RPMs (common when cold), causing the surge and poor running. This is more likely to be a problem as the pump ages.
* Fuel filter: A clogged fuel filter restricts fuel flow, leading to similar symptoms. This is a cheap and easy part to replace.
Ignition System:
* Distributor cap and rotor: Worn or damaged components in the distributor can cause misfires, especially when cold. Cracks in the cap or worn contacts can disrupt the spark.
* Ignition wires: Cracked or damaged ignition wires can lead to poor spark delivery, resulting in rough running.
* Ignition coil: A weak or failing ignition coil struggles to produce a strong spark, especially when cold.
* Temperature sensor (CTS): The coolant temperature sensor (CTS) tells the computer the engine's temperature. A faulty sensor provides inaccurate information, causing the computer to adjust the fuel mixture incorrectly when the engine is cold. This will lead to rough running until it warms up and the sensor reads correctly.
Other Possibilities:
* EGR valve: While less likely to cause *only* a cold-start issue, a sticking EGR valve can contribute to rough running, particularly at lower temperatures.
* Vacuum leaks: Leaks in vacuum hoses can disrupt the proper operation of various systems, affecting fuel delivery and ignition timing.
* Throttle body: A dirty or sticky throttle body can affect the idle and contribute to surging. Cleaning it is often beneficial.
* Mass airflow sensor (MAF sensor): This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can lead to a poor fuel mixture, causing rough running.
Troubleshooting Steps:
1. Check the basics: Start with the simplest checks: fuel filter (replace), visually inspect the distributor cap and rotor, and look for obvious vacuum leaks.
2. Fuel pressure test: A fuel pressure gauge will determine if the fuel system is delivering adequate pressure.
3. Scan the codes: Use an OBD-I scanner (appropriate for a 1990 vehicle) to read any diagnostic trouble codes (DTCs) stored in the computer's memory. This can point directly to a failing component.
4. Check the CTS: This is a relatively inexpensive sensor to replace and is a common cause of cold-start problems.
It's best to tackle these in order of ease and cost-effectiveness. If the problem persists after checking the simpler items, more advanced diagnostics or professional help may be needed. Remember that multiple issues can sometimes exist simultaneously.
