* Ignition System: This is a very common cause. While driving at 65mph, the engine is under less load and the ignition system is stressed less. However, a failing component might only manifest under higher RPMs or under load (like acceleration). Consider:
* Distributor Cap and Rotor: Cracks, corrosion, or worn contacts can cause misfires at higher speeds.
* Ignition Coil: A weak coil might not provide enough spark consistently at higher RPMs.
* Ignition Module (or Control Module): These control the ignition timing and spark. A failing module could cause erratic timing or missed sparks.
* Worn Spark Plugs and Wires: These are essential for proper combustion. Worn or damaged components will lead to misfires.
* Fuel System: Issues here could also cause sudden problems:
* Fuel Pump: While less likely to cause a sudden problem *only* at speed, a failing fuel pump might not be able to keep up with demand under load, but then perform sufficiently at lower RPM cruising speed. It's less likely than ignition.
* Fuel Filter: A clogged fuel filter restricts fuel flow, causing erratic running, especially under load. Though this would generally affect performance more broadly.
* Fuel Injector(s): A clogged or failing injector can cause a misfire in one cylinder.
* Sensor Problems: Several sensors provide vital information to the engine's computer:
* Mass Airflow Sensor (MAF): Measures the amount of air entering the engine. A faulty MAF sensor provides inaccurate data, leading to a lean or rich fuel mixture and erratic running.
* Throttle Position Sensor (TPS): Tells the computer the position of the throttle. A faulty TPS can cause erratic idle and acceleration problems.
* Crankshaft Position Sensor (CKP): This sensor tells the computer where the crankshaft is in its rotation. A faulty CKP sensor can cause erratic running, misfires, and starting problems.
* Vacuum Leaks: A significant vacuum leak can dramatically affect engine performance. Check all vacuum hoses for cracks or disconnections.
Troubleshooting Steps:
1. Check for obvious things: Start with the simplest things first – inspect spark plug wires for cracks or damage, visually check the distributor cap and rotor for wear, and listen for any unusual noises coming from the engine.
2. Listen for misfires: A misfire often sounds like a popping or sputtering noise from the engine. This helps pinpoint which cylinder(s) are having issues.
3. Check codes: If your S10 has a check engine light, use an OBD-I scanner (appropriate for 1989) to read the trouble codes. These codes will point to the most likely problem area. Keep in mind, OBD-I is much less sophisticated than modern OBD-II.
4. Test the basics: Test the spark plugs and wires with a spark tester. Check fuel pressure using a fuel pressure gauge.
It's difficult to pinpoint the exact cause without more information or a proper diagnostic, but starting with the ignition system and vacuum lines is a good starting point given the situation. If you're not comfortable working on your car yourself, take it to a mechanic experienced with older vehicles for diagnosis and repair. They'll have the proper tools and expertise.
