* Transmission Issues: This is a prime suspect. At 45-55 mph, the transmission is likely in a specific gear, and problems in that gear (worn components, low fluid, incorrect fluid, internal valve body issues) could manifest as rough running, hesitation, or loss of power specifically in that speed range. Check your transmission fluid level and condition. A slipping transmission is a common cause of this type of problem.
* Torque Converter Issues: Similar to the transmission, problems with the torque converter can cause issues around specific speeds and loads.
* Fuel Delivery Problems: While less likely to be speed-specific, issues with the fuel pump, fuel filter, or fuel injectors could cause poor performance under load (like at highway speeds). A clogged fuel filter would restrict fuel flow more at higher demands.
* Ignition System Issues: Worn spark plug wires, a failing distributor cap or rotor (if equipped), or weak spark plugs could cause misfires, especially under load. These misfires might be more noticeable at higher speeds and engine loads.
* Exhaust System Restrictions: A severely clogged catalytic converter or muffler would restrict exhaust flow, leading to reduced power, especially at higher speeds.
* Engine Issues: While less likely to be *only* in the 45-55 mph range, problems like a failing oxygen sensor or a vacuum leak could contribute.
Troubleshooting Steps:
1. Check Fluids: Start with the basics. Check your transmission fluid (level and condition), engine oil, coolant, and power steering fluid. Low or dirty fluids can cause various problems.
2. Listen for Unusual Noises: Pay close attention to any unusual noises coming from the engine, transmission, or exhaust system while driving at 45-55 mph.
3. Check Engine Light: Is the check engine light illuminated? If so, get the codes read using an OBD-I scanner (for a 1991 vehicle, this is likely a different system than modern OBD-II). This will give you valuable clues.
4. Professional Inspection: Given the age of the car and the speed-specific nature of the problem, taking it to a qualified mechanic is highly recommended. They can properly diagnose the issue, which may require specialized tools and knowledge.
Don't continue driving the car extensively with this problem, as you could risk further damage to the transmission or engine.
