2018 Jeep Grand Cherokee Fuel Economy Dropping? Here's How to Fix It

The Basics

Poor fuel economy, or getting fewer miles per gallon (MPG) than you should, is one of the most common and frustrating car problems. It hits your wallet directly every time you fill up. While many things can cause it, the core issue is usually that your engine isn't running as efficiently as it was designed to. This means it's burning more fuel than necessary to produce the same amount of power. Based on an analysis of 100 owner discussions, this problem often stems from a mix of mechanical issues, faulty sensors, and even unexpected electrical drains.

Common Indicators

Owners reporting poor fuel economy typically notice a few clear signs. The most obvious is simply needing to fill up the gas tank more often for your regular driving routes. You might see your trip computer's MPG display drop significantly, or you can calculate it manually and find it's well below the EPA estimate for your vehicle. Some drivers report their engine feels a bit sluggish or that it takes more throttle input to maintain speed. In several discussions, the check engine light was also illuminated, often with a specific code like P0128 (Coolant Thermostat).

What's Behind It

Based on confirmed fixes and owner reports from the forums, the causes are varied. A frequent mechanical culprit is a failing thermostat that's stuck open, preventing the engine from reaching its optimal operating temperature. A cold-running engine uses more fuel. Another major category is parasitic battery drain from aftermarket accessories or malfunctioning factory systems. Owners specifically mentioned issues with heated seats, the Uconnect infotainment system, and aftermarket lights like a 20-inch light bar staying on or drawing power when the vehicle is off, which can strain the charging system and impact efficiency. Faulty oxygen sensors and dirty air filters were also commonly cited reasons for the engine computer to miscalculate the fuel mixture.

Identifying the Cause

Pinpointing the exact cause requires a systematic approach. Your first and best step is to check for diagnostic trouble codes (DTCs) with an OBD2 scanner. A code like P0128 points directly to a cooling system issue. If there's no check engine light, you need to start observing and testing. Listen for unusual sounds from accessories and check if things like the radio power button or interior lights behave oddly after you shut off the car, which can hint at an electrical drain. A simple visual inspection of the air filter can tell you if it's clogged. For electrical drains, a multimeter test at the battery can measure if there's excessive draw when everything is supposed to be off. Tracking your driving habits is also crucial; aggressive acceleration and heavy loads will naturally lower MPG.

What Actually Works

Here are the solutions ranked by frequency of success and cost, starting with the simplest and most affordable.

1. Replace a Stuck-Open Thermostat: This was a top fix, especially when code P0128 was present. A thermostat that won't close keeps the engine too cool, causing it to run rich. Replacing it is often a low-to-moderate cost fix that restores proper operating temperature and fuel economy.
2. Address Electrical Parasitic Drains: In numerous discussions, fixing an electrical gremlin solved the problem. This involves tracking down what's drawing power. Common culprits were faulty heated seat modules, the Uconnect system not fully powering down, or poorly installed aftermarket accessories like light bars. Disconnecting or repairing these components stopped the drain.
3. Replace Faulty Oxygen Sensors: The upstream (pre-catalytic converter) O2 sensors are critical for the engine computer to adjust the air-fuel mixture. Worn-out sensors send bad data, leading to poor efficiency.
4. Perform Basic Maintenance: This is always the best starting point. Replacing a dirty air filter, ensuring tires are properly inflated, and using a fuel system cleaner can provide modest but immediate improvements with little investment.
5. Review Driving Habits & Aftermarket Modifications: If no mechanical fault is found, assess how you drive and what you've added to the vehicle. Aggressive driving, roof racks, large off-road tires, and heavy light bar assemblies all increase aerodynamic drag and rolling resistance, which tanks MPG.

Recommendation: Always start with the cheap and easy checks: tire pressure, air filter, and driving habits. Then, scan for codes. If you find P0128, the thermostat is your likely target. If not, but you suspect an electrical issue, investigating parasitic drain is the next logical step.

Repair Costs

Costs can vary widely based on the cause and whether you DIY.

• Thermostat Replacement: The part is inexpensive ($20-$80). DIY labor is moderate, often requiring draining and refilling coolant. A shop will typically charge $150-$400 total.
• Electrical Drain Diagnosis & Repair: This is highly variable. Diagnosis alone at a shop can run $100-$200. Fixing a faulty heated seat switch or Uconnect module could mean a new control assembly ($100-$300 for the part) plus 1-2 hours of labor. Fixing a DIY light bar wiring job might just cost your time.
• Oxygen Sensor Replacement: A single upstream O2 sensor costs $60-$200. DIY replacement is usually straightforward. Shop cost is typically $200-$400 per sensor.
• Basic Maintenance: An air filter costs $15-$40. A bottle of fuel injector cleaner is under $20. These are prime DIY tasks.

Preventive Measures

You can't prevent every sensor failure, but you can minimize your risk of poor MPG. Stick to your vehicle's scheduled maintenance, especially air filter changes. Be very cautious with aftermarket electrical installations; have them done professionally or follow wiring diagrams meticulously to avoid parasitic drains. Use a scan tool occasionally to check for pending codes before the check engine light comes on. Most importantly, adopt smooth driving habits—gentle acceleration and coasting to stops are far more efficient than aggressive driving.

Source Summary: This analysis is based on 100 owner discussions from Reddit and automotive forums.