Engine Running Rich vs Lean: What's the Difference?

Knowing if your engine is running rich or lean is key to optimizing its performance.

Running rich vs lean depends on your car’s ratio of fuel-to-air consumption. All combustion-powered gas engines burn a combination of air and fuel. As these enter the combustion chamber, a spark ignites them to produce power. Different ratios of air to fuel can cause your car to run rich or lean. Neither is good for your vehicle’s performance. So, what's the difference between them? And how do you know if your car is running rich or lean? It’s easy to tell once you know the signs.

Engine Running Rich Symptoms

When a car is running rich, it means the engine is receiving too much fuel and too little air. If your car is running rich, you'll likely experience these symptoms:

• Low gas mileage
• A strong smell of gasoline, especially when idling
• Check engine light warning
• Engine performance problems
• Emissions test failure
• Ruined or sooty spark plugs
• Black smoke from your exhaust
• High carbon monoxide content

When left unchecked, a rich-running engine can degrade your car's catalytic converter. It does this by clogging the converter with sediments made from burning excess fuel.

There are many causes of a rich-running engine. Dirty air filters are one of the most common. Filters capture and remove sediments before they enter the engine's combustion chamber. If the filter is dirty, air can’t pass through the filter, thus preventing it from reaching the engine. Here are some other causes of an engine running rich:

Fuel injectors stuck in the open position Faulty MAF sensor Bad O2 Sensor Faulty MAP sensor Faulty engine coolant temperature sensor Bad intake temperature sensor Faulty fuel pressure regulator Faulty injector

Engine Running Lean Symptoms

Running lean is the opposite of running rich. The term “lean” refers to when your car's engine is getting too much air and not enough fuel. When this happens, your car, truck, van, or SUV will experience one or more of these symptoms:

• Trouble starting 
• Weak acceleration
• Clean spark plugs/no residue on the tips
• Check engine light comes on
• Stalling engine
• An overall decrease in the vehicle’s performance
• Clean spark plugs/no residue on the tips
• Check engine light comes on
• Stalling engine
• An overall decrease in the vehicle’s performance

Lean-running engines can be caused by clogged fuel injections. A bad fuel pump or a vacuum leak can also cause this problem. Fixing the issue means doing a variety of repairs that include:

• Replacing the fuel pump
• Replacing the fuel filter
• Replacing the fuel pressure regulator
• Replacing the powertrain control module
• Replacing one or more injectors
• Replacing oxygen sensors
• Replacing the mass air flow sensor
• Repairing a vacuum leak

Running Rich Vs Lean: What to Do About It

It’s best to start by checking your vehicle’s engine control unit (ECU) with a vehicle diagnostic tool. Depending on your car and the sensor you use, you might be able to diagnose what’s causing the problem. To fix it, you might need a spark plug wrench, an oil filter wrench, and a variety of other hand tools to fix the issue. JB Tools carries the biggest selection of these tools at the best prices. Professionals and DIYers alike turn to us for low prices on the best automotive tool brands.