7 Common Engine Running Rich Symptoms

A vehicle’s fuel mixture refers to the ratio of fuel and air in the combustion process. When you have too much fuel and not enough air, your car is considered to be running “rich”. When you have too much air and not enough fuel, your vehicle is considered to be running “lean”.

The engine computer of the vehicle controls the fuel mixture through the use of oxygen sensors, fuel injectors, emissions sensors, and air flow sensors. If your car has a rich fuel mixture, there are certain signs you’ll notice:

Top 7 Symptoms of an Engine Running Rich

#1 – Check Engine Light On

If you use an auto diagnostic tool to scan the trouble code from your check engine light and it shows P0172, this means the exhaust gases have an abundance of gasoline in them as they are coming out of the combustion chamber.

There are a lot of instruments used in the engine control unit (ECU) including oxygen sensors, the manifold absolute pressure, and the mass air flow sensor. The manifold absolute pressure is used to watch the engine’s air-fuel ratio. These sensors tell the vehicle’s ECU there is a problem which then causes it to turn on the check engine light.

#2 – Smell

One of the first things you may notice is a strong smell of fuel or rotten egg smell from the exhaust. This is due to excess fuel not being burned properly in the combustion process and escaping into the exhaust manifold and ultimately out of your tailpipe. The job of the catalytic converter is to burn off the extra fumes but when the fuel mixture is way too rich, even the cat can’t burn the excess off.

#3 – Bad Fuel Efficiency

bad fuel economy

If you notice that you’re not getting as much gas mileage out of your tank as you used to get, it could be a sign of your engine running rich. You’re essentially burning more fuel than you actually need for the car to properly operate. Note that in cold temperatures, a car will generally run a bit richer than usual so if your gas mileage is slightly worse in winter than summer, this is actually normal.

#4 – Poor Engine Performance

Power in a gasoline engine comes from a combination of fuel, air, compression, and then a spark. Without these four requirements, you won’t have power being generated from your engine. So, weak engine performance would mean that a problem must exist in one of these areas.

You may have compression that is normal and a spark that is normal but still have low engine power. This could only mean that the mixture of fuel and air either has too much fuel or too much air.

#5 – High Carbon Monoxide Emissions

high carbon monoxide emissions

While an exhaust naturally expels a certain level of carbon monoxide, if your engine is running too rich, this level will be higher than usual. This is often the case why a vehicle will fail to pass a state emissions test.

In addition, too much carbon monoxide being expelled needs to be taken seriously because it can put your health at risk, especially if it starts to seep into the car while driving. If you were to just breathe these fumes in for a few minutes, it could cause serious neurological damage.

#6 – Rough Engine Idle

A fuel mixture that is rich could be the cause of rough idling. This means the vehicle will actually vibrate and feel a bit rough as the engine is running. While the car is not moving (idle), you may notice that the RPMs on the tachometer behave erratically, slightly jumping around. Sometimes the vibration in the engine can be felt while driving.

#7 – Clogged/Fouled Up Parts

fouled up spark plug

Two big signs of your engine running rich have to do with the condition of your spark plugs or catalytic converter. When running rich, the bottom of your spark plugs can get fouled up with a dry, black soot. This is known as carbon deposit and will affect the performance of your engine.

As noted earlier, the catalytic converter’s job is to burn excess fuel before it reaches your tailpipe. If it continually has to burn the excess fuel, it will eventually cause clogging up of the catalytic converter and degradation of the honeycomb design within it. Once this happens, a costly catalytic converter replacement will be necessary.



  1. I do not have any check engine lights. The only symptom I’m having is smell. Thank I’m going to pull the plug and check that first. Also make sure my air cleaner filter is good. Was wondering if I could run a hotter plug?

    • If you know what you’re doing, yes running a hotter spark plug could help. I can’t recommend this for everyone since it’s possible to cause major engine damage if the wrong plugs are put in.

    • Only symptom I have is smell but that is only at 3/4-wide open throttle, otherwise there are no other symptoms of running rich. Maybe my car just runs too rich at higher RPMs? Maybe I need a dyno to tell me

  2. I have no check engine light, OBD scanner shows rich condition (O2 sensor value is around 0.9V for Toyota Passo during idling). What to do? how to induce lean condition to check O2 sensor

  3. I have custom catless downpipes for my 5.5 litre biturbo and I haven’t tuned it yet. I’ve come across almost all the symptoms mentioned above and this is how I’ve been driving for last 40k kms. How do I avoid these symptoms. Should I tune it? Or make it back to stock?
    Thank you

  4. Only symptom I have is smell but that is only at 3/4-wide open throttle, otherwise there are no other symptoms of running rich. Maybe my car just runs too rich at higher RPMs? Maybe I need a dyno to tell me

  5. I have explorer 2006 spending lots of fuel , so i had code p0171 and 0174 , then i found a leak from EGR gasket when I fixed that I don’t have engine light any more but my LTFT for bank 1and bank 2 fluctuating between +10 in ideal and – 10 in 2500 rpm , can any one support me with that please and tell me what is the issue . Thanks

  6. I feel there is air leakage in intake manifold downstream of MAF sensor, check;
    1- check connection and damage in vacuum hose pipes (e.g. downstream of Evap.
    purge valve, hose to break booster etc.)
    2- Check engine block head gaskets
    3- Check faulty MAF & MAP sensors

  7. i have tata indigo diesel cr4 ,my car rpm fluctuating at idle when clutch is pressed its upto 850 to 1100 , its also when engage in gears sometime. i notice black smoke also above 2000 rpm and tailpipe shows black soot inside. there is no any warning light on dash.
    car tata indigo ecs vx cr4 diesel 2011 model and running is 60000km
    what should be the cause?????

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