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”. On the other end of it, when you have too much air and not enough fuel, your vehicle is considered to be running “lean”.

This article will show you the most common signs that your car is running rich so you can get the problem resolved and stop wasting fuel (because gas prices are high enough as it is).

engine running rich symptoms

Top 7 Symptoms of an Engine Running Rich

#1 – Check Engine Light On

check engine light

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.

See Also: 6 Reasons Your Oil Smells Like Gas

#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

reasons and causes of car not accelerating

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

rough idling

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. A car backfire is often a result.

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. 

Mark Stevens


  1. My 2003 chevy blazer has had a small leak in gas line since I bought it a year ago and check engine light was on upon purchase but now it’s idling and check engine light is blinking and I ran a diagnostic at auto zone and it said cylinder 2 is misfiring and system is running rich. What could this be do you think? You think it’s simply fixable by replacing the gas line that’s been leaking? Is too much air getting to the engine and not enough fuel? It’s hard to start too. Have to turn over at least 3x to get it to start up.

    1. I would certainly start by replacing the leaking gas line. If the system is running rich, it’s too much fuel not too much air.

  2. Hi.

    I have a 2008 C280 Mercedes.

    Recently i had the check engine light come on, and it was diagnosed as a faulty thermostat stuck open. They replaced this and all was well, but I still get fault code P0172 and P0175 – Running to rich on bank 1 and bank 2. I reset the EML and every two days the same codes appear.

    I have just today changed the air filters, and the sparkplugs to see if that makes a difference, and reset the EML.

    Are there any other things this could be? I dont get any smell of fuel and the car runs like a dream, maybe ever so slight rough idle occaiionally when at traffic lights or stationary, but not all the time.

    Its driving me nuts as to what it could be, i was thinking nect inline to check / replace would be the O2 sensors and MAF???

    Any help would be much appreciated!


    1. Checking the O2 sensors is a good idea. You could have a leaky injector or other component in the fuel system that is causing too much fuel to be injected. Have you noticed any change in your fuel economy since those codes showed up?

  3. 2000 Ford F150 failed smog from HC at 81 at 15mph from max 61 and at 62 HC at 25mph from max 45.
    Car runs smooth. I have access to a bidirectional scanner what can I check as far s live data?
    O2 values indicting bad O2 sensors? Fuel trims?
    What other things should I look for to pass the smog?
    Thanks in advance.

    1. Yeah, check fuel trims to see if it’s running rich. I would bust out the factory service manual and look at any sensors that are on the high side of normal (particularly the O2 sensors). In other words, look for reading that are not quite abnormal enough to trigger a check engine light but still a little suspicious, if that makes sense. Good luck on your search.

  4. Hi I’ve been having a lot of probs with my fiesta 2012 with the engine light coming on every time it’s been a dif code so got the work done now I’m without a car again light has come on & stays on my garage have said it’s running rich on the plug in & they they are waiting for ford to send a code for them to reset my ecu but do ford do that


    1. I’m not sure. Sounds like more diagnostics may be needed to determine why it’s running rich, unless a bad ECU is a common problem on these Fiestas and Ford knows that’s the fix for this issue.

  5. I have a 04 trailblazer and cut the cat and straight piped it and left the 02 sensor. First I was having a po420 code now im having a po171 code so can removing the cat make the 02 sensors think its running to rich

  6. What about carburator jets. In my case a rebuilt 350 chevy that the owner bought too much carb. Holley 650. A 550 would suit it better. I think I need to reject it smaller?

  7. I have check engine But i forgot the code. I just remember what the mechanic said, it was oxygen sensor, recently I replace my stock air filter with an k&n cone air filter, i deleted my catalytic and replaced with an stainless pipe, my resonator was deleted and replaced by scuv/scav as a resonator and a muffler. But there are no symptoms of a bad oxygen sensor, maf/map sensor. The accelaration is good and so smooth. What could be the problem why my check engine light is on?

    1. Was the oxygen sensor deleted along with the catalytic converter? The check engine light is probably caused by the deleted catalytic converter.

  8. I installed a HKS BOV in my 2008 2.0l (ej20) subaru forester and after some 30km drive it developed some problems like emit more smoke from the exhaust sometimes the engine will switch itself off and not start, what would be the problem and what should I do for the engine to accommodate the BOV?

    1. Has the engine been tuned for the BOV? Subarus are very sensitive to engine calibrations and require a tune if you change practically any part of the engine.

      Generally BOVs are avoided on Subarus because most Subaru engines use MAF sensors (measuring mass air flow) instead of MAP sensors (measuring speed density). BOVs vent air to the atmosphere that has already been measured by the MAF sensor. Your car expects that air to be there, so your car will run really rich after the BOV opens.

      Consider uninstalling the BOV and returning the factory BPV and see if the problem persists. Check out this helpful NASIOC page for more info on BOVs. https://forums.nasioc.com/forums/showthread.php?t=468038

  9. Am having troubles with my 2002 dodge cara van and I was told there too much fuel that going into the engine I got high tech checking the wiring what does that got to do with?

    1. If the technician thinks the rich fuel mixture is caused by a faulty sensor, he may be testing sensors to see if their signals are in spec, checking for voltage drop between two points, or looking to see if there is a broken wire somewhere.

      If the wrong signal is sent to the injector or the ECU, it could cause a rich air fuel mixture.

  10. i have multiple issues i recently purchased a 2004 jeep liberty 4wd 3.7 it had a terrible oil leek changed the valve cover gaskets and cured that issue i have then moved on to change the spark plugs and noticed i am lean on passenger side and rich on the driver side, what would you recommend i do next? i already have coil packs on order i was told to start there. Or should i just give up and look for another vehicle?

    1. Do you think you’re running lean on one side by the condition of the spark plugs or data from an OBD2 scanner? If the vehicle hasn’t been scanned yet, I would do that. A good scan tool may be able to give you more info, for example if you’re only running lean on one cylinder bank and the other bank is fine.

      If you’re running lean on one bank, this likely rules out the spark plugs as the root cause. I suspect the ignition coils are also not at fault because your Jeep has a coil on plug setup, meaning each cylinder has its own ignition coil.

      If you haven’t used the coils yet, do some more tests before replacing them to make sure it isn’t going to be a waste of money. Throwing parts at the problem gets expensive very quickly.

      Lean running conditions are often the result of leaks. This leak could be from a vacuum hose or the intake manifold gasket, for instance. You can test for leaks by performing a smoke test.

      If you are running rich on the other bank it may be a result of the ECU trying to compensate for the lean condition on the passenger side. This is why I would start by figuring out why it’s running lean on that side first.

  11. Thanks for the article….my forester SG5 turbo shows black soot at the exhaust pipe, no check engine light. Changed the Plugs recently. What could be the issue?

    1. Is this one of Subaru’s direct injected motors? Direct injection tends to produce a lot more soot out the exhaust than port injected motors, and this is not cause for concern.

      It’s possible you’re running a bit rich, or maybe you have some carbon build up. Many vehicles (especially turbocharged ones) run extra rich for safety at full throttle. If there are no other symptoms, this is likely not cause for concern.

  12. P2097 is the engine code currently showing for a vehicle I’ve been working on. It means there’s a rich fuel trim error in the air/fuel system.

    It’s primarily caused by exhaust leaks, bad or failing oxygen sensors, clogged catalytic converters, bad or failing fuel injectors, fuel regulator problems, dirty MAF or MAP sensors and even restricted air flow due to a dirty air filter.

    All of these issues should be considered and check thoroughly to remedy the code. The electrical connections should be inspected as well; for dirty, broken, burned or frayed wires.

    If you suspect the O2 sensor is the problem, carefully check the voltage readings at the suspected bad or failing O2 sensor. One or more of these problems can contribute to the referred code.

    The code P0172 is a direct code this article is about but I thought I would address a similar problem concerning rich fuel symptoms with a different code.

    1. I have my engine light coming on and off and I had it checked and it came up with the code P2198-P2196.
      Can you help me with this and tell me what may be the problem? The thing is it come on for a few days and then goes off for a few days. My son was going to put a new sensor in thinking that it may not be working. I have been told that it could be the flow of stored fuel vapors from the charcoal canister to the intake manifold.
      thank you

      1. The O2 sensor is measuring a rich air/fuel ratio. Possible causes are a bad O2 sensor, bad or dirty MAF sensor, bad charcoal canister, a leaking fuel injector, exhaust leak, or vacuum leak. If this occurred shortly after filling up the gas tank, fuel may have actually entered into the charcoal canister preventing fuel vapors from being purged.

  13. check engine does not show up, I diagnosed but there are no code but still the plugs coated with black soot, what should I do? my car is Mazda dy3w

    1. Is there any oil in the spark plug well? If yes, it could be leaking valve guide seals or piston rings. If no, could be a faulty/leaky injector but all of them at the same time would be unlikely. Also, make sure you’re using the correct spark plugs with the correct gaps according to manufacturer’s specs.

  14. I have a 1997 Z3 BMW I experience rough idle and when I excellerate there is jerking and trouble getting up to speed. First mechanic put in a catalytic converter. BMW mechanic changed air filter and fuel filter. Still have the problem $2700 later. Help!

    1. They replaced the catalytic converter? Your symptoms don’t sound related to that part. I’d look at the MAF sensor or see if you have a vacuum leak somewhere.

  15. 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?????

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

    1. Pull a vacuum hose, like the brake booster line & throttle it with your thumb to keep the engine running… it will run lean.

  21. 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?

    1. 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.

    2. 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

  22. I dont have a catalac converter,but has the systoms that was mentioned above..So do you think its still the O2 sensor?

    1. If your check engine light is on, you should get it scanned for any trouble codes. It’s likely P0172 shows up. In that case, this page has more info about causes: http://cartreatments.com/p0172/
      Yes, the O2 sensor could be the cause since it’s located before the cat location.

      1. How could it have a “check engine light” but no catalytic converter? This is obviously an Old pre-80’s car.

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