Anonymous asked in Cars & TransportationMaintenance & Repairs · 1 month ago

Unburnt fuel smell from exhaust?

When car is idling, I can smell the unburnt fuel smell from exhaust that irritates eyes. I changed plugs last week but still no change. Acceleration is great, no misfire or any hesitation. Codes stored are P0401 (Egr restricted flow) and P0135 (O2 sensor heater circuit malfunction). I cleaned maf sensor and tb and still no effect. I thought of replacing downstream o2 sensors but surprisingly there were no downstream o2 sensors. There are plugs on both sides for sensor but they are zip-tied and there is no hole/place in pipe for sensor. I'm not the first owner so I'm not sure if the car came this way from factory. I'm getting 450-500 km on full tank inside city. My car is Mercury Grand Marquis 2005.

6 Answers

  • M.
    Lv 7
    1 month ago

    "There are plugs on both sides for sensor but they are zip-tied and there is no hole/place in pipe for sensor." 

    It sounds to me like the exhaust system was replaced by somebody who didn't understand that the O₂ sensors are a necessary part of the vehicle fuel delivery system. 

    So you are missing that part of the feedback system, and the car would likely run on the rich side of the ideal mixture.  If it ran on the lean side of the ideal mixture, the engine exhaust valves would burn and fail.  It's fail-safe programming. 

    If a car didn't need wiring for O₂ sensors, there would not be zip-tied wiring.  It would be non-existent. 

    Talk to a dealership or competent mechanic or exhaust repair facility about getting some O₂ sensors installed. 

    When I bought my old Ford Ranger, there was a downstream O₂ sensor after the converter but no wiring harness.  I researched the electrical wiring in Alldata and found what needed to be added.  My missing wiring had been clipped at the engine harness several feet away.  I got the needed wiring and connector from a scrapyard car and soldered it in.  Then the Check Engine Light shut off and it ran properly. 

    Rich exhaust will burn your eyes.  It's also wasted fuel. 

    Your codes: 

    Because the O₂ sensor is missing, the computer thinks the O₂ sensor heater is burned out. 

    Because you don't have TWO O₂ sensor signals that show DIFFERENT inlet and outlet oxygen levels, the computer thinks that your catalyst is no longer working.  The inlet and outlet readings should be unequal. 

    So, as I said before, get the two O₂ sensors fitted into the exhaust system, and if your catalytic converter in fact operates, all your codes and rich fuel condition will be repaired.  If the catalytic converter is no good, get a proper one that will work with the computerized system on your car, not a $39 Chinese clone.

    -Engine overhaul mechanic and general automotive mechanic since 1972

  • 1 month ago

    OKAY THEN>FIRST of all you have been using BAD engine oil in there! NEVER use VALVOLINE< an oil NO FACTORY on earth will use cuz it does EXACTLY what you descried here! IT makes ACIDIC VAPORS and FUMES that get into SENSORS over time and ROT PCV hoses and gasket and seals from the INSIDE OUT! IF you have NO O2 sensor you have a REAL PROBLEM as the AIR FUEL mixture cannot adjust properly! You can ADD them by WELDING! These cars are EXCELLENT in every way!>> get a can of AMSOIL POWER FOAM and use it top clean the THROTTLE BODY and the IDLE AI CONTROL valve to SHOWROOM CLEAN! The FINAL AIR"FUEL mixture is adjusted HERE by the computer based on O2 signals! and TEMP signals and other sensors as well! Remove ether PLUGS and replace with DENSO TIWN TIP IRIDIUM LASER plugs There is NOTHIGN better on earth! USE PENNZOIL ULTRA PLATINUM OIL! and get a warranty our to 500,000 miles! This oil is made from NATRUAL GAS and NOT crude oil! It CLEANS as you drive and e]will remove JUNK baked into your OIL PAN< CAM covers, FRONT cover OIL pump and LIFTERS! A good WELD SHOP can install the BASES for there O2 sensor to attach.  GOOD LUCK! Most engine issues BEGIN with BAD OIL CHOICES! WAX BASED Oil and ASH based OILS are JUST not good enough! VALVOLINE makes Sludge and always VARNISH deposits and even COKE when it hardens! SYNTHETIC OIL make almost ZERO fumes or vapors and keep EVERYTHIGN CLEAN inside and PROTECT a converter, if you HAVE one! GOOD LUCK! be for now. remove AND clean THE egr VALVE WITH amsoil power foam AS WELL! i SSUALLY DISCONNECT THEM ANYWAYS! they SUCK old burnt gas BACK INTO THE intake SYSTEM! this may IMPROVER nitrogen CONTENT, BUT dens0 twin tip iridium WIL DO THE same thing WITH no HOT EXXHAUST TO reburn< ADN KEEP THE ENGIEN cooler AND THE plugs CLEANER! i AM IN Detroit! you CAN text me IF YOU ARE anywhere NEAR HERE! I HAVE CONNECTION WITH ENGINEERS AT ford AS WELL

  • 1 month ago

    Remove the egr and clean out the carbon restricting it. Put it back on with a new gasket.

  • 1 month ago

    For cats to work they actually need some unburnt fuel in the exhaust. Reacting with that is what allows the catalyst to get hot enough to burn off all unburnt hydrocarbons, nitrous oxides and carbon monoxide.

    The oxygen sensors (a.k.a. lambda probes) are what tells the engine management just the right amount of excess fuel is needed to get the catalysts working without adding so much that the cats overheat and destroy themselves internally.

    So if someone has deliberately fitted boxes lacking holes for the oxygen sensors and has taped up the cables and connector plugs, AND you have any smell of unburnt fuel then it’s a fair guess that the last owner carried out a “cat delete” which is illegal almost everywhere.

    Your O2 sensor heater circuit malfunction is correct. The malfunction is caused by no sensors. I can’t suggest a reason for the EGR restricted flow message, but fix the obvious fault first and then sort out what remains. 

    It’s not going to be a cheap fix as you straightaway are looking at one cat per side and the oxygen sensors. You may be able to fit salvaged parts at lower cost but not if you live in California where they must be new parts. Be aware that cats are quite fragile and so it is not unusual to find that secondhand ones have been severely damaged internally by rough handling: so only buy used if you can absolutely trust the supplier and the delivery courier.

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  • Barry
    Lv 6
    1 month ago

    Sounds like a replacement exhaust pipe with no threaded holes for O2 sensors. A dealership parts dept should be able to tell you what should be there.

  • 1 month ago

    Maybe your cats (catalytic converters) are not working anymore. The may be plugged.

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