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  1. #1
    Enthusiast Vipervin's Avatar
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    Running Rich with HFCs

    I have high flow metallic cats on my 2003 and I've noticed a rich, fuel smell after driving it fairly hard. The car also throws a check engine light every so often for the cats. I tried clearing with a OBD2 device, but it returned a few weeks later.

    Is a tune the best solution for avoiding the check engine light and getting the most performance out of the cats? What's the most popular canned tune and flash device for these in the most recent years?

    Thanks

  2. #2
    You still have all 4 O2 sensors hooked up?
    You can teach a dog to fetch, therefore you teach a potato to dance! “Amazingly Bad Analogy”

  3. #3
    Enthusiast Old School's Avatar
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    Raw fuel and cats don't work well together. If you are smelling raw fuel in the exhaust, that will destroy your cats. You need to correct that first before going any further.
    Last edited by Old School; 06-18-2021 at 05:34 PM.

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    Supporting Vendor JonB ~ PartsRack's Avatar
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    CATS dont cause a car to run rich, but running rich can kill your cats
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  5. #5
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    Quote Originally Posted by JonB ~ PartsRack View Post
    CATS dont cause a car to run rich, but running rich WILL kill your cats
    Fixed it for you.

  6. #6
    Enthusiast Vipervin's Avatar
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    Still have all 4 sensors plugged in. Should I unplug the post-cat pair?

    I don't always smell the gas smell, but I notice it sometimes. I replaced all plugs and wires recently, but I'll review for any other issues

  7. #7
    I was just asking, you do need all 4.

    Quote Originally Posted by Vipervin View Post
    Still have all 4 sensors plugged in. Should I unplug the post-cat pair?

    I don't always smell the gas smell, but I notice it sometimes. I replaced all plugs and wires recently, but I'll review for any other issues
    You can teach a dog to fetch, therefore you teach a potato to dance! “Amazingly Bad Analogy”

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    do you have ability to read the front O2's? are they loose? pcm needing a reset?
    I don't think the Rears affect fuel trims
    Last edited by uvbnbit; 06-21-2021 at 05:37 AM.
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    Several things here. I think your tune is off. If you're getting a gas smell, you're not burning the fuel correctly. I am not sure if Prefix does a race controller for the gen 3. That's the only way you're going to get rid of the cel light. The handheld units cannot legally shut off the rear 02's.
    If still the original 02 sensors, change them out with factory units only. Bosch units suck. That swap alone might cure your problem.
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  10. #10
    OK, stock engine with HFC, should not need a tune, you will get a CEL. Check the Air temp sensor, and the coolant temp sensor. If one is sending a cooler temp to the ECU, you get too much fuel. After that it is time to flow test the injectors, one may be sticking open/leaking. Last is the ignition system, you could be loosing a single cylinder and the unburned fuel is what you smell. This should give you some things to look at. Good Luck!
    You can teach a dog to fetch, therefore you teach a potato to dance! “Amazingly Bad Analogy”

  11. #11
    Running high flow cats can definitely give you a raw fuel smell , but it's going to be much better then without cats like I'm running.

    My vette has high flow cats and it smells like fuel sometimes as well

    And Sorry to thread hijack , when I bought my 05 it already had a check engine light because they deleted the cats and rear 02 sensors ( because of the header install )

    Am I safe running eith a cat delete ? On my vettes its perfectly fine , not sure about vipers

    Am I better off getting cats and 02 sensors installed or just having them tuned out to get rid of the CEL?

    Thanks alot guys

  12. #12
    Enthusiast Old School's Avatar
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    I don't know how anything about the cats (high-flow, stock or none) could cause a raw fuel smell, they are only there to reduce emissions and stay legal. If raw fuel is reaching the cats, it will destroy the cats.

  13. #13
    yes they reduce emissions by burning excess fuel in the exhaust

    on any car if u remove the cats u will get a much harsher fuel smell, whether its a stock toyota camry or a high performance sports cars, no cats = much stronger exhaust smell.

    Im running catless on my viper and i get a strong fuel smell when stopped at a light or idling , i dont think my car has anything wrong or is running rich, its just a lack of cats.

    If the smell bothers you i would just go back to stock cats and see if that solves the problem, if not then you know the car is in fact running rich
    Last edited by 1Willy1; 06-21-2021 at 10:53 PM.

  14. #14
    Quote Originally Posted by SNKEBIT View Post
    ...If still the original 02 sensors, change them out with factory units only. Bosch units suck. That swap alone might cure your problem.
    The NTK units suck too ..ask me how I know. Discovered this fact the long-hard-$$ way. The cost of education I guess.
    These little after market bastages work in a very insidious way ..they connect to the ECU just enough to prevent a DTC/CEL.
    But they don't actually communicate properly ..so the required data sample set is never collected by the ECU and the O2 Monitor never gets set to the Ready state.
    Its criminal that they're advertised as "compatible" ..when they are definitely NOT.

    Only MOPAR O2 Sensors will work on the Viper.
    Last edited by viperBase1; 06-22-2021 at 07:24 AM.
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  15. #15
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    Quote Originally Posted by viperBase1 View Post
    The NTK units suck too ..ask me how I know. Discovered this fact the long-hard-$$ way. The cost of education I guess.
    These little after market bastages work in a very insidious way ..they connect to the ECU just enough to prevent a DTC/CEL.
    But they don't actually communicate properly ..so the required data sample set is never collected by the ECU and the O2 Monitor never gets set to the Ready state.
    Its criminal that they're advertised as "compatible" ..when they are definitely NOT.

    Only MOPAR O2 Sensors will work on the Viper.
    I thought NTK was the maker for the factory ones???
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  16. #16
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    Agreed the Bosch units suck. I made the switch to the wideband Holley/NTK 534-194 more than a decade ago.
    Solved my problem of constantly having to replace the Bosch sensors on my catless Roe charged RT.
    These have since been discontinued but replaced by Holley/NTK 554-100.
    https://www.summitracing.com/parts/hly-554-100
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  17. #17
    is it possible to just get the sensors tuned out ?

    would that affect performance in any way ?

  18. #18
    Supporting Vendor JonB ~ PartsRack's Avatar
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    Quote Originally Posted by 1Willy1 View Post
    ...Running high flow cats can definitely give you a raw fuel smell , but it's going to be much better then without cats like I'm running.

    How? Raw, unburned fuel makes the raw-fuel smell. Not the cat!!

    Why is it not burned? Bad Plugs, Wires, Coils, injectors, etc.

    But a CAT can only smell like raw fuel IF RAW FUEL GOT TO THE CAT from some incomplete combustion issue.
    Dont blame the cats, unless they are burned out and non-functioning caused by RICH-RAW FUEL
    Jon "JonB" Brobst ~~~~:<~ JonB@PartsRack.com


  19. #19
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    Quote Originally Posted by 1Willy1 View Post
    is it possible to just get the sensors tuned out ?

    would that affect performance in any way ?
    There are two questions here:

    1. Can you tune the O2 sensors out? The answer to that is yes, but please see the next question below.
    2. Can you tune the sensors out on a Gen III Viper? I don't know the answer to that. Gen III aftermarket tuning solutions are very limited, so the tools might not exist to do it with the factory PCM.

    Running a car open loop can certainly be done...it used to be done all the time. Would it affect performance? Depends on your definition of performance.

    If done properly, the car would run just fine, so from that perspective, the performance wouldn't be affected. WOT operation is already open loop in these cars. But (and that's a big BUT), it's going to depend on the PCM's ability to estimate airflow properly. You'd want to be absolutely sure that the PCM could handle wide swings in the environmental conditions (temperature, barometric pressure, etc.) to make sure your tune is robust...can it handle you driving your car to the top of Pike's Peak where the air is thin? Does it matter if you never intend to do that? Car manufacturers have to care about it, because they don't know how the vehicle is going to be used once it leaves the factory. But you know how you will and won't use the car, so you at least have that going for you.

    Where it would most likely change would be from an emissions and fuel economy standpoint. The O2 sensors are used to adjust fuel trims, which makes sure you have as little gas left over after the combustion process as possible. If you take away the PCM's ability to adjust the trims, then you are at the mercy of whatever lookup table(s) are being referenced to figure out how much fuel gets injected based on the estimated airflow coming into the engine. If the calibration is spot on, you'll be burning close to stoich at idle and part throttle, so it won't be a problem. But we don't live in a perfect world, and no calibration is ever going to be able to account for all of the possible environmental conditions you'll encounter, which is why the trims exist in the first place. It's a checks and balances sort of thing; by removing the O2 sensors from the equation, you are taking away a big check that helps balance the imperfect nature of engine calibration.

    I've personally owned and tuned a previous vehicle to run open loop speed density (MAP vs. RPM, no MAF). It ran well enough, but I noticed pretty wide swings in the cruising AFR depending on the weather conditions - even summer vs. winter fuel blends made a big difference. The reason I did it was because I didn't really fully understand how to compensate for moving the pre-cat O2 sensors further downstream than the factory locations due to running long tube headers. The sensors ran cooler, which affected their switching, which led to codes. I got tired of the codes, so I just turned everything off and said screw it. It wasn't the right answer, and I regretted it up until the day I sold it when it became someone else's problem. I decided I would never get to that point with this car, and I've held to it, which has left me much happier in the end.

    Some of this is also going to depend on the mods you intend to do...if you want to run a big honkin cam with lots of overlap at idle that'll make little kids cry when you drive through the parking lot of your next local cars and coffee, the narrow band O2 sensors are pretty much useless. If I recall correctly, the Gen 4 and 5 folks running the Prefix 9L Extreme packages with their calibration run open loop all the time just for that reason.
    Last edited by Steve M; 06-22-2021 at 02:39 PM.
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  20. #20
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    Quote Originally Posted by JonB ~ PartsRack View Post
    How? Raw, unburned fuel makes the raw-fuel smell. Not the cat!!

    Why is it not burned? Bad Plugs, Wires, Coils, injectors, etc.

    But a CAT can only smell like raw fuel IF RAW FUEL GOT TO THE CAT from some incomplete combustion issue.
    Dont blame the cats, unless they are burned out and non-functioning caused by RICH-RAW FUEL
    This is also true based on my understanding of how everything works, with the exception of a cold start when the PCM is dumping extra fuel in to get the cats to light off. The raw fuel smell should be expected under those circumstances, but should only last a few minutes.
    2008 SRT-10 Coupe - Venom Red Metallic w/ White Stripes - 1/4 Mile Passes (YouTube)
    High Flow Cats | Gen 5 Intake Man. | UDP | HPT | 3.55s | MCS | DSS Axles | Nth Moto Clutch | 11.027 @ 130.06 (Vid), 1.78 60' (+1,622 ft DA, w/ 3.55s & Hoosier Drag Radials)

  21. #21
    Quote Originally Posted by SNKEBIT View Post
    I thought NTK was the maker for the factory ones???
    That's what I thought too ..read it here on this forum somewhere. But I've also seen posts saying MOPAR ONLY.

    Bought the NTK's off of RockAuto that were clearly being sold as compatible for the GenIV Viper.
    After almost 2 years learned they never freakin' worked.

    Thanks to the Torque app. on my phone and a bluetooth OBDII scanner, recently captured the traces clearly demonstrating they were not functioning correctly ..if at all. Swapped in the genuine MOPAR units and shazaam ..all four sensors have comms with the ECU transmitting data and getting perfect traces that make sense.
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  22. #22
    Enthusiast Vipervin's Avatar
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    Thanks for the input and troubleshooting recommendations. I originally had a clogged cat when I bought the car. I found that oil was dripping into one of the spark plug receptacles and the boot was arcing to ground. This was probably causing a misfire in that cylinder and the clogged cat was also on that side. I corrected the oil drip issue, swapped all plugs/wires, and the car ran better. I put on high flow cats recently and the car runs great and now has the performance I would expect.

    The rich smell was pretty mild, but I also want to ensure that there's not another issue that could clog these, so I'll do more troubleshooting. I'm also thinking about just swapping the pre-cat O2 sensors to be safe. I have a bluetooth scanner and plan to use it with the OBD Fusion app to look for anything strange, also.

  23. #23
    Supporting Vendor JonB ~ PartsRack's Avatar
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    Quote Originally Posted by Steve M View Post
    This is also true based on my understanding of how everything works, with the exception of a cold start when the PCM is dumping extra fuel in to get the cats to light off. The raw fuel smell should be expected under those circumstances, but should only last a few minutes.
    Agreed. Normal cat function.

    [Remember when carburetors and NO cats ruled the day? Before that manual chokes ruled, and "CHOKE" was a good name for them.... Fuel Stink galore, and clouds of mosquito-fogging out the back.]
    Jon "JonB" Brobst ~~~~:<~ JonB@PartsRack.com


  24. #24
    Quote Originally Posted by JonB ~ PartsRack View Post
    How? Raw, unburned fuel makes the raw-fuel smell. Not the cat!!

    Why is it not burned? Bad Plugs, Wires, Coils, injectors, etc.

    But a CAT can only smell like raw fuel IF RAW FUEL GOT TO THE CAT from some incomplete combustion issue.
    Dont blame the cats, unless they are burned out and non-functioning caused by RICH-RAW FUEL
    Quote Originally Posted by Steve M View Post
    This is also true based on my understanding of how everything works, with the exception of a cold start when the PCM is dumping extra fuel in to get the cats to light off. The raw fuel smell should be expected under those circumstances, but should only last a few minutes.
    Well maybe vipers are different, but I've had two catless vettes that would make your eyes water the smell was so bad..... put cats back on and the smell is gone

    I'm not saying I'm right or anyone else is wrong, I'm not a mechanic, but high flow cats will leave you with SOME obnoxious exhaust fumes and in my opinion theres a possibility nothing is wrong with the OPs car , and the smell could simply be the high flow cats

    Quote Originally Posted by Steve M View Post
    There are two questions here:

    1. Can you tune the O2 sensors out? The answer to that is yes, but please see the next question below.
    2. Can you tune the sensors out on a Gen III Viper? I don't know the answer to that. Gen III aftermarket tuning solutions are very limited, so the tools might not exist to do it with the factory PCM.

    Running a car open loop can certainly be done...it used to be done all the time. Would it affect performance? Depends on your definition of performance.

    If done properly, the car would run just fine, so from that perspective, the performance wouldn't be affected. WOT operation is already open loop in these cars. But (and that's a big BUT), it's going to depend on the PCM's ability to estimate airflow properly. You'd want to be absolutely sure that the PCM could handle wide swings in the environmental conditions (temperature, barometric pressure, etc.) to make sure your tune is robust...can it handle you driving your car to the top of Pike's Peak where the air is thin? Does it matter if you never intend to do that? Car manufacturers have to care about it, because they don't know how the vehicle is going to be used once it leaves the factory. But you know how you will and won't use the car, so you at least have that going for you.

    Where it would most likely change would be from an emissions and fuel economy standpoint. The O2 sensors are used to adjust fuel trims, which makes sure you have as little gas left over after the combustion process as possible. If you take away the PCM's ability to adjust the trims, then you are at the mercy of whatever lookup table(s) are being referenced to figure out how much fuel gets injected based on the estimated airflow coming into the engine. If the calibration is spot on, you'll be burning close to stoich at idle and part throttle, so it won't be a problem. But we don't live in a perfect world, and no calibration is ever going to be able to account for all of the possible environmental conditions you'll encounter, which is why the trims exist in the first place. It's a checks and balances sort of thing; by removing the O2 sensors from the equation, you are taking away a big check that helps balance the imperfect nature of engine calibration.

    I've personally owned and tuned a previous vehicle to run open loop speed density (MAP vs. RPM, no MAF). It ran well enough, but I noticed pretty wide swings in the cruising AFR depending on the weather conditions - even summer vs. winter fuel blends made a big difference. The reason I did it was because I didn't really fully understand how to compensate for moving the pre-cat O2 sensors further downstream than the factory locations due to running long tube headers. The sensors ran cooler, which affected their switching, which led to codes. I got tired of the codes, so I just turned everything off and said screw it. It wasn't the right answer, and I regretted it up until the day I sold it when it became someone else's problem. I decided I would never get to that point with this car, and I've held to it, which has left me much happier in the end.

    Some of this is also going to depend on the mods you intend to do...if you want to run a big honkin cam with lots of overlap at idle that'll make little kids cry when you drive through the parking lot of your next local cars and coffee, the narrow band O2 sensors are pretty much useless. If I recall correctly, the Gen 4 and 5 folks running the Prefix 9L Extreme packages with their calibration run open loop all the time just for that reason.
    Hey Steve

    Really appreciate that kind of detailed response.

    My cars fuel mileage is pretty bad in the city , like 150 miles per tank , but from my research it appears to be quite normal. ( I think) I can get 400+ miles per tank on the highway which to me is great

    I might look into just installing cats and 02 sensors rather then tune them out, local performance shop says they can do it but I think I'd rather get rid of the cat less smell , although not overpowering it bothers me after a while.

    Going to reread your post in detail on my desktop, lotta good info there thanks again

  25. #25
    Enthusiast Vipervin's Avatar
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    Just to confirm. If I want to replace the O2 sensors, I should order the front two pre-cat sensors and get the OEM Mopar models? It sounds like the post-cat sensors aren't as critical for adjusting fuel trims?


 
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