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Thread: Poor gas mileage

  1. #41
    Join Date
    Oct 2017
    Posts
    122
    The P0420 shouldn't affect anything. All that light on the dash is there for is to make the gubberment happy. It means the car is, or could be, polluting more than it is supposed to. That's it. The catalytic converter isn't doing its job right. I find it ironic that Asian cars have, supposedly, more precious metals in them and so on and bring greater scrap price, but they more commonly go bad than that of a North American car. I would recommend a MIL eliminator for that problem.

    Don't get a remanufactured air flow sensor if you get one. The Delphi unit is only (*only*) about $75 off of the internet. With the exception of brake calipers and stuff that I have no other reasonable choice, I have pulled the plug on remanufactured stuff. It is all garbage. Not worth the risk, not worth the time. I would be more tempted to throw $$$$ at the oxygen sensors first. Spark plugs and air filters won't help at all as long as it is already running just fine. That's old school thinking. A hotter plug may change things, though.

    To figure fuel economy, there's all sorts of things to consider.

    Tires: Not all tires roll equally. It can be significant. I've noticed new rubber yields less mpg than a tire that has aged awhile as well.

    Tire inflation: Don't be afraid to dump 40+ PSI into a tire. That's about where I run mine. Ever ride a bike on a flat tire? Yeah, that's a lot of work.

    Ambient temperatures: Often, people let their vehicles warm up before they drive them. It also takes longer to reach operating temperature where it is most efficient. Thicker bearing grease, denser air is harder to push through, tires are also hard to roll when it is cold. Ever push a car on a zero degree fahrenheit day? Well, they just don't move well.

    Oil viscosity: Thick oil is harder to pump. That's why lots of vehicles now specify 0w20 oil or maybe even thinner now. You do not need 40 psi of oil pressure at idle. You are wasting energy by having that.

    Ethanol content: They say "up to 15%" but.... well.... test the fuel. You will see how much it can vary. Ethanol-free yields 3 mpg more in my SX4.

    Engine health (A compression test can reveal some things). Cleanliness of fuel injectors. Oxygen sensor feedback. Engine coolant temperature readings. Plugged exhaust. Adaptive fuel trims.

    Brakes. Brake dragging is a big one. Lots of times, I see brake pads shoehorned into the caliper bracket so that they don't float around like they should. You then get dragging brakes. Caliper slide pins stick and do the same thing. As can faulty brake hoses and caliper piston seals.

    Missing splash shields. Yeah, they can affect your aerodynamics.

    Take all of this little stuff and add it up and you can get a few mpg difference. You can get that 25 to 28.

    Mine has been getting just a bit over 29 MPG or 8L/100km with the 5spd. It is all highway with a very significant portion being 78+ mph every day.


    Ever notice that some new pickup trucks now have liquid cooled differentials? That's not there to keep it cool. Well, it sorta is. Long story short, they want the gear lube to be thinner for less drag. Thinner oil has less protection, so it needs to be cooled to keep it from getting too thin. Thicker oil needs to be heated so that it isn't so darn thick. Under light loads, it won't heat up well, so they use wasted engine heat to warm up the differential for less drag. If you can keep the temperature more consistent in the differential, you know what oil viscosity needs to be in there and you can maintain that. How about that for ways to squeeze a lemon?

  2. #42
    Join Date
    Dec 2011
    Location
    Chicoutimi, Quebec, Canada
    Posts
    463
    Quote Originally Posted by phantomrt View Post
    ..Ever notice that some new pickup trucks now have liquid cooled differentials? ...
    OK..from whay you say, they have a heat exchanger that uses engine coolant to keep differnetial at constant temp. Interesting. Maybe not relevant here...but what pickup truck uses that ?
    2011 sx4 JX AWD CVT 32000km Also drives 2015 KIA Sorento AWD 2.4

  3. #43
    Join Date
    Feb 2018
    Posts
    40
    Thanks for the detailed note below. Lots to consider, and lots I don't feel confident around doing anything myself. I just got new all-seasons, and am off the winters. Thus far there's no improvement. Perhaps this is because they're new tires, as you allude to below.

    If the O2 sensor is the problem with P0420, I understood this could affect fuel efficiency - and the wider internet suggests that ignoring a cat code can ultimately be bad for the motor. I don't know the truth of this.

    I finally figured out how to get the MAF sensor out of the car, and so I opened it up. It looked clean as a whistle to me, but given that I'd removed it already, and had the can of cleaner, I sprayed it anyways. Waiting for it to dry now, and will put it back in. Just like everything in life, once you know what you're doing, it's pretty easy. Hopefully I didn’t break anything.

    Photos from before.

    Quote Originally Posted by phantomrt View Post
    The P0420 shouldn't affect anything. All that light on the dash is there for is to make the gubberment happy. It means the car is, or could be, polluting more than it is supposed to. That's it. The catalytic converter isn't doing its job right. I find it ironic that Asian cars have, supposedly, more precious metals in them and so on and bring greater scrap price, but they more commonly go bad than that of a North American car. I would recommend a MIL eliminator for that problem.

    Don't get a remanufactured air flow sensor if you get one. The Delphi unit is only (*only*) about $75 off of the internet. With the exception of brake calipers and stuff that I have no other reasonable choice, I have pulled the plug on remanufactured stuff. It is all garbage. Not worth the risk, not worth the time. I would be more tempted to throw $$$$ at the oxygen sensors first. Spark plugs and air filters won't help at all as long as it is already running just fine. That's old school thinking. A hotter plug may change things, though.

    To figure fuel economy, there's all sorts of things to consider.

    Tires: Not all tires roll equally. It can be significant. I've noticed new rubber yields less mpg than a tire that has aged awhile as well.

    Tire inflation: Don't be afraid to dump 40+ PSI into a tire. That's about where I run mine. Ever ride a bike on a flat tire? Yeah, that's a lot of work.

    Ambient temperatures: Often, people let their vehicles warm up before they drive them. It also takes longer to reach operating temperature where it is most efficient. Thicker bearing grease, denser air is harder to push through, tires are also hard to roll when it is cold. Ever push a car on a zero degree fahrenheit day? Well, they just don't move well.

    Oil viscosity: Thick oil is harder to pump. That's why lots of vehicles now specify 0w20 oil or maybe even thinner now. You do not need 40 psi of oil pressure at idle. You are wasting energy by having that.

    Ethanol content: They say "up to 15%" but.... well.... test the fuel. You will see how much it can vary. Ethanol-free yields 3 mpg more in my SX4.

    Engine health (A compression test can reveal some things). Cleanliness of fuel injectors. Oxygen sensor feedback. Engine coolant temperature readings. Plugged exhaust. Adaptive fuel trims.

    Brakes. Brake dragging is a big one. Lots of times, I see brake pads shoehorned into the caliper bracket so that they don't float around like they should. You then get dragging brakes. Caliper slide pins stick and do the same thing. As can faulty brake hoses and caliper piston seals.

    Missing splash shields. Yeah, they can affect your aerodynamics.

    Take all of this little stuff and add it up and you can get a few mpg difference. You can get that 25 to 28.

    Mine has been getting just a bit over 29 MPG or 8L/100km with the 5spd. It is all highway with a very significant portion being 78+ mph every day.


    Ever notice that some new pickup trucks now have liquid cooled differentials? That's not there to keep it cool. Well, it sorta is. Long story short, they want the gear lube to be thinner for less drag. Thinner oil has less protection, so it needs to be cooled to keep it from getting too thin. Thicker oil needs to be heated so that it isn't so darn thick. Under light loads, it won't heat up well, so they use wasted engine heat to warm up the differential for less drag. If you can keep the temperature more consistent in the differential, you know what oil viscosity needs to be in there and you can maintain that. How about that for ways to squeeze a lemon?
    Last edited by mikefly; April 19th, 2018 at 02:32 PM.
    Suzuki SX4 | 2008 | hatch | auto | AWD | 135,000km
    Subaru Forester | 2014 | auto | AWD | 70,000km

  4. #44
    Join Date
    Oct 2017
    Posts
    122
    The new Ram pickups have a "cooled" rear differential. I think they are the first to do it. It is all in the name of lower rolling resistance. There may be some side effects such as a bit longer lasting ring and pinion, but there are also side effects of potential cooler failure and coolant / gear lube mix.

    75+ years of solid axle rear differentials have proven to be able to outlast the rest of the vehicle. It has to be for lower rolling resistance. Why else do it?

    If you broke the airflow sensor, it may have been on its last leg anyway. It wouldn't be the first part I touched that went bad afterwards. For example, I killed about 4 ignition coils in a ford 5.4 engine. It was running fine prior to me removing the intake, and when I put it back together, 4 ignition coils didn't want to work. Great. Customers don't like it when stuff like that happens.

    A poorly functioning catalytic converter can be for a number of reasons. One reason is poor flow. It could be partially plugged, thus not working right, and the untrained "seat-of-the-pants-o-meter" wouldn't necessarily know a restricted exhaust from another. It gets restricted, you drive hard down the highway one day, detonate the snot out of the engine, and... well, you get the idea. A leaking exhaust can cause a catalyst efficiency code as well, but I'm not sure exactly how. My Jeep has had a cracked manifold for... well, they all crack. No catalyst efficiency code after 20 years and 247,xxx miles. My '03 Sonata had an efficiency code and a leaky exhaust upstream of the converter. Another customer of mine has an '03 Audi A6 with... leaky exhaust right at those beloved European inspired flex joints. Catalyst efficiency code. There have been more over the years and I tell people to pretty much just ignore the light or spend the $1,300+ to fix it.

    When I got my sx4, the rear brakes were dragging. The pads were also worn to nothing in about a week of owning it. I got new pads and installed them. They were tight, so I ground them down. Fast forward to last week, and they were already half gone. And sticking. I had to grind more off of the pad so it fits loose in the caliper bracket, so it releases the rotor. My last tank of gas calculated to 31 mpg. but it is also about the season where summer gas blends come out. That makes a difference. I do not know what the difference is, but something happened. The weather warmed up, too.

  5. #45
    Join Date
    Feb 2018
    Posts
    40
    Update. Now that it’s warmer, the windows have been down, and the squeal I thought was coming from the serpentine belt seems more likely to be coming from the rear brakes. So maybe there’s a slight rub as cman (and others) hypothesized.

    Sending car for three month oil change in a couple weeks - will ask them to check.
    Suzuki SX4 | 2008 | hatch | auto | AWD | 135,000km
    Subaru Forester | 2014 | auto | AWD | 70,000km

  6. #46
    Join Date
    Feb 2018
    Posts
    40
    Second update for anyone searching this topic in the future. I reset the OBD codes after I cleaned the MAF sensor, but the P0420 code came up pending shortly thereafter. The light never came on, so I checked it again - and it's no longer pending (or active). I *hope* this means the issue is gone. If so, I'm pleased as punch. Crossing my fingers.

    EDIT - haha came on today! Active and pending.
    Last edited by mikefly; May 2nd, 2018 at 05:01 AM.
    Suzuki SX4 | 2008 | hatch | auto | AWD | 135,000km
    Subaru Forester | 2014 | auto | AWD | 70,000km

  7. #47
    Join Date
    Nov 2013
    Posts
    14
    Changed my MAF with Denso brand and P0420 code went off along with the check engine light. That was about a week ago but now my check engine light came back up reading P0171 and P0300.. Engine running to lean for some reason? I also checked for leaks and heard or seen nothing. P0300 states that my cylinders are misfiring but i don't notice any difference in how the car performs or runs.
    Interesting thing is that about 3 years ago Suzuki changed out my whole engine due to cylinders running and under 30%. I wonder if they just reused parts from the old engine?
    Last edited by sx4miami; May 7th, 2018 at 02:06 PM.

  8. #48
    Join Date
    Feb 2018
    Posts
    40
    Update - rear brake was not rubbing - the rotor rattle clip (or brake pad reed sensor?) was rubbing a bit. Fixed, should not have affected fuel efficiency.
    Last edited by mikefly; May 19th, 2018 at 08:19 AM.
    Suzuki SX4 | 2008 | hatch | auto | AWD | 135,000km
    Subaru Forester | 2014 | auto | AWD | 70,000km

  9. #49
    Join Date
    Feb 2018
    Posts
    40
    1st full tank post MAF cleaned and tires changed = 11L/100km
    Suzuki SX4 | 2008 | hatch | auto | AWD | 135,000km
    Subaru Forester | 2014 | auto | AWD | 70,000km

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