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Help me troubleshoot poor fuel economy

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  #51  
Old 07-17-2017, 09:41 AM
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Nope. Not yet. Found the leaking head gasket so that became priority 1. Then mechanic replaced fuel filter when he did the suspension - immediate need after head replacement.

I didn't notice anything on the MAP sensor by the temp sending unit. I didn't pull the manifold MAP when I did the head but I can this week and double check.

I put up a thread for injectors, but now I'm hesitant to spend the money. I've spent the purchase price of the car in repairs so far... That's a lot in 4 months or so and most of it was unexpected. I can't afford to just throw parts at it, especially $150/each sensors, especially when there's 4 of them, especially when they're not common failures, and there's no other real symptoms and no CEL. WTF? F this car. I'm so ridiculously frustrated with this thing.
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Old 07-17-2017, 10:06 AM
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There are two 02 sensors in this car and the upstream one is the critical one to inspect.

Coolant can contaminate the sensor causing it to become lazy. Similar to the way sparkplugs get fouled. One option is to buy an inexpensive one and see if it makes any difference? Amazon has
one one
for less than $14. No idea whether or not it's a quality product though.
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  #53  
Old 07-17-2017, 10:26 AM
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Quote:
Originally Posted by Zsm View Post
There are two 02 sensors in this car and the upstream one is the critical one to inspect.

Coolant can contaminate the sensor causing it to become lazy. Similar to the way sparkplugs get fouled. One option is to buy an inexpensive one and see if it makes any difference? Amazon has one for less than $14. No idea whether or not it's a quality product though.
I was under the impression we didn't have any cheap alternatives. There's a couple options I'll look into. I'll pick up a pair soon for ~$40 and see what's what.
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  #54  
Old 07-17-2017, 11:32 AM
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We have the sensor linked back on the first page and some are around $100. Just watch the universal one as noted back there on the link as they sometimes don't fit/work. I never heard of that brand on amazon. All the ones we sell are good and have no issues. Also, we are here if you need anything.
Good luck.
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Old 07-17-2017, 12:01 PM
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If my o2 sensor was fouled, though, wouldn't I be receiving a CEL?

Also, how would o2 sensor symptoms show up? My economy increases at lower speeds. It's really only highway speeds that the economy sucks.
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  #56  
Old 07-17-2017, 12:03 PM
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Its doesn't at first, mine did this. Fuel economy went to the toilet overall(which then you look at plugs/filter/etc..), then a CEL light came on a couple weeks later. Then I confirmed it with the schwaben scan tool.
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Old 07-17-2017, 12:15 PM
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Just confirmed the CEL with the scan tool? Or did you have something for data logging and recording AFR's?

I've been driving this for a few months now, no change in performance so not sure how long he's had this awful fuel economy. This supports the notion that I probably SHOULD change the o2 sensor, but it also supports the notion that it's fine because I still don't have a code. Or can they just perpetually be lazy and never throw codes?
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  #58  
Old 07-17-2017, 12:28 PM
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Confirmed with scan tool when the CEL popped up. I could see my gas milage drop from week to week and thought it might be plugs, wires, etc... and then it was the dang O2 sensor. I did the front and the rear at the same time because in my other MINI i had the rear go only to have the front go 2 weeks later.

I have owned 3 MINI to date and over 360K miles driven between the 3 you can almost know whats going to happen.
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  #59  
Old 07-17-2017, 12:30 PM
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Was it fouled or just old and failed?
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Old 07-17-2017, 12:49 PM
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It failed. Cant bring them back to life. After so many miles they just fail.
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  #61  
Old 07-17-2017, 02:13 PM
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When the heater circuit fails in the 02 sensor, that usually triggers the CEL. Lazy 02 sensors still technically work since the circuit is still intact.

FYI, here's my son's car. Tips show some pretty normal soot at the bottom of the tips. This is about 350 miles including the teen car clinic where the car was, ahem, flogged spirtedly.


Car has 224+K Miles, getting mid-20s avg. I'm planning to change the upstream 02 sensor this fall. No idea how old it is.
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  #62  
Old 07-17-2017, 02:27 PM
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You guys have convinced me. Before injectors, I'll do upstream o2 sensor.
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  #63  
Old 07-17-2017, 02:43 PM
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The only other thing I can think of is whether or not you have monitored coolant temp or double checked the coolant temp sensor?

Coolant temp sensor plays a role in determining fuel enrichment.
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Old 07-18-2017, 01:19 PM
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Woah dude, just stumbled across this thread--holy cow, our cars are experiencing the same exact, painful issues at the same time. I smelled fuel while bleeding coolant yesterday, and then as I was driving around, a check engine popped up. Stopped at O'Reilly's...upstream O2....WHY ARE OXYGEN SENSORS SO EXPENSIVE???

Seriously considering buying two of
these these
and hoping they work.

Last edited by sarom058; 07-18-2017 at 01:35 PM.
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  #65  
Old 07-18-2017, 02:09 PM
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After reading a few threads on o2 sensors here, as well as anecdotally from other forums, I hear this isn't really a spot to skimp. I just ordered the NTK o2 sensor for $99 or something from Amazon. Only 1 for pre-cat since the 2nd one only states whether the cat is functioning so I'm not as concerned with that one as it doesn't affect economy.

I have no codes, smooth idle, no hesitations... Just god awful fuel economy.

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
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  #66  
Old 07-18-2017, 03:03 PM
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Looking forward to seeing how this works out.

My son's car is now averaging around 21mpg since he's doing shorter trips up and down the hills here. I'm thinking a little car like this ought to be a little more efficient than this.
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  #67  
Old 07-18-2017, 07:27 PM
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I'll let everyone know. If this doesn't significantly help then I'm at my wit's end with this thing. I'm still getting low 20 mpg, no more than 300 miles to the tank with about 275 of that as highway driving. That won't cut it for road trips.
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  #68  
Old 07-22-2017, 01:03 PM
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How the F do you get at the precat o2? EVERYTHING is in the god damn way. Holy F! Been working for 90 minutes and have made exactly 0 progress. F'n Christ. There's legitimately no way to get at it. Tried from above, tried from below, tried with open ended wrench, tried with o2 sensor socket, tried with and without cheater bar. I'm stumped. Legit going to sell my car. So fed up with this thing. NOTHING is easy with it.
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  #69  
Old 07-22-2017, 01:30 PM
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90 minutes later... got that bastard out. Seems pretty alright... /sarcasm
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Old 07-22-2017, 01:43 PM
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Nicely done.

Now go fill up your tank, reset the trip meter and avg mpg and have fun. 😁
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Old 07-22-2017, 02:13 PM
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So how long until the ecu adapts to the new o2 sensor. Logic would say immediately, but display is the same. Also, is there a possibility that something is wrong with the ecu that no cel was displayed between this and a head with 4 cracks in it, blown head gasket, and misfire in cyl 1?
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  #72  
Old 07-22-2017, 06:15 PM
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It's not going to instantaneous. ECM is going to have adapt to the new values coming out of the sensor. May take a few drive cycles (4?) before you start to see a change.

My son's R53 took almost 4 days before the new post cat CEL went away after installing a new NTK sensor. Took another two days before the pending code finally resolved.
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Old 07-22-2017, 07:47 PM
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Really? I'd imagine that o2 sensor feedback would be almost instant. I don't know how much the ecu will adjust fuel trims and at what intervals, though.

Anecdotal evidence that it's improving economy: Filled up and estimated range was 289 miles. I've now driven 61 miles and my estimated range is 295 miles. I know you're all reading this thinking I'm a fool for looking at that, but it's behavior my ecu/car hasn't displayed before now. Typically, when I get on the highway, I'll have say a range of 150 miles, then 5 miles down the road I'm down to 135 miles of range. So it seems the ecu is aware that it's using a ton of fuel. The fact that my range has adjusted and has occasionally increased as I drove today is a definite step in the right direction.
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  #74  
Old 07-24-2017, 06:53 AM
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So for what it's worth, ALWAYS second guess yourself. Yesterday I happened to look at my exhaust and found that I am/was most definitely running pretty rich... Last week when I cleaned the tips, they both looked like the right side where I wore away some of the black coating.


So this morning I cleaned off the tips again and will continue to monitor. If I'm still rich it'll be injectors and then the only thing I can do is a tune... Gods, I hope I don't need to keep digging.
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  #75  
Old 07-24-2017, 10:16 AM
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Maybe, if you don't already have this, get an OBD monitor you can use while you drive? The BMW guys seem to like OBD Fusion.

Would be an easy way to monitor things like fuel trim, 02 sensor activity, etc...

Looking forward to hearing how this week goes for you.
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