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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Sun, Sep-23-2018, 06:27:54 PM   #1
ATB88
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Default Abysmal gas mileage, need diagnosis help

Background:

-2002 with 58xxx miles
-VANOS fully Beisann'd 5k miles ago
-6MT swapped from SMG 5k miles ago (with full 6MT software flash, and gear position sensor)
-Both engine bay TPS sensors replaced 5k miles ago (OEM from FCP, but not OE)
-ICV cleaned 5k miles ago
-Throttle bodies cleaned 5k miles ago
-Fuel filter replaced 10k miles ago
-MAF replaced 500 miles ago (OEM bosch)
-Intake air filter replaced 500 miles ago
-Adaptations reset 500 miles ago
-ran two containers of techron fuel system cleaner 10k miles ago
-valve adjustment 5k miles ago
-coils and plugs 5k miles ago (although I forgot to use the supplied dielectric grease when installing them... is that a big deal?)

As you can see, my car has had a lot of work done in the last little while, and many of the things that can commonly cause poor fuel economy have been recently addressed (based on my googling for common causes of this issue for our cars).

My fuel economy is terrible (12-13 MPG with 70/30 city/hwy driving), and I don't drive like a goon (mostly staying below 3k when going around town).

While hunting around in the engine bay for causes, I found that the oil return line that hooks up to the bottom of the intake manifold and feed back down into the oil pan was disconnected, whoops. I figured that was the source of my problem. So I hooked it back up, and reset adaptations, and two tanks of gas later, it almost seems like it's even worse than before... go figure.

Anyone have suggestions before I go and get this thing smoke tested for vacuum leaks or something? Fuel pump? injectors? Mileage is pretty low, so I'm hesitant to throw money at those at this point... One thing is that when I got the car, it had only been driven <1k per year for several years in a low. Did a lot of sitting around. Maybe this could have caused some gremlins in the fuel system that need to be worked out (old fuel sitting in the lines for a long time).

Thanks for any ideas!!
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Old Sun, Sep-23-2018, 06:51:31 PM   #2
HassanEido
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Default Re: Abysmal gas mileage, need diagnosis help

Ok..
You either:
A-have leaky fuel injectors (unlikely)
B- Bad o2 sensors. *mostly this*
C- A terrible tune.
D-clogged cats (unlikely at your mileage)
I would start at B .
You mentioned you flashed a 6mt tune? Did u just change the parameters or did u install a completely different bin.
If u want great economy, go for the euro software . It'll make 25+mpg mixed. For now check your o2s, leaks, injectors and verify what software u have.
__________________

Mods:
-Custom CF airbox
-SSV2 catless headers, Custom 2.5" dual sec1, Custom sec2 2.5" dual with e9xM X pipe, SuperSprint Race inspired custom section 3..
-Cat Cams 280 272.
-Custom dyno tune
-Tms pulleys and electric fan conversion
364.1 whp 287.8 wtq

Custom exhaust thread and dyno http://m3forum.net/m3forum/showthread.php?t=569086
Cams thread and dyno http://m3forum.net/m3forum/showthread.php?t=554439
Custom cf airbox thread http://m3forum.net/m3forum/showthread.php?t=595180
Why not to use after market bearings with surfacetreatments and /or increased clearance
http://www.m3forum.net/m3forum/showp...4&postcount=10

Last edited by HassanEido; Sun, Sep-23-2018 at 06:58:33 PM.
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Old Sun, Sep-23-2018, 06:53:57 PM   #3
Maximz
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Default Re: Abysmal gas mileage, need diagnosis help

I would start with monitoring air/fuel mix ratio, then refurbish injectors, fairly cheap <200$ it cost me when I did that, I normally run in 20-22 range, a lot of highway driving but also a lot of stop and go highway traffic, I’m sure someone much more experienced will chime in, besides cleaning/changing seals of the injectors (preventative) I haven’t messed with the fuel system.
Also what type of gasoline are you using? I’m sure you are well aware of the 93+ but which gas station, as many companies use different fuel detergents, just curious
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Old Sun, Sep-23-2018, 07:04:00 PM   #4
ATB88
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Default Re: Abysmal gas mileage, need diagnosis help

Thanks for the suggestions!

1. How do I troubleshoot O2 sensors? I have no CEL on or codes, I was given the impression that O2s last 100k? I have access to inpa and dis

2. My tune is the most recent updated bmw factory 6MT tune for US spec cars, flashed the whole bin, didn't just edit my old smg

3. I'm in California and run 91 almost exclusively from Chevron

Last edited by ATB88; Sun, Sep-23-2018 at 07:18:15 PM.
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Old Sun, Sep-23-2018, 07:24:04 PM   #5
HassanEido
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Default Re: Abysmal gas mileage, need diagnosis help

If they're not throwing codes then they're probably ok .
Do you have access to a lambda sensor you can use to monitor ur afrs on part throttle?
Does your car stumble when u start it in the morning ? If u have leaky injectors that would be one symptom. Monitor your afrs as a start.
Vacuum leaks would cause drivabiity issues but not excessive fuel consumption in my experience .
As a side note;
US maps run richer to save the in header cats. Euro maps run much leaner and get significantly more gas mileage. Night and day difference.
__________________

Mods:
-Custom CF airbox
-SSV2 catless headers, Custom 2.5" dual sec1, Custom sec2 2.5" dual with e9xM X pipe, SuperSprint Race inspired custom section 3..
-Cat Cams 280 272.
-Custom dyno tune
-Tms pulleys and electric fan conversion
364.1 whp 287.8 wtq

Custom exhaust thread and dyno http://m3forum.net/m3forum/showthread.php?t=569086
Cams thread and dyno http://m3forum.net/m3forum/showthread.php?t=554439
Custom cf airbox thread http://m3forum.net/m3forum/showthread.php?t=595180
Why not to use after market bearings with surfacetreatments and /or increased clearance
http://www.m3forum.net/m3forum/showp...4&postcount=10
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Old Sun, Sep-23-2018, 09:37:37 PM   #6
ATB88
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Default Re: Abysmal gas mileage, need diagnosis help

Thanks for the tips! Might switch to a euro tune once I get everything sorted, but I do want to make sure my car runs as it should on the US tune first so that the euro tune doesn't just cover up existing issues.

By access to a lambda sensor, do you just mean something that can show me lambda readings? INPA can do this, right?? If so, what should I be looking for in my AFRs?

As far as how my car behaves in the morning: starts right up, and then I usually let it idle till the revs drop below 1k. Idle hunts sometimes, and also, on a cold start, in the time I'm waiting for the revs to drop, sometimes I feel what I would describe as slight "knocking" or "tapping" with an erratic rhythm, coming from the engine bay (vibrating through the floor). But car doesn't ever stumble when I drive it or hit the throttle.

Drivability is fine, so, I guess maybe we're looking at injectors.. wouldn't be shocking to me if the O-rings weren't looking great after 16 years.. If that's the case I feel like I might as well just send them off for a rebuild while I'm taking them out.
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Old Mon, Sep-24-2018, 04:48:25 AM   #7
HassanEido
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Default Re: Abysmal gas mileage, need diagnosis help

INPA can't give u AFR readings. u will need an wide band AFR sensor.
I wouldn't jump to injectors yet. Idle hunting shouldn't happen btw, do u have an aftermarket intake/filer
__________________

Mods:
-Custom CF airbox
-SSV2 catless headers, Custom 2.5" dual sec1, Custom sec2 2.5" dual with e9xM X pipe, SuperSprint Race inspired custom section 3..
-Cat Cams 280 272.
-Custom dyno tune
-Tms pulleys and electric fan conversion
364.1 whp 287.8 wtq

Custom exhaust thread and dyno http://m3forum.net/m3forum/showthread.php?t=569086
Cams thread and dyno http://m3forum.net/m3forum/showthread.php?t=554439
Custom cf airbox thread http://m3forum.net/m3forum/showthread.php?t=595180
Why not to use after market bearings with surfacetreatments and /or increased clearance
http://www.m3forum.net/m3forum/showp...4&postcount=10
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Discussing E46 M3 Market Sales Tracker: Please Report Your Purchase or Sale! in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)