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E46 Xi Forum
The E46 XI was produced from 01-05 in sedan and touring body styles. Powered by either a 2.5L inline 6 in the 325xi or a 3.0L inline 6 330xi. Discuss all thing about BMW AWD E46 'Xi' here.

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Old 02-16-2015, 02:29 PM   #21
Dev02
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My understanding was that the instrument cluster instantaneous MPG gauge is driven off the injector duty cycle and info comes from the ECU. I'm pretty certain it's not vacuum based.

BK
Damn...
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Old 02-16-2015, 03:57 PM   #22
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I don't know how abnormal your MPG or exhaust vapor trail is, given your climate, but the ECU calculations for instantaneous MPG depend on a known fuel pressure and injector delivery volume per pulse. If you're not throwing codes, operating temp is normal and it's running OK you're likely fine. I presume your fuel filter has the air line like mine. Did you make sure that it's not compromised on either end? That could screw with the fuel pressure. I changed my fuel filter two days ago and the short rubber flex line at the filter was complete goo. Just tossing some suggestions out there...

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Old 02-16-2015, 09:11 PM   #23
White_Knuckles
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Originally Posted by bk_856er View Post
My understanding was that the instrument cluster instantaneous MPG gauge is driven off the injector duty cycle and info comes from the ECU. I'm pretty certain it's not vacuum based.

BK
If that gauge is tied into ECU logic it then definitely sounds German. But as many point out, the gauge acts like a vac display. It stuffs the needle on throttle application and dives when you let off. Just like Vac gauges. Where did you come across this? I've mainly seen posts where folks were saying it's way off, stupid, etc., but no one reporting a wack gauge or even a dead one. They just seem to work. But no one has mentioned how they work.

The thing is, if it's a true "injector" monitor the car would be fussing and surging as the needle bounced around. The same would hold true if vac is jumping around - the car would be responding. Dev only sees the fluctuation at the gauge as the car rolls on smoothly. Back to gauge issue.

D> can you safely have an assistant shoot a video of the gauge while you're driving her? I'm unclear what you're seeing? Post that thing up here or try a more detailed explanation. I watched mine it's very damped not able to make needle craziness jacking or releasing the gas. Nice slow tapers.
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Old 02-16-2015, 09:34 PM   #24
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Bk, I will check that hose, and others, this week. I made sure it was connected well when I installed the filter, but I might have messed up the upper part earlier when dismantling the stuff in the upper bay. That was when the gauge started flipping out.

Knuckles, I will get a video of the gauge tomorrow. Makes a lot of sense.

I've been doing a lot of searches trying to find out exactly how the gauge works to see if that helps me troubleshoot, but so far no luck. One thing I did notice is that it only acts weird at low speeds, after 30 or 40 it seems to be right on and react instantly with no lag.

Thanks!
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Old 02-16-2015, 09:45 PM   #25
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I'm assuming BMW didn't dumb down the design over the years. The instantaneous MPG meter exists in my '85 E24 and also my '03 E46 and visually behaves the same way. I intimately know my E24 and there are no vacuum lines or sensors to the dash or anywhere else! When guys uprate their injectors or modify fuel pressure with an adjustable regulator the MPG meter is rendered whacky (because the injector delivery/pulse is now different). In my case I have a AFM-to-MAF conversion on my E24 and under moderately heavy load I can actually see the dash MPG meter oscillate with the O2 sensor closed loop operation as the fuel mixture dithers around stoichiometry - same frequency as my lambda meter that I can simultaneously see. Indirect evidence, but I stand by it. I do agree that the meter "behaves" like a vacuum gauge. Just not sure I'd put much faith in its diagnostic utility due to the unknown filtering and damping of the needle. Might point to the need to capture some live data and trims from OBDII.

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Old 02-16-2015, 09:50 PM   #26
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Disconnect the air hose in the engine bay and blow in it - should be sealed and no air should flow. I call it an air hose because I don't think it's actually under vacuum, but it looks like a vacuum line. Not certain about your car, but I believe it's connected to the "F" connector on the rubber bellows. Someone will correct me if I got that wrong.

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Old 02-16-2015, 10:53 PM   #27
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The one downstream of the MAF? Leave the other end connected to the filter and try blowing through the top?

I have a scanner coming and will check the codes when it gets here, but I did go yesterday to an Autozone and they said there were no codes coming up.
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Old 02-16-2015, 11:34 PM   #28
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Yes. I'm not 100%, but I believe that line leads to the pressure regulator on the fuel filter. Careful those lines can get brittle with age and snap when you look at them wrong. Mine has a nice flexible blue silicone section connecting the plastic hard line to the F connector, so it's easy to slip on/off.

Certain conditions need to persist for a while before the code gets triggered with a corresponding Service Engine Soon light. A code scanner is a useful thing to have around, especially if it's one that can read live data and you have the know-how to interpret it.

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Old 02-16-2015, 11:50 PM   #29
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I've never used a scanner before and am completely unknowledgeable about them, but I'm a quick study. If the vac line is bad I have a bit of good tubing here. I'll check them in the morning.

BTW, what scanner do you use?
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Old 02-17-2015, 12:06 AM   #30
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For routine work I use the Peake Research R5. I recently added the OBDLink SX (USB) to my toolbox for its additional features, but I haven't messed with it yet.

BK
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Old 02-17-2015, 01:34 AM   #31
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BK your prior experience tells the tale. I had it backwards thinking the vintage cars had vac gauges and they carried on the tradition with some elect/vac hybrid these days. That poor gauge is often bashed as lame due to damping and the silly scale. I've read posts where owners were trying to maintain a certain mileage watching the scale driving. Really? Anyway, good to know it's more sophisticated. But we're still stuck on the posters issue described as "erratic". I'm pretty sure it's not a good diagnostic tool as well. If the gauge is functioning properly then the car should have a strange response when its "erratic". Interesting.
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Old 02-17-2015, 09:15 AM   #32
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Knuckles, I'll get a video over lunch when I run errands and post.
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Old 02-17-2015, 04:10 PM   #33
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Ok, here is the video (I think...);


I'm sure there's a way to link to the video and have it play here but I seem to be YouTube challenged (or... in the preview it IS working so maybe I am good).

Also, maybe how it is acting is normal and I am worried for no reason. I could swear it was giving me instantaneous response before but who knows...?

The gauge actually seemed to be working less weird than it has, and the steam out the back was a bit less also. I attribute this to the car knowing it was being filmed. The tick in the engine seems super loud in the video but is barely heard in person.

Thanks!
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Old 02-19-2015, 02:24 AM   #34
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Dev, that looks very close to mine. I had to make a run so I watched mine after seeing yours. Discovered it doesn't move at all when in park and rev it to 1500. It swings hard right on take off or any pedal feed even small increments. It only lands in the mid range as you taper the throttle and lay back in cruise (30+). Quick pedal transitions can duplicate a bounce where it spikes up and rolls back down quickly. Seems like it may be RPM related as the tach needle moves almost proportionally to the MPG. When the revs jump up the MPG jumps etc. Mine is always stable when you hold the pedal steady at cruise. It's not predictable, it needs to settle out first. Possibly useful at higher, steady speeds? Does not register when creeping.

I honestly can't determine if yours is abnormal but great video effort! Let's see what others say.
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Old 02-19-2015, 10:49 AM   #35
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Thanks for checking! I'm staring to think I AM overly sensitive, as you suggested earlier, and cannot remember exactly how it acted before the Klima-clean and the fuel filter change. I don't think my old 320 had one of these (might have, that was awhile ago) and the '02 certainly doesn't, so it IS a new gauge to me. Mostly what has me thinking it's not behaving correctly is when I come to a stop and it jumps up to the top with no throttle input. Maybe that's a math calculation by the computer saying "if you keep using the gas you are, AND you keep going slower, you will get poor mileage"?

Whatever the case, if yours is acting the same it makes me feel much better.

I still plan on doing some things once it warms up;
Check for vac leaks (can't hurt)
Check codes with new toy (come on, it's a new toy!)
Check and change DISA
Clean MAF (don't think this can hurt either)
Do the CCV (when it actually gets warm, not just above freezing)

I'll update after doing all this since I hate when I read a thread with a problem I'm having and they never come back and say "Hey, this fixed it!" Might take awhile though... -9 this morning and not looking cheery anytime soon.

As stated multiple times to all of you, thank you SO much!
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Old 02-19-2015, 11:29 AM   #36
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White Knuckles, was that 60's Pontiac a Gran Prix? We had a 63 that had the gauge, not much of a concern when gas was less than $0.30/gal.

About the MAF, I just disconnected mine while trying to start the car when my pump died as a test. The car fired on the last legs of the pump but only stumbled and died, but it ran long enough to throw a code.
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Old 02-19-2015, 11:08 PM   #37
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imo69 >> Ha! Yep that would indeed have been a 63 (or so) Grand Prix with an auto, groovy console, a bright white interior. and that gi-normous vac gauge. That thing responded similarly to our altimeters (or whatever they claim) when pedal pokin'. I never really spotted it out of the Red zone come to think of it. Hey, gas was cheap and it had a big-old motor! BAAWAAAAAAA

One troubleshooting trick is to remove the MAF connector forcing the car into a default loop. I get closed/open loop mixed up but I think when disconnected it's a "closed" condition? Anyway, she should run fine on the factory fuel/timing settings to get you home at least. It may be a little rich but should not fall on its face. I left my MAF unplugged once then started and moved the car which seemed fine running. But - the trans error lamp came up also tripped a code that scared the **** out of me. Plug her back in, reset the code and whew, all's good.

Dev> You may still have an unusual deal going on with the gauge? The "stop and then it jumps to the top" symptom is not consistent with mine. But think about what the car would be doing should it be a fuel demand spike at a stop. It would be grabbing great gobs of fuel to um, idle? No, the car would fuss and shake going flood, crazy rich.

Advise trimming a proper chunk of black Duct-Tape to place over that devil removing it temporarily from your view. If ya can't see it, it ain't broke.
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Old 02-20-2015, 10:18 AM   #38
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imo69 >>Dev> You may still have an unusual deal going on with the gauge? The "stop and then it jumps to the top" symptom is not consistent with mine. But think about what the car would be doing should it be a fuel demand spike at a stop. It would be grabbing great gobs of fuel to um, idle? No, the car would fuss and shake going flood, crazy rich.

Advise trimming a proper chunk of black Duct-Tape to place over that devil removing it temporarily from your view. If ya can't see it, it ain't broke.
Yeah, I am wondering if there is excessive vacuum happening on braking (I believe the 330 uses a vacuumed booster) and that somehow effects things.

I COULD ignore the gauge and keep muttering "there's no place like home, there's no place like home" but what fun would that be? I am fairly convinced this is not a driveability issue at this point and that the car is running well, so it is now a puzzle I get to play with and solve. Admittedly I have to wait to play till it is slightly above what-the-hell-are-you-thinking cold, but I will get to the bottom of it. Or if I don't I'll at least know the car a lot better.

Thanks again and would love to see pics of the Grand Prix.
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Old 02-20-2015, 09:41 PM   #39
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Yeah, I am wondering if there is excessive vacuum happening on braking (I believe the 330 uses a vacuumed booster) and that somehow effects things.
Truly an oddity. We don't want to blend theories here. I guessed from other posts and folklore that the bugger was vac operated. It has now been brought to our attention that it may be an ECU output with the computer sending varying voltage levels to control its motion. It's an analog meter device that may be defective where the ECU levels are fine? An unhappy gauge could exhibit strange needle behavior.

But vac via braking affecting this? - not so much. All power brake systems have a vac booster. (The big round can right below the windshield line, drivers side) An excessive vac condition wouldn't effect a voltage operated gauge. Actually I've never experienced excessive vac with any car.

Click your heels together two times and the gauge changes to an AM radio signal strength meter. Handy touring on the Autobahn.
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