325iX mileage

[SRR2]

I just compiled the mileage data for the 325iX (M20) for the last two+ years. I was kind of shocked to see that the mileage has steadily deteriorated from around 26MPG to around 23MPG over about 30,000 miles. The decline was gradual and pronounced. I have enough data to determine that it's statistically significant.

During that period I replaced the O2 sensor, but there was no visible effect on fuel consumption attributable to that. It also had about half the wear of one set of tires, and about 5000 miles on a new set, also without any visible effect. Recently the AFM was acting up such that the idle and low speed operation was seriously affected. I was able to open up the AFM and clean the element which restored what seems like correct operation. The ECU isn't throwing any codes, stomp test gives 1444. Valves are in good adjustment. Everything seems normal and in good condition about the engine otherwise. There's no leaks, oil consumption, or cooling problems. Fuel filter and air filter have under 10000mi on them. The plugs, thermostat and water pump were replaced a short while ago without any effect on mileage. Tire pressure is maintained at about 31f, 33r.

The car is used fairly consistently on 15 mile trips where it has opportunity to warm up completely. It's rarely used on short trips, so that isn't the problem.

I recently had the alignment checked and found that the rear toe was slightly out of spec. It was something like .8 degrees. I can't picture toe changing so gradually as to produce a steady decline in mileage though so that's probably not it.

So I'm wondering what's up here. Two possibilities come to mind: 1) the AFM is operating, but out of spec, causing a rich condition. 2) The Cat is failing and/or clogged and restricting the exhaust. Or what...?

Appreciate any ideas about the cause or troubleshooting suggestions.

[Kalevera]

Mine gets low 20s, a 325iX/4 5 speed, but it's got 100k miles on yours and the 6 button OBC, so who knows. I just drive it.

best, whit

[SRR2]

Hmmm. I'd like to know why the steady decline from a consistent 26-27 down to a consistent 23. Does your car have the original AFM? Do you know what kind of mileage it delivered in its youth?

What do you think of the cat-degradation notion? Is there any reasonable way to determine if the cat is still flowing freely? I suppose full-power performance would be one way. As far as I can tell, WOT produces approximately the same acceleration as it always has.

I'm talking myself into a AFM problem. The design they use just sucks to high heaven (yes, I know this weakens my usual "BMW engineers know what they're doing" line. Lets blame it on Bosch then.). Instead of using the pot in 3-wire operation (sensing the wiper position by calculating the proportion of end-to-end element voltage at the wiper), they use the pot as a variable resistor. 30+ years of experience as a professional EE has taught me a few things, one of which is -- you *never* use a pot as a variable resistor if you can avoid it, which you almost always can. The reasons would fill a couple of pages and I'll skip the dissertation. So, in addition to the klugey nature of that AFM, they used the worst kind of transducer in the worst electrical arrangement of it. Let's suppose for the moment that the element has degraded through wear and perhaps the contact resistance (and CRV) has also increased because of wear. The AFM will report airflow too high and the mixture will be too rich. But then, why wouldn't the O2 sensor correct the mixture downward? I wonder how much authority the O2 sensor really has in setting mixture. Is it possible that the engine can run consistently 5% or 10% rich? Last time I pulled the plugs, they looked like they were running rich. Sort of dirty and dark looking, though there was no buildup of carbon. They weren't the nice light tan I usually find.

What's your experience with M20 and M30 AFMs?

[Kalevera]

Hi Steve.

Yes, I believe it's the original AFM, but have no idea what kind of economy it offered when new. My experience with the E30 iXes is limited: beyond doing some serious work on my own, we only have one automatic iX in the customer base. Ditto AFMs and electrical engineering, I have limited knowledge of both fields.

I thought about incrementing drag on the drivetrain when I first read your post, perhaps in the form of a bad viscous in either the transfer case or the rear differential. I would think that a clogged cat would exhibit different operating characteristics, and an engine operating condition would have PA emissions inspection data from the past two years to consider (unless you're one of the lucky people who doesn't have to do that kind of stuff). I suppose it would be possible for the VC to lock up over time, as they seem to do on the New Process 249 and derivatives found in various Jeeps. I'll have to research the procedure for checking the tcase...I assume it relates to driving in figure 8s on dry pavement and listening for tire chirp. Not sure that such wear would have a dramatic effect on gas mileage, but who knows. Plus, it seems like the typical failure points for the ZF transfer case are the front driveshaft splines stripping and slipping chains, although we all know that a VC isn't to be trusted. When the VC in the rear end goes, it's supposed to apply no lockup at all; the BMW approved testing method for these rear ends is to check the density of the fluid -- the silicone or whatever's in the VC will separate from the gear oil. Perhaps they assume that the only way they fail is with a broken seal. Again, just a theory, could be toast or not, I'm not sure

best, whit

[SRR2]

The PA emissions requirements changed (yet again) two years ago. No more tailpipe tests on older cars. Visual inspection is it, so not much help there. I suppose I could ask my guy to pull out his equipment and do the test anyway. The test is performed at idle though so it may not show too much involvement of the AFM.

However, now that I think about it, my wife mentioned that she thought the car wasn't rolling as freely as it had. There's a hill here on I78 that we drive on virtually every day. The Porsche will roll down at 83-85, the 540 at just over 80, but the 325 won't make it much over 70 any more. So you may be on to something here. It also sounds like something that could have degraded gradually. The front and rear differentials were serviced with new Red Line ATF (or whatever they recommended for differentials) about 15000 miles ago and got a clean bill of health from the tech, who reported that the fluid that came out looked new. But the fluid coupling is a horse of a different color. It bears further research, for sure.

[JonE]

How are the brakes, particularly those small "parking" brake shoes?

[SRR2]

Don't know exactly what you mean. The brakes are in good condition, have had reqular fluid changes and replacement parts as needed. They don't drag if that's what you mean. The brake performance on this car has always been horrible. The brakes are so undersized and weak that they can't, and have never been able to, generate enough braking force on a dry road to lock the tires (or wake up the ABS if you want to get technical). The parking brake is a bit loose. I'll have to deal with that when the weather improves.

[JonE]

Hmm, seems as if the brakes should work better than that. I can get the abs to activate on my 325iC, '89. They seem to work quite well, in fact. Previously, I was referring to the small brake shoes that are activated by the parking brake lever. At least on my e30 they are integrated into the rear brake rotors, and needed to be serviced/replaced. Just another thing to consider with your gas consumption issue. If they're shot, frozen, rusted, etc., they could be supplying significant "drag", as you say, even though the regular brake pads are working "fine" and not dragging.
I am just offering this up as another possible factor related to the consumption issue. Certainly seems an easier issue to check than all that all wheel drive stuff...
JonE