Terrible Fuel Mileage

[Scorpius90]

Hey all,

Have a 95 525i Auto that I daily drive. Was doing some quick research, and I get some pretty bloody terrible fuel mileage. To a tank of fuel (89 Octane as recommended) I'm getting 400KM, or 250 miles for you American folk. Need some ideas as towards why this would be happening, I'm planning on switching out the plugs on it for some new OEM ones, my tires are filled as per the door card recommendation. I bought this car as a daily cause I figured it would be better than my big turbo Supra on fuel, but my Supra literally gets better fuel mileage in the city than this thing does.

Any ideas?

Thanks for the help!

[theBMWbeast]

Vacuum leaks, ignition tuneup, simple stuff usually gets you.

[moroza]

E34 are not known for good fuel economy. If that's the reason you bought it, I recommend something else. 400km out of an 80l tank is entirely possible even on a perfectly healthy car, with short trips, cold weather, lots of idling, and a heavy foot. That said, it is quite low for a 525i. Possible causes include: bad O2 sensors, dirty injectors, bad ignition parts (spark plugs to use are NGK BKR6EK), intake leaks, fuel leaks, bad coolant temp sensor, stuck open thermostat, dragging brakes or wheelbearings... Is your Check Engine light illuminated (and does it work)? Any other symptoms such as misfiring, smoke, smell, or noises? What kind of consumption do you see at highway speeds?

my Supra literally gets better fuel mileage

As opposed to figuratively getting better mileage?

[Scorpius90]

I just swapped in those plugs today, did a quick Google search and I was told those were the plugs to put in. The plugs that were in it were absolutely terrible condition and were NGK BKR6IEX Iridium plugs. No fuel leaks or open thermostat, the car never came with a thermostat. So I had that done and the shop I took it to put the aluminum housing on along with it. The Check Engine Light isn't illuminated, so I wouldn't think there are any codes being thrown. It's funny you mention misfiring, that only JUST started happening. Even that is on a cold start and has only happened twice so far. Hasn't happened yet with the new plugs, but if it does I'll suspect the ignition coils. I did check the wheel bearings when I put some better OEM wheels on it and they felt pretty solid. The car only has 214,000KMs on it, so the car for the most part is in relatively good shape. The brakes aren't dragging either.

Basically as far as I can see there aren't any out of the ordinary symptoms that the car is giving off. Consumption at highway speeds is better(ish). A 170KM trip takes about 25% of the tank. Which I assume is higher than normal.

Would it help to run some injector cleaner through it? I know that stuff is absolutely terrible, but I'd rather not have to rip into my daily driver and change out the injectors.

[Scorpius90]

Edit: Double Post.

[fo3]

What's your OBC saying for consumption?
Mine gets 9l/100km highway and 13l/100km city. I mostly do suburban driving with lots of traffic light stop/starts so mine averages at 12.6l/100km currently.

I went through the same thing as you, vac leaks, plugs, ego sensor but it is what it is - a small, low torque 6cyl engine in an automatic and heavy car that really suffers in stop start driving.
Highway economy is only good due to 5spd auto and not having to move a heavy car from a stand still all the time.
I can't tell you how much I get from a tank because I rarely fill it up - a full tank costs me about $90 or more at $1.50/l. I think from memory the only time I ran a full tank or close to it was when I was getting the suspension and alignment stuff done maybe? I think it was about 550kms from full to empty but don't take my word on that and as I was driving to mechs or parts shops there was some highway/freeway driving.
Mostly I fill 1/2-3/4 tank ($30-50), and get about what the ODB says - 13L/100kms, ie 200-300kms out of 1/2-3/4 tank top ups.

E: On re-reading, yours seems about right to me 1/4 tank (20l) = 170kms. Though the fuel gauge in mine is screwey (maybe it is in yours and everyone's? IDK), I get a different needle position every time I look at mine or start the car. Always starts high then drops while driving but sometimes goes up again. Always then goes up again next time I start the car.
My previous cars are a rx3 (15l/100km), 1997 aussie ford falcon sedan (exactly the same economy as my e34, even though it had a 4L engine - was 100kg lighter, 20kw more power and waaay more torque so a lot faster and more fun) and a 2006 ford falcon ute - same weight as a e34, maybe more as it had the back full of tools and gear, 182kw 4L i6 and did slightly worse at 14l/100km

[Scorpius90]

My OBC says average fuel consumption in the city is around 15L/100KM and around 10L/100KM on the highway. I don't know maybe you're right fo3 in that it's just a part of driving something with no torque with a huge car and an automatic transmission bolted to it. Doesn't help with the fuel economy of everything.

[moroza]

US and I think Canadian 525iA had a 4-speed auto. Australia and the rest of the world got a 5-speed. A 525iA is about 1500kg/3300lb; I wouldn't call that huge or heavy. 10l/100km highway and 15 city sound about right, maybe a little low.

[fo3]

US and I think Canadian 525iA had a 4-speed auto. Australia and the rest of the world got a 5-speed

Yeah I know, I was saying I only get as good as 9l/100km on the highway because I got the 5spd. I expect worse with a 4spd/

A 525iA is about 1500kg/3300lb; I wouldn't call that huge or heavy.

1500kg is up there with the heaviest cars I owned (falcons), which are physically bigger and a larger heavier engine (which has 100nm more torque way down low in rpm that helps with fuel economy) 4spd auto, 9l/100km highway, 13l/100km city in them. Same economy as my e34, ~same weight, the fact the e34 has a smaller motor does not help as the game is all about weight and torque.

My OBC says average fuel consumption in the city is around 15L/100KM and around 10L/100KM on the highway.

Highway seems right so there doesn't seem to be anything hugely wrong, but 15l/100km city is a bit high. Maybe do some more poking around for air leaks and sensors and the other things moroza mentioned. oxy sensor and coolant sensor are the obvious ones to me that would explain 2l/100km higher than normal city consumption

[CarterH]

You'd be surprised in the city. In endless stop start traffic, my 535 I was sitting around 19L/100km. My combined of city and highway/country on average was between 12 and 13L/100km which is about what you'd expect considering the age, engine size and weight of the car.

[fo3]

Yeah, the economy of the 535 scared me before buying one as I heard don't expect under 15l/100km in the burbs/city and I'd had enough of that with my falcons. I was hoping to get a little bit closer to the official euro figure of 9.9l/100km - allowing a little bit for weariness.

If you're talking city, like a huge one like Melbourne with traffic lights every 500m I don't doubt 19l/100km. Luckily I'm in the outer suburbs of a smaller city and sometimes I can drive even up to 1.5-2km at 60/70/80 km/hr using 8-10l/100km on flat stretches without a stop
No doubt those decent runs help offset the 25-30l/100km acceleration from standstill consumption I get from all the traffic lights I have to suffer through on my short commute. I drive extremely economically where I can and even worked out the best route in order to limit stops and maximize cruising.
Cars with twice the power do a lot better in fuel consumption these days. Even e34/e39 540i drivers can get better figures due to more torque.

To be honest I expected someone to post something saying the OP is doing something wrong and maybe I'd learn something I expected to OP to be doing better than me in the outer city/burbs as well, after all wasn't that the point of vanos? I've got larger cams with more overlap and expected a later vanos build to be doing better on economy - at least in the city where the 5th gear doesn't mean anything. We're both a far cry away from the official 9.9L/100km mixed figure - I've not done better than 11.9l/100km truly 1/2 - 1/2 city/highway mixed

[Scorpius90]

Yeah the 4-speed automatic doesn't help at all in terms of fuel economy. I really wish I could find a 5-speed automatic and get away from the 4-speed. but I would need the trans ECU at that point too.

I think I have a bit of an exhaust leak so that might be hurting fuel economy too, I can hear the distinct clicking of an exhaust manifold leak. I'll have to go through the sensors, I tried doing the gauge cluster test yesterday, and no codes popped up. But I guess I'll have to hook up a scanner to it and see what's going on.

[moroza]

You could try a taller final drive. The 5-speeds had a 3.23 vs. the 4-speed 4.10, though the latter's top transmission gear is actually slightly taller, at 0.72 vs 0.74. I picked up ~10% economy on the highway, with no apparent change in the city, after replacing my 544iT's 3.15 with a 2.65. I find most BMW geared too short.

My 525iT with a stock 4-speed driveline gets about 9.5l/100 at 120-130kph. That's with tall squishy tires, worse aero, and more weight than a sedan. Mixed tanks... depend entirely on the mix; it's possible to get exactly 0mpg by idling the tank empty without moving. (I wonder how long it'd take to do that?) My closest guess to 50/50 nets about 11.9, driving moderately.

[Scorpius90]

Mixed tanks? What do you mean by that? A 50/50 mix of what?

I mean a taller final drive would be sweet for sure, but I don't think the M50TU in my car has the torque to push a heavy car like the E34 around with such a tall final drive.

[Honolulu]

Another data point from Honolulu, Hawaii, as I've been really disappointed with my '94 525iA. After 170,000 miles, and around town, I get about 16 mpg (US) which equals 14.7 liters per 100 km. BMW was never known for good mileage, and never will be. The company pays the highest fuel economy penalties in the world, for all makes, just to sell in USA. It must be working for them... but not me. I know the PO had the trans rebuilt years ago. The takeoff from stop is really poor and maybe the trans is at fault, but the federal mileage rating in city driving is about 16 mpg, so I'm getting about all that can be expected. Good thing I got the car for $0. Better thing that I sell it ASAP, but my bike is on the fritz and I need transportation, so it's the E34 for me, for a while.

My wife's '98 C280 Benz (of course also an auto trans) does better, and has about 100,000 miles. It runs much stronger too.

[tombo]

My car is a far different beast from your M50 powered cars, but my 1992 535iM with a 3.91 LSD just returned 27.5 MPG on a cross country road trip. Granted when I'm in town I hoon about like a hooligan and get 17-20 mpg, but I figure it's totally worth it for the fun. I think that works out to like 10l/100 (ish) highway and 14-17l/100 city for you metric brethren.

[fo3]

Yeah, twin exhaust pipes all the way to the rear muffler and large cams/valve overlap - these engines weren't made for torque - rather the scream up top with the most power they could get. Not enough torque, not enough down low to push a 1500kg car in stop start driving economically.

If it makes any e34 owner feel better even a new toyota camry has the same problem these days. They got as heavy as a e34 and if you have one with the smaller 2.4l engine (similar power and torque) they do similar economy around town, 13-15l/100km I've read. People that got the larger v6 3 or 3.5l toyota motor get more power, torque and economy in their camry/aurion in city driving.

So the answer is stroke and boost your 525i

[Scorpius90]

Gonna be honest, I've been thinking of switching things up a bit and getting a later facelift E38. There are a couple in the states I could import to Canada. Would be a royal PITA, but would be worth imo.

I was also considering slapping on a small Borg Warner 56mm but realized that I already have a Supra that soaks up enough of my funds lol.

[moroza]

50/50 city/highway. E38 will help neither the fuel consumption nor the funds soaking. What about a 525tds?

[Honolulu]

my ''87 735i got 27 mgp at freeway speeds.

11 around town.

[fo3]

Gonna be honest, I've been thinking of switching things up a bit and getting a later facelift E38. There are a couple in the states I could import to Canada. Would be a royal PITA, but would be worth imo.

I was also considering slapping on a small Borg Warner 56mm but realized that I already have a Supra that soaks up enough of my funds lol.

I think you're looking the wrong direction. A e38 is 300kg heavier so even if it has more torque or a more efficient engine it's still auto and you're not going to get better than 15l/100km in one of them IMO. Power to weight ratio if you aren't looking at an e34 with a bigger engine and man trans is probably a manual e46 330i m54b30, which should get closer to 11l/100km in the city and has 170kw/300Nm so it's more fun (E: even if it weighs close to a e34, E2:and even if you're going to be open to a whole lot of "it's a e46" issues with regard to switches and rear strut mounts)

[Scorpius90]

Hmm, so swapping a V8 with a manual transmission into my E34 will help with my fuel efficiency is what you're saying. Now that can be a real possibility.

[CarterH]

A swap would be the cheapest option. A manual swap isn't too difficult. The only issue would be that TRANS PROG error but I recall that can be fixed with a simple wire under the shifter console. There are a few guides around about it.

A 7 series would be a true gas guzzler since they're not built with economy in mind at all and aren't exactly sporty either, so it would be a large slow fuel hungry car. Excellent but very hungry. The thing about older cars is that fuel economy is going to be difficult to achieve, if at all. The guy I sold my 535 to has to put in about $20+ in fuel each day for work thanks to the stop start of the city and it being a big old heavy car.

[moroza]

The thing about older cars is that fuel economy is going to be difficult to achieve, if at all.

Actually, a lot of the most efficient cars made are from the 80's. While there have been some mild improvements in engine efficiency since the early days of EFI, it's been more than outpaced by weight gain, such that US average fuel economy peaked in 1985, followed by a decline until 2005 or so when hybrids started to get common. E34 don't get mediocre economy because they're old, it's because they're 6 or 8-cylinder, RWD, not especially light, and not especially aerodynamic. A newer car with similar dimensions including displacement and cylinder count, such as an E60 535i, gets about the same mileage - 2010 535iM 16 city, 26 highway - albeit with a lot more power (nothing a turbo M30 can't do). An 80's Civic will get better economy than any petrol BMW, and likely better than a new Civic for that matter. You want city economy? FWD, light weight, small and few cylinders, diesel, driving style with a lot of coasting. You want highway economy? Same deal, except gearing matters more, weight matters less, and aerodynamic drag matters a lot more. Want better economy without driving an underpowered tin can? Turbodiesel. Hence the 525tds suggestion.

[bigsixe34]

I think I got around 19 MPG (12.4L / 100km) combined in my 525i, and around the same with an S52.


Owning an m30 car was the worst by far in that department. Don’t think I ever did much better than 14 MPG (a whopping 16.8 L / 100km!!)


The OBC in my e53 w/ m54 always ends up back at 20.1 MPG (11.7L / 100km). That’s about 90% city. On the interstate I’ve reset and gotten as high as 27 MPG (8.7L / 100km)


If you’re top priority is fuel economy an e34 is not your car.

250 miles on a 21 gallon tank is 12 MPGs, i.e. something's not right....