Gas Mileage is terrible.

[Dom]

I have a 1991 525i, black on black in very good shape, 170,000 miles. I take care of it and baby it. I guess you get the point this is not a beater.
My problem is that I'm only getting 13.8 mpg city driving. When I bought this beauty it got around 18-20 around town.
The plugs are maybe 30,000 miles old, do oil chages twice a year. Tires are in good shape and checked. It passes the smog tests but I'm not sure how much my smog friend changes things. Air cleaners are good.
Anyone with an idea?
Love the E34, wish it was a M5, best looking car bmw made.
Thanks Doug V.

[shogun]

Make a stomp fault test, details on my website, or if you have a code reader like the Peake fault code tool, easier.
Ever changed the O2 sensor?
O2 sensors do wear out and eventually have to be replaced. The performance of the O2 sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce its ability to produce voltage. This kind of deterioration can be caused by a variety of substances that find their way into the exhaust such as lead, silicone, sulfur, oil ash and even some fuel additives. The sensor can also be damaged by environmental factors such as water, splash from road salt, oil and dirt.

As the sensor ages and becomes sluggish, the time it takes to react to changes in the air/fuel mixture slows down which causes emissions to go up. This happens because the flip-flopping of the fuel mixture is slowed down which reduces converter efficiency. The effect is more noticeable on engines with multiport fuel injection (MFI) than electronic carburetion or throttle body injection because the fuel ratio changes much more rapidly on MFI applications. If the sensor dies altogether, the result can be a fixed, rich fuel mixture. Default on most fuel injected applications is mid-range after three minutes. This causes a big jump in fuel consumption as well as emissions. And if the converter overheats because of the rich mixture, it may suffer damage.

You can read the O2 sensor's output with a scan tool or digital voltmeter.

A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum (0.1 v) to near maximum (0.9v). Making the fuel mixture artificially rich by feeding propane into the intake manifold should cause the sensor to respond almost immediately (within 100 milliseconds) and go to maximum (0.9v) output. Creating a lean mixture by opening a vacuum line should cause the sensor's output to drop to its minimum (0.1v) value. If the sensor doesn't flip-flop back and forth quickly enough, it may indicate a need for replacement.

If the O2 sensor circuit opens, shorts or goes out of range, it may set a fault code and illuminate the Check Engine or Malfunction Indicator Lamp. If additional diagnosis reveals the sensor is defective, replacement is required. But many O2 sensors that are badly degraded continue to work well enough not to set a fault code-but not well enough to prevent an increase in emissions and fuel consumption. The absence of a fault code or warning lamp, therefore, does not mean the O2 sensor is functioning properly.

copied from http://www.autotap.com/techlibrary/understanding_oxygen_sensors.asp

[Dom]

I didn't even think about the O2 sensor. It was last changed at about 90,000 miles if I remember correctly.
Thanks, I really appreciate the help.

[632 Regal]

Hi Dom,

Good to hear from you agian!


I didn't even think about the O2 sensor. It was last changed at about 90,000 miles if I remember correctly.
Thanks, I really appreciate the help.

Shogun is correct, do a stomp test and see if you get any codes. That is the first thing to do. On O2 sensors we have different opinions and while he is correct... I dont think the peak tool checks faults but resets them. Correct me Erich (Shogun) if I am incorect.

I have never changed O2 sensors 280k, 220k, but V8 models and there are 4 of them. I'm just like that. Doesn't make me smart or anything.

Pull the cover and see if you have a bunch of oil in the spark plug wells. What plugs are in there? I have seen people replace them with single ground plugs that widen from the heat to +.090 and simply only fire on small load. My sons E36 was an example, between .065 and .090 and ran weak. I pulled the plugs and smacked the gap to .030 and it was a whole new engine. Never could tell there was a miss before I re-gapped either. Another thing to consider is the plug boots. Any oil leaks?

Intake air leaks cause meyhem along with the PCV valve being bad. Before sinking hundreds check the basics first. If it gradually changed mileage since you bought it my first guess is the plug gap widened or the wells are full of oil. My guess is the plugs. Check all the screw clamps on the intake system too. Look at the vacuume tubing for cracks and leaks.

Let us know.

[shogun]

I changed 2 O2 sensors some years ago on my 750, could gain a fuel savings of abt. 2 liters/100 km. 2 liters sounds a lot, but the 750 is a gas guzzler and for city driving accdg to the owners manual it consumes 20 liters/100 km, actually more. But I could notice the difference with the new O2 sensors.

Peake tool: yes, the R5/FCX-3 is a Scan and Reset Tool. Functions:
Scan/Reset Tool for BMWs
ML reset (resets "check engine" or "Service engine soon")
Oil service reset
Inspection reset

So for O2 sensor one could read with the Peake as fault: oxy sensor heating relay, or oxy sensor.
But oxy sensor does not automatically mean the oxy sensor is the problem, the oxy sensor just reports back what comes out of the engine and not what all comes into the engine, like for example faulse air, as Regal already mentioned. If the air leak is between MAF and intake, it is not measured.

Lifetime of oxy sensors: normally heated sensor lifetime is typically 100,000 miles (160,000 km).

[genphreak]

On an M50, the fuel pressure regulator has a little air line to under the manifold that can go brittle/crack up. It's only 8" long or so but causes mayhem if it's going bad. Also, check the main intake leakage points- probably due for an ICV connector repalcement under the intake manifold behind the throttle body, its clip/lock breaks and it has a viton o-ring/seal- plus the two ICV pipes of course- and check the ICV is moving freely/not sticky.

[Yanga26]

Hi,regarding this issue,i will suggest to check /replace the ignition coil.hope this help!
BTW, can someone tell me if it is posible to use ,let say,e39 540i ignition coils pack on an '95 e34 540i?
Thx in advance........Yanga26

[shogun]

Use www.realoem.com and find the part number from an E39 540, copy that and go back to the first page of realoem and paste the number there in search by part number and all BMW with the same part number will show up.
I once tried to use for a test ignition coils from an E36 on an E34 540, they did not fit, length was different.

[632 Regal]

Hi,regarding this issue,i will suggest to check /replace the ignition coil.hope this help!
BTW, can someone tell me if it is posible to use ,let say,e39 540i ignition coils pack on an '95 e34 540i?
Thx in advance........Yanga26

As Shogun mentioned, cross reference them at realoem.com

http://www.bimmernut.com/forum/showthread.php/45049-Gas-Mileage-is-terrible?p=348706#post348706