Why is my MPG surging?

[jog5000]

MY mpg guage seems to be surging.
I can be drving along, my foot not moving on the accelerator, and my mpg guage is moving up and down up and down... not a lot, but enough to get me thinking.....

http://www.geocities.com/jogsbmw320i/dash.avi Thats a video of it. Look carefully, you can see it fluctuating. I am not moving my foot, the car isnt surging forward..... Any help please!

[SAltonS]

If you are going up an incline or going down an incline, then the motor is using more or less power to keep the car moving. That might be the reason. Or am I totally out there.

[jog5000]

Totally out.... it happens constantly - up, down, flat..... constantly moves like that....

[amrrashad]

I'd turn on the cruise control on a flat piece of road just to make sure the movement is not due to your foot

[PhilipWOT]

I'd turn on the cruise control on a flat piece of road just to make sure the movement is not due to your foot

+1. Beat me to it.

[jog5000]

I would also turn on my cruise control..... only problem - I dont have it! The previous owner also told me about it, and described it exactly the way I have.

[qt02747]

Hi, my 318is (1998) have exactly the same problem, when coasting (not moving your foot) the MPG gauge surges and you can feel the power from the engine also doing the same. I do not know why no one else have experianced this with a salution to the problem. I have already checked my idle control valve, it is not the cause. The moment I find the cause I will post it. Maybe it is unique to South Africa.

[jbomz3]

This "eco-meter" or "mpg-gauge" as anybody ever done a blanking plate or cover for it ?
I absolutely dispise it , keep banging on about it my mz3 didn't have one thank god , but i find it really annoying having a needle moving in relation to how far my slipper is enbedded in the carpet . Even the high spec' stuff used to have them - totally beyond me someone spends £50k+ on a car why would they be interested in a contraption fastened to the throttle cable/flap . :dunno: . JJ .

[jason_recliner]

The guage will go up and down slightly even with cruise control on.
Don't worry about it, it's only an indicative reading anyway.

[jog5000]

The guage going up and down is not the issue - the issue is while the guage is going up and down, the motor is revving faster then slower at the same rate of the guage going up and down...

[RAFVC10]

As of yesterday I'm having exactly the same problem. Did anyone get a solution?

[Foxider2012]

I would say maybe your Fuel Pump is goin or the fuel pressure is being disrupted some how... Maybe a dirty fuel filter

[BMR_LVR]

I would say maybe your Fuel Pump is goin or the fuel pressure is being disrupted some how... Maybe a dirty fuel filter

I could be wrong (wouldn't be the first time), but I believe the MPG gauge operates on engine vacuum. It may be that you have a leak in the vacuum system. I’ve read a lot of posts here about vacuum leaks and the problems it causes.

[Landshark 328]

I am also out there but try this. Go to your obc and while driving reset your average mpg gauge. When first being reset it will fluctuate just as much as the instantaneous gauge. See if that fluctuates as well. If this does not fluctuate then this could be an indication that your guage is messed up. Do you know what your car is averaging on gas? Fill it completely up and drive 100 miles/ kilometers(whatever applies) then fill it up after driving that distance and use the gas you put back in to calculate your average mpg. If its is around 23- 24 I wouldn't worry about it

[Landshark 328]

the fuel pump +1

[zachiepie]

I dont often trust the MPG gauge and the OBC MPG rating that often. It gives you a good idea, but its a lot more reliable to do the math yourself. Say you gassed up 15.5 US gallons and only drove 410 miles on that tank, you only get 26.7 MPG (or about 9 L/100km if my math is right). If the MPG gauge is fluctuating all over the place and not giving you accurate readings, try doing that math just to be safe. If it lines up with what the gauge is telling you, its probably a vacuum leak somewhere.

Thats just a thought, anyways :)

[BMR_LVR]

The guage going up and down is not the issue - the issue is while the guage is going up and down, the motor is revving faster then slower at the same rate of the guage going up and down...

Now I get what you're saying ..... not just an issue with the guage, but the engine RPM's are fluctuating. Knowing this, I would say that the MPG guage is simply reacting to, or indicating if you will, the relative pull on the engine as it should.

Erratic RPM's are fairly common on the E36. I wish I had the knowledge to point you to the exact problem, but I don't. Try searching this forum for "surging RPM's" or the like.

Let us know what it was once you get it fixed.

[lakers2k177]

Im am having the same problem on my '03 325i, however my MPG gauge surges as the car is idling when im slowing down to come to a stop. The RPM's is about 600 and that gauge doesnt move only the MPG gauge will move from the 50 (all the way to the left) mark to about 30 or so then go back all the way to the left. It has never done this. I just passed 100,000 miles. Dont know how serious of a problem it is but it definetly is a problem or an indication that something has failed.

[MrAbbs325is]

Ever try reading your codes? It could be a number of things from a mis-communicating O2 sensor to a throttle position sensor to a MAF sensor, vacuum leak, and a fuel filter. Your check Engine light will not come on for every code if it is sporadic.

[jog5000]

Had the codes read, doesnt throw any out there. Still been bothering me - cleaned the ICV as well, still nothing... It sometimes feels as if a brake is dragging slightly - when you are cruising at highway speeds, and you can actually feel that you are moving slightly forward in your seat, as if you are ever so slightly braking.....

At the moment, judging by my OBC, I'm getting 11.4 l/100km.... thats with rather spirited driving/heavy foot...