Erratic mpg gauge!

[JJ26]

Does anyone know if there are any common faults to look for on the mpg guage under the rev counter. Mine just flicks endlessly from one side to the other when your above or below 3000 rpm. Does this gauge work electrically or is it some kind of vacum system?

[Brianmoooore]

The gauge is electronic, and in spite of what some say, is accurate. Far more so than a vacuum gauge.
I wouldn't say the failure you have is common, but I have seen it before.
The effect you are seeing is caused by lack of speed data, and on one I tried to repair once, I found a burnt resistor in the path between the speedo and the economy gauge. Unfortunately, replacing this resistor did not cure the fault, so at this point I just replaced the cluster, as I have when I've come across this since.

[dark_sounds]

i too have a similar problem, if im sitting at stand still it flicks about about 5mm to a 1cm just randcomly up down up down, not doing anything to the engine, and when i am cruising , it also does it sometimes, has been getting worse recently...

[JJ26]

When the gauge is working correctly does it give you a reading from idle up or only from 3000 rpm upwards? My list of parts for this car is growing by the day

[E30BeemerLad]

My list of parts for this car is growing by the day

Welcome to E30 life fella
At least parts are easily available and in most cases not too expensive, put a wanted thread in the relevant section, someone will come to your rescue for most parts

[DazzyA]

I've only had my cabby about 3 months and have got through 3 clusters so far. The present one seems ok, but has 90k's more than the original.

[craigw8]

My gauge started flicking around randomly and the speedo and tacho stopped working. After looking through this forum I checked out the sensor on the rear diff and found that the cable had been cut and wired back together by idiots. Could be a similar problem on yours.

[TheDutch]

I knew someone who had a similar problem to this where everytime they touched the throttle the gauge would swing all the way to the right, and if they lifted off it would go all the way to the left. After ages of checking all sorts of stuff for months we happened to notice it was a 320i and was, in fact, all working normally.

However, I notice that this is not an M20 afflicted car, and I have to concur with what Brianmooooore says. Also, if there is no problem with your speedo then the diff sensor is presumably fine.

Does the economy gauge not derive its level from the diff signal and injector signal? Food for thought perhaps, but I think I'd look for another cluster if I was you, they are common enough.

[JJ26]

Everything else on the dash works properly and has show no signs of failure, the only symptom i have that could be related is when cold starting the car from cold it floods terribly untill about 2500 rpm. if you dont touch the throttle it will idle fine, but soon as you just touch the throttle (enough to knock the idle switch off) it pretty much stalls, hold it a bit more and it chugs like a car with too much choke. I screwed the mixture down a few turns to compensate after repairing a split intake hose. This made it a better for a while but now has gone back as bad as it first was a few weeks ago. Sound like the engine temperature sensor is on its way out?. Note the car is fine once run for 5 mins, although quite thirsty.