1996 C200 Fuel guage readings

[johnnyj]

Hi all

Still getting to know my new C200, hence this is my third post on "oohh should it do that" type issues.

My car's fuel guage reads a bit oddly. Basically, the reading is never the same between engine switch off/on. For example, I can have a 1/4 tank when I park the car up for a the night, then in the morning (with no intereference at all) the dial reads something different (usually lower). There are no leaks and nobody is pinching the fuel.

It is a bit disconcerting as one minute I think I have plenty of fuel, and the next moment the warning light is on. As a result, I am filling up all the time. £20 of bog standard unleaded lasts for about 150 miles, based on putting more petrol in when I get nervous about the guage.

Any ideas?

Many thanks

John

 

[sunking101]

If it's definitely not registering correctly fit a new sender unit. The mileage you're getting seems about right so if you've no leaks it must be the sender.

 

[jberks]

I'm not so sure - it depends on how different. The fuel gauge does average out the reading to some extent - otherwise it would be bouncing around as you go over bumps, round bends etc, so the 'average' is always going to be a bit higher at the end of a journey that at the start of the next one. Personally, once the level drops below 1/4 I start to trust the gauge less and less as it falls no matter what car I'm driving. I have frequently parked up with around 1/8 of a tank showing on the guage and returned to find the light stay on as I start up. You generally know yourself how long its been down there so you have a pretty good idea whether you're pushing your luck or not.
I would doubt the sender would be responsible as it would be consistent even if its wrong. When my sender failed, it just wouldn't go to full, I've had a ford one that refused to go below 1/4 (I though I was getting great MPG! - till I got suspicious and put 9 gallons into my 1/4 full 8 gallon tank). At the end of the day its just a float attached to a position sensor, which is little more than a piece of resistance wire.

It could be lower electrical current in the morning causing it to misread,- the pick up is voltage/resistance related- but I would suspect you would have other problems too as MBs don't tolerate poor batteries too well.

Does it rise as you drive along, or stay lower?

 

[996jimbo]

This is perfectly normal, at least I hope it is as mine does it (C43) and various cars I've had in the past have done it to a greater or lesser extent. On some cars the guage goes up, to do with fuel returned to the tank from the system when switched off. The guage dropping might be something to do with priming the system when you start up (taking a big gulp of petrol out of the tank) combined with the guage taking an average reading.

Fuel guages are pretty approximate - you will no doubt have noticed the amazingly good fuel consumption you get from the first 1/4 tank and watch in horror as the needle gathers pace on its downward plunge to empty.

 

[johnnyj]

Fuel guage

Thanks all! Yes, I put £20 in and feel very smug for about 2 days, then start to worry. It has got to the stage where I look at the half full guage on parking up and think "mmmm, you won't be there in the morning...."!!! and sure enopugh the light is on, saying "fill me up".

I guess I have been a bit spoilt as a previous diesel Mondeo owner.

Thanks again

JJ

 

[jberks]

If anyone has looked at a late 1980's 3 series (E36) you'd notice that the first 1/4 segment is far smaller than the last 1/4 segment - I guess this is how BMW got around this. I personally find the 1st quarter to disappear more rapidly than the last one - drops off full almost immediately.
If you can, fill it up and then re-top it whenever it reaches 1/2 (costs the same after the initial pain of filling it up) - that way you know you've always got fuel. I believe its actually cheaper to do this in the long run as the less free space in the tank, the less air there is for the fuel to evaporate into and dissapear down the vent.
This may even be part of your problem?

 

[996jimbo]

...Except of course that the bigger fuel load you carry the heavier the car and therefore the more work it is doing. Mind you, I'm getting a bit Formula 1ish now - the rubbish I carry around all the time weighs more than the fuel I put in it.