How Linear Is The Fuel Gauge

[starman]

I've had my 1995 3door petrol Rav4 for just over a week now and I have noticed that the fuel gauge is erratic to say the least.

When I bought the car the gauge needle was about half way up the first quarter zone. I understand the tank is 13 gallons so each quarter segment should equal just over 3 gallons. I put in 8 gallons and it moved to halfway up the 4th quarter indicating about 11 gallons which was believable. Over the next 100 miles the needle did not move (except for going up slightly when driving uphill and down slightly when going downhill). Over the next 40 miles it fell quite quickly to just under the 3/4 mark (just inside the top of the 3rd quarter). After a halt at some lights when I started off again the gauge went back to halfway up the 4th quarter. At this point I lost confidence in it and decided to top up. The tank took 6 gallons to fill until the pump cut out. The gauge was then off the scale and I had done 160 miles since the original fillup. If the car came with 2 gallons (3 at most) and I have put in 14 but have 13 left then I have used 3 or 4 gallons to do 160 miles which I cetainly don't beleive.

Could anyone with an early petrol Rav tell me how they interpret the fuel gauge. I suspect mine is faulty, maybe a sticky float lever. Is this accessible without removing the tank?

Thanks

Dave

[bothwell_buyer]

A subject close to my heart.....

The 1994 RAV I have will probably have the same tank as you have. I ran the car dry of petrol yesterday and filled the tank up today - and it took 53 litres to the first click/cut-off.

I replaced the tank a year ago as the previous tank had been used by someone to jack the car up. The sender unit and float can be accessed by removing a plate on the floor under the rear left seat. This gives you access to the top of the tank and the screws that hold the assembly onto the tank itself. Lucky you though. There is another fuel sender unit beneath another plate on the rear right side under the seat!!

The tank straddles both sides of the car and thus over the propshaft hence the 2 senders.

from what you say, it sounds as if one of these is maybe faulty. When I removed that tank, there was a lot of corrosion on the pipes and a lot of muck around the electrical connections...it may be that its an electrical connection fault.

My car had been off the road for more than 2 years which didn't help.

Anyway, now the gauge is slow to get to the full position, and is not showing full til well after leaving the petrol station. On the gauge accuracy, given the simplicity of the gauge and the complexity of the tank with 2 senders and various baffles, I don't pay any attention to it until the warning light comes on - by which time I reckon I have a gallon or 2 left to get to the garage. If its the original tank, as you know its always better to avoid emptying the tank in case you begin to suck up debris from the bottom. I don't bother as I washed the replacement tank out.

So there we are - fairly easy to check the senders, but I think you'll need to use WD40 to soak the 8 screws first. Then if you do that take care in replacing them to ensure a petrol tight fit.

Take care you don't destroy the fuel outlet pipe/ banjo thing!

I have the minibus running on propane and propane gauges are notorious for being a waste of time... so I use the trip meter on that one - the tank takes 80 litres maximum, and no matter what way I drive it, I will get at least 240 miles on that. I normally get more but its easier to refill with propane after 240 miles. (Mileage should be a lot better but despite a clutch of engine tuning specialists they can't find the reason behind the rich mixture).

happy motoring...

[starman]

Hi Bothy and thanks for the info. It does raise some more questions as to the way the system works.

So if i'm understanding this correctly the tank has two compartments (one either side of the propshaft) which are "bridged" over the propshaft.

So when I fill with fuel the nearside one fills up and then spills over to the offside one? I thought I read somewhere that the fuel pump is in the tank, if so there would have to be one in each compartment. If the fuel pump is external I guess the feeds from each compartment would "T" together somewhere and fuel level in each tank would balance via this T. (Hope that makes sense) Are the two level sensors wired in series or parallel?

I can't wait for the workshop manuals to arrive (I've ordered four) so that I can get my head around things like this.

Dave

[anchorman]

You are in the hands of the 4.1 guru with the info bothy has given you but I thought I'd just chuck in some general information.

It is not uncommon these days to find tanks that straddle the prop shaft as the safest place for the tank is ahead of the rear axle in the event of a severe rear end shunt. The tanks do not usually rely on "spill" from one side to the other, there is either a low point or a pipe to feed the lowest point which is a well containing a permanently submerged electric pump. This pump will stay submerged even if the tank runs out to reduce the risk of arcing inside the tank. Diesels just draw from the high pressure pump on the engine. I should point out that there are sometimes two sumps and two pumps, i.e. everything is duplicated but fuel is pumped to the front through a common pipe.

The reason for two senders is to reduce the amount of innacuracy if the fuel is surging from side to side during cornering and the level is an average of the two (or to cater for the less common "two sumps". The fuel sender is just a rheostat (a set of windings accross which a contact fixed to a float arm moves) on the earth circuit of the fuel gauge - again to reduce the risk of a spark in there! Its a bit like a dimmer switch on a domestic light and the more fuel in the tank will turn up the "brightness" of the gauge to show more content of fuel. These are fairly crude devices and although they can be "weighted" to take the shape of the tank into account they are seldom accurate. Lots of cars have a slow half of the tank where they seem to go for ever on an amount of fuel then you can watch the thing go down - or vice versa. For that reason most modern cars (and you will have to tell me in the case of a 4.1) have a low fuel warning light to assist and by noting when this comes on it is possible with use to get a fairly accurate idea of range when this comes on.

With regard to the manual don't be too dissapointed if the info on the tank is fairly vague unless you are lucky and petrol tanks are the authors specialist subject!!!