Fuel guage

[Bob and Carol]

Hi all,

Got myself a new 420 and just working through a few anomalies with the car.

I find the fuel gauge drops quickly and the tank does not take much to fill up. So ran the car until the pointer was almost at the bottom of the red then siphon out 9 liters. With a 36 liter tank I thought this was poor.

My questions are,

Is this normal?

Is there a way to correct this?

Does anyone know what a safe amount would be in such a wide tank so as not to have a miss fire and damage the Cat.

I did a search on “fuel gauge” but it did not come up with anything, as this is my first post so it may be me.

 

[Jonathan Kay]

Welcome.

Yes, it's common for the gauge to be very inaccurate.

There's a variety of interventions:

Use distance travelled since last fill rather than the gauge. But two hints. Carry a spare can while you're running it low to see how far it will go! And some don't recommend ever running a tank to empty because it might suck crud.

Add new markings on the gauge by adding known volumes of fuel from empty upwards. Same two hints.

Add an electronic widget which remaps the resistance shown by the sender to the resistance seen by the gauge.

Bend the float wire in the tank so that the resistance shown by the sender bears a closer resemblance to the amount of fuel remaining.

John Vine's excellent and detailed experiments are the best starting place.

Jonathan

[Jonathan Kay]

"I did a search on “fuel gauge” but it did not come up with anything, as this is my first post so it may be me."

It isn't. The site offers two different search functions, both linked from this page. I get much more helpful responses by using an external search engine with site restriction. 

That's discussed in this thread: Search facility. And you can try it from the link in my Signature below,

For fuel gauge that gives: https://duckduckgo.com/?q=site:caterhamlotus7.club+fuel+gauge.

Jonathan

[Bob and Carol]

Thanks Jonathan.

[ChrisC]

Welcome.
If the tank is bone dry (ie new build) it won't start with 5lt, a second can is needed.  So not sure where the "run out" line is but at 9lt I think your pushing your luck.    So your fuel gauge is sort of accurate to my experience, and that's not surprising given the pickup/fuel pump/sender is all one unit.  
That said they fuel pump unit can be fitted incorrectly, but this normally results in a low full reading. 

I think your fuel gauge action sounds normal to me.  Try it on a track day and see how fast it drops

[Geoff Brown]

The 'guess the fuel-o-meter' is a very common problem.

With experience I have learnt that from a brimmed tank with the fuel just visible in the fill tube the needle is on 'full' & remains there for at least 20-30 miles when on the move. When driving indication at  7/8ths mark = full. At  5/8ths mark = 3/4. At 3/8ths mark = 1/2. At 1/8 mark = 1/4. All indications are approximate but 'close enough for Government work'.

At zero there remains approximately 1/8 of a tank left which for my R400D is about one gallon or 22 miles. How do I know ? Returning from Quay For My Car a few years ago SWMBO delighted in telling me as we entered Marlborough after a good Blat over Salisbury Plain that the fuel gauge needle had been resting on zero 'for a while'. 

With Swindon 'only' 12 miles away I eased off the gas a bit. Got home OK & refueled car next day. The familiar sound of fluid on bare metal told me that I had cut it fine. The quantity put in to brim also indicated how close we were to an unscheduled stop at 23:00 in the middle of nowhere !

So ever since fuel anxiety sets in at an indicated 1/4 even though I know I have more fuel than that available.

Having the 'calibration' in my head I see no reason to interfere with the indication system.

BUT on longer journeys I always carry 4.5L in the boot - obviously in a fuel container 

 

[John Vine]

Yes, this is a well-known problem, certainly in pre-2015 cars. 

Due to the design of the tank and the position of the pump, there's at least 6L of unreachable fuel (more in an SV).

To improve gauge accuracy on my 2008 car (specifically in the Empty to 1/4 range) I simply bent the float arm.  See this Guide.

Whether this works with a new car I'm not sure.  The pump is a similar design, but of course the gauge has changed over the years.

The alternative (which I tend to use anyway) is to drive on the trip.  I fill up when reaching 180 or so, and reset the trip to zero.

JV

[ScottR400D]

My fuel gauge, as well as being inaccurate, is inconsistent. 

I filled it last weekend. I topped it off with a jug and it was brimmed. The gauge showed just over 3/4 full. 

Took the car out and it was showing just under full. Stopped at car meet. It was still just under full. 

Left car meet and it immediately dropped to 3/4 or so.

Back at home it now shows the same as when I left after about 80 miles.

I've given up worrying about it, I just fill up asap after it drops below 1/2. I don't think I've ever put more than 22-23 litres in and it's supposed to be 38 full. 

[Golf Juliet Tango]

I'd just drive and become familar with things. Use the trip meter every time you fill up and see what the car will achieve.

[Bob and Carol]

Thanks everyone, I will go for the “trip reset” with each fill.

[StevehS3]

I used John's guidance above on my 2016 car and it worked well. The sender is designed for a different fuel tank (incorporating a well for the pump/sender unit I think) so it couldn't be accurate without modification to the float arm. I now have confidence in the gauge. My 310R has a large variance in fuel consumption between leisurely touring, spirited blatting and track so I personally wouldn't want to rely on mileage alone. 

[Geoff Brown]

Agree with comments in#11 best to understand what the gauge is telling you than going by the trip as in mixed driving consumption will vary & not necessarily according to the trip.

My personal calibration in #6 works every time. If I had taken notice of trip instead of gauge then there would have been a few long walks or recovery depending on the circumstances.

[John Vine]

I agree, Geoff.  My personal solution is a combination of:

1. Making the gauge more accurate in the low range by bending the float arm
2. Adding a little label to the gauge to remind me how much useable fuel remains at 1/2, 1/4, top of "R"
3. Monitoring the trip reading

JV

[StevehS3]

When I fill up I check the gauge and guess (sorry estimate!) how many litres it will take to fill the tank. I have got quite good at it but what an awful admission :)

[John Vine]

I have got quite good at it but what an awful admission

I do exactly the same!  Nerdy, or what?

JV

[StevehS3]

Glad I am not the only one, John!
Anyone else play 'What's left in the tank'? 

[mph]

I realised very quickly that my 2012 R400 gauge was largely nonsense  I fill up when the trip is over 150 miles, and it pretty much always takes circa 22 litres at that. I guess that 180 would be about empty - which fits with JV's fill up prompt. 
 

I once ran the gauge to empty, did a further 30 miles, and it still only took 25 litres
 

[Drumster]

My 29 year old fuel sender and gauge seem pretty reliable....

Full down to 3/4 the needle is steady

3/4 to 1/4 needle is trying the break free from the dial as it swings about

1/4 to E needle remains steady again and it means I have 14 litres left.

 

[Golf Juliet Tango]

Does anyone bother to check the detailed accuracy of the fuel gauge on their tintop? Or are the obsesions because we are dealing with a much smaller tank?

[anthonym]

the smaller tank.

"I fill up when reaching 180 or so, and reset the trip to zero."

me too - and in France never let a filling station go by (R500 K) where 98 is disappearing.

ANTHONY

[heptagon]

I always seem to be filling mine up but don't pay much attention to how much I get into it - perhaps I should, I thought it was empty at the weekend and I got only 22 litres in. 
The fuel gauge works on the basic principle of Full, Full, Full, Empty which can give you a bit of a fright if petrol is not nearby - such as the middle of Wales on the Taffia run.

[Paul D]

My 2008 fuel gauge is consistent and steady but not my definitive indication of when I need fuel.

I know what mpg I get on both gentle and fast runs, and as long as I fill up by 220 miles I can only put mid-20s litres into it, so I use the trip and start planning at about 180 miles indicated.

[Golf Juliet Tango]

I put 41 litres into the 21 today.  On the road I'm driving as gently as I can most of the time.