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Fuel tank gauge - help!`


Stu_420R

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Hello, 

It seems my fuel gauge is showing empty when there is still 10L in tank. I have a 2016 S3 420R.

I will be driving the car down to Donnington next weekend for the trackday next weekend - does anyone know any place nearby that can take a look? I seen a post somwwhere with an owner with a similar issue who got it fixed quite quickly and easily...

Thanks in advance, 

Stewart

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Where are you?

...

Does it show Full correctly when filled, where does the needle go when you switch off the ignition, and does the needle behave sensibly... steady reading etc?

All other instruments OK? Any recent work?

Do you feel up to a few simple checks with a multimeter and wiring diagram and BlatChat?

Jonathan

PS: John Vine's Guide. Your set-up may not be identical but this explains what might be wrong and how to fix it.

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I am in NE Scotland, Aberdeen but planning to travel to Donnington next weekend for LOT day. The fuel gauge appears to act normally but the temp gauage seems to be quite erratic, i.e. it sometimes only goes to normal temperature under throttle. 

I only recently got the car but I am aware it has had a new windscreen as it was cracked when I viewed it.

Thanks for the guide and links, I haven't spent much time looking around the site yet - I am upto the attempting to check whats wrong myself but quite short on time this week.

Cheers, 

Stewart

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When you say the temp gauge does to normal...?

Are you meaning the temps rides whilst idling and drops rapidly as the revs are applied...?

If so that's normal for the Duratec as CC use the OE crank and water pump pulley sizes which is fine in a saloon car but means the engine can be overcooled due to excessive pump speed whilst 'enjoying the cars potential' or on the track, this can lead to cavitation in the cooling system and localised overheating.

Two solutions, either an oversize pump pulley or a smaller crank pulley, the later means the alternator can run at a lower speed too, though never seen a problem on mine as that's what I run.

Fuel gauges are known to need at least 10L in the tank before the register, and they are not renowned for their accuracy either, 

Recently drained the tank on another members car....... after a 'fueling mishap'  *rofl* and after the Stack showed zero we pulled out another 9L.

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In the last journey at idle the temp I believe was falling drastically (to 40c!) until I rev'd it up or drove away, I think. (this was a week ago and my memory isnt great!). 

It sometimes moves quite erratically when moving. I am concerned about localized overheating, overheating full stop or the car running too cool so it seems wise to adopt one of your options.

Perhaps my gauage is the least of my concern...I should gauage it on milage if it is a 30L tank, I can just go on mileage. 

Thanks!

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#4: agreed.

I don't think that there's any reason to think that this is connected with any other circuits, and a lot of Sevens seem to be running cool nowadays.

..I should gauage it on milage if it is a 30L tank, I can just go on mileage. 

That's what I'd do with a week to go, but remember the extra consumption on track! I certainly wouldn't pull the sender and float before then.

But you could check the gauge reading with the sender shorted and then on open circuit. That should narrow down the fault in preparation for when you have more time.

Jonathan

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Just to clarify things re the fuel gauge:

What does the gauge display:

  1. When the ignition is off?
  2. When the tank is full and the ignition is on?
  3. If you continue driving beyond "empty"?  Does it remain resolutely on "empty" or does it drop into negative territory?

In an S3, there are about six litres of "unreachable" fuel sitting below the pump, and the minimum for the pump to work is somewhere around eight.

Driving on the odometer is what I do.  Even after correcting my float problem, I use a combination of gauge reading and odometer reading.

JV

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Thanks John

Unfortunately I am away from the day so not sure what it says when it's off, I am quite sure it is empty, although not certain.

When it is full, it shows full. 

I haven't been brave enough to try #3 yet! I will do that with a gerry can in the car!

What mpg basis / range do you have? I believe I have a 35L tanl.

Thanks

Stewart

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The fact that your gauge will actually reach "full" suggests that the sender float is working as designed, so there's probably no point in removing the pump to inspect or alter the float arm.  I suspect your best move would be to run the tank dry (with gerry can on board!) and note the new "empty" gauge reading.

I typically get between 26 and 33 mpg.  The average since new is 28.5, including trackdays.  Sometimes I get as much as 35, usually when cruising (relatively) gently all day down empty French D-roads.  On track, it drops to 15 or so.  Not too bad overall for an R400D on roller-barrels and a well-worked gearbox.

I tend to fill up at 180-200 miles, although I also carry a 5-litre can at all times.   My S3 tank is 36 litres, of which only around 30 are useable.

JV

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I understood my 2015 R400D had a 38l tank but I really don’t have any idea. I have certainly run it to zero, run it on a while and still not hit more than 30l or so in.  

I would suggest carrying a can and putting it in at the first sign of running low. I’ve found that happens early on track with corner g but not so much on road. 

As for temperature, it seems the Duratec will have an 82c or a 100c+ stat depending on where it was built  

IMO the latter is too hot for every reason except economy which I doubt bothers many 7 owners.

Mine has an 82 and typically runs around that point; a few degrees higher in summer traffic. It gets up to 80 quite quickly once moving. If I cruise along for a while, then boot it, the temp falls to around 75 then slowly creeps up again.   

It’s never run anywhere near 40 except just after start up. 

How old is your car? I understand later cars are fitted with the 100+ stat. It hasn’t been removed has it?  

 

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A 2016 420R should have a 100C+ stat, from the factory.   If the temperature is fluctuating like that I would suspect either an airlock or, and in my opinion more likely, a poor connection to the gauge temp sensor in the submarine hose.    The gauge and the ECU use a different temperature sensor, if you can find a local club member with Easymap you will be able to confirm if the ECU matches the gauge.   If they are the same then an airlock is more likely.  If they are different then connections to the gauge sensor would be where I would look.

 

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Ahh, mine’s an SV hence 38L possibly. 

Reference airlocks, I have never had the slightest issue with that, having drained and refilled several times to change and repair rads. Pour the stuff in with the top rad bleed open, lock it up when it tops out, fill to level and off you go. Maybe top up a few ml after a short run  

I know some engines suffer but not the Duratec installation, IME.  

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... it seems the Duratec will have an 82c or a 100c+ stat depending on where it was built  

My R400D (2008) has the longer thermostat housing, which I've always taken to be the 100+C one.  However, the car runs at 80C or lower on the temp gauge.  Could the longer housing nevertheless have the 82C stat inside?

...the tank pump / sender not being correctly oriented.

Given that Stewart's gauge will read "full", I don't think that poor pump orientation is his problem.

JV

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John as you sure you gauge is correct.   The only temperature I trust is the one from the Ford sensor used by the ECU.   My 360R was an early example so still had the 1/4 sweep gauge of the Sigma engined cars, but the reading was always lower than the ECU.   Another problem I had (but this could be an early model thing) was the ECU switched the fan on at 97C, but the thermostat didn't open until 102, which meant my fan ran constantly.   I swapped to an 82 which solved all my problems.   I always assumed that this meant the ECU settings and stat combination must have changed during the introduction of the 420/360R in 2015(ish), but I know a 2010 build that had the same problem and was solved with the same solution.    

I still think the sender orientation may be the problem, if the float is bottoming out on the slope in the tank.   I seem to remember reading somewhere the 3/4 sweep gauges learn the lowest and highest reading, and display based on that, but could be wrong on that one. 

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Re #22:

...as you sure you gauge is correct.

I can't guarantee that my temp gauge is correct, but I also use it for oil temp (via the usual two-way switch), where it shows oil at 80-95 depending on conditions.  So it does seem reasonably accurate.  Next time I fire up the car, I'll take some Easimap temp readings.

...if the float is bottoming out on the slope in the tank.

IME, the pump unit rests on the bottom of the tank.  There's a spring loaded section at the base (containing the filter) that is pushed upward by the tank base as the pump is installed.  With the earlier pump (like mine), the float does not reach the bottom of the pump. In the later model, it does.  So yes, depending on which pump the OP has, you could well be right -- in which case, lifting the boot floor would be a good idea in order to check pump orientation. 

I wasn't aware of any learning mechanism.  That's an interesting one!   Anyone know more about this?

JV

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I always assumed the problem with the water temp reading using the sensor in the submarine was the earth connection,   Not sure how good the earth connection is on the dry sump tank, maybe its better :-) 

Can't remember where I read that about the gauges, so don't take it as gospel.    

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