I always like to run my tanks as low as I can before refilling, please dont try to debate the point, we are all different and I respect everyone elses choices.

 

Its important for me to know how much capacity or how many mile/kms remain when the trip computer says O miles or to know how many miles I can get to a tank before running dry, it avoids the anxiety if you know as I did with my last 2 Skodas that there is actually 5 or 7 litres remaining equating to 70 miles (example) of cautious driving.

 

On my MK1 Octavia I ran dry enough times when the fuel guage played up to know its reserve, on the MK2 with its PD engine I just ran it dry on the first tank with a 5l jerrycan in the boot.

 

On my Yeti with its common rail engine I believe doing that is likely to do a lot of damage, and its for that reason thats its all the more important for me to know the actual reserve capacity to avoid ever actually running dry.

 

My initial idea was to stop the vehicle at home as soon as possible after zero miles remaining on the trip meter, to disconnect the fuel feed hose from the fuel filter and to pump the tank dry using VCDS to activate the scavenge pump & then measure the volume of fuel removed.

 

Is this feasible and have I missed something obvious like its the scavenge pump that breaks up when it runs dry or that it will overheat doing what i propose or that it will not prime again afterwards?

 

Presumably removing the fuel line will be no different to changing the fuel filter, is there a procedure for priming/bleeding after doing this?

 

Thanks in advance.

I'd be tempted to go in via the cover under the back seat and just siphon it out, preferably not by mouth-sucking.

 

Can't see running the in-tank pump 'til it starts to aerate doing any harm though. If you ran it dry and then kept going at it for long enough trying to get every last drop you could maybe overheat it, but I'd imagine you'll be listening/watching well enough to avoid that.

 

Going in via the cover would confer the additional benefit of seeing any debris on the tank floor, but I doubt there'd be much/any.

Thankyou for the simple & common sense method (something that usually evades me) that is certainly what I will do, not having lifted the underseat trim and seen the access hatch it didn't spring to mind but I'm sure its the same as the Octavias were.

 

It has a saddle tank but the fuel must balance between the two sides if the pick up is via the sender unit like the Octavias.

 

I only have another 20 or so miles to go to find out.

 

Is it catastrophic if you run the CR engine dry? I can only find generic warnings on the web for all modern diesels.

Edited 23 August, 20205 yr by J.R.

If you use Wino's method of drawing the tank by syphon will you get more fuel out than you would with the pump? I am not familiar with the design of the tank but wiil the pump run dry earlier going up hill than down and if it does run dry temporarily will that introduce air bubbles into the system?

I found with Skoda diesels which I ran upto 2015 that if I refuelled with 5 miles left on the maxidot there was normally about 1.5 litres left in the tank giving an actual safe range left to cover at least 15 miles hence so people brave enough can drive a few miles with the maxidot showing zero.

Only 1.5 😯

 

You clearly were not brave at all and never got to see just how far you could have gone, maybe this one will prove to be different but even Skoda quote a "reserve" of 5 litres or reduce the stated tank capacity by that amount and its been more like 6.5/7 litres in my previous tests.

2 hours ago, Liger1956 said:

If you use Wino's method of drawing the tank by syphon will you get more fuel out than you would with the pump? I am not familiar with the design of the tank but wiil the pump run dry earlier going up hill than down and if it does run dry temporarily will that introduce air bubbles into the system?

 

What I would really prefer to do is simply run it out of fuel but read of so many scare storys of the entire fuel system being wrecked which is why i am asking the question specifically of the C.R. VAG engines, clearly people do run them out and not pay the price but if there is a real rather than hypothetical risk I am not going to do so, the there is also the repriming of the system which I could probably due by carrying the computer and VCDS.

 

It really would be my preferred option, doing it on roads like I used last time I could shut down immediately there was any hesitancy and not run the pumps dry.

So I have some info, the testing did not go as planned.

 

Access to the fuel pump & sender access hatches is harder than on the Octavia, rear seats removed (simple on Yeti) and seat mounting rails removed then solid moulded carpetted trim tray can be removed, access is via plastic push in caps rather tha screwed metal plates & they have let a lot of road dust enter & no doubt are responsable for the rear axle noise that I hear.

 

LH float level sensor has a feed & return pipe going to the pump on the other side which prevents it being withdrawn, its a telescopic spring fit so sits tight to the bottom of the tank as does no doubt the pump so the system draws and returns fuel from the very bottom of both sides of the saddle tank.

 

I was on a very slight incline but there was only 10-15mm of fuel showing at the front, not enough to siphon & I would have needed to do the other side as well so I decided simply to measure the top up as I have done before.

 

It should be noted that with the fuel pick up points at the front of the tank when you are very close to running out the first indications of fuel starvation will be when going uphill.

 

So I drove to the filling station and carefully brimmed the 60 litre tank, I had driven approx 3 miles since the maxidot showed 0 miles remaining.

 

I brimmed it at 55.5 litres exactly as the time before on zero remaining miles, I then very patiently tried to squeeze every last drop in and got it to 56.5 litres with the fuel to the top of the filler neck, I did not do this last time so 55.5 litres is the figure.

 

That means a one gallon fuel reserve when zero miles showing which is a lot less miles on this vehicle than my previous Octavias 44 miles, maybe 50 if I drove exceptionally slowly.

 

So I have my answer but not through my preferred method, its consistent with my 2 previous Skodas and I will continue to fill up as close to zero miles as I can and to note how much fuel goes in to the same level. I know that if zero miles comes up with 435 on the trip counter I have to fill up before 480 miles.

 

On the Octavias with a smaller 55 not 60 litre tank my range was 700 miles, some progress 🙁 I guess I have the emissions fix to thank for that and the DPF & regens.

 

 

Edited 25 August, 20205 yr by J.R.

The main risk with running out of fuel in any diesel car is damage to the fuel system components that depend on the fuel for lubrication i.e. pumps and injectors. On old VE cars the lower operating pressures meant lower clearances and greater tolerance of low fuel lubricity or dry running. PD cars would have been more sensitive but owners' experiences suggest it's still fairly tolerant. In the CR engines, the common rail and injectors are working up around 2000 bar, which is generated by the high pressure fuel pump. These don't tolerate lack of lubricant very well at all, something which caused no end of grief for US owners when the CR engines launched there and HPFPs were failing left right and centre due to the low quality of US diesel. HPFP failures tend to send metal filings into the injectors, and also back down towards the tank in the return line, meaning most of the fuel system ends up needing replacement or extensive cleaning following a failure. Lots of €€€€!

 

Going by the service manual, the Yeti seems to share its fuel tank with the Superb 2. There are two fuel gauge senders, one on each side, so I suspect the instrument cluster is reading both individually and averaging. The official procedure for draining the fuel tank is to hook a battery directly to the fuel pump in the tank and use that to drain out the fuel. Taking out the fuel delivery unit requires a special too (T30101) which lets you unscrew a lock ring using a ratchet/t-bar.

 

As to your question, there is an adaptation channel that allows the fuel gauge to be adjusted. It used to be that you drained the tank, refilled it with 9 l of fuel (I'm not quite sure of the origin of this figure and the service manual doesn't shed any light on it) and set the needle to a certain point by tweaking the adaptation channel. From digging into it now, for anything based on the Golf 1K, the channel adjusts a resistance value used by the cluster. My guess here is that it sets a reference value for the resistance output of the fuel level sensor and generates gauge position and distance to empty based on that and the current resistance. It's possible that it's the resistance corresponding to that 9 l of fuel. There's almost certainly a map of resistance to needle position somewhere in the cluster EEPROM too if you have any tool that can read that. I'm trying to think out the experiment that's needed to get the calibration right via VCDS. In the real world probably something like:

• Note the current value of the fuel gauge adaptation channel
• Drain the tank completely and see what resistance the level sender is showing (hopefully it's exposed in VCDS measuring blocks)
• Refill to 30 l and read the resistance (if you can get away with using laptop/phone on the forecourt to monitor filling)
• Refill to 60 l and read the resistance again

You could also keep an eye on it as you work down through the tank - IIRC VCDS does read out fuel remaining in measuring blocks, so you could log that against distance remaining and MFD average economy for the tank and see if the data makes any sense. I wonder does 0 km remaining => 0 fuel left?

Thanks for the response.

 

Yes it did look like a 2nd sender on the other side, 7Zap had shown it as something else (the fuel pump?) so I removed the one on the left, the one on the right looked identical but only had 1 cable, the second one on the other sensor I believe is for the low fuel level light.

 

I am not looking to recalibrate but to have the exact answer to your final question, 0km never means zero fuel as they have always quoted a reserve capacity, I am firly sure now that mine is 4.5 litres 1 gallon, after 20 years i am used to the peculiarity of the fuel guage, ignoring the "miles remaining" maxidot display the fuel guage is super accurate, you just have to understand that full is full and empty is not  empty but a reserve capacity.

 

The maxidot shows an accurate miles remaining on fill up based on previous average consumptions, its dead accurate down to just under half a tank and then it gradually removes miles from the remaining figure until it shows zero when there is one gallons worth less, if you drive till it runs out you will hot exactly the original range figure shown on the maxidot.

 

I did this on the first tankfull on the MK2 Octavia, ran it dry (70 miles after 0 miles remaining) and then put 5 litres in from a jerrycan, the range went up to 40 miles which is pretty amazing for probably 1cm of fuel depth, I drove to the nearest filling station, after 1 mile the range had dropped to 20 miles, after 2 down to 5 miles & after another hundred yards it showed zero miles, so even with only 5l in the tank it still followed the same algorithm to give me a 4.5l reserve capacity.

 

I wont be screwing with it!