Fuel Economy Drop After Tank Drain

Mullie

I recently put petrol into my diesel car but realised before starting the engine.
The car was taken to a dealer garage where the tank was drained.
I keep a record of the fuel economy and average approx 4.5 - 5.0 L/100 km.

For the tank following the fuel drain, the economy spiked to 6.9 L/100 km!!
The only conclusion I can come to on this is that when the car arrived in the garage they started the engine to drive it into the workshop and ran the petrol through the engine.

Am I jumping to conclusions here? Its the only thing that would explain a spike that large.
Is there any easy way to check if petrol has been run through the engine ie. Residue on a filter somewhere?

tobsey

My car (Octavia) stores trip information and also more long term information.l If you press the button on the wheel it flips between the two. It stores consumption, distance, avg speed etc. If yours is the same then check what the distance is that corresponds to the 6.9. It may be that they drove it a short distance at high revs to give everything a good clear out.

Anan1

Exactly how did you calculate consumption for the tank following the drain?

Mullie

This was the consumption measured over the whole length of the next full fill after the drain.
It was reset on the fill on the car computer and I also tracked it using the FuelLog app. On both, although they didn't match exactly, there was a huge spike.

Harcrid

I would give it a bit more time to let the figures settle.

Anan1

That fill would have been bigger, as the tank would have been completely empty. The only sure way to measure consumption is to brim the tank, zero the trip meter, and calculate from the mileage and litres when you refill.

Matt Simis

Mullie wrote: »

The only conclusion I can come to on this is that when the car arrived in the garage they started the engine to drive it into the workshop and ran the petrol through the engine.

Am I jumping to conclusions here? Its the only thing that would explain a spike that large.
Is there any easy way to check if petrol has been run through the engine ie. Residue on a filter somewhere?

I wonder on your logic there.. how exactly do you see the two things as related?
Petrol in a diesel will damage the fuel pump(s), filters and fuel injectors, due to lack of lubricity. Ive seen a modern diesel that was run on petrol thanks to stupidity and it was f)cked, you wouldnt be complaining about a couple of L/100km if that was the garages solution. Trace amounts of petrol likely did get pumped through, but thats pretty much unavoidable.

Wait a while longer and bear in mind that resetting the figure will usually result in higher short term consumption till you put some more time in driving.

Mullie

Matt Simis wrote: »

I wonder on your logic there.. how exactly do you see the two things as related?
Petrol in a diesel will damage the fuel pump(s), filters and fuel injectors, due to lack of lubricity. Ive seen a modern diesel that was run on petrol thanks to stupidity and it was f)cked, you wouldnt be complaining about a couple of L/100km if that was the garages solution. Trace amounts of petrol likely did get pumped through, but thats pretty much unavoidable.

Wait a while longer and bear in mind that resetting the figure will usually result in higher short term consumption till you put some more time in driving.

Thats my question, are the two things related?
I'm seeing a big change in the economy figures based on 2 calculations:
1) The onboard computer - the average consumption figure based usage of the full tank of fuel used immediately after the drain.
2) An app that calculates the fuel based on litres and mileage tracking from one fill to the next

Ok, I guess that trace amounts will have got through the system, so I assume that is what is has caused the increased usage.
Just hope its not long term!

Thanks for all the responses folks

ianobrien

Don't forget that the volume of fuel in the tank has probably changed during the drain (decrease more likely). The car is now suddenly seeing that X litres of fuel is gone and thus the fuel consumption has increased.

Also, the garage might have drained the lines as well (to make sure) and bled them, also increasing the amount of fuel that the car thinks was used driving.

The car was also probably left idle and ran under load for a while afterwards to check if there was ill-effects and to check for fuel leaks.

Edit - there are more informed on here than me but maybe the garage had the battery disconnected for so long that the ECU has to "re-learn" the optimal settings. I know that some ECU's change their settings based on how the car is driven and that your car may be doing that at the moment.