Fuel pressure sensor (Nissan) - technical advice please

herman2811

mikey72 wrote: »

The Qashqai has a 14 gallon tank, and is supposed to give 42mpg, so that's about 590mpg to completly empty. It's more likely there'll still be at least a gallon or two when you fill up , even if you fill it to the brim.

So the old figure seens in the right ballpark, the new one seems high.

If the pressure sensor is faulty, it can "overfuel" - inject too much fuel as the engine runs. The ecu can try to cope, but if the pressure is too far out, it's beyond the capability of the map in the ecu to correct. It may not be enough to set a warning, but it may be running richer, and using more fuel.
You would struggle to prove anything, as it's still within any tolerence from car to car.
The trip will read incorrectly as it measures the time the fuel is injected for, and works on a standrad pressure to then calculate the amount of fuell used. If the pressure is wrong, the calculation is wrong.

Has it had an mot yet?

If it has, do you have a copy of the emission test results?

I see what you're saying, but my figure of approx 600 miles is based on it running to completely empty. I've recently run it very close to that, so actually this new figure seems to be in the right ballpark.

Yes it had an MOT during the summer, but this was after the part was replaced.

herman2811

Notmyrealname wrote: »

They tend to only give the average over the previous 50 or 100 miles or whatever, not the entire tank.

I could be wrong, but my understanding is that the trip computer gives the average since the last time it was reset.

mikey72

Unless you have a record of the amount you filled the car with, and the milage you had done each time, each time you filled it, I think you would be hard pushed to prove anything now I'm afraid.

PollyOnAMission

I've done some tests to investigate the accuracy of my Qashqai mpg computer and it is very variable.

Are you only using the on board computer for your fuel consumption figures, or are you also doing manual brim-to-brim fill up calculations?

herman2811

mikey72 wrote: »

Unless you have a record of the amount you filled the car with, and the milage you had done each time, each time you filled it, I think you would be hard pushed to prove anything now I'm afraid.

I know - there's absolutely no way I can prove anything, but I am currently dealing with someone at head office about other issues, so I am hoping they might show some goodwill about this as well. Doubt it though - as I said, they have already indicated they don't believe the part can affect fuel economy.

PollyOnAMission wrote: »

I've done some tests to investigate the accuracy of my Qashqai mpg computer and it is very variable.

Are you only using the on board computer for your fuel consumption figures, or are you also doing manual brim-to-brim fill up calculations?

Yes, the mpg part of the trip computer can be very variable. I therefore always, after filling up, reset the mileage counter so I can continually monitor the estimate of how many miles will be done on that particular full tank - ie, miles done on counter + distance to empty according to computer = total/tank.

I accept this will be pretty inaccurate when the tank is full, but it does improve during use. And I generally let the tank get quite empty before re-filling. So for example, the counter might say 570 miles, with the computer telling me I have 30 miles to empty - at that point I'll probably fill up again, reset the counter, and start again!

Before the part was changed, I was probably generally filling up when the counter said 470 miles and 30 miles to empty.

PollyOnAMission

Your best bet, for more accurate fuel consumption figure, would be to keep a record of how many litres you put in every fill-up with the odometer reading for each one. Always fill to the brim, or else keep track of partial fillups too and add these to the next fill up amount.

Your first fillup will give you a baseline, but by the second one you will 'have number of miles' and 'amount of fuel used', from which you can calculate MPG.

MPG = (miles/litres used)*4.55