Fuel pressure sensor (Nissan) - technical advice please

herman2811

I have a 3-year-old Nissan Qashqai+2 (bought from new) which recently had a recall on it's fuel pressure sensor - ever since this was replaced, I am consistently getting an extra 100 miles (approx) out of a full tank. It was literally like flicking a switch!

Does anyone know if a faulty fuel pressure sensor can cause such a drastic change in fuel consumption?

I am in contact with Nissan head office about a number of issues, and am seeking compensation regarding them, including the extra expense I believe I have incurred due to this faulty part.

Based on the mileage I have done, how much it costs me to fill up, and the difference in fuel economy I am seeing since the part was replaced (~600 miles/tank now compared to ~500 miles/tank for 3 years before), I have very roughly calculated that over these 3 years I have spent about £667 more than I should have!

Unfortunately though, the lady I am dealing with has spoken to her technical team, who have advised her that this faulty fuel pressure sensor would absolutely NOT cause such a difference in fuel economy.

I just can't accept that though because I am quite pedantic about fuel economy generally and have always monitored it religiously - and as I say, immediately after replacing this part, there has been a very dramatic improvement, leading me to think it DOES affect fuel consumption.

Can anyone who knows about these kinds of things please offer some advice?

Thanks in advance

Notmyrealname

It shouldn't make much of a difference if at all and certainly not 20% unless it was leaking. All that would happen is that if it was over-pressuring, the ECU would compensate by reducing the amount of fuel delivered by the injector so the same amount of fuel would be delivered to the cylinder in order to maintain the correct air/fuel mix. Any excess fuel would be returned back to the tank.

Now if you were talking about early mechanical injection systems or carburettors then yes, I would expect it to happen. However this is a closed loop system where the exhaust emissions are monitored by the ECU which then adjusts the fuel delivered.

herman2811

Notmyrealname wrote: »

It shouldn't make much of a difference if at all and certainly not 20% unless it was leaking. All that would happen is that if it was over-pressuring, the ECU would compensate by reducing the amount of fuel delivered by the injector so the same amount of fuel would be delivered to the cylinder in order to maintain the correct air/fuel mix. Any excess fuel would be returned back to the tank.

Now if you were talking about early mechanical injection systems or carburettors then yes, I would expect it to happen. However this is a closed loop system where the exhaust emissions are monitored by the ECU which then adjusts the fuel delivered.

Thanks for that. Am quite confused now though - as I said, there was a very clear and dramatic improvement on changing the part. I've not been told that they did anything else at the same time.

mikey72

Is it a petrol or diesel?
Were there any warning lights on (even intermittently) before the new sensor was fitted?
Have you changed your driving style, for example does the car pull better so you don't need to accelerate as hard now?

What was the old mpg compard to the new?

herman2811

mikey72 wrote: »

Is it a petrol or diesel?
Were there any warning lights on (even intermittently) before the new sensor was fitted?
Have you changed your driving style, for example does the car pull better so you don't need to accelerate as hard now?

What was the old mpg compard to the new?

It's petrol. No warning lights at all.

No change in driving style at all. Was getting about 40mpg (or just under) before the change, now seems to be about 41mpg. But as I said, getting a definite 100 miles extra per tank.

mikey72

Just under 40 to 41mpg can't be 100 miles per tank.

On a ten gallon tank it would be 10 to 20 miles extra, even on a 15 gallon it could only be a maximum of 30.

To get 100 miles more on 10 gallons, that's an extra 10mpg.
So thirtyish mpg before.

herman2811

mikey72 wrote: »

Just under 40 to 41mpg can't be 100 miles per tank.

On a ten gallon tank it would be 10 to 20 miles extra, even on a 15 gallon it could only be a maximum of 30.

To get 100 miles more on 10 gallons, that's an extra 10mpg.
So thirtyish mpg before.

I see what you're saying, but the figures I've quoted ate what the trip computer tells me. I've always monitored it religiously and always reset it on filling up each time. I always try to drive as economically as possible and almost play a bit of a game to try and get the mpg as high as possible each time.

mikey72

herman2811 wrote: »

I see what you're saying, but the figures I've quoted ate what the trip computer tells me. I've always monitored it religiously and always reset it on filling up each time. I always try to drive as economically as possible and almost play a bit of a game to try and get the mpg as high as possible each time.

How many miles do you get out of a tank of petrol roughly, then and now?

I suspect it's been overfuelling in the past.

herman2811

mikey72 wrote: »

How many miles do you get out of a tank of petrol roughly, then and now?

I suspect it's been overfuelling in the past.

I used to get about 500 miles/tank, it's now about 600.

What do you mean by "overfuelling"?

Notmyrealname

herman2811 wrote: »

I see what you're saying, but the figures I've quoted ate what the trip computer tells me.

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

mikey72

herman2811 wrote: »

I used to get about 500 miles/tank, it's now about 600.

What do you mean by "overfuelling"?

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?