Dear Forum,

I recently replaced the dreaded 'Valley Hose' on my 2005 XJ8L N/A. During this process I managed to break the plastic 'nipple off of the Fuel Rail Pressure Sensor (the nipple to which the vacuum pipe attaches). I replaced the sensor with a new one, but now my car is reluctant to start and has codes P0172 & P0175. Since these are running too rich codes, they cannot be caused a vacuum leak from (say) my reseating (on new gaskets) of my inlet manifold?

Curiously, if I look back at previous trims, my fuel pressure showed as 380-420 kPa; now, it shows as 12 kPa when the sensor is plugged in and 483 kPa when it is unplugged! My STFT's & LTFT's are ALL off the negative scale and my car (when I can get it to start) is running extremely roughly, smoking at the exhausts, Restricted Performance and reeking of petrol......

I removed the new pressure sensor to see if it had the same Ford part numbers on its back as the sensor it replaced, but it was unmarked. However, it came from British Parts UK and they have always been reliable? I can suck or blow air through the old sensor, but on the new one I cannot? Does it open to the vacuum electronically? Also, when I removed the new sensor, a jet of fuel vapour shot out of the Rail with some considerable force?
I really do not know what to try next? Have I forgotten to plug something in when reseating my Inlet Manifold that would cause this problem?

Please see the Freeze Frame DTC below and please help me, I am stumped!!


Freeze Frame 26.10

 

12kPa is near none - far too low.

483 is huge - too high.

Whether it's a bad sensor I can't say, but something about it or its circuit is badly wrong.

With a low value the car will be desperately trying to add fuel, of course.

 

Thanks JagV8. I am puzzled by the 483 kPa - This is what my code reader shows for fuel pressure if I unplug the sensor? Where does it get this figure from, I wonder? Also, the car seems to be desperately adding fuel when it shows as low AND when it shows as high?

Here are the Fuel Trims etc at idle and when revved up:

At Idle

Revved Up

 

Fuel Rail Pressure Sensor - Unplugged

 

I would recommend trying another sensor. Although when I had the issue, I actually had a code come up for Fuel Rail Pressure Sensor Circuit it when I scanned. I am pretty sure the one I replaced it with was a FoMoCo, and its worked well for the last 2.5 years.

 

Thanks Jazzyjags. In fact, P0193 was the first code I got, so I cleared it to see if it would return and it has not done so?
Do you know whether the sensor is closed until it is opened by a combination of vacuum and electronic signal, or open until closed by those factors, please? I ask, because I can blow/suck through the sensor I replaced, but with the replacement sensor I cannot?
I suppose logically, it should be closed until actuated, otherwise fuel rail pressure would escape through the vacuum pipe into the inlet?

 

JagV8 - I am starting to confuse myself! If the car is adding fuel, doesn't that mean that there is too much O2 in the exhaust, so fuel is being added to burn it and if so, isn't that a caused by a vacuum leak? But then, the Fuel Trims are very high negatives, so they indicate an attempt to take fuel away? I am confused!
If (and it is a big if) the over-fueling IS being caused by a vacuum leak at my inlet manifold, wouldn't that show up as a lower MAP reading? I ask, because my MAPs are the same now as they were before I lifted and resealed my inlet manifold and I am hoping your answer will not cause me to revisit my inlet manifold removal/reinstatement!

 

I never tried blowing into the sensors, so I wouldn't be able to tell you. But if you got the code, I am fairly confident that's it. the small vac line that connects to it is not broken, is it? That's the only other thing I can think of.

 

Thanks again Jazzyjags. Cost me £40 for the sensor and another £25 to ship it here! I am going to do the pinpoint tests tomorrow to rule out circuit problems. Don't know if there is a way to test the sensors actual function.....That 12 kPa looks set in stone with this new sensor - Shouldn't it vary a little? Like it used to? Perhaps the true pressure is not being relayed to mission control and the fuel pump is running riot? Gonna try to find out before I spend another £65!!!!!!

 

I keep thinking that prior to my Valley Hose leak, I had EVERYTHING running beautifully? So what has changed....I have removed and reseated the inlet manifold and replaced the fuel rail pressure sensor that I broke. I have had all 8 of my injectors cleaned, tested and o rings replaced. I now have no coolant leaks but my engine is a mess!
Manifold vacuum leak? A sensor not plugged in? What, PLEASE?

 

I suspect with a bad pressure reading it just cannot get fuelling right so is doing the best it can and then flagging codes.

Even a sensor from a scrap car might be better!

 

Thanks JagV8, I am sure you are right. Unfortunately, there is no access to scrap cars here and I can see no sense in paying yet another £65 for yet another brand new sensor. I still wonder how my computer can deduce that the fuel pressure is only 12 kPa when this sensor is connected and a massive 483 kPa when it is not? What fuel pressure sensor is it using to get this figure, given that the fuel pressure sensor is disconnected? according to the DTC code, failure of this sensor results in:
ECM Default:
– Default value of 3.80 BAR
(55.11 psi) used
– Fuel pump feedback control
inhibited
So, how about I try disconnecting the sensor, performing a hard reset and then try starting the car with the sensor still disconnected? If the ECM sets a default of 380 kPa (it must use some other pressure sensor to establish this default), the car should start and at least idle okay? The trims should also be within the realms? Also I should get a pending code P0191,2 or 3 to tell me that the sensor is faulting? (Bear in mind I only had the P01913 code once and it has not reappeared)? If I then reconnect the sensor and try starting and it fails to start or starts but with great difficulty and with off the scale negative trims (as it does at the moment), then surely I will know that the problem is the sensor? What do you guys think?

 

I'm new to Jags but it seems there needs to be a "fuel pressure regulator". Is the fuel pressure sensor also the regulator?

 

My understanding is yes; the fuel pressure sensor measures the pressure at the injectors and combines this with the amount of vacuum present at the intake and uses these measurements to regulate the speed of the fuel pumps (and therefore the flow and the pressure) via the ECM.
Other Members with far more knowledge than I will correct me if I am wrong!

 

483kPa = 70.05 PSI which is too much of a round number to be a coincidence I think.
I suspect that is the end of the scale so with a faulty or unplugged sensor, it's maxxed out the reading as some sort of default.

The fuel pressure sensor is a differential sensor so it's actually measuring the pressure in the rail, minus the pressure in the intake.
You should not be able to blow through one of these fuel pressure sensors, each port is sealed up against a diaphragm, if you can blow through it then something is wrong there...

With a naturally aspirated engine this means there is a vacuum in the intake, with the supercharged motor there is either vacuum (at idle) or pressure when it's on boost.
The same pressure sensor is used in both engine types. ECU has a target fuel pressure of 55PSI, above the manifold pressure under normal conditions.

12kPa (less than 2 PSI) sounds to me like the sensor is stuffed.

The sensor p/n is AJ87977, or 3R3-E9F972-AA, the 2nd number you'll find used on Ford Focus, and some other Ford models. It can also be purchased as a Land Rover part, p/n 4458309.

What pressure reading to you have with just the ignition on? Without the engine running.

 

Thank you for your post, Cambo.

I have fuel pressure 12 kPa and MAP 100 kPa.

British Parts UK have assured me that this is the correct sensor for my Vin? You are correct, I cannot blow through the replacement sensor. If the sensor is faulty, shouldn't the ECM set a default of 380 kPa (although I have no idea what sensor it uses to do so)? Could the reading without the engine running be the last reading recorded by the ECM when it was running?

 

Pinpoint Test G206886p2

The fuel pressure went up to a princely 21 kPa (for a while, then back to 12 kPa) when I started my car today!

I need to access the ECU connector to check the signal circuit for high resistance?

The voltage between sensor connector pin 03 and ground appears to be 5.03 volts, which exceeds the 3 volts it should read? The three wires are in very good condition until they disappear into the wiring loom, then after that I obviously cannot see? Please, how do I repair a short circuit? Do I have to remove the three wires all the way back to the ECU to try to find a break? How do I get at the ECU plug? What do I need to remove to access the ECU connector?
As you can probably tell, I have never ventured close to one of these ECU thingys!

What is really gripping me is that if I look at my All Sensors for October 6th my STFT Bank 1 was -0.78 and LTFT +0.78, my STFT Bank 2 was 0% and LTFT -4.69% and my Fuel Pressure was 381 kPa. My engine was running perfectly and then I had the Valley Hose coolant leak! Okay, I have had to fit a new Fuel Rail Pressure Sensor, but the same connector is connected to it - so (presumably) this sensor must be receiving the same voltage etc as the previous sensor? So why the huge disparity in performance??

 

The power supply to the fuel pressure sensor from the ECU is 5V

The output from the sensor is 0-5V, not sure of the measuring range but i'm going to guess it's 0-70PSI

I guess a short of the supply wire and the signal wire could explain why the ECU is determining that there is 70PSI in the rail when it's unplugged.

With the sensor plugged in a short between those two wires in addition to the load of the sensor could pull it down to almost nothing...

Pin 1 is the 5V supply in, Pin 2 is the ground, Pin 3 is the signal out.

If you are getting 5V between Pin 3 and Pin 2, with the sensor disconnected, then there has to be a short somewhere in there...

 

Thanks Cambo, but the 5.03 volts is between Pin 3 (Blue) and Ground; the Book says it should be no more than 3 volts? CORRECTION (Apologies) Ignition on I have 5 Volts between Pin 3 (blue) and Ground. I have 5.02 Volts between Pin 1 (Orange/Yellow) and Ground. I have zero Volts between Pin 1 and Pin 3. Ignition off, I have 5.01 Ohms between Pin 3 and Ground, 0.08 Ohms between Pin 1 and Ground and 5.09 Ohms between Pin 3 and Pin 1.

 

What book?

This is what it says in the wiring diagram