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Cambo, thanks for staying with me! It must be getting late there!!
I am looking at Pinpoint Test G206886t8 in the X350 Workshop Manual 2004 - Check the FRP Sensor Signal Circuit For Short To High Voltage. (addition) There are no volts between Pin 1 and Pin 3, ignition on.
If you're getting the same 5V between pins 1-2 and 3-2 (and it reads like you are) then there is a short between 1-3, the very low resistance you measure between pins 1-3 of 0.08ohms is probably the resistance of your test leads.
You measure 0V between 1 & 3 because it's the same voltage, like putting both probes on the + of a battery, it says zero
I suppose they say "greater than 3V" because the target 55PSI of fuel rail pressure is less than 3V output from the sensor.
5V on pin 3 certainly explains why the ECU sees the fuel pressure pegged out at 70PSI / 483kPa.
Cambo, I have read your most recent post 10 times and each time my mind plays out a different scenario of what has happened here! I wish I had some of the clear, analytical thinking that you seem to bring to the subject!!
My mind game problem is that I need to understand the problem before I can decide what to do about it when it really would be simpler for me (and for you) if I just asked you what I should do....
Please bear with me while I try to explain the two horns of my dilemma. Firstly, below are two sets of fuel trims and pressures etc. The first is the current set and the second is for 16th October. All that has been physically changed on my car between these two sets of figures is that a leak in the coolant pipe to the Throttle Body has been fixed, the Inlet Manifold has been reseated, all 8 injectors have been professionally cleaned, tested and fitted with new O-Rings and the FRP Sensor has been replaced with new.
My reward for all this effort and expense is contained in the first set of figures. The new sensor is using the same wiring and connector as the previous sensor, so (presumably) it also operated with the same set of supply and signals - Yet it did so perfectly (it would appear by the second set of figures below)?
Secondly, If the ECU is being incorrectly told (but perhaps correctly based on the massive over-fueling going on) that the fuel rail pressure is too high, why is the fuel pressure being shown as only 12 kPa and why does the ECU not instruct the fuel pumps to slow down? The ECU is telling the injectors to provide less fuel (high minus STFT & LTFT), but they are obviously not doing as they are told? I apologise for the length of this post, but the short of it is that I cannot decide whether to buy a small bearing puller to get my windscreen wiper arms off so that I can get at the ECU, strip all that wiring out and look for a short or throw even more money at the problem (perhaps with no return) and buy another sensor?
It seems to me that every time I try to fix something, I break something else!! So, after subjecting you to all of that, I come back to my original point; what do you think I should do? (incidentally, in order to improve my understanding and when you have any spare time, I really would also appreciate your comments on the questions I ask in my last paragraph above).
As an afterthought, how about I try a hard reset before I try anything else? EDIT: Another afterthought; could it be that (as you describe in your last post) because Pins 1 & 3 both have 5 volts, they cancel each other out and are read as zero volts at the ECU when the sensor is connected and this manifests itself as no rail pressure (or 12kPa) which is very nearly no rail pressure, so the ECU's response is to demand more from the fuel pumps?
All that has been physically changed on my car between these two sets of figures is that a leak in the coolant pipe to the Throttle Body has been fixed, the Inlet Manifold has been reseated, all 8 injectors have been professionally cleaned, tested and fitted with new O-Rings and the FRP Sensor has been replaced with new.
You've unplugged a number of connectors in the process of doing all that, moved some wiring harnesses around. Things can happen on old cars.
Originally Posted by EsRay
The new sensor is using the same wiring and connector as the previous sensor, so (presumably) it also operated with the same set of supply and signals.
All bets are off with aftermarket components. You haven't got an original FoMoCo sensor (the lack of markings on it confirms that) BUT the previous measurements you took suggest that it is not a problem with the sensor, but the wiring harness to the sensor.
Originally Posted by EsRay
Secondly, If the ECU is being incorrectly told (but perhaps correctly based on the massive over-fueling going on) that the fuel rail pressure is too high, why is the fuel pressure being shown as only 12 kPa and why does the ECU not instruct the fuel pumps to slow down?
You're looking at it the wrong way around. The ECU is being told there is only 12kPa pressure in the fuel rail, which is way too low when it has a target of 380kPa, so it's trying to increase the fuel pressure, which likely contributing to the over-fuelling.
Originally Posted by EsRay
The ECU is telling the injectors to provide less fuel (high minus STFT & LTFT), but they are obviously not doing as they are told?
The ECU can only do so much with the fuel trims. If the fuel pumps are running flat out because the sensor is saying 12kPa, but in actual fact there might be 400-500kPa pressure in the rail, then no amount of fuel trim is going to help.
Originally Posted by EsRay
Another afterthought; could it be that (as you describe in your last post) because Pins 1 & 3 both have 5 volts, they cancel each other out and are read as zero volts at the ECU when the sensor is connected and this manifests itself as no rail pressure (or 12kPa) which is very nearly no rail pressure, so the ECU's response is to demand more from the fuel pumps?
Pretty much this.
Next step is to unplug the harness at the ECU, and look at the resistance between the two wires; the 5V sensor supply, and the signal return to the ECU. There should be no continuity between them when the ECU is unplugged. If there is, well that's your problem...
EDIT
What happens when you run the engine with the fuel pressure sensor unplugged and it shows 483kPa as the fuel pressure?
Cambo, Thank you very much for all your excellent advice. You had it nailed back at your post number 23...!
As my afterthought showed, with your input I eventually managed to figure out myself what is happening......You obviously already knew.....Thank you again!
What happens when you run the engine with the fuel pressure sensor unplugged and it shows 483kPa as the fuel pressure? (quote from Cambo)
Please see below:
I have started to strip the loom from the FRP Sensor back towards the ECU. I cannot check the continuity between the supply and signal wires at the ECU until a small puller I have had to order for shipping arrives and enables me to pull my wiper arms off. The stripping back is a nightmare; the (presumably) heat resistant tape binding the plastic harness tubes has amalgamated in the heat and either breaks off in little pieces or has turned into a sort of resin shell and has to be cut - Obviously with great care given my recent propensity for breaking things!
So far, the wiring within looks pristine!! However, surely there MUST be a short somewhere between the sensor and the ECU with 5 volts showing at the sensor connectors signal pin? I still don't understand why the Pinpoint test asks for voltage at this connectors sensor wire pin of not more than 3 volts - Surely the voltage at this disconnected sensor pin should be zero? Unless the ECU uses the difference between 3 volts and 5 volts to measure the pressure? I just have this horrible feeling that I am going to sweat my way all the way to the ECU and find nothing!!!
Please, would you kindly reassure me that I should continue to strip out my wiring harness!!!!!!!!
Cambo, got the Wiper Arms off at last - Just trying to figure out how to separate the Cowl Vent Screen from the windscreen? Then I am at the ECU Connector!
Do you really think my problem is 3 volts good, 5 volts bad? (sounds a bit Orwellian, doesn't it)?
Originally Posted by EsRayAnother afterthought; could it be that (as you describe in your last post) because Pins 1 & 3 both have 5 volts, they cancel each other out and are read as zero volts at the ECU when the sensor is connected and this manifests itself as no rail pressure (or 12kPa) which is very nearly no rail pressure, so the ECU's response is to demand more from the fuel pumps?
Pretty much this.=left
=left Next step is to unplug the harness at the ECU, and look at the resistance between the two wires; the 5V sensor supply, and the signal return to the ECU. There should be no continuity between them when the ECU is unplugged. If there is, well that's your problem...
Okay (at long last) Cambo, there is no continuity between the 5V supply and the signal return?
So, Pinpoint test G20688t7 - Check FRP Sensor Signal Circuit for high resistance - Is it greater than 5 ohms? Answer, no, 1.0 ohms. Pass
G206886t8 - Check FRP signal circuit for short to high voltage - Is it greater than 3 volts? Answer, Yes, 5 volts. FAIL
G206886t9 - FRP Signal circuit to Ground - Is resistance less than 10,000 ohms? Answer, Yes, Was 5.01 ohms when ECU was plugged in; now shows no reading with ECU unplugged? Fail
Cambo - Thank you for all your help - Please continue to help me when you can. I have drifted quite far from my original 'Running Rich' post, so I have opened a new post specifically about my Fuel Rail Pressure Sensor:
10-27-2018, 08:17 AM
Another afterthought; could it be that (as you describe in your last post) because Pins 1 & 3 both have 5 volts, they cancel each other out and are read as zero volts at the ECU when the sensor is connected and this manifests itself as no rail pressure (or 12kPa) which is very nearly no rail pressure, so the ECU's response is to demand more from the fuel pumps?