Hello All! I'm working on a 99 XJ8 4.0 V8. CEL keeps coming on and scans to a P0453 code. My research tells me its the fuel tank pressure sensor sending too high of a voltage to the ECM. I replaced the sensor with other OEM one, but the output voltage back to the ECM is still the same as before, 3.3 volts. I read on one of the posts that the sensor reading needs to around 2.5 volts, so I installed a couple of in-line resistors on the voltage return wire from the sensor. This dropped the voltage down to 2.5 volts, but the check engine light still comes back on with the same doggone code P0543 after I cleared it several times !!! I have already ripped out what little hair I had left on my head... Any help would be greatly appreciated, along with a wiring diagram of the evaporative pressure sensor circuit.

 

Hey there qsagher, hiding out on the NE side of town I see....I'm on the E side, off Curry Ford & Dean.

So, ...here's some procedures to check...I know you said you replaced the sensor, but I figured it couldn't hurt to include the whole thing.

Using an scan tool access FTP sensor value with Key on Engine off (KOEO). Normal value is usually somewhere near 2.5 Volts at atmospheric pressure (may fluctuate some due to altitude). It shouldn’t ever be above 4.5 Volts.
1) If it is close to 2.7V with the gas cap off, the problem is likely intermittent. Using a Digital Volt Ohm Meter, measure the voltage on the signal wire while trying to induce the problem by wiggle testing all the wiring at the FTP sensor. If the voltage fluctuates when wiggle testing, check for connector problems; i.e. water in the connector, broken or chafed wiring.
2) If the value shown on the scan tool is above 4.5V, unplug the sensor (if possible) and check for voltage again. If the high voltage is still present when unplugged, inspect wiring harness for a short to voltage on the signal wire. If the high voltage disappears when unplugging the sensor, check for a good ground to the sensor and proper reference voltage. If you have a good ground and 5 Volts reference voltage, replace the FTP sensor since it’s likely shorted out.
Here's a couple more...

• Unusually high pressure in the fuel tank as a result of a blockage in the EVAP purge hose or overfilled fuel tank
• Loose or damaged FTP sensor connector
• Loss of ground to the sensor

I attached the 1999 Electrical Guide for your viewing pleasure.

 

Hello HighHorse thanks for sending the info and the wiring diagram. It's good too meet a fellow Jag enthusiast on the forum who is local to the Orlando area.
. Here's the latest on my dilemma: Unplugging the fuel tank pressure sensor did not make the CEL go away. So I bypassed the sensor with a couple of resistors thereby dropping the voltage on the green/red wire going back to the ECM down to 2.5 volts, which is the perfect value that the ECM is expecting. However the pesky light still stayed on! As I mentioned before, my generic scanner does not read detailed ECM info, hence I traced the green/red wire back to the ECM at connector EM81 terminal 16 using the wiring diagram you sent. Unbelievably, the voltage at this ECM input wire was 4.4 volts, while at the other end near the tank it was only showing 2.5 volts! Obviously there's a short somewhere, causing the cel to stay on. Now I'm facing the dreaded task of manually following this wire all the way back from the trunk, up to the ECM in the front. Any tips or shortcut advice would be greatly appreciated.

 

Other than that blasted EVAP canister valve on the purge canister. This cured my 455 after I replaced all else related to it. Its real easy to get to and about 20 minutes of work, mostly getting the car up. Back the car onto a ramp on the RH side, or put it on a stand...your option,..and the valve is pretty much right there. If I recall it was something like a $20 piece.

Yes, this is the XK, but its the same.

 

I will definitely check out the CCV under the car and the attached hoses as well. However, the P0453 code is specifically linked to a high voltage input to ECM, so I am still suspecting a short in the signal wire from FTPS to the ECM. The wires from that sensor go to a large connector (BT4) mounted in the trunk underneath the parcel shelf. Do you by any chance know if the other side of the connector is inside the car and routes the harness beneath the liner of the parcel shelf? Or is the connector BT4 simply a junction that keeps the wiring inside the trunk and routes it under the back seat to the front of the car? This will help greatly so I might avoid removing the inside parcel shelf liner. Thanks!

 

Have you checked the hosing for blockage, thus the reason you'd get a high pressure signal?...and/or ensuring a good ground?

Sorry for the small detail, its all I could find which showed it this simply.

 

Thanks for the EVAP hose diagram. However my problem is that even with the sensor completely unplugged, or with me replacing the sensor at the fuel tank harness connector with a couple of resistors to reduce signal voltage to 2.5 volts, I am still getting 4.4 volts at the ECM input terminal. To me it indicates that somewhere along the wiring from the trunk to the ECM, there's a short to another wire causing the high voltage reading at ECM. I guess I will have to endure the tedious task of tracing the wire all the way, which means pulling out the parcel shelf liner, rear seat etc....

 

The things I see for the code are possibly ...a bad FTP sensor (which you say you've swapped), ...damaged sensor connector, ...or shorted signal wire (which your keen to and collates with having signal when unplugged), ... and/or poor ground circuit. Reviewing the EVAP connections for damage/corrosion and output voltage is recommended.

If you can't resolve this, I know a very good Inde' Tech, ...Andy, The Jaguar Doctor off Forsyth in Winter Park.