Does this car have a fuel pump controller? Most fuel injector cars do. Before you put out cash to buy and install the pump... double check that it might not be a control module.

I am guessing on this one, but if you can get 55 psi at idle, what's up with 13 at cruise? If the controller sets pump pressure based on TPS voltage, then it stands to reason that controller is bad, not the pump. Like I said.. guessing on this one.

 

Good point on the pssible bad fuel pressure control module. I called the dealership this morning and they can't get me in to Wednesday and wont be able to provide me with a loaner (they generaly do - although I don't know why as I didn't buy the car from them). I know if I take it in Wednesday it will be there at least a week, thats just how it works with them.

Anyhow, I decided to research and try tackling this myself. I found some good articles and threads on some Ford forums on the module, and one that give step by step troubleshooting for the Ford module, but the pinouts appear to be different than my module.

Does anyone know if there is, or have, a step by step diag for the Jaguar FPDM? I dont want to just throw parts at it. I suppose it could be a bad sender at the fuel rail, bad wiring somewhere, or bad module. BTW, I checked the vacuum line from the sender to the throttle body, seems to be in excellent condition, not cracked or kinked.

Thanks in advance for any input.
Steve

 

I had a fuel pressure sensor crack apart, and when I ran my garage autopsy, the internal board was highly corroded. Other than the advice above, is your fuel filter relatively new? I don't recall reading that above.

My new sensor is from a Lincoln LS by the way. I googled the part number, and it was cheaper than the jag dealer, and in stock at Ford/Lincoln.

 

Thanks for the reply Matt. The filter is a year old, about 5000 miles on it. You wrote about the fuel pressure sensore, you are talking abou the one at the right front fuel rail? Did you have any symptoms prior, or did it just fail suddenly?

 

This link is the fuel system for an xk8 http://jagrepair.com/images/AutoRepa...FuelSystem.jpg see if it helps. As for the regulator I do not have any info on it right now.

 

Update - Car went to the dealer. They said they replaced the MAF and it cured the stumbling problem when applying half throttle as descibed.

They admitted there were no stored codes. To be sure, they put the bad MAF back in, same issue. They used a pressure gage connected to the shrader valve after the repair and saw no fuel pressure problems.

I think it is a good use of time to try to speculate why the fuel pressure appeared to drop when I stepped on the throttle and the engine stumbled. Based on my limited understanding, it seems possible that if the MAF was not reporting to the computer the increase in airflow, then the computer may not tell the fuel pump controller/fuel pump to send more fuel, so the pressure drops because now more fuel is used, but more is not being sent. Again, just specualtion of course, but I wonder why there were no codes.

Related - I have seen some posts where one bank was showing rich long term fuel trims without codes and it was a bad MAF, but it doesn't make sense to me why only 1 bank would be afftected by a single/common upstream sensor. As I wrote a few months ago in this post, my bank 1 is 9+% rich and bank 2 is about 5% rich. It will be interesting to see if my LTF's even out now, or maybe mine always wont be equal, but will be closer to zero.

I want to thank everyone that has helped me try to fix this, even if I ended up having to bring it to the dealer. Particularly, thank you to Gus for your inputs and thoughts over the past few months, and most recently, Steve (whiteXKR) who seems always willing and ready to help with technical/electronics advice...I thank you Steve for being so responsive!

 

If I read your trouble shooting correctly with the old MAFS installed the fuel pressure fluctuated to a loss of pressure when accelerating and with the new MAFS the pressure was consistent.

 

Glad you got it sorted Steve.

Your theory sounds plausible, but I too wonder why no codes. My guess is that the elctronics in your MAF had some atypical failure mode that the system is not programmed to detect.

A hard reset might not be a bad idea to speed up removal of junk data in the ECU, plus it would let you see the LT fuel trim changes without waiting.

 

Typically the MAF sends a voltage signal to fuel pressure regulator or fuel controller based on volume of air flow passing over the hot screen or wire. If the MAF is not properly measuring the total amount of air passing over the hot wire; the the voltage signal sent to order up more fuel will also be low for the given volume of air actually being sucked into the engine. Consequently, the air/fuel ratio is way too high. e.g. Too much air and not enough fuel. Since the MAF indicated that less fuel was needed, the controller dropped the fuel pressure accordingly. ... or something like that ;-)

 

Exactly Gus. And thanks again.