XK8 / XKR ( X100 ) 1996 - 2006

Fuel Pressure Rising

Old 04-14-2019, 09:35 PM
DigitaL Dave's Avatar
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Default Fuel Pressure Rising

Greetings gentleman . Its been some time since my last post but only because most of my Jaguar questions have already been answered in these forums , a little searching will most certainly yield a variety of information and quite often a few gems of knowledge .

I'll dispense with the pleasantries and get to my issue ;-)

2003 XK8 4.2 naturally aspirated ( Fuel pressure rising after shutdown )

code: P0193 "fuel pressure sensor circuit high input" .

Layman's Diagnostics - I'm popping fuel pressure sensors like popcorn , 3 in the last 6 months . Two cheap eBay sensors and 1 OEM sensor .

I'm on my 4th sensor now . Steady 55psi while engine is running . When I turn the engine off the pressure slowly rises topping off at 80psi , stays at that reading for several minutes before dropping very slowly , somewhere around 5psi every couple of minutes or so .

I put an old sensor back on the vehicle and capped the vacuum port and plugged the vacuum line . Fuel pressure needle fluctuated rapidly at 80psi while engine is running . At shutdown pressure dropped at the same rate as described above .

My questions are as follows ;-)

Is there a fuel "line" pressure regulator on the pump ? If so, is there a difference in pressure regulation between supper charged 4.2 fuel pumps and naturally aspirated 4.2 fuel pumps ?

What is the proper fuel pressure readings for a n/a 4.2 after turning the engine off ?

What would cause this 25psi increase in pressure after shutdown ? Could this be the reason I'm blowing these sensors ?

Any advise would be greatly appreciated .

Old 04-19-2019, 12:25 PM
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I replaced the fuel pump about six months ago with a used pump . Everything seems right except for the 25psi rise in fuel pressure after turning the engine off , that and I keep damaging fuel pressure sensors and pulling fuel into the vacuum line .

I'm open to opinions and suggestions ;-)
Old 04-19-2019, 02:47 PM
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There was a long thread a while back about this. Suspicion was that the XKR pumps have a higher "release" pressure than the XK8, basically another part#. No firm resolution that I can recall. Maybe search by the code (P0191, or P0193) for details.On that thread, the poster was describing rising fuel pressure on deceleration, as the fuel injection stops, and the pressure is supposed to be released at the pump.

I would start with a reputable fuel system cleaner (Techron?) in case this release valve is gummed up somehow. These pumps are EXPENSIVE, so you have to rule out other problems first.

I would also look at reading the fuel temp sensor. It is supposed to assist in keeping high fuel pressure when a hot engine shuts off to avoid vapor lock.

Best of luck, keep us posted.
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Old 04-20-2019, 10:21 PM
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Thanks for the feedback fmertz . I hope to have it resolved soon . I will keep you posted .
Old 05-05-2019, 09:25 PM
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So the weather here on the east coast is warming and my garage queen XK8 ... ( I call her pretentious ) , is ready for a spring drive .

I finally had the time to remove my recently purchased (used) fuel pump and replaced the pressure relief diaphragm with one from the old pump I pulled out about a year ago. If you remember, I had an issue that caused fuel to drain back into the tank after the vehicle sat for a few hours causing hard starting and several key turns to prime the fuel line before it would start .

For this article I will refer the original pump as pump#1 and recently purchased pump as pump#2 . The pressure relief "diaphragm" will be referred to as a "valve"

While inspecting both pumps I noticed that the older pump ( pump#1) was missing one of the four tabs that hold the valve in place . This was enough to lead me to believe that someone had at least attempted to replace this part before . The valve even looked newer than the one on pump#2 .

This was a chilly day , the plastic tabs holding the valve felt very stiff with very little flex . To prevent further damage I used a heat gun on low setting and warmed the plastic cup assembly focusing mainly on the tab areas . I did this until the plastic was warm to the touch . The tabs then retracted from their position quite easily , edging the valve up just slightly to rise above each retracted tab until it is above all tabs then pull straight up and the valve is removed .

It was at this point I noticed the broken O-ring in pump#1 , which explains why the fuel was draining back into the tank . At this point I'm convinced that someone had attempted to replace this part and botched the job ... then sold me his "problem car" at a great price ;-) No worries though , "pretentious" is worth every penny .

I did a little research on these pressure valve model numbers , and I do mean a little . Looking online for new fuel pumps for a (naturally aspirated) 2003 xk8, I found 2 fuel pump pictures that had the same numbers as the valve on pump#1 ( 107775 2696 V4 ) . The numbers from the valve on pump#2 are ( 96624022 0165 NK ) .

So rather than replacing the O-ring on a pump with a broken tab, I replaced the valve on pump#2 with the valve from pump#1 . After numerous test with both valve model numbers I have at least some conclusive evidence .

The "pop-off" or pressure release from valve number ( 96624022 0165 NK ) is 80psi

The "pop-off" or pressure release from valve number ( 107775 2696 V4 ) is 70psi

I'm hoping the 10psi reduction in pressure is enough to keep from blowing fuel pressure sensors every two months .

I should note , if someone does decide to replace a pressure valve on one of these pumps , there are a couple of things you should remember .

1: Warm the plastic before attempting to pry the plastic pegs back to prevent damage .
2: Do not place the O-ring on the pressure valve .

Use a drop of 3&1 oil on the O-ring and insert the ring into the body of the pump, seated at the bottom . The O-ring makes sealed contact at the tip of the valve port as the valve is pressed inward locking under the plastic tabs. If you place the ring on the tip of the valve port itself, it will most likely roll upward on the port shaft and guarantee a pressure leak , that's if your able to get any pressure at all .... ask me how I know this .

I should also note that, while changing out and testing multiple pumps modules sounds like an egregious task It wasn't really that bad . When I removed the fuel pump the first time nearly a year ago, I attached a 2' foot length of flexible high pressure fuel injection line from the fuel tank to the end of the fuel line , now , I simply remove the tank panel and fuel inlet and slide the tank back .

Catch ya in the next thread .
The following 6 users liked this post by DigitaL Dave:
ccfulton (05-06-2019), clyons (05-06-2019), DavidYau (05-05-2019), fmertz (05-06-2019), JimmyL (05-06-2019), RJ237 (05-06-2019)
Old 05-05-2019, 09:40 PM
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Well done, good right up,
Glad you got it sorted......
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