Hey all, quick background here... Bought the car not too long ago, it's an 01 XJ8 Vanden Plas. Car starts and idles smoothly, but when you go to rev the engine, upon depressing the throttle, it drops about 200 rpms before revving up. It idles smooth and steady with no misfire or shake, and this hesitation during rev or rapid throttle change doesn't seem to really affect anything when driving.

The main issue is intermittent...I will go to overtake someone from a cruise, randomly the car will have ZERO power, which causes the transmission to progressively downshift into a gear lower than it should be just to overtake. When this happens, you can feel the power come on, then disappear, then come back again. It feels like a misfire however there are no codes in the computer and no dash lights or messages other than the ABS and Trac Control being inoperative (I gather they haven't worked in years from speaking to the previous owner).

This issue comes and goes and it's SO drastic that I would estimate it to be a loss of about 100 horsepower. Unfortunately I don't have access to a scantool with Jag software that can pull Misfire counters or advanced data.

One thing I did notice is that when filling the gas tank, I can usually only get a gallon in at a time before it begins burping up through the filler neck, so pumping gas takes some time. The other day when I did manage to get 1/2 tank into it, the car ran far worse than usual until the fuel level got near the bottom of the needle, where it's improved some.

My guess at this point is that liquid fuel is being drawn through the EVAP system, though I wonder if it doesn't have a bad coil pack or two and possibly a bad throttle assembly. I'm having a hard time finding an EVAP system diagram so I can check the vapor line at the engine for signs of raw fuel...any help?

I've got to start somewhere because it's running so poorly right now and the computer seems to think everything is A-Okay.

Any input? Sorry for being longwinded but I wanted to provide as much info as possible because troubleshooting online sucks.. I've done some forum research as well and have found several similar complaints resolved with various fixes such as coils, MAF, etc...

Any help is appreciated and if you've made it this far reading this, you have my applause!

Basics: Replaced Fuel Filter
Replaced Intake Manifold Gaskets
Cleaned TB and Intake Manifold off-car
Smoke-tested Intake Manifold, no leaks found

Verified through DLC: Good TPS voltage, good MAF "stab-test" response, good CTS reading, good IAT reading, good Upstream O2 cycling, less than 5% +/- Short Term & Long Term Fuel Trims & achieves Closed Loop operation.

 

Hi, my guess would be low fuel pressure, there is a schrader valve on the fuel rail, hook up a test gauge, tape it to the outside of the screen and go for a test run.
It is essential to have an OBD reader in order to maintain these cars.
Here is a link to a list of those that are known to work properly.

https://www.jaguarforums.com/forum/g...oved-**-33347/

Most are less than $100

 

I have a pretty high-end fuel pressure gauge and kit but it's so badly gummed up with Ethanol sludge that I'm afraid of it leaking while on a test drive.. but I'll try to give that a shot. Thanks

Any input on it running worse after pumping gas into the tank?

Also, I have a Matco OBD-II Code Reader that also pulls Live Data and has a Graphing function, but it's pretty limited. That is how I was able to confirm several things like MAF output, TPS, etc...by driving with the Code Reader plugged in and propped against the radio controls as I monitored things the best I could while still driving safely. It isn't setting any codes either, even when it's acting up badly. Not even a Limited Performance message. Only lights on are the Yellow ABS light, ABS Inoperative, and Trac Control Inoperative messages.

EDIT: Fredd, what I meant was that I have a scantool that does Global OBD-II, but it doesn't have specific tests such as a Misfire Counter, Cylinder Contribution, Solenoid Activation, etc.

 

The link below might solve your fuel fill problems, and may solve driveability problems too. The tech named Brutal has good information.

https://www.jaguarforums.com/forum/x...uel-tank-9803/

 

The Matco should do fuel trims and bank misfire.

Anything unusual in those?

 

It's a step or two above Matco's entry level code puller. Paid $150 for it. It does Generic OBD-II Code Reading, Domestic and Asian MFG-Specific codes, single graph functionality with a total display time of maybe ten seconds while sampling 1x per second.. No BCM/TCM/ABS/SRS, etc functionality.

I tried using our shop's Snap-on Modus but all I got was Generic OBD-II, no specific software. I used it to do multiple graphs at a higher sampling rate but it won't give me Cyl Contribution test, a Misfire counter, etc.

It has no stored misfire codes, but again I can't count misfires cylinder-by-cylinder. Fuel trims are right where they should be. I should note that the fuel pump check valve is faulty so the car was hard to start and would over-run the fuel pump, but I installed a Check Valve on the pressure side of the system *just* before the fuel filter, which fixed the hard start and gave me a significant improvement in driveability and power. Due to these things though, it's possible that it's over-running the fuel pump again, *BUT* even part throttle and roll-on is lacking power now too, not just under heavy load.

Gonna check all of the Ignition Coils tomorrow to see if maybe one is arcing or bad.