Supercharger boost with floored pedal missing

Hi all

After my 1995 XJR / US version had noticeable slowed down and started making noises, my mechanic found the supercharger (Eaton M90) to be the culprit and I got it replaced with a used one.

Car is clearly faster again, but I miss the very noticeable acceleration boost (you could hear it too) when the gas pedal is fully floored. Which was always very helpful during overtaking. For example, the car needed only 6.5sec from 0 to 100kmh.

This is now missing. What could it be?

The throttle is a little harder to set properly on the TRACTION CONTROL cars.
Check to see if the throttle is opening all the way when the pedal is on the floor.
Check physically and look at the TPS signal in the ECM data.

Thank you!
Will tell mechanic to check that, and report back here for update.

My suggestion would be as per motorcar man. Check the calibration/settings of the throttle body. Can make a huge difference.
Did the replacement supercharger come with the same size pulley as your original one? Because if it did not, that could be the problem too. A larger pulley will get you less air. The standard drive belt can take up a bit of difference in pulley diameter. But the telltale is the wear indicator on the tensioner. If that is way off, you might have a different pulley, or a very worn belt, of course.

Good luck

My mechanic says he can’t read the ECM data/computer data of my XJR. How could he access it?

thank you!

This can be done with a regular OBDII scanner with live data. (Available at harbour freight if you’re in the US). You can see on it the actual opening of the throttle in % during moving of the pedal. This helped on my XJR which had the throttle cable not properly adjusted and never had a full throttle opening.
Another thought could be the s/c tensioner-pulley which is made from ponsy cheap plastic(!) It’s bearing failed on my car, melted plastic around it and finaly the s/c was not driven at all.
Best regards chris

The car is 1995. Only 3 items are OBDII, among them emissions/catalytic converter, the rest is all OBD.
So I drove up to New Hampshire where jag-lovers user John6 has a PDU, that can read this data. it/s a monster of a computer, but he was able to check the Throttle data, and it only showed 33% with fully depressed pedal. We traced it around the engine, and found the culprit, a loose throttle cable due to a broken plastic piece right where it comes off the Traction Control Unit, mounted at the passenger side to the right where the fuseboxes are, near the firewall. We temporarily stabilized the broken plastic with wire and duct tape and that was it. Wow.
(The PDU also showed that the ABS front sensors were out, and that seems due to broken off ground wires at the wheels.)
What a different driving experience with 100% throttle!

Reg the S/C: I had bought a used one from Ebay for $500, but it makes a roaring noise when the car is cold. So I am having my original one rebuilt by Magnum Powers in Oregon (recommended somewhere here), and then swapping it back in.

Thanks for all the feedback!

Here is the broken yellowish plastic piece, which made the throttle cable loose. As you can see the piece has been repaired before with putty.

Here is the fully depressed throttle data with loose and tightened cable

Here is the torn off ground wire for the ABS on front right wheel, the other end is hanging around nearby.

Now I am trying to hunt down this Throttle cable:

can anyone confirm if it’s the part number MNA4620MD, THROTTLE CABLE ACTUATOR TO ENGINE, for a 1995 XJR Supercharged with Traction Control?

Hi- it was nice having you at the barn And I’m glad we found your problem, and instituted a temporary workaround.

The broken plastic part which allowed slack in the accelerator cable might be part of the traction control actuator, not the cable itself? Check out MNA5930AA. On some the plastic part is black not white and might represent a product improvement? Good luck!