I know someone has an answer to my problem. I recently aquired a 1997 XJR (6CYL) The problem is this. The car has poor acceleration and will not get above 4000 RPM. Now for whatI have done to try and fix the car.
1 Replaced both fuel pumps.
2 Replaced the fuel filter.
3 Replaced the supercharger and bypass valve in addition replaced all O Rings
on duct from s/c to intercooler and replaced the intake manifold gasket
4 Cleaned the throttle body.
5 Replaced the plugs and gaped them correctly..
6 Replaced a faulty injector.
7 Checked the camshafts for correct timing.
8 Compression test result 150, 145, 130, 125, 130, 125
9 Replaced fuel pressure regulator.
10 Checked all relays for proper operation including the relay for second fuel pump.
11 Cleaned all grounding points.
12 Checked all fuses.
13 Replaced the exhaust and checked the front down pipes for blockage, no blockage.
14 There are no OBDII Codes showing.
15 At 2600 RPM showing only 5 lbs of boost checked all hoses and clamps on TB
16 Previous to my purchase the transmission was replaced.
17 At idle in Park shows 17 hg of vacume, and steady.


The car starts and just fine and idles around 675 in drive. It does not miss, overheat , or stall ever. I just has no pep what so ever. an as I said cannot get above 4000 RPM. The car has just over 76,000 miles on the odeometer.

If any one has an idea please let me know. Thanks for taking the time to read!

 

You don't mention any testing except compressionand this may be why you have had to do so much work. With XJR's, the symptom of poor acceleration at high rpmis caused by low fuel pressure from a dead second pump or inop supercharger bypass valves.I suggest getting the ECU scanned in real time to monitor the sensors to see if you can find one out of range. Then I would get another compression test done as yours seems too varied for accuracy. I wouldmake a proper fuel pressure test rig and check it at all speeds. The mixture control systems may be malfunctioning, this means the MAF and O2 sensors should be watched in real time. Although they should set codes if out of range, sometimes they don't in certain circumstances. Finally, certain tranny codes will restrict engine performance to preset rpms (although it's 3000, not 4000), check for these codes. The 4L80E tranny does have some of these parameters although usually when it happens, the tranny light comes on.

 

what was the solution to this?

 

Don't know. It was 4 months ago.

 

I'm having a similar issue, but not so serious. my '95 XJR accelerates fine throughout the rpm band, but it should be faster for a supercharged motor. My naturally aspirated '96 is quicker. It's throwing two codes for O2 sensors, and doesn't seem to like to hold high speeds. Perhaps poor exhaust flow is a factor?

pancho