Bought this car about a year ago as a non-runner, with help from this group (many thanks!) I got the car running. I have spent the last year rebuilding the front/rear suspension, Supercharger, chain tensioners, brakes etc. etc. Car now runs well apart from a kickdown issue, If I floor the throttle at ~70mph, it drops down a gear, revs increase to around 4,000 then it bucks and jerks with no increase in speed, similar to the rev limiter if you try to go above 3,000 in Park. If I drive at 30-40 shift to 2nd gear the revs go above 4000 without issue. So it seems to be an issue with the kickdown or possibly the higher speed.

Perusing the archives there seem to be a number of possibilities

Fuel pumps: fuses ok, Jumpered each pump with car off, each deliver ~ 45-50psi at fuel rail and a steady flow of fuel

new spark plugs

new battery

Cleaned MAF sensor

New fuel filter

Fresh premium gas in the tank

The CEL is not illuminated and there are no codes set (Icarsoft reader)

Any suggestions greatly appreciated!

 

Possibly an ignition coil. When you installed the new plugs, was there any oil in the spark plug wells?

You’ll need to provide fuel trims while the misfiring is occurring. If it’s rich more so on one bank vs the other, the ignition misfiring (if that’s what’s happening) on the rich side.

 

I had the cam covers off to replace the tensioners, all new gaskets so no oil in the spark plug wells.

Done some more testing, took the car out for a run, problem occurs when I accelerate around 60mph and the car drops down a gear, that can be from kickdown out just acceleration.
The car jerks and will not acclerate.

Hooked up the scan tool to look at short term fuel trims. Just looked at left and right bank, not the upstream or down stream ones. when driving they jump around between -10% and +10%. When the problem occurs they both drop to zero. Does this mean no addiotnal fuel is being added?

I did the acceleration test a few times to confirm the fuel trims were dropping to zero. The car then went into "engine failsafe mode" with a code of P1252 - no other codes

"vacuum switching valve stuck on vacuum to atmospheric/vacuum to vacuum"

code cleared itself after stopping for gas, not quite sure what this means

 

Looks like the DTC 1252 is cruise control related -

 

The 1999 XJR has AJ26 engine and management. The throttles did have issues.(as did the early AJ27 throttles)

Some info we got decades ago at the dealer.

 

Stuizzy, I ran into this problem on my 98’ XJR years ago ‘assuming’ you have the same problem. Pay extra attention to post #9 and #11.

https://www.jaguarforums.com/forum/x...ncerns-159443/

 

Addicted to boost Thanks for the info. I had similar problems when I first got the car, it would not rev past -3000 rpm when driving. As you said turned out to be the grease in the bearings had dried out. New grease fixed them. Maybe there is still an issue. Will take a look at the TB tomorrow.

 

Taking the air intake off prior to starting the engine first thing should be an indicator of what’s going on with your car.

 

Addicted to boost- Pulled the air intake off to look at the butterfly, As the accelerator moves so does the butterfly, with to pedal to the floor the butterfly is 100% open. As I recall when I first got the car even with the pedal to the floor the butterfly was not fully open - as you found the partially seized bearings were preventing it from opening fully. Adding new grease to the bearings fixed this for me. So I think the throttle body is working correctlty and is not the cause of the problem

Motorcarman Thanks for the documentation - will be working through it.

As the problem only occurs under acceleration I am wondering if there is some issue with the supercharger - maybe the wastegate is opening? I may remove the supercharger belt and see how the car runs - any comments on this idea?

 

I have figured out what the issue is, I took the ca out with both fuel pump relays removed but the and jumpered the connections so the pumps run continuously, Car performs perfectly no hesitaion of bucking as the revs increase to arouns 4,000 rpm. So the issue is fuel startvation

I have done some more investigation and have concluded that fuel pump 2 is not being commanded on by the ECU

WIth the car idling if I remove the relay for fuel pump 1 the engine dies. it is my understanding that if pump 1 is not running the ECU should swithch over to fuel pump 2
I have checked the relay for fuel pump 2 - all OK. There is power to the coil for the relay it is not earthing to energise the relay. If I connect pin 2 on the relay to earth it energises and the pump runs. This earth should come from the engine ECU. If I earth the appropriate pin at the ECU then the pump runs.

So it would apear the the ECU is not commanding the pump 2 on if pump 1 is not running. Probably not commanding pump2 on at high revs as well.

Is there anything else that would prevent pump 2 being commanded on?

 

I've heard of failed connections on the back of the fusebox. I'd start there before looking at the ECU.

Sounds like you're on the right track but I will say when I got my XJR the 2nd pump was seized & didn't work. The car didn't buck or do anything strange, just had less power at high throttle/revs than it did after the pumps were replaced.

 

You can check it by removing both FP relays, disconnecting the 6 pin connector on top of the tank, use a voltmeter to check power and ground for both pumps at the connector, use a jumper wire where both FP relays were at. You don’t need the key in the ignition at all for this test. If you look at the relay, pin 3 & 5 is what you need to jump in the fusebox. Also, did you check the fuses?

 

To summarize what I have done so far:

If I bypass the relays both pumps will run. To me this means the pumps and power supply are ok. Also tried earthing the relay solenoid, the relay clicks and the pumps run. So the power to the solenoid is ok. I.e. power supply and fuses are ok
The car accelerates beyond 3500 rpm with both pumps hardwired and no bucking. Hence my issue is pump 2 is not running at high rpm.

The ECU can only detect if a relay is bad and not if a pump is bad. I tried removing the relay for pump 1 to see if the ECU will fall over to pump 2. It will do this if you leave the ignition on for about a minute before starting the car. The car the runs on pump 2, so the fall over is working. Interestingly if you now put the relay for pump 1 back in both pumps now run all the time. This clears itself with just pump 1 running after 10-20 minutes driving the car. Just leaving the ignition on for a minute does not seem to clear this.

On startup pump 1 should run continuously and pump 2 for a short time to pressurize tho fuel rail. I hooked up a multimeter to the pump 2 power supply and could see a short pulse on startup.

As redundant test I also checked that both pumps would run by earthing the appropriate pins on the ECU

To me all this means the pumps are behaving as expected except that pump 2 does not come on at high engine rpm > 3500 which is my problem.

Next plan is to display rpm on my scan tool and take it for a drive to make sure the ECU is seeing the correct rpm. Can’t imagine it isn’t as I think it gets rpm from the crank position sensor if that was going bad the car would not run.

Another thought is that the ECU may only turn on pump 2 if more conditions than rpm > 3500 are met. Not sure what those other conditions might be. Maybe another sensor is bad ? I am seeing no other symptoms or codes.

I don’t think any of this implies issues with fuse boxes or wiring. Please correct me if I am wrong.

 

Watching this with interest as very very similar to my issue but ~I do not get any codes. Have you checked evap functionality? When I pulled my fuel cap it may have been a vacuum in tank issue as air rushed in. Not been able to do much on mine recently as the UK weather isnt fun to work in... way too much rain.

I'm thinking something potentially along the canister closed valve may be playing a part. If you go for another test run, see what the fuel cap is like afterwards too? Try taking it off and see..

 

You can pop the cover off that valve, tip the guts out & put it back empty. Keeps the system looking complete with the canister doing it's job much the same. That was my fix as I had two valves go faulty in not very long, it's been fine ever since.