CHECK TROUBLE CODES FIRST!
Usually when you see that warning there will be a P0121 code. Most of the time it is the result of moisture getting into the plug of the throttle position sensor at the throttle body. It is the four wire plug. Push in on the lock and unplug it. Spray something like WD-40 on the four prongs and into the plug. Plug and unplug it several times to clean the connections. Mine cleared the trouble light after I did this. This is a very common problem and this cheap fix is the best thing to do first.

 

Hi,

Did you get any information about how to check for the broken spring? It could be as easy as pressing the throttle plate to see if it returns. Or, may be a little more complex.

Please let us know.

Thanks,
Jeff

 

If you get a trouble code of P0121, the Jaguar manuals tell you to first check the wiring which is what I suggested and it worked for many of us here. The dealers can get you for over $2,000.00 by just replacing the throttle body. If what I suggested that you try first, which works in many cases doesn't work, then the problem is in the throttle position sensor. It is complicated how it works and you just can't replace it on your own.

DO NOT TRY MOVING THE THROTTLE PLATE!! IT IS CONNECTED TO A MOTOR THAT MOVES THE PLATE AND YOU WILL WIND UP HAVING TO CHANGE THE THROTTLE BODY.

A rebulit throttle body ready to go can be bought and changing them isn't all that big of a deal. Check the venors listed at this site.

 

This is a VERY VERY common problem many jag xj8 owners have. I have it. It is a TPS problem. The only true fix seems to be to replace the TPS, which means replacing the TB. I thought at first that it was a MAF problem as well. SO, I replaced it.

NO CHANGE!

I researched and diagnosed the issue with my TPS.

Compatibility problem between the throttle control and the TPS.

What I found was degraded wiring.

I replaced ALL of the wires from the engine compartment wiring harness to the throttle body. Using very high quality materials. Silver plated wire, gold pins, NO BUTT CONNECTORS (always solder).

I did this. The problem still was there but it rarely ever happens now.

I do know that the wear of the TPS is the cause of mine to fail. By dramtically lowering the resistance of the wiring, I dramatically improved the issue.

I will still be replacing my TB in Feb.

 

Check the wires at the connector for the throttle position sensor. It's completely unreasonable how the computer reacts to such a seemingly small issue, but it was 100% the issue that solved the exact same symptoms. One wire was loose.

 

The problem is back.

I am noticing a ROUGH idle.

My TB is noisy.

I am going to go ahead and replace the TB and the MAF next week together.

Being that I have already replaced the wiring...

I do realize that the local gas station I typically pump my fuel from has 91 octane. Could the 2 octane loss be that big of a deal?

Any Ideas? It does seem that when I use the Shell 93 octane, things seem to be better. That is just thinking back though. I have yet to have tested between the 91 and 93 octane to see if it causing this or not.

 

Use fuel that is recommended, but I don't think that's your real problem. Before you spend all that money, go to autozone and get a new filter (if it doesn't look so good) and some "mass air flow sensor cleaner". It's an inexpensive attempt at least.

 

l don't understand the excitemnet to change the MAF. If you want to do a quick check, turn the engine off, open the hood, unplug the MAF and start the engine and let it idle. Notice that you get a different warning. Turn it off and plug the MAF back in and restart. It should clear.
You can buy scan tools for under $100 that will give you live data from the car's computer. Then get a book like HAYNES TECHBOOK OBD-II & ELECTRONIC ENGINE MANAGEMENT SYSTEMS. This is a generic book and it will get you started in being able to read what the engine is doing.

 

I attempted the disconnect the MAF while engine is off, then start it idea.

The same fault condition occurred. I reconnected the MAF, fault disappeared until it faulted again on its own terms.

I have had 91 octane in for about 3 weeks. The problem has been so bad the entire time. I didnt realize that the premium they were selling on base was 91, until the other day, when I asked about the octane.

Yesterday, my tank was finally near empty of the 91, I put $10 of 93 in, so I could run it out fast.

As soon as the engine started, all conditions ceased. I let it run to 0 miles range. I put $10 of 91 in. The car went completely insane. I filled up with 93. It has faulted once within the past day, compared to the literal 15-20 faults per day.

I have a hypothesis. What if my MAF is bad enough to cause this issue. What is the operating condition of the engine using 91 octane is actually screwing the car up.

I am not saying the 91 is causing the problem. I am wondering if the condition of the MAF is causing irregular readings in conjunction with the other engine sensors that work along side the MAF.

I will just go ahead and change the MAF. $75, big deal. I still believe a TB is in order. We'll see, as the saying goes, "Don't fix what is not broken" right.

 

I am experiencing this same problem and will take the steps suggested. My first question is - where is the TPS?

Mine is an '04 XJR - supercharged, and I see lots of wire connectors, but none that look as described. Can someone point me in the right direction?

Meanwhile, I am off to get some fuel conditioner and fill the car with gas from a different station than the one I normally use.

Thanks!

 

before you do anything or spend any money put HIGH GRADE GAS in. this car will give all sorts of problem codes if any, I mean any lower grade gas is present in the tank. The timing is upset and the computer reads that and sends out a problem signal.

 

Start at the air cleaner and follow the tubes to the back of the engine and the throttle body is at the back of the supercharger. Look for the four wire connector. You will not find a throttle cable as the computer controls the throttle plate. Don't try to move the throttle plate by hand.

 

So, in attempt to make my jag run better, with the current issues discussed in this thread. I did a very in depth tune up.

New plugs and fuel filter were the highlight of this attempt.

The plugs I removed were bosch platinum +4. They were in terrible condition. I replaced them with NGK Iridium plugs.

I replaced the MAF.

The one thing I did, that made me the most interested was my fuel filter.

I replaced the STOCK filter, keep in mind my Jag has 103,500 miles on it.

When I took the filter off, the fuel that drained from both ends of the line connected to it, it let drain into a glass bowl.

The fuel was a very brown. There was obvious signs of water as well. I put a bottle of Heat in the tank to assist removal of water and other contaminates.

I put the new filter on, primed the system and started the car.

With the combination of all the parts I replaced; air filter, fuel filter, plugs, MAF, and cleaned the throttle body, the Jag is running nearly perfect.

The idle was very steady for a long while. It faulted with the faults discussed in this thread, soon after doing the tune up.

I drove it around. Got on her a few times. All seems well. The faults have yet to have returned. The idle did begin to fluctuate, while parked. I could here the plate in the TB close to far making the intake sound as if it were being starved of air. It makes a deep, quiet grown when this happens.

yet, it did not fault as it usually would during the fluctuations. I hope for the best.

I am holding off on the TB for the time being. I want to see if the condition improves. Perhaps, my dirty fuel tank is causing so issues at hand here. I want to burn out this tank first.

I will keep in touch.

Please, tell me you insight, anyone. What do you think?

 

The throttle body is available from JPAM (Gaudin) Jag parts in Las Vegas for about $650. Changing it is not hard for the shade tree guy, its easy to get to and requires no adjustment. This is what its going to take to fix this problem permanently. Every 4.2 car out there is going to face this thing eventually. Cars that run the freeways will go longer, in town will go shorter. The dual TPS wipers inside the TPS body rub the goody off the potentiometers and they poop out.
Paying the dealer in the thousands to do this is sad, I hate reading that folks have paid this. If you can screw in a hinge, you can change this part.

 

I finally bought a Throttle body today. I get it tomorrow. I am done fighting this stupid issue!!!!!!! ugh. lol

 

quick question though.

I have heard alot during my quest for a new TB. I heard that once installed, the ECM must be flashed. Something to do with calibration.

The question is, is a replacement TB plug~n~play?

 

I don't think you have to do anything with the ECM, unless you want to do a hard reset to clear the trims.

To 'calibrate' the new TB with the TPS and PPS, turn the ignition to II, then depress the accelerator slowly to the floor, the return it to rest. Do that 5 - 6 times, it'll sync the new TB sensors with your pedal. Good luck.

 

Thanks for the help buddy.

I got the new TB, threw it in. Did as you said, then started her up.

I will say, she is running better than she has in the last 2 years. My happiness with my jag is back.

It is really nice being able to confidently drive my car without the worry of it dying on me.

NICE!!!!!!

 

That's good to hear. Treat her well!

 

Well, last week, me and the old jag went on a nice long trip after the repair. Toured the smokey mountains, etc. The Jag ran beautifully the entire time. No issues.

Of coarse, it should drive without issue after the TB R&R.

It is just nice, I have never been on a trip with that car without some kind of issue ruining the trip.

I drove 1900 miles in total, averaged 31.7 mpg highway. After my first fuel fill up, from empty, i managed to get 621 miles out of that tank.

great trip

so glad to have this issue resolved!