Yesterday while taking a nice long drive in the country all appeared quite fine with the Jags operation. On the back leg of the trip home I tried to pass a car with accelerator to the floor and after the initial shiftdown that was expected the car bogged and kinda scared me that I was not going to be able to pass. (not sure of speed at the time, probably about 70, temp was normal)
After that the normal partial foot down acceleration no problem.... kick down and move... but full throtle not normal bogging down. No warning lights, no check engine light appeared. The car is fitted with ASL (automatic speed limiter) but it was off (supposedly).

Any thoughts or cures. Has this happened to anyone?

 

Sounds like a fuel pressure problem. Use an OBD reader to watch fuel pressure while flooring it, should maintain pressure at around 50 or so psi. Mine, and some others with the return-less fuel system system on 2003 and up MY would do this, and pressure would drop to around 10 psi when floored. Was a bad fuel pump. That's where I would start.

 

I'm watching this thread. I have the same problem. I understand these cars have two fuel pumps, one of which only activates when the car is at nearly full throttle.

By pulling some fuses you can check to see if one of them is dead. I however don't know the full procedure or how it works. Hopefully someone more knowledgeable will chime in.

 

03 and newer have only one fuel pump. 02 and older xk8s have only one pump, the xkr has two. others will confirm.

my guess would be a clogged fuel filter, gummed up throttle body, or possibly something else that gives you alot of fuel or alot of air when you need it the most...maybe clean the MAFS, check the air filter, etc.

 

I have already changed the fuel filter, cleaned the MAFs and intalled a clean air filter. My throttle body's butterfly looks pretty darn clean, so I can assume the rest of the TB is also clean.

Gotta check that fuel pump.

 

I would take another look at the throttle body, since it is easy to do. If you only looked at the butterfly, you may have missed the more important section of the TB wall below the butterfly. That, and the underside of the butterfly, is where the gunk likes to build up.

The first time I cleaned mine it didn't look that bad, but tons of stuff came off on the rag when I really got to it. The cleaning also removed a ring that had formed around the base of the butterfly that caused it to stick occasionally.

 

Alright. I should blast it with carb cleaner, right? Do I have to remove the throttle body for this task?

 

Update on actual fuel pressure readings from my car with elm327: in psi, my pressure fluctuates from 54.8 to 56.6, and this is idle, partial throttle/load, or full throttle/load. As I stated in my earlier post, when my original pump was going bad, after checking many things, I finally was watching real time fuel pressure at full throttle and watched it drop to 10 psi. This was the only way to get the dealership to agree the pump was bad because at idle the pressure was normal. The much missed Reverend Sam, a former XK8 owner and forum member also had the same symptoms on his 2003 XK8 and he pulled his fuel pump himself and found that a plastic tab was broken that holds the internal regulator in place was broken, and under high volume flow, fuel was squirting out back into the tank rather than into the fuel line, following the path of least resistance. I suspect that same happened with mine.

 

I use a CRC product specifically for throttle bodies. You do not have to remove the TB. Just prop open the butterfly and spray a clean rag with the cleaner. Swab out the body paying attention to the area where the butterfly is when closed. Get the edges of the fly as well.

Gus' site also has a cheater method: JagRepair.com - Jaguar Repair Information Resource

p.s. this is a good time to check your throttle cable tension and if your butterfly is opening properly. My cable was loose enough that the fly only opened about 65%.

 

I read somewhere that carb cleaner can remove the coating down the walls of the TB, I used TB cleaner because it said on the can it was just for that.

This might help...
Worth checking out before using the wrong stuff. TS

 

Thanks for the info, but haven't done any of the suggestions yet. I took the Jag out of the garage and did a visual check of accelerator cable and connections including the gas cap. Last week I had the car smog certification check and thought that the cap might be loose. Didn't appear to be.
So.... Took it for a drive to see if I could duplicate the bogging down issue again...... no such luck:-) Ran beautifully without a hitch. Could say, better than ever.

You got me!..... I have no clue........ (Will clean the TB tomorrow anyway)

 

Is the car bogging down at around 2500rpm’s? If it is it might be going into Restricted Performance with a temporary fault. Install your OBDII reader and take the car out for a run when it bogs down check your code reader it should capture any temporary faults. The fuel pump or filter could be the problem but I would suggest doing this first.

Another way to clean your TB http://www.jagrepair.com/ThrottleBod...ing1999xk8.htm

 

No, at about 5k under heavy acceleration with no flashing messages.

 

Install the OBDII reader and drive the car and see what comes up.

 

I don't have one.

I bought TB cleaner and I'm going to do that tomorrow when the engine is cold. I remember the last time, the top of the butterfly looked a little dirty, so I'm guessing the inside is worse, and when I pulled on the throttle cable it didn't open all the way. I hope that fixes it.

Also, when driving back from the store, I floored the car. I saw the rpms climbing but the car didn't want to move quickly however as soon as I let off the gas everything seemed normal. It'll accelerate well until I push it at about ¾ of the throttle.

 

If the RPM’s went up that meant that the engine was responding to you request to go faster. If the car did not respond to that then you may be looking in the wrong place.

 

So you're saying cleaning my TB might be a good place to start? Sorry, ESL here.

 

What I am saying is without the codes it is difficult to pinpoint the problem. No codes no direction. With the RPM responding I would say the engine is responding. Why is it not going faster? It is never easy to trouble shoot when the car is not in your hands.

 

Last time I checked the codes, the car was already doing this, no codes appeared. The situation hasn't really gotten better or worse since that last time.

Can I clean the TB when the engine is warm to the touch? I'm bored, and although it's almost midnight, it doesn't seem like too much trouble.

 

Yes you can clean the TB with the engine warm. As for the OBDII codes you can have a temporary code that does not set a hard code but could affect the operation of the car. You normally do not see the code but it takes place and the only way to capture it is for you to plug in the OBDII reader and drive the car. Yes you may not see a code on your car just standing still… it is a temporary code/problem….. I am going to bed!