This is for a 97 XJ6. The car seems to be fine driving around town at speeds of 55mph or less.

This issues arises when I take the car for a highway drive. At speeds of about 60mph or more, the car gives a clear jerk when stepping on the gas pedal, even if I depress the pedal only very slightly. Then, as soon as I fully release pressure on the gas pedal, the RPMs seem to hang stationary for about 1 or 2 seconds, then the same car jerk, and then the RPMs begin to fall. Even if very gently and slowly on and off the gas pedal, the car jerks.

Even worse, after this starts happening on the highway, when I slow the car to about 30mph, the car continues to give itself gas and move itself when I'm not pressing on the gas pedal. This makes it difficult to stop the car to 0mph, as the brakes are fighting the engine. The car will continue to drive itself sometimes seemingly indefinitely. For example, last night the car was doing its jerking on the highway, then I got off the highway and took back roads home. For the 15 minute drive home the car gave itself gas to about 30mph and I didn't have to press the pedal. Then this morning its driving fine around town again. If I take it up to highway speeds this issue will arise.

I was thinking this is something mechanical with the throttle body, so I gave it a good wipe out, and now its spotless. I removed it and had given it a full cleaning about a year ago, and the car has driven fine up until recently. The throttle body return spring seems to be strong and snaps the butterfly shut. It doesn't seem there's any excessive slack in the throttle cable when I touch it or push the gas pedal. Idle is fine, but only before I bring the car to highway speeds. For example, the other day when this jerking was occurring on the highway, I popped the car into neutral and slowed to a stop. The RPMs were bouncing between about 2k RPM and 3k RPM. The engine was literally pulsing about every second between 2 and 3k... I opened the hood, thinking the throttle cable was stuck, or the throttle body spring wasn't shutting the butterfly valve all the way. The throttle body seemed closed and the spring seemed perfectly fine.

Right now I'm worried that this jerking and constant throttle during shifting is beating up the transmission. Please let me know if you have any ideas as to what it might be.

Thanks

 

Does this post sound like what you have?

Crazy/no idle and dash lights.

 

Now, this sounds like Toyota... Sounds a bit like engine management + transmission problem...

 

no, its not like the other thread referenced. I dropped it off at the mechanic today so we will see what he finds. ironically it finally threw a TPS error code on the way to the mechanic, though I don't understand how a faulty TPS could cause the car to drive itself. Its a P0121 code if I remember correctly. I found this thread, and it seems a bit similar but has only a little info.

 

My car has had this problem of and on for 2 years no one has been able to diagnose it
I have had no fault codes will be watching with interest please post what happens or how it gets fixed

 

You all should read ALL THE TSBs for this model.

Here are a few.

The engine breather mod is NLA.

bob gauff

 

Hey Bob thanks for the TSB,s already done checked and investigated all of these my Indie over here in Australia is the specialist on these cars only indie with the factory diagnostic equipment
He is a 4 hour drive from me plenty of local guys where I live but none of them could fix my car that's why I take it to him I have even videoed my car when it does it to show him and he is baffled as well and I am not ready to give him a blank cheque yet to explore
My last guess is the adjuster on the throttle cable into the cruise control traction control unit its been repaired but pretty ugly I am getting one of the cable brackets from Jerimiah Jaguar will advise if that fixes it not holding my btreath

 

I've been dealing with this and trying to narrow it down over months and probably a few thousand miles of driving. Maybe I have found the issue?? Here's a recap of the problem and what I've done. Maybe a bit lengthy, but I want to share as much as I can think of for others as this has stumped me.

The car felt like the transmission was kicking at highway speeds when I would get on and off the gas. Initially it would be fine the first few minutes on the highway, then it would get progressively worse over the next 20 minutes until it would kick every time I feathered on or off the gas.

I took it to my jaguar mechanic and he changed the transmission fluid and put on a new filter since I don't know when it was changed last (mileage about 105k). He mentioned the fluid looked ok and there wasn't any sign of abnormal wear of metal parts in the fluid.

I drove the car home and after about 45 minutes it started kicking again. Over time I noticed that it gets worse and worse the more I let off the gas pedal. If I drive on the highway for a full hour without letting off the gas pedal at all, the car will not be kicking. Then if I pop it into neutral before letting off the gas to slow down, the RPMs will not hang or pulse and they will drop down to idle normally.

I swapped mobil 1 fully synthetic ATF into the transmission, then it started kicking every time I would get to highway speeds. No wait, it would just start doing it. I think this is because the synthetic fluid is thinner than the regular stuff and flows differently. Just a hunch... Pretty much convinced its a transmission issue with the valve body or something, I put in a bottle of lucas automatic transmission fix. Things got better almost immediately. With the fluid thickened up, I could feel the car shifting slower, but it was not as bad with the kicking. I was pretty much babying the car when driving it at this point. I've driven like this for maybe two months, and dealt with it. I would stay on the gas as much as possible when on the highway, and before getting off the gas pop it into neutral. This was a fair workaround for a while... As of recently the issue has shown itself more and more often even when trying to work around it. Sometimes it will happen around slower back road speeds.

When it happens, the engine drives itself. The transmission will do this apparently if the right things go wrong. The RPMs will initially pulse between 2000 and 2500 for about 10 - 20 seconds, and then its as if something gives and the RPMs hold at 4000 and will not go down (this is when in neutral). When in gear the car just wants to take off and the brakes are fighting it. Sometimes when I turn the car off and back on it will turn back on with the same problem. Numerous times I've gone under the hood when its doing this and found the throttle body to be mechanically CLOSED completely.

So now as this issue has recently become more prominent as what I suspected was the lucas transmission additive working itself down, I called my mechanic again to discuss and see about a transmission rebuild or replacement. He has some x300 parts cars, so we discussed changing out a few things such as the ECU, transmission control unit, etc before taking out the transmission. He mentioned disconnecting the idle air control valve to see if that was any help, as sometimes it can get stuck open. I thought it to be a complete long shot to be honest.. Then yesterday when the car acted up again after a short highway drive, I went under the hood and disconnected the plug to the IACV while the car was revving itself to 4000 RPMs. Nothing happened, just as I expected I thought...

Then later that night before about an hour + highway drive, I figured what the heck... So I started the car, drove a few minutes down the road, then when it was warm I pulled over and disconnected the IACV. Then I got on the highway and drove it all the way home without any issue. I wasn't bashful being on or off the gas at all. I pupped it into neutral a few times while driving and expected to see the RPMs slowly drop down or hang up as usual. With the IACV they dropped right back down to idle as I have longed to see for a long time! This is like day and night behavior.

This morning I started the car cold with the IACV disconnected and it would not idle. I plugged it in, started the car until it was warm, then disconnected the IACV plug and it drove great. Not problems with a weird idle range with it disconnected as I had expected.

That's where I'm at so far... It puzzles me that it looks like all this strange behavior is a faulty IACV.

 

I replaced my IACV valve with a new one very costly after having it checked and put on the factory diagnostic it still does it every now and then
I haven't driven the car since october but will be getting it out as soon as summer goes its to hot and humid up here in summer and rains all the time winter is the exact opposite dry temps in the mid to low 20,s Celsius the car just goes better not to mention tourist season is over so the roads are not congested with daydreaming holidaymakers perfect time to get the XJR out and have some fun
Keep us posted I had basically given up on remedying the auto acceleration problem till you reposted

 

Has there been any ideas on this problem
I just drove my car first time since October and realised why I hadn't been driving it the throttle still sticks very painful just went through all the posts relating to this issue tried them all bar just replacing everything

 

I replaced my IACV today. I pulled the old one out and gave the throttle body a good wipe. It was not all that dirty. I wiped out the IACV cavity and tunnel also. The IACV itself was caked with crud. With the new one in and everything buttoned up, I started the car. It idled where I would expect and warmed up. I started blipping the throttle slowly and the RPMs hung. I took it out around some back roads and the RPMs kept getting hung at various places and the car continued to drive itself. When I popped it into neutral it would rev over 3K RPMs at times. When stopped, and its revving itself up, if I blip the throttle again the RPMs will drop down to the normal idle.

I put the battery cable ends together to clear the ECU, it was a long shot. No change. I disconnected the IACV as I had been doing previously - no change. Now I can't even get out of my driveway without the RPMs taking off and hanging...

Any ideas? Was I supposed to calibrate anything after installing the new IACV??

 

This problem sounds alot like a problem that I experienced some time back. My car would run without me stepping on the gas, fuel trims were high and the car smelled of gas. I found the problem to be the pressure control valve located by the charcoal canister the proper name is the "rochester valve". this valve regulates gas fumes that go back to the intake manifold in order to relieve gas tank pressure. when it goes bad additional fuel is sucked into the manifold, this causes alot of problems. take a look at my thread about the rochester valve, pics and in depth explanation is available at this thread. and the best part is this fix takes 10 min and costs about 25.00 dollars.

 

This may have already been covered, but......

.....have you checked your throttle cable adjustments?

A few years ago I went thru hell and high water trying to fix a high idle/hanging throttle problem and, in the end, slackening and re-adjusting the cables solved the problem

Cheers
DD

 

I think I've finally sorted it out and I just got back from a lot of back road and highway driving without any issue. I think the problem as a whole was combination of a sticky IACV and the cruise control arm to the throttle body binding and not closing fully. After replacing the IACV the RPMs were hanging again as mentioned. I went under the hood completely puzzled and felt the top of the throttle body again. The cable attaching to the throttle body has good tension. The throttle body arm attaching to the cable would close fully when the cable tension was let off. This lead me to believe the throttle return spring was fine also.

I fiddled with the top part of the throttle body, that which has an arm connecting to what I think is the cruise control actuator, and the RPMs dropped. I found that this arm was somehow binding up and the cruise control part of the throttle body (top part again) was not closing fully. Literally held open only 1/8th or 1/16th of an inch each time I'd apply only a small amount of throttle to get the car moving from stationary. The arm itself looked clean so I'm not certain why it wasn't closing on its own. I suspect any amount of crud is farther down into the housing, but I couldn't see an easy way of getting in there and cleaning it without taking the whole top part of the throttle body off and disassembling it.

I went to an auto parts store and picked up a package of 5 or 6 throttle return springs of different sizes and tensions. I hooked one end of the spring to the cruise control part (top) of the throttle body. I hooked the other end of the spring to the post where the throttle cable runs into its rubber hose. This way when the throttle cable pulls the butterfly valve open, the cruise control (top) part will remain closed. What's ironic is that the design of the throttle body and the throttle cable post look like there should be some sort of return spring or cable attached there.

The first spring I put in was too tight. I could feel the tension in the throttle cable and the car felt sluggish. When I floored the gas pedal, the throttle cable was having to fight the spring and the car would not drop a gear in sport mode.

The next spring I put in felt a bit too lax and I wasn't certain if it was going to work, but its turned out to work perfectly. There is very little tension on the throttle body, so the butterfly opens with ease, but as the butterfly opens more and more, the spring continues to hold the cruise control part closed. There is enough tension on the cruise control part so that it gets held at fully closed when the throttle is eased off and the butterfly comes to a close or slams shut.

I have a few pictures below. All-in-all the pack of springs cost less than $6 at Advance Auto Parts, and I'm only using one of the five or six spring sizes that came in the pack. If the car starts acting up again, or I find that I'd like just a little bit more tension on the spring, I'll just turn the spring around the throttle cable post a few times. For the moment I have no reason to change anything.

Note that I have not tried operating the cruise control yet. I have a few hours of highway driving tonight, so I'll give that a shot and see how it goes. Sometime in the next few weeks I'll try to figure out how to take the whole throttle cable and cruise control attachment assembly apart to clean it up. For now I'm going to put some more miles on it and see if there are any problems.

The picture on the left shows the return spring that turned out to have too much tension and fought the throttle cable. The middle picture shows the good return spring. The picture on the right shows the return spring under tension and holding the cruise control closed.

 

What a nuisance this has been. I hope you can finally solve the problem without the added spring, but if not, let us know if the cruise control still works.

 

Back when the TSBs were released and the cars were coming in for the sticking/hanging throttles I used to fix this fault by removing the throttle spring and unwinding it (bending) 1/2 turn with pliers.

This left a longer 'tail' for the end of the spring. I cut off an inch or so and then wound the spring 1 additional turn when replacing it. The end needs to be cut because it will be too long to fit back in place.

It is best to have an extra spring in case you butcher the original with experimentation.

I have also used XJ40 components to make a 'helper spring' as it is used for the AJ6 engines.

Either way, the throttle sometimes hangs open slightly with the original spring and even the updated one.

bob gauff

 

well good news. after about 4 hours of highway driving and some back road driving this past weekend the cruise control appears to be fully functional. the car changes gears very smoothy with a working IACV, even better than with the IACV unplugged. An added bonus is that the "instant fuel" reading is actually working now. I had previously looked at it as a bit of junk as it never seemed to make much sense. Now its stable when I'm on the throttle, and when I let off the throttle the reading goes to 99 by the second update.

 

My Rochester valve has been replaced couple years ago
just wondering how long they last

 

My 97 XJ6 was doing almost the same thing. The Idle Air Control had come apart allowing intake air to by-pass the butterfly valve through the orfice. Of course a new IAC fixed it. Located top of throttle body. Arch