Didn't know quite what to title the thread but today I had one of those particularly exceptional driving days in terms of the car's performance. I have had this sort of thing happen on the last few cars I have owned and I always thought I was imagining it but it was definitely weird. What happens is for some odd reason the stars all align right and the car drives like it is on steroids. Today was the first time this occurred with the Jaguar but it used to happen with my old Vette every so often and I am not sure what triggers it but it only seems to last for a particular driving cycle. Once you shut engine off the next time you start it seems normal again.

Specifically what happened today was I started the car at the end of the day to return home and didn't quite hold the key long enough so it died. I started again and suddenly the car is responsive like I never experienced. It takes off with barely a touch of the throttle and drives like it has at least 50 more horsepower all the way home. I was reluctant to shut it off since it is unlikely this fluke will occur tomorrow.

I think the same type of startup hiccup (i.e., not quite starting the first time but quickly starting the second) was the same thing that would cause the Corvette to run more briskly.

It is almost like the car wakes up on the right side of the bed, for lack of a better analogy. Has anyone else had a similar experience where for some odd reason your car behaves this way?

Doug

 

I think I maybe able to help you with this. Explanation first. Your car basically learns how you drive. The ECU teaches itself. What you have done is basically "unlearn" your car briefly. Now you need that permanently.

The best way to get this back permanently is to disconnect the battery (ensure you have your radio codes first) and leave it for some hours. Before anyone pops up and says you only need to do it for half an hour sorry but I have tried that sort of time and it does not work. Anyway leave it disconnected for at least 3 hours, pref overnight. Then re-connect the battery. Teach your windows to open and shut again to one touch. See how it goes. If there is no change then its only cost you a battery disconnection. If its back to its old zooomy self then great.

Please let us know how you got on. I would be very interested to know if that fixes the problem for you.

Frank C

 

It's hard to see an immediate connection, but folks report the same change in the feel of the car after adjusting the throttle cable. As you know, this car has throttle-by-wire, meaning the gas pedal provides input into the computer through a variable resistor. The resistor is connected to the pedal with a cable, and that setup has an adjustment (it is right there, next to the brake booster, under the driver side cover). Once the slack is removed from that cable, the car feels different. To me, it felt like it shed 300 lbs. It is hard to see how your failed start/restart sequence would have somehow re-adjusted the cable, but is it true that the manual calls for a "relearn" of the resistor range by the computer after the cable is adjusted. You are supposed to press and release the gas pedal through the entire range of motion (I did this with the ignition on, engine off). Maybe you should look into this some more, there are many posts. Just a guess, I suppose.

Poetically, I see your point too. There seem to be these drives once in a while where everything seems perfect. The car responds to everything you do, everything feels right, there are no suspicious noises, folks seem to telepathically know you are there and somehow leave the road to you. Magic that makes the stuff we put up with completely worthwhile.

Best to you, keep us posted.

 

Frank--what you say makes sense but I think I recall days with older cars I have owned (pre-ECU) where the engine would just feel peppier for a driving cycle. We used to be able to force this by playing with the distributor and advancing the timing a hair. I thought something similar was going on when this would happen with the Corvette that had OBDI engine management. I am hesitant to disconnect the battery--partly for the annoyance of having to get the radio set up again but also partly I would be concerned that, with the electrical gremlins in these cars, I would screw something else up. But I might give it a try. I think there are some gizmos that you can plug in to the cigarette lighter that will maintain radio settings when you disconnect the battery--yes? Also, are you talking about a simple disconnect or the other type of reset that folks here talk about when you touch one cable to the other terminal briefly?

Mertz--I am certain it is not the cable since I have already gone through fiddling with that a few times. Moreover, it was not just the throttle response that was different but the engine just felt stronger at all driving conditions.

Doug

 

I've always found engines run particularly well just before they blow up.
I'm sure that's not the case here though.

 

temps drop recently? I know when the cool temps first hit, specially in the morning when the car is cold also, that they seem to have a little bit more power.

 

in the xkr you have 2 fuel pumps... and they both suck and they are both in the tank

the normal operating mode requires that the ecu control relays to operate the on/off of the pump.

on startup with the key in position 2 before cranking the ecu commands the 2nd pump (the supercharger fuel pump) on to prime the rail for a few seconds.

then when the car is running the primary (pump 1) is on constantly in a return fuel system on the early 4.0 cars..

the second pump kicks in only after 3000 rpm or so.

what happens is that the supercharger pump.. that only runs periodically usually goes first and almost always un noticed. you will sometimes get issues like hard/long cranking and sometimes stalling.. when the primary finally goes you will be left stranded.. ask me how I know..

the pump may fail mechanically or sometimes its just the relay.. sometimes the wiring harness / electrical connections.. etc..

moral of the story.. its a fueling issue perhaps.. and the xkr does not monitor fuel rail pressure or pump activity... just relay position.. so it does not throw codes...

ymmv

 

This is only true up to the 2002 model year. Doug has a 2005 XKR and it has a completely different fueling system with one pump.

 

I think Frank was closest to the right explanation in that the startup hiccup temporarily screwed with the ECU. The effect lasted for the entire drive home. When I started the car (normally) this morning the magical effect was gone--just drove as it normally does.


Doug

 

y'all are correct.. I missed the year of his car.. indeed a different system..

when I bough my car I think the supercharger pump was already on its way out and the car definitely had 2 personalities depending if the pump kicked in or not and it sounded just like mine.. jumped in too early.. does the 2005 not provide additional fuel pressure during supercharger operation?

 

Yes, I believe it does, but the pressure is closed loop regulated electronically by the ECU and the pump output is controlled by pulse width modulation.

 

I don't think this is an issue with the later year North American cars. My 2001 doesn't need a radio reset when I disconnect the battery, just needs a reset of the windows.

As Frankc noted, the ECU "learn" function will indeed affect the driving. A reset as he suggests can make a big difference, but keep in mind that it forgets lots of engine management stuff. So it may run a little worse for a few miles before it matches the more aggressive driving you are looking for.

 

yeah the pwm controller is behind the seat somewhere.. could be fuel pressure if that pump is unable to keep up as it ages, clogging fuel filter (also on the pump I believe?) a failing pressure regulator (any of the fuel regulation in the 2005 use vacuum for instance) or sensor giving feedback to the ecu and pwm controller, or worn points on the relay not permitting the fuel pump to pull the full 30A the circuit is rated for.. just a thought.. or it could just be a computer gremlin like you guys think.. but in my experience if the computer is working its working as advertised or its not... usually these things have a more physical solution .. it something is wearing out and not performing as designed

oh. i have also seen failing batteries do weird stuff in cars built after 2000

 

hey whitexkr you have been around these parts forever and I have seen your name pop up I think with head gasket and timing threads.. can you check out my head gasket diy help thread I put up today.. thats deeper into an engine than I have gone before and looking for veterans to weigh in on it...

thead jack over..

 

if you have a good scan tool.. unlike the 4.0 I believe the 4.2 reports fuel pressure .. you can log it the next time you drive to see if the pump is keeping up and you put your foot into it..

 

Along with my window reset I have to reset the radios bass,treble and sub settings.

 

Blimey, not much response then a flood.

Yes I am talking about a hard re-set. I heard about touching the cables together to discharge the capacitors etc. Personally I have not tried that method myself, just seems a little counter intuitive touching positive and negative together. Though it probably would work just as well. Personally I would stick to disconnecting the negative for a few hours.

I read only recently about someone, on another forum, who did this and reported excellent results. Sure it may not work. Sure it may have to "re-learn" stuff. But what have you to loose?

 

Just had a look through the 'other' forum.

Here you go Hard reset, interesting results.

and for the guy who messaged me yes it is me from the other forum. I assume you mean this Franks XKR Story.

Hope you try the reset and let us know, give it a few days for your car to relearn again. Should take around 3-6 cycles, i.e long drives.

 

Sorry, missed your message when I skimmed through the replies.

Just to clarify things. Mine too does not need radio codes and just a window reset after disconnecting the battery. However when I did the headrest repair I had the battery disconnected for over 3 hours then I had to enter the radio codes etc. That is why I suggest disconnecting for over 3 hours.

 

Might give a try and am still wondering if I could use one of those cigarette light gizmos to retain the radio settings or will using this counteract the computer relearn?

Anyway I did not even though there was an "other forum" and clicking on your link it looks like a virtual mirror of this forum. How did that happen?

Doug