Hi everyone.

Problem: When I turn the ignition for the first time in the day, the butterfly plate in the throttle body opens and closes rapidly while making a loud tapping noise. This goes on for 1 to 2 minutes and then it stops. If I wait to start the car until after the butterfly tapping has stopped, the car starts and runs perfectly. When I start the car later, the tapping does not reoccur. It only seems to happen on the first start. If I start before the tapping goes away, the engine jumps to 4000 rpms and runs runs roughly for several minutes.

Other Background: 1 year agi, the car had no problems. After sitting for 6 months, the battery drained so I recharged it. Then the car started and ran normally. Another 2 months go by and the batter is low again. I recharge it and start it up, Hear the tapping noise, start before the tapping goes away, the engine jumps to 4000 rpms and rus roughly for several minutes. After awhile, the rpms drop to 800 and the car runs normally again. My first fix was to replace the battery. Then I did a hard reset. Still the tapping issue persists.

Any thoughts as to the root cause. I have several guesses: bad/dirty throttle body. bad or poorly calibrated TPS, or problem with the ECU itself or its values.

Thanks

 

Thom4782,

As I'm a cheap skate, I always go with easiest fixes first, so as cleaning the butterfly is a simple 10 min job, it wouldn't hurt to try that first. Then if you're sure the battery is fine, it's probably the throttle body's TPS. It's on the right hand side and AFAIW should be available as a separate part. In case you didn't know, just turning the TPS position, in the hole, left and right "tunes" the idle position.

 

Thanks, I will clean as best I can the butterfly and what's accessible without removing the throttle body, I'm really curious, however, what causes this to happen only on the first start of the day. It's like the car when cold doesn't know for a moment or two what the right opening is for the butterfly. Could this be an idle control valve issue?

 

Thom, doesn't sound like anyone was anywhere near it, but parts of your description sounded an awful lot like what happened when I adjusted the throttle cable connection to the throttle body too tight.

Like David said, looking at the cable tension is free, so I thought I'd suggest it. When properly adjusted: no slack in cable; cable does not move butterfly until gas pedal is pressed.

I remember a TSB that describes how the throttle plate or butterfly opens slightly with key in ignition on but I can't recall the TSB Number. FWIW I'm not thinking of the TSB that describes throttle body replacement and points out the sticker on replaced throttle bodys / advises against cleaning those with the sticker. BTW general concensus here is go ahead and clean them, be gentle. I just did mine = all ok & she starts instantly now. Maybe it'll spark someone else's memory so you can review it.

John

 

Thanks, I'll check the cable tension just to be sure.

I do see the butterfly slightly open with the key in the ignition. The rapid opening and closing of the butterfly plate starts when I turn the key so the car electrics are on and the car goes through its system checks. As mentioned, this only happens with the first start of the day. The opening and closing starts lasts 30 seconds to 2 minutes and then stops. It's like the car is search to set the 'right' butterfly positioning and is having trouble doing that. After the opening and closing stops, the car starts and runs perfectly for the rest of the day, This means it fires right up, idles at about 800 rpms, runs smoothly, accelerates and decelerates as expected.

I'm beginning to think if this is an idle control valve issue

 

There is a gas pedal sensor reset procedure you can try: Ignition on, engine off, slowly go through the ENTIRE gas pedal travel, back and forth a few times, including kick down switch if you have one. This is supposed to re-teach the ECU about the range of the gas pedal sensor. Free to try, unlikely to hurt anything.

Probably would not hurt to get some "numbers" too. A cheap ELM327 and a mobile device app would let you read the live throttle position (a percentage, I believe). This is a standard OBDII PID, so no need for a Jaguar-specific reader. Maybe the gas pedal sensor is available too (this is drive-by-wire, so pedal and throttle positions are not necessarily identical). Again, doing a full travel of the pedal might show some unexpected numbers.

Next step would be to pull the intake tube and try and figure out if there is a visual confirmation of the throttle position with its "number" as seen by the computer (ignition on, engine off). It sort of sounds like the throttle position sensor is no longer providing a smooth reading of the actual position. As this setup has to be driven by feedback, it seems like a loop of over-corrections based on poor feedback. Not sure you can do this, but if you can unplug the stepper motor to the throttle plate, and hand actuate the throttle, you could confirm the sensor is giving reasonable output.

FWIW, problems with the throttle body are somewhat common on the 4.0l cars. I believe they can be sent out for professional service. Also, I believe alternate sensor part numbers have been identified over time for a replacement if that is the way to go.

 

Yesterday I tried fmertz's suggested pedal reset procedure. This morning there was no tapping sound when I turned on the ignition. Then I started the car and it ran normally, I'm keeping my fingers crossed that that the reset procedure has fixed the issue. It's similar to what I needed to do with the windows after the battery died.

As time goes on I continue to be impressed by the XK8, It is an incredibly wonderful car to drive and it's sophistication is amazing. On the other hand, the amount of knowledge one need to have to address questions like this is stunning. This forum is a wonderful source of that.

Thank you all.