Ever since I purchased a sweet ‘99 XK8, it frequently dropped into limp mode. I’d explored various fixes and crossed my fingers, but , alas, it came back to haunt us without warning. It happened more frequently to my wife. She says it hates her and refuses to take it out any more. From service records, the TPS and VVT systems cost the previous owner dearly with no cure so I inherited the problems

I was convinced the replacement TPS sensor itself was probably not the root cause of the limp modes and played with the throttlebody as a whole to become more familiar with the intake menagerie.

Since the accelerator pedal and Pedal Position Sensor feed the ECU with critical information, posts on that subject from this forum had me address that first. Bluetooth ODBII scanner revealed the butterfly is 5.5% @ zero throttle reading and a fully depressed accelerator pedal produced 90.2% @ full throttle. With this model year, the cable spool and sensor are attached to the throttle body. The cable external sheath is constructed of spiral wire so the mount uses this feature for cable adjustment. A thin, square plastic nut is secured in the steel bracket and rotating that square nut adjusts the inner cable tension. The square is retained in the bracket with a tie wrap and cutting this permits removal of the plastic nut, rotating it in 90 degree increments. It takes a few reinsertions and readjustments to ease up to but not beyond the 100% throttle reading. Tightening the cable too much will pull on the spool and give a false signal indicating the pedal is depressed. The fact that the throttle position registered 5.5% showed me my pedal is back to the original setting and not pulling on the cable... good to go. Do not use my value of 5.5% as a target. The throttle can and does close further. After all my tweaking on the TPS, 5.5% is no longer this ‘99’s cold closed throttle position.

I’m breaking this TPS & PBB project into separate submissions with associated images, so tune in to the next episode for more.



Original Zero throttle position reading cold on this engine. Jot down what the position is on your engine and if this reading is higher after PPS adjustment, then the cable is too tight.

 

I had played around with the relatively new TPS that came with our ‘99 XK8 and have a feeling the sensor it is still good but it was maladjusted. I came up with a logical approach to tweak it, as well as the Pedal Position Sensor, using a simple OBDII scanner. If someone else has revealed this procedure before, I don’t want to step on anyone else’s toes or be blamed for plagiarizing their work, I just haven’t run across it before on the internet.

Since the price of replacement TPS is now down to around $20, I figure, what the heck, throw a new one in. However, most of the Throttle Position Sensors on Ebay are mismarked for the model years or, they send the wrong one even with the correct item pictured. The first two received were for 2002-2006; took over a month before obtaining the right one. The one on the left in the photo with the small L bracket that engages the throttle is the correct sensor for the 99-02 Model Years, 4.0L, and the one on the right with arms diametrically opposed is for the 4.2L, 03-06 MY.

Before making changes, it seems imperative to document physical details and equipment reactions beforehand. Many have noted the clicking and humming of the throttle body when turning the key on and off. I can’t turn the key and look down the throttle body at the same time so I hooked up my cell phone borescope and witnessed the butterfly flapping in the breeze while turning the key. When key is turned to “run” position, the butterfly closes down tight, then reopens. Mine returns to 5.5% throttle position cold, ambient temp currently 40F, but that’s the same value when playing with it in warmer weather, too. Using a feeler gauge, that gap is approximately 25mm/ 0.10inch. When the key is switched off, it closes down to zero gap, returns to slightly open position, opens much wider and then returns to original marginally open position again.

I have to wonder if the ECU is closing with the key on, to reestablish ZERO opening/closed point. If there is a method to this madness, it only goes to show there should be a delay between key insertion and start. I always give the ECU a chance to run it’s course and let the instrument cluster do its thing before engaging the starter.

In preparation to fiddle with the TPS, the throttle body intake and the breather tube have to be removed which is a simple and quick procedure. Fortunately, for this vehicle, the original tamperproof screws securing the TPS to the body had been replaced. The worst part of a TPS swap can be trying to remove the screws without the correct screwdriver. Once the throttle body is opened up, part of the experimentation is closing the throttle by hand and watching the OBDII screen. After turning the key on, it seem the throttle motor refuses to allow forcing the butterfly closed, but after some piddling, there was only spring force to overcome, closing the butterfly. My #1 objective was to set the Position Sensor to where the scanner tells me it reaches zero just as the butterfly touches, no sooner nor any later. It can be stimulating rotating the sensor back and forth and observing the numbers rise and fall. After a few minutes, my fingers told me an open setting of 7.1 was a good number, but it wasn’t like it could be dialed in precisely, the values jump a few tenths of a percent at a time. When this playtime ended, it was time to get serious and take a shortcut for a more accurate setting. With the sensor rotated completely counterclockwise, the values maxed out. Then, the scanner read 8.6% open and 1.6% after it was forced down. Didn’t need a calculator to figure out setting the TPS to 7.0% should bring the brass disc in contact to the throttle body with zero percent reading and zero clearance simultaneously. I had to settle for the 7.1% reading as it jumps in small quantum leaps rather than dials in smooth analog scale. That is probably not the behavior of the sensor but the ECU's numerical assignment for that location. That still ought to make the ECU happy.

After the adjustment, the gap was remeasured and the 25mm strip barely slipped through again. This is not the best technique for measurement, just a good check. That technique likely isn't sensitive enough to register the microscopic increase in throttle. Just interesting to know the gap is still in the same ballpark as before. Fine wires would be better than a wide strip. I often use a set of tip cleaners and micrometer for measuring small gaps or carb jets, but there wasn't a decent set of tip cleaners at home.

 

After replacing the intake tubes, the initial startup was nothing to brag about. Engine shook like a cylinder or two were not firing, and it was in reduced performance in a few seconds. That was interesting.

Time for a hard reset. After a few minutes of intermission, it was time to give it another go. Engine rolled up to about 1500 rpm instantly, with no muss, no fuss and then slowly settled down to a slow idle in about a minute like one would expect. I wasn’t surprised to see the throttle closed down to a mere 1.2% after it warmed up for a few minutes as the ECU has finer idle control with the Idle Air Control servo but seeing it down to a very smooth, quiet 650 rpm did make me take note. The engine idles extremely smooth, not a quiver. Check out the image with 649 rpm and 1.2% throttle opening. Notice the nickel standing up in the lower left corner. It runs as good as it idles, too. Never better and more power with that last 10% throttle at the pedal.


Youtube Jaguar XK8 V8 engine idling balancing a nickel

 

The nickel is not on the engine its effectively on the body.

 

Didn't say the nickel was on the engine. Toughest part wasn't balancing, but finding a level surface where the coin didn't roll away. Oil filler cap was close enough.

oil filler cap turned out to be level enough
Youtube Jaguar XK8 engine idling, balancing a nickel- on the engine

 

The one I saw was on the water expansion tank...only joking mate ...amazing this superglue.

 

no glue needed for my sticky Avon CR6ZZ tires to pick up some spare change. Those tires are the best street legal tires I have ever put on any classic. Instant transformation,, the car rides beautifully and on rails too. A bit pricey at $1,600 + a set. But what fun !

Z

 

$60 on e bay delivered ..Works for me

 

You say you replaced the intake tubes, what do you mean by that exactly? Nice idle by the way! Mine has had a "roughish" idle since i got it, and ive tried several things to no avail

 

Just meant reinstalling the intake and breather tubes, from air filter & MAF sensor to throttle body after playtime was over.
Please note, this was on a '99 XK8 that has a touch different TPS than the '97. We have a '97 XK8, too, as well as '06 XKR but I can't recall the similarities and differences without raising hoods.

 

These motors really do run like sewing machines when they're 'on song'.

@Z - I love that Mustang

 

thank you ! Here’s the inside story.

Z

 

Which OBD tool are you using? I am looking to solve my P0121 error on my rebuilt throttle body from AutoECU last July and I need to get a better scan tool.

 

Cheapest bluetooth scanners using the free Torque lite app on cell phone. I have a plug in scanner but the bluetooth scanner makes it completely portable and you can set up the display with numerous simultaneous readouts large enough to read from a distance.

 

Thanks, I had similar results. Unfortunately I am still going in to fail safe mode. One under hard acceleration and once when hitting a bump. Does anyone know of a specific connect or ground that is known to be a problem?