Hi folks, for my first real post it’ll be a rather long one, and a head-scratcher, so buckle up. This is a story about my 1997 Jaguar XJ6/X300 with 113,000 miles on her. Enjoy.
THE PROLOGUE:

For months I’ve been chasing a high idle issue (I know, me and every other lovely schmuck on this X300 forum). The car sits around 1100 rpm in idle, and revs up to 1500 and stays there with even the gentlest touch on the pedal. I have to brake so much to compensate for the high idle, that I’ve worn down a set of brake pads in only a year of driving.

I’ve replaced the throttle body, gone through 2 “new” IAC valves (one aftermarket, one used OEM), replaced the MAF sensor, the crankshaft position sensor, the fuel pump relay, checked the fuel pump fuse, replaced the spark plugs, 4 out of 6 ignition coils (I ran out of spare ignition coils), and the air filter. I also replaced the EGR valve around 3 or 4 years ago. To hunt for potential air leaks, I’ve sprayed starting fluid around the bolts that connect that throttle body to the engine, the IAC valve, the MAF sensor, and the EGR valve… no amount of spraying starting fluid changed the revs, so I’m assuming those areas aren’t having air leak trouble.

Strangely enough, after replacing the throttle body, the problem appeared to be “fixed” for exactly one drive around town, and I was delighted to be driving normally for the first time in a while. Upon starting up the car the next day, the problem returned, and hasn't shown any signs of resolving ever since. I think some of my hair went gray from that.
THE BEAST:

Three days ago, my car was having an immensely difficult time starting. Occasionally, it pretended to start then stalled out within seconds of sputtering (fuel tank was half full). When, after much prayer and divine bargaining, it did finally start, I was greeted with a new problem after waiting for a minute or two in my car. As I was starting it cold, and the car has a high idle issue, it began at 1500 rpm, which is all fine and dandy– but it then did something very out of character, and the revs fell all the way down to 500 rpm. Good news, yay, a normal looking rpm! Bad news, the idle was rough– shaking my butt, shaking my hands on the wheel rough– and the exhaust was burping, pop-pop-pop-pop, not smooth at all, both sides. Engine misfire! Woohoo! Gosh, I just replaced those coils. Anywho.
I took the ol’ girl out for a spin down the street, around 25 mph, and she seemed to be driving like normal, no loss of power (I love this car very much), and in fact still exhibiting the over-eager revving that I described in the prologue. However, when I came to a stop sign, the awful shaking returned, and the revs dropped again to the 500-600 rpm range. I thought she’d stall out, so I turned right on around and drove back home.
I hooked up a code reader, and crickets. Nothing relevant. So the first thing I attempted was to identify which of my ignition coils was the problem child… I approached this by unplugging them one at a time, with the engine running, to see if anything changed. Unplugging the ignition coil closest to the windshield/firewall revealed something rather disturbing and odd…
THE PHANTOM MENACE:

When I unplugged the ignition coil, I heard the engine immediately rev UP. When I poked my head in to peek at the rev counter, I found that it read 1500 rpm. Now, I’m no car savant or anything, but that’s really counterintuitive and strange, right? Out of curiosity, I swapped out another ignition coil in that same spot, repeated the experiment, and found that the exact same thing happened. With all the ignition coils plugged, rough idle, 500 rpm. With that one pesky ignition coil unplugged (no matter what coil I slammed in that hole), the revs shot up to 1500 rpm. Am I looking at a wiring issue? An act of divine intervention to personally inconvenience me? This remains unclear. If any of you folks have seen anything similar, sound off! Especially if you managed to fix it.
I sort of suspect that my current shaky situation is somehow connected to the phantom revving caused by that one ignition coil. Alternatively, I’m scared that my fuel pump might be failing, as replacing that for this particular model seems like a real PITA (for me, because peep the username, not a car guy).
I really love this car, but this particular problem is driving me (no pun intended) very close to giving up on it, despite years of really beautiful memories traveling across the country with it. Plus it’s just such a sexy vehicle, and I’d hate to part with it.
TL;DR engine is misfiring, and I’m seeking ways to determine whether my problem lies with (1) wiring related to the ignition coils (2) the fuel pump (3) the fuel pump regulator or (4) something other arcane issue I’ve never heard of.
Additional ammo I possess: a spare ambient temperature sensor and coolant temperature sensor… I am getting a little tired of throwing random parts at this mystery problem though. I am going to purchase an entirely new set of ignition coils and slew of other sensors if I can find them for reasonable prices.
Cheers, I’d appreciate any support. A pat on the back. Maybe some advice would be nice.

 

To fix your idle issue, you need to get the TPS reset. That can only be done with a device loaded with Jaguar software on it. The dealer should be able to perform it. If not, call around to "british car experts". Someone in your neck of the woods should have the device capable of plugging into your OBD2 port and resetting it. PDU, WDS, VCM, Mongoose....probably some of the fancy expensive all-in-one computers might do it too?

Now, I'm not sure about your new idle issue, but I would put the original throttle body back in the car and see if you're back to your same old idle issues, which is easily solvable with the TPS reset.

Also, I would pull the spark plug in that one cylinder and replace it. (maybe replace all six?) You want the cheap Champion 71 spark plugs, or you can use the cheap NGK BKR5e plugs. Perhaps your plug has failed, which is why any time you reinstall a coil, the idle goes bad?

 

The TPS and the ECU are ' matched " at the factory by a process called orientation or TPS reset

This is an electronic telling the ECU what the mechanical idle stop TPS value is

To get this reorientation done requires special equipment

luck of the draw if swapping parts around and have the same match

That being said do not adjust the TPS , or the mechanical stop screw inside the throttle body throught until full understanding

Further reviewing your observations

By placing your hand on the throttle above the butterfly and manually twisting closed does the idle come in range ?

 

Vee,

Your IAC valve exercise / reset ?

While you have your TB off might as well reposition the coil spring to the alternate stronger preload position , whether or not the spring has been upgraded part as it would not hurt , this is 1 of the 2 alternate solutions for throttle not returning to idle stop that I'm aware of

 

This is a pic of the 2rd alternate solution of an external spring found in different sizes at the non - big box hardware stores

very high idle in neutral - Jaguar Forums - Jaguar Enthusiasts Forum

Still looking for pic of the first alternate using the original factory coil spring alternate stronger preload position

 

When the idle spring became 'back-ordered' during the recall, I simply 'modified' the original spring by winding it 'one-extra turn' and cutting off the extended 'tail' that was the result of the tight coil.

If you wind it tighter it will be too long to put back together.

You simply cut the excess off and finish the install of the upper throttle body actuation section.

Nobody ever complained and it saved several trips to the dealer.
The throttle potentiometer adaptation must be performed as the spring will only keep the 'blade/butterfly' from 'floating' during high airflow/vacuum situations.

Recall R493 has the explanation.
If the idle will return to 'normal' with a slight tap on the throttle pedal, then the spring is at fault.

If the high idle is not caused by the spring, then a TPS adaptation is in order.

 

The coils can be tested by a Innova 4400 machine at the auto parts store ( O' Riely's ) if they still have them in the back room , call first

The coils are very high above average 35, 000 volts because your Jaguar is special , just ask her

You will have to bring heavy leather gloves so the store knows what your doing , the power switch is on the back

My original coils at 110 K miles tested good

On car codes identifying cylinder ?

This test is not real world of heated coils and a common degraded coils ( cracked cover ) shorting out to valve cover coil wells , you may see some arcing corrosion on the well walls

Motorcarman has a way of observing while running them on car in the dark to see more easily , high humidity the better

There is the common crossing of the coil wires at the aft 2 cylinders

The aft most is Light Green / Purple , and the next is Light Green / Orange

I'll need to double check that

I always had a problem if the coil connectors are seated fully

 

Fuel supply squaring up :

In the beginning as you rotate the key the fuel pump will come on for 3 seconds only then off

You will have fuel pressure trapped in the fuel line by a check valve inside the fuel pump and the fuel pressure regulator up front ,

The pulsing of the injectors will bring the pressure down and hopefully by then the starting will carry on forward as the crankshaft position sensor sees engine rotation and turn the pump back on for the duration of your drive , safety design , do you see 300 RPM on starter rotation , this can show and still be a bad fuel pump enable use of the single CKPS signal wire

Things that can go wrong is the CKPS can get grime on the face of the sensor effecting the signal , whipe clean

The CKPS can fail in this one aspect of fuel pump enable ( along with your erratic ignition timing ? ) and not give code P0335

A good CKPS will read 1300 ohms as a basic meter reading , part bad from store ?

The pressure in the fuel lines in the initial 3 second " charging " can bleed off too quickly by leak in the fuel pressure regulator ( wet fuel in the vacuum line short hose on the top of the FPR )

Check valve in the pump or stuck open fuel injectors

You can jumper the fuel pump relay to run pump directly jumping between socket 3 and 5 with blade connector ends medium gauge jumper wire , this will run at all times and drain the battery with keys in pocket , a auto parts store can make one for you with a kit of tools and wire behind the register

The MAF will read on paper 1.2 volts DC at the proper idle speed of 700 RPM , the value will climb toward 5 volts on throttle up ; this is the middle signal wire on the MAF connector , the engine valve cover is not a good meter ground

Vee has a suggestion of giving the pins / blades of the MAF connector a slight twist for better socket contact , maybe the same for IAC

A sign the fuel pump is failing is burnt sockets at the pump motor connector and burnt power contacts inside fuel pump relay , You can swap the relay for a better chance but would show pump is on the way out

The coils get power at all times " sitting " on the common to all 6 white / Pink wire , the coils will fire when the ECU provide a ground on the 6 Independant Light Green wires to the ECU

ECU connector check ?

You can see your connector socket numbers in the pic far below

So your CKPS sockets on the ECU connector are Black 23 and Black 26 , the signal is protected by a mini coax shield which has a ground post on the rear engine firewall other than the 2 large positive power post with the battery cables on it

Your injectors return to ECU ground wires :

1 Black 2 Fwd most
2 Black 15
3 Black 14
4 Black 13
5 Black 27
6 Black 25 Rear most

Your coil return to ECU ground wires :

1 Black 10
2 Black 8
3 Black 6
4 Black 5
5 Black 9
6 Black 11

 

In the month following this post, I replaced all the coil packs and spark plugs, air filter, another IAC valve (along with the plug connecting to the IACV, because my wiring looked shoddy and some pins were bent), and the EGR valve. Some combination of these things made the misfiring go away. Great!

However, the high idle issue persists. I haven't gone down the route of a TPS reset as Vee suggested, but I did pick up a spare ECU with the code LNB1410AD, which matched my original. The swap is straightforward, as the ECU in my car sits just under the glovebox behind a panel.

Good news, bad news: the high idle issue went away, but my engine began to intermittently misfire again. For now I've got the car hooked back up to the original (because I'd rather drive with high idle than with a near constant misfire issue).

This ECU swap experiment appears to point to the ECU as the culprit here (I imagine it's communicating incorrectly with the IACV in some way or another)... but I realize now that the route of plugging in secondhand ECUs into my car is very much a crapshoot as it seems most listings aren't tested.

At this point, I'm gonna hunt around for more matching ECUs, but my hopes aren't terribly high. The two minutes when I thought my car was finally running correctly was euphoric though

 

The coils and injectors I believe have a splice in the 2 different 6 set , split into 3 each power "sitting " on the coils and injectors . So would be a power starved situation to getting full firing including injector fuel amount

The injectors get their power sitting from the large # 5 right ECU Controlled relay , the coils get their power siting from the fuse # 12 right engine bay fuse box ( Papa Indy 1 connector pin 4 White / Pink wire involved )

The ECU provides the timed ground to fire off these items , and these ground wires have no splice

I only read of one example where the splices were bad but worth a check , mine where great

Researching

 

So in the wiring diagram pic ( easier / simpler / cleaner representation ) the SC version same for normally aspirated

These splices should be under the long front to back plastic cover on top of the engine

The 2 / 11 wire splices in reality are broken down into many more in " standard practice " as max 4 wires in a splice , if you piked up a check engine light code for misfire you may be able to better decern which splice

I do recall someone had an issue inside the ECU as a control component but that may have been on a different V - 12 ECU

 

Further detail I should've included, I read through this thread, and came to a couple of semi-promising conclusions. First, that I needed to determine whether my car is SWB or LWB... I'm 90% confident it's SWB, considering the lack of rear leg room. Second, that the spare I swapped for my original LNB1410AD (that my car didn't like very much) was in fact meant for a LWB jag...

I failed to mention that the spare, although it was labeled with LNB1410AD, had an additional label reading LNB1410FE -- which indicates that the ECU was sent in to get its chip swapped, from what I understand. This new designation ending in FE is specifically for LWB jags, which partially explains why my car didn't react well to it.

I have since ordered an ECU labeled LNB1410AE, which... assuming it's not junked, might fix my high idle issue?? Fingers very very crossed.

 

having had some issues and swapped an ecu myself i can confirm regardless of if the ecu is the same or not the idle may or may not be right.
it is a matter of over time TPS changes, and the ECUs adapt.
so when you replace an ecu it is very very very likely (more likely than not) that it comes from a car where the tps read slightly different.
even if the ECU is exact same part number.

also i don't see why a LWB would have specific ECU. only difference is a longer cabin.

 

I'm having high idle issues currently and it might be the linkage for the S/C bypass being a bit sticky, or its return spring being weak

 

The SWB is 36 inches along the bottom of a rear door window glass itself and the LWB is 42

David ,

On the SC can you manually with your hand twist the throttle body butterfly to closed position and make an idle difference ?

For both there is the issue of if the TB butterfly does reach the idle mechanical stop screw

When purchasing a difference ECU there is an issue of the TPS and ECU matching a null value ( 0.60 volts DC on the middle TPS wire ) of the correct arrived to mechanically the idle stop ( and some that don't make it to the idle stop )

More later and the R - 493 TSB which in the 3rd part involves a stronger throttle return spring and work arounds ( on this forum ) in not able to obtain newer spring

This TSB being accomplished on a different ECU may account for the ECU part # change as the full TSB being done involves changing the PROM chips out on their holders

The engine ECU should have nothing to do with LWB or SWB just regions of the world emissions requirements , TSB done , and if supercharged or 3.2 liter inline 6

 

Notice page 16 parts information and a differentiation for LWB and SWB ( which still is a spot of stumble under my bonnet )

05.1-26 amend4 (R493)

 

I've not tried with the throttle butterfly (not removed anything yet) but if I mess with the arm coming out of the S/C bypass actuator I can move it and let go and both recreate high idle and stop the high idle, depending on what I do. Used to be that a light tap on the throttle pedal would settle it back down. Now it's just as likely to ramp it up.

 

Sounds like the throttle butterfly needs to be cleaned from sticky gunk that collects there?

I know that's a pain to do with the s/c, but its most likely what creating this mess. What's worse is that you may need a TPS Reset once you're done cleaning the throttle body. That can only be done with a device with jag-specific software loaded on to it. A dealer, if they can perform it, will likely charge you one hour of labor for the 15 minutes of time they'll end up spending.

 

I don't have the SC and your TB is rotated 90 degrees from the normally aspirated

You can see your SC on page 88

801S TITLE

Your SC bypass valve is on page 105

See page 108 for an oily breather gas passage that can get clogged later deleted at VIN # X To be covered in a TB interior butterfly cleaning , do not remove the TPS and the IAC valve mount bolts can snap off without heat from the inside , 5.5 mm socket or maybe 6.5 can't recall but it is a something .5

 

Parker, assuming I do manage to find someone capable of performing a TPS reset on the jag, will it be immediately evident that the idle returns to normal, or will the system need to "relearn" everything? I've left a voicemail at a nearby dealership I suspect would be able to do this for me.