That's an interesting thought, Brewtech. I think you're right that John should first confirm that the TPS is properly adjusted before touching that screw.

JagFix38 posted a good explanation here:

https://www.jaguarforums.com/forum/x...e-idle-118636/

Which reminds me, any time you're having idle issues it's worth cleaning the Idle Air Control Valve stepper motor pintle and port. Be sure to disconnect the stepper motor electrical connector before removing it, and wait to reconnect it until it is reinstalled. If it is accidentally powered while removed, the pintle can thread all the way out.

Cheers,

Don

 

Yeah, no wish to adjust anything that I have not yet disturbed as the car was running fine before.

Not sure when I will get time again, but next will be the TPS adjustment, then testing of as many other sensors as I can locate and can find test methods for. I have reread most of the Haynes manual, and the linked page above.

I ordered a new rotor cap and rotor arm, not that I think they are particularly the problem, but might as well have a new set installed and a used set in the trunk as spares. I might order some Iridium XP plugs, as there is a good rebate on them, even though the Champions are only one year/1500 miles old. Still concerned by the yellow spark, sure it should be blue, and assume this is not helping get the best out of the engine. I see the IACVs are only ~$10, maybe a good idea to order a spare now too, although it hurts to replace parts with no evidence they are faulty.

 

Don't fit any fancy plugs, they can cause problems and the car will run better on those old Champions (which also last forever on the AJ6).

Have another look for a vacuum leak, usually that is the cause of high idle. Your stepper motor is supposed to take care of idle speed but if there's a vacuum leak and she's getting unmetered air, the stepper motor IACV won't be able to adjust for it.

Sometimes the air bleed screw Brewtech mentioned gets sloppy and allows extra air down beside the threads, teflon tape helps out here.

Larry

 

I do want to make it clear that if there's any deviation in the TPS position, the values being produced will be reflected as either a high idle or poor running condition. I currently had a ton of issues with my XJS, that were all caused by a fouled, and incorrectly set TPS. Cleaning, it with electronic cleaner and a proper re-set, I suddenly had a high idle. 1000RPM at a red light sounded cool but, its not right. So I turned the idle screw and it has been smooth ever since. But intake leaks are the first and easiest to rule out! Good luck!

 

Hi all, late to the party again- I will throw in 2 thoughts, worth 1 cent each for my total 2 cents worth- Part of the problems I was having with my 92s surge and rough idle turned out to be a faulty valve on top of the evap can (at left in front of the wheel). The valve had split in half and was a permanent air leak. My other thought is that on my 94 after the head gasket change, I also ended up replacing injectors. I had one that was not closing so that the car would start wonderfully cold and slowly get rougher. Then, after sitting at work it would barely start once the injector had filled that cylinder with gas. (Yes I changed the oil). I have a "redneck-y" jig made up to try to clean the injectors but it didn't work for me, and I decided that a fresh set would (probably) eliminate any...... doubt.
By the way, that tps sensor should have elongated bolt holes so that you can install it but leave the bolts loose to adjust while running it. ( i'm sure someone said that but I didn't see it, Sorry)
I hope you get this sorted soon, driving season is upon us!

 

Still waiting on parts arriving, hoping to get some time to check/re-calibrate things this weekend.
One observation is that there continues to be a lot of blue smoke on initial start-up, along with lumpy idle, that clears and she settles into a fast idle.
Air leaks are certainly something else for us to focus on, if TPS and IACV overhaul does not help, but wondering if there is something in the emissions that has failed and is allowing her to draw in more oil?

Rotor arm, cap and coil arrived and have been fitted, with unsurprisingly, no improvement in the current issue.

 

In the midst of it now. Cannot see that the cap will prise off my TPS, it looks to be all molded in one piece. Might risk tiny holes in it, but worried I damage the internals.
The hose from the oil fill tube was very gunky and there was oil in the bottom of the throttle body and inlet manifold again. Cleaned the hose from the air inlet elbow to the little plastic fitting in the hose that goes to the oil fill tube - what is that device? It looks like a copper plate in the bottom of the hose union, with a two wire connector. Not a valve of any sort.

Installed new IACV today too.
Will report back after setting the TPS etc.

 

Dont press hard when drilling. Its plastic, not concrete. It should tap in like butter

 

Small holes in the range of 3/32 to 7/64 will accommodate the straw nozzle from an electronic cleaner spray can. I suggest positioning the holes in the corners of the TPS cover that hang lowest when the TB is installed to promote good drainage long-term. If you drill in the corners there is very little risk of coming into contact with any of the internal components.

Sorry that the large images still haven't been transferred to the new servers at Jag-Lovers, but you can see my thumbnails and descriptions here:

XJ40 TPS Drain Holes

Cheers,

Don

 

Perhaps the throttle plate was at 0.004" at the widest gap, perhaps not, it is even more closed now.


There was fresh oil in the throttle body when I took it off. and in the manifold. Car started fine first thing, into the garage and apart it came. Drilled 2.5 mm holes in the TPS, stopping when the resistance changed as it cleared the plastic. I note that the throttle body has a small (~2 mm) hole above the mounting for the TPS that would drain oil away as it approaches the TPS.

Remounted it to the throttle body, but did not mount the throttle body on the manifold. Made the electrical connections and back probed the voltages. 4.8V supply, and adjusted the TPS position such that it was around 0.5V with throttle closed and confirmed it got to around 4.7V on wide open throttle. Reassembled the air intake, after cleaning the heater plate on the unit between the oil filler tube and the throttle body.

Car started fine when cold, but during a run it started to act up again, idling too fast if put in neutral, huge delay/almost dying when pressing the gas pedal. Limped home. Would not restart after switching off- no throttle was just constant turning over, WOT gave the almost starting but not running- exhausts were puffing clouds of black gas laden smoke.

Removed one plug, it was black, sooty. No gas spraying out of plug hole when trying to start that I could see.

Cleaned all six plugs with wire brush, still would not start.

Disconnected plug to the water temp sensor and away she went, idle smoothly around 900 rpm. Fiddled with the IACV adjustment procedure (new IACV installed today) but cannot start car with water temp sensor connected and it runs far too fast when I do get it started and then reconnect the temp sensor.


Went for a drive with the water temp sensor disconnected, all was well, car performed as expected. Returned to garage, connected water temp sensor and idle rose to 1800 rpm and throttle response on blipping had a huge delay again. Disconnected water temp sensor and all was well again. Changed temp sensor for another new one (Uro brand this time, second new one I tried) and it seemed no different. Went for another drive with it disconnected and all was fine, returned home, used allen key to adjust idle air adjustment screw again to bring it up to around 900 again, then reconnected water temp sensor and all seemed well. Stopped and started engine a few times, all seems well.

Not claiming victory until it behaves tomorrow, and the rest of the week, but certainly seems to be something related to the water temp sensor.

Is there perhaps an air leak somewhere, and rich cold temp starting compensates for it, but it cannot handle it when warm?

 

John,

I can't recall whether we've talked about your Coolant Temperature Sensor before, but you can measure it's resistance and compare it to the chart on pdf page 51 in the Engine Management Manual, which you can download here:

AJ6 4.0L Engine Management System / Diagnostic Guide

If the CTS has failed open or at a high resistance, the ECM will apply cold-start fuel enrichment even after the engine is at full operating temperature, leading to rich running, hard starting, rough running, poor fuel economy and sooty spark plugs.

Cheers,

Don

 

Don, I recall CTS as one of the first things I changed, but I had checked the original and it gave appropriate resistances when tested, so was not surprised the replacement made no difference.

Not sure why, was it the new IACV or the removal off carbon deposits on the idle adjustment screw, or the further closing of the throttle plate or the TPS adjustment, but the car ran well again today. 15 miles/twenty minutes to the next town, parked for an hour drove home. No problems.

Well maybe a little less eager to start, but only just.

Next weekend I should be able to optimize the IACV now that she does start when hot. I do need to source a new hose from the oil filler tube to the heat plate thing, it is brittle and cracking.
And the gas tank is still leaking vapours.
Will try for daily use this coming week and see how she does.

 

I think your RPMs at idle in D should be around 650-700rpm. Bring it down a bit. I bet shes peppy! Mine was maladjusted also and had about 1000rpm at red lights.

 

from the oil filler tube to the heat plate thing, it
If that has any cracks in the pipe I think from memory that it feeds into the inlet elbow after the MAF. That could be a source of extra air in the engine.

 

A bit slow with updates, sorry.
Car ran well for a few weeks, then one time stalled while edging into the garage, so cold engine, but running. Reluctant to restart. Battery was low, so charged it, and of course car sat and got colder, then it restarted no problem.
Autozone said battery was good, but low in charge (2%!) so could not check alternator. Charged battery overnight, next test said voltage regulator was dead, so we have a new alternator and three drive belts (California car, so has air pump). That was a much harder job than expected, took much longer. Problems included the air pump restricting access, had to remove fan as air pump belt is very short and would not slip over the blades, and the alternator pivot bolt would not slide through the mounting bracket so that needed to come out too and have the bolt hammered out.
Fitted new fuel filter too. Gas in the inlet side of the old one was a little dirty, but not horrendous, no rust particles etc.

Hopefully all is well now, I guess only time will tell.
Need to sort out the clock that still loses time when parked overnight.
Idle is set to 700 rpm, seems quite smooth

 

As an update to this thread, the car got rear ended at a stoplight this morning.
Beyond my ability to repair, never was good at bodywork.
I will go to marketplace and list the car, might be of use to some people.

I will start looking for an X300 to replace it.

I am beyond gutted. Thirty one years old and barely a scratch and some young kid takes her away in an instant. All the work I did on it now wasted.

 

Oh John,

I am so sorry to hear this, but thankful that you are alright. What are we going to do about all the distracted, oblivious drivers on our roads?

Please keep us informed.

Cheers,

Don

 

Thank you Don, it is heartbreaking.

It was actually my daughter driving, on her way to watch the sunrise with friends on their first day of senior year. She did comment that the girl behind had her phone in her hand when she got out of the car, but who knows if she was texting or not. Another girl ran into the back of the other girl too. My daughter said it was not as if they had to stop quickly, it was just a normal light change, slowing down, cars in front of her, no panic, just two kids not paying attention.

No one was hurt and hopefully this will let an entire class of youngsters see what can happen if you do not pay attention.
Something needs to change though.

 

A few years ago texting while driving was expressly outlawed in Tennessee, and recently the laws were revised again to prohibit cell phone use by drivers except via wireless interface such as a bluetooth headset or connectivity with a vehicle's audio interface. Driver's are to never have a cell phone in their hands while driving. The law is new, but I'm still seeing drivers with phones pressed to their ears or held at steering wheel level while they read or type texts. I would assume California laws are even more stringent.

 

Im sorry for your loss and glad no one was injured. You obviously cared for this car. i hope you can find a serviceable replacement.