Hi All, Hi GTJoey,

You will be pleased to hear the first item to arrive was the bottle of techron! Saturday morning it is going straight in the tank!

Cheers,
Nick

 

Go get em tiger!
put it in sport mode , shift the lever to the left and hang each gear till it screams!
once at 90 mph shift to regular drive and drive for about 20 miles!
top off with more fresh gas and repeat without the chevron
it will clean out
im breaking in the new baby!

 

Yes!

 

Yeah baby!
Gtjoey1314

 

Hi All,

An update: I replaced the thermostat which was a little bit more tricky than on a N/A car as someone stuck a supercharger in the way! Whilst doing so I saw how horrid my coolant was, so I drained that, flushed it several times and replaced it with Jaguar D542 coolant. I also replaced the coolant temperature sensor.

I also put some technron fuel additive as per GTJoey's advice.

The thermostat change definitely helped; the dashboard needle got to just under N, it never did before, the ODBII scanner showed the coolant temperature got to 88-90 degrees C, before it would stay at 80C and that was it.

I took the car out on the motorway this afternoon and drove it hard as per GTJoey's instructions. Did not see any smoke, it was getting dark, but it seemed to drive well and be a bit more responsive and did not have oscillating revs at idle, so far... but it is too early to say.

One thing I have not done is change the ambient temperature sensor, apparently this goes on the air intake on an XJR near/on the MAF but on the underside of the car. I could not locate or get to it from the top/engine bay. As per my previous posts, with the ODBII scanner connected, the outside temperature oscillates from -40C to +120C and the fuel trim still looks bad around +10% or more so perhaps the temperature sensor and its wild readings are making the car fuel itself rich...

I will try and change the air temp sensor or get my mechanic to do so on his ramp.

Cheers to everyone for their advice, things are moving in the right direction...

PS - GTJoey are you now the proud owner of a McLaren, you posh pant you !!

Nick

 

Enjoy the good life my friend....make sure you use the whole bottle and run it up again, then fill up with premium grade petrol.
It will help...
Good luck, right before n is the sweet spot for the temp, make sure your throttle body is clean as well
Good luck!
GTJOEY1314

 

Hi Nick,

Congratulations on your progress! The thermostat was definitely a big part of your problem - your engine was running cold for sure.

Just for clarity, there are at least two air temperature sensors. The Intake Air Temperature Sensor (IATS) measures the temp of air as it enters the engine air intake plumbing. Its signal is a secondary input the Engine Control Module (ECM) uses for correcting ignition timing rather than fueling.

The Ambient Air Temperature Sensor (ATS or AATS), along with the Solar Sensor, provides an input to the Climate Control Module and has nothing to do with engine fueling.

Your scan tool may be able to show the temperature readings of both of these sensors, so confirm which sensor's signal is fluctuating so much. It is normal for the IATS reading to warm up significantly as the engine air intake plumbing heats up from a cold start - the temperature of the air in the intake will almost always be warmer than the ambient air temperature. You can test the IATS according to the table on this page from the Engine Management System manual:



Cheers,

Don

 

Hi Don,

Thanks for your reply and sorry for the confusion in my original text! The sensor I am referring to is indeed the air intake temperature sensor. I can see from under the hood, but need on the XJR6 it is accessible from underneath the car and I don't have the means to get it in the air to fit it myself. I have booked it in with my mechanic for him to replace it (he has the facilities!).

Do you think crazy reading from this sensor (-40C to +120C and nothing in the middle) will cause issues?

Cheers,
Nick

 

Hi all,

The intake air temp sensor on my car has now been replaced and according to the car mechanic its fuel trim readings are now as expected. It is much more responsive. The lambada / o2 sensors were also checked and working as expected.

However....

The dreaded osciliating revs has returned...

When the car is at a standstill in D every now and then the revs dip. If you put it in neutral the revs osciliate quite wildly and the car will eventually stall.

This car does not like me!

Any ideas what could be the problem?

Please note I have new coils (made in Japan ones) and Champion spark plugs.

Thanks,
Nick

 

The electrical connector on the transmission will get dirty and effect the idle , especially coming out of park . You have the GM transmission on the S/C engine different then the ZF transmission on the N/A engine so this may not have the same history of a causing this problem . An easy thing to square up without jacking up the car . Besides it's something to keep in good condition like a ground stud . I believe on the ZF transmission that it's a bayonet connector that releases like a air hose coupling and not by twisting .

The Idle Air Control valve may be acting up and might be able to see the command signal with a ELM327

The injectors are ground seeking provided by the ECU . Clean all the engine ground studs including the case ground of the ECU other then the 2 wire ECU grounds

The IAT sensor can be read from the ECU connector without removing it and if possible wiggle the connector to see if you have a problem there . Getting wire positions from wiring guide

Red 1 to Red 30 as 30 is the positive lead on your meter reading resistance comparing to the chart that was provided earlier

The IAT sensor is electronically tied into the camshaft position sensor and the EGR temp sensor so you can remove those sensors connectors and see if they are sucking the signal down . Easier said then done under the S/C intake but I believe the S/C EGR is off to the side .

You can verify the sensor by removing it ( the 2 wire one ) and dipping it in ice water 32 degree and boiling water 212 degree .

 

Will the idle oscillate when cold, or once the engine is warmed up?

 

Just a wild hair guess on the idle oscillations one of the sensors can be way off range and once the ECU sees that and rejects that signal and goes to a different set of maps in the ECU then after a certain period of time the ECU goes back to a full sensor operation then rejects the sensor again .

The ECT and IAT sensor has an effect in seeing cold if bad ( high resistance or open circuit ) of enriching the fuel mixture thereby increasing the RPM . The map shifting when the signal is rejected would bring the RPM down . in a cold engine scenario this my be enough to stall the engine .

Idle Air Control valve not hitting the ECU commanded target as it is a multi position stepper motor ?

Just a guess as I had a bowl of Fruit Loops this morning

 

I suspect that if the car was rejecting sensor data for being out of range, it would return to open loop operation. There would be no oscillating....unless it was constantly trying to switch from open to closed, back to open, then closed...

If the oscillations occur when cold, it can only be four components that the car reads before going closed:
1. TPS....easy to read through the OBD2 port. (Is the % open steady at idle?)
2. Coolant Temp Sensor....waits for a certain temp before going closed. If it was flip flopping data, It could cause trouble.
3. Idle Air Control Valve....this could be the culprit...
4. MAF...not sure how to diagnose this other than disconnecting it to see if anything changes.

If oscillations occur when engine is cold, it’s likely one these items. If happens when hot, it could also be these items as well as several other items. I’m just trying to help narrow down the options.

 

Hi Vee and Don,

The car is going to the mechanic on Monday and I will print off this forum post for some background reading.

Vee the osciliating starts to occur when the car is warmed up.

When I last connected my ODB2 reader the TPS is steady at idle.
The coolant temp sensor has been replaced (I replace it and the thermostat) and all reading are as expected, before my car would not heat up to operational temperature.
Idle air control valve - have not investigated that.
MAF, is new, but same occurs with the old one.

Thanks for everyone's comments.
Nick

 

 

Hi All,

An little update for you. Took my car to the mechanic. He services all makes of cars but claims Jags and other classic as a speciality. It was reassuring to see an X308 and two S Types either waiting for their appointments or being worked on.

On the way there I had to travel along a couple of brief bits of motorway and used the opportunity to give the car some stick. To my disappointment the car seemed to miss a beat / hesitate under hard acceleration. So my problems are not restricted to idle...

The car is still with the mechanic, but I called today for an update and here it is.

1. There are no leaks - as expected.
2. All coils and spark plugs tested - all good.
3. Crank Position Sensor replaced (probably not the issue), but has not been changed for 60K miles.

No change to the issue. However, the mechanic has seen it first hand and tested the coils whilst this is occurring and confirms no weakening of spark/coil performance when this happens.

Next he is going to diagnose the operation of the throttle position sensor.

However he is saying the wiring/signal to the injectors could be the source of the problem (he gave me a more technical description but I cannot relay it professionally!) and that will be the next thing to look at.

He also mentioned the possibility of it being a fouled valve due to the low mileage / low use history of the vehicle, but he doubts this is the problem is not overly apparent (apart from idle and under hard load).

I am a bit disheartened about this and if there is no progress this session I probably will take it to a 100% dedicated classic Jag specialist near London where I used to live.

Note (to save you reading this long thread):
1. Car has new MAF
2. O2 sensor are working fine and have been shown to correctly operate via garage's diagnostics (I have the print outs).
3. Intake Air Sensor replaced.
4. Thermostat replaced.
5. Coolant Temperature Sensor replaced.
5. Tehron (one bottle of) has been applied.

There is not much car left to replace...

Any thoughts would be much appreciated.

Cheers,
Nick

 

How bad are the oscillating revs and are they regular eg in 800-900 range? If its a lot and irregular then I'd look at TPS, if its little and regular I'd look first at o2 sensors orientation(plug them in differently), or even unplug them completely and see whether it happens too(it should run in open loop all the time).
Then a standard coil test, swap one at a time or when it starts to oscillate unplug one at a time see whether oscillating disappears.
You can test the injector signal quite easily with a set on noid lights(they are cheap and widely available). It will be trickier to test the injectors.
Also does the problem appears quickly as soon as it gets to temp and switches to closed mode or later on?
You could also ask the garage to test fuel pressure and have a look at the ECU plug, in case some water got in there(this happens quite often).

 

Hi Katar83 and company,

To answer your questions

The osciliating revs visibly occur when the car has warmed up and is in idle. It will idle fine for a few seconds, miss a beat fall from 800-900 to 600 and then be fine and then the same thing. If you are reversing and this happens (with foot not overly on acelerator/gas) the car may stall. When driving it is not apparent unless you boot it and then it feels like a flat spot.

The problem appears as soon as car gets to temp and goes into closed loop.

I have asked the garage to look at the ECU plug (this is something I can also check myself)

Thanks for your feedback, any additional thoughts are always appreciated.

Cheers,
Nick

 

Something you can try as you drive round at it's point of failure you can record live data as a film and review later and see what goes out of parameter . Keyword being PID's and get the one an Ebay that includes free software . 12 bucks free shipping .

Watch the whole video

 

When TPS fails the oscillating revs would be in a wider range and random so I guess this is ok although worth checking, since its fairly easy using OBD.

I'm guessing the flat spot is around 3k RPM when you boot it? It does sound like the engine stops firing on one cylinder when this happens, so I'd concentrate on figuring out which one.

Since this happens when it warms up and get into close loop, unplug the o2 sensors, see whether it starts misfiring in a permanent open loop.

I assume you do have a spare coil that you could test, I know you have new ones but I'd still check it just to be sure and yes, have a look at the ECU plug. Its not uncommon for this to fail.

Its probably not that but mine would do this when one of the vacuum ports in the intake was open, without a blanking cap(disappeared one evening). There are two ports, behind the fuel pressure regulator, one should be capped. Might be tricky to see, being the R, but have a look just to be sure.

Checking injectors - I'd leave this to your garage, unless you can get the noid lights.