Right, voltage across the TPS wires is 0.69v. Idle remains at around 900 immediately after startup, then rises up to 1050 ish a few moments later. Engine is warm. I’ll try moving the TPS a bit.

 

Excellent. If you take it below 0.60v, you've gone too far. No need to rotate it past that point. Perhaps you want to be at 12.5%?

0.69v is probably too high. 0.63v-0.65v should hopefully bring that idle down to an acceptable level. I think you're close.

 

Dropped down to 12.5%, no change in idle even after restart. Voltage is 0.68v
Dropped down to 11%, voltage is 0.62v. No change in idle after restart.
12.2%, 0.66v, 0.65v
11.8%, 0.62v

Idle remaining at 990-1050 whatever the TPS is set to.

 

OK. No more than that. We're done fooling around with the TPS. Lock it in where it is, and move on. We might not get around a TPS reset after all.

Now I need a reading on the Coolant Temp Sensor please.

 

Maybee still high has to do with the fuel mixture rich ( ECT sensor ) , the next step after sensors check would be vacuum leak and cracked exhaust manifold that will bias the o2 sensors to regulate a rich mixture , common

Maybee check the IAC valve 4 wires voltage after IAC test command puts it in apparent correct position

Compare the with the readings after you cycle through and idle goes bad

 

No, the exhaust manifolds, unless cracked enough to shove your fingers inside will not create this problem.

I am assuming you have checked for a vacuum leak. Basically pull out the MAF, wrap one end in plastic and put it back in. Then pull off the hose connecting to the intake elbow and blow into the elbow. You should not be able to blow continuously. It should block up quickly.

Forget the IACV. It's fine, we have checked it out. It can normalize your idle, but it isn't because there are other issues at hand. (I still think the TPS reset is inevitable)

Coolant Temp Sensor is still plausible, as are the oxygen sensors.

 

Not sure if this is fixed, but I did the ISCV test again just out of curiosity and it wouldn’t run without a little bit of throttle. Oh well, I thought, and plugged the ISCV back in.

Well, I was very surprised when I started the car up and it idled at 750! Seems all the adjustments finally got through to the car. I’m scared to touch anything now, so I’m leaving it for the night. Hopefully in the morning it can start from cold and maintain a low idle. TPS is currently set at 12.2%, which will do for me.

I took it out for a test drive, and immediately noticed how nice it was not to have to ride the brakes at slow speeds, how quiet it was when in neutral, and there was an increase in fuel economy too. I really hope this lasts until the morning, but I suspect it won’t.

If we’re back to square one tomorrow, I’ll try the next steps that you suggested. For now, goodnight. And thank you all again!

 

That is promising. There's nothing wrong with 12.2%! The key is the voltage, and at 0.65-0.66v, that might be good!

One day...someday, find a forum member in the UK with a PDU/WDS/VCM, etc. to do a TPS reset for you!

 

when i was having the issues with mine i though i had it sorted at one point or another but then after turning off and on a few times i was back where i started.

hopefully you have it sorted but likely it won't be until a tps reset.
its such an easy job, 5 minutes and done.
the only thing can be finding someone to do it.
bt really it should be the first port of call before doing anything else.
its just easy and maybe eliminates the need to do anything else.
i just speak from my own experience with high idle and trying this and that cus i couldn't find someone to do the reset.
as soon as i found someone, i went, spent $50 and 10mins mostly chatting with the bloke and then felt like i had just wasted all my time and energy trying anything else.

my tacho is for some reason out of sync and it appears to idle at 800 in park now so i thought it was a little high, but it is infact idling at 700.
in gear my tacho reads about 675 but the car actualy idles pretty much at 600

 

I’ve asked my two local Jag specialists if they can do one, haven’t heard back yet. I have a hunch they don’t have the right gear still.

My tach reads 900 ish, which would fail the MoT again as it needs to be 600-800. But according to the scanner it’s at 768 rpm at idle. So hopefully they’ll let that pass.

 

You can remove the CKPS and clean the face of the sensor of grime ( can contain metalic debris in the case of like wheel sensors ) and then the connector

Grounds cleaning including the large engine ground strap from the starter mount bolt

Large external ground strap on ECU case ?

 

Started this morning to drive to work, was idling at around 1400. 30 mins later, it’s settled at 1150 when fully warmed up. So the car has reset whatever setting I managed to change yesterday, as expected.

What do you suggest I try next?

Parker, these ground straps, do they affect idle? I’ll check the ECU strap and engine bay ground anyway.

 

See what that " setting " is before ( good ) and after ( bad ) as the voltages on the 4 wires to the IAC valve

8 volts dc ?

Confirmed valid ECT sensor readings of 88 C for closed loop engine regulation ?

 

There is a specific ground stud to protect the Tach signal ( wire shield ( a mini coax cable ) ground )

PIG153L which is located ....................

Rear engine firewall cars left as the small studs different than the larger stud ( positive battery cable )

 

would want to hope they accept what the scanner shows as that is accurate and the cluster is not.
unless there is a requirement that the tacho is correct nd accurate. in which case regardless of what idle is you would still have a fail.

wonder if there is a way to reset/align the tacho reading.

hope you find a specialist with a scanner soon.

 

We have never confirmed that the Coolant Temp Sensor is working accurately.

Ultimately Spud Maat is correct, the reset is the best way.

Coolant Temp Sensor needs to be verified.
Vacuum Leaks need to be confirmed.

After that, the reset is the best medicine. I was sure a manual adjustment would work....I can't figure out how the car knows to override that, unless there is a bad CTS or vacuum leak.

 

Def check for vacuum leaks.
disconnect either the hose off the inlet manifold or from the brake booster.
the idle should change.
place your finger over to block the leak,
unblock the leak repeat a few times.
if the idle is staying the same regardless you likely have a vacuum leal somewhere.

a way to check for a leak around the inlet manifold is to spray some carb cleaner around where the inlet manifold attaches to the block.
if that causes a change in idle then you have a leak.

just two quick backyard ways that can help diagnose a leak.

 

I’ll have a go at checking for vacuum leaks later today, and another try at reading coolant voltage. I did spray carb cleaner around hoses and the intake manifold with no result, but I’ll try pulling the hose off and doing the MAF plastic wrap up thing.

Good news, I’ve heard back from a very well regarded semi-local Jag specialist who say they’ve done throttle pot resets before many times. They reckon they can squeeze me in sometime next week, which should give me time to check whatever’s left for me to check before falling back on the TPS reset as the last option.

 

Removing vacuum line from brake booster caused the idle to jump up about 200 RPM, so I assume that rules out vacuum leaks.

Getting 0.55v across the coolant temp sensor wires, so I think that means the car is reaching >88 degrees and the coolant sensor is reporting accurately.

Besides O2 sensors, which are unknown, and the TPS, everything seems to be functioning properly then.

 

ECT , 0.55 volts dc on one single wire to a good ground ?

The other wire should read 5 volts to ground

Your mechanical coolant thermostat should keep the coolant slightly above 88 C or about 90 C ( 0.65 volts )

So 0.55 volt is a Temp above that , but not a worry ( if correct reading method )

On the brake booster hose , it does not prove a whole engine or brake booster absolute no leak at that brake booter test point , only that the leak ( ? ) increases with an open hose

You could be going from no leak to a leak

Or a small leak to a leak

There is a hose on the top of the fuel pressure regulator and a hose attached to the underside of the 2nd or 3rd from the front intake manifold , look for hoses on the EVAP components under the engine air filter box

Which brings us to FPR leaking extra fuel into the intake by way of the vacuum hose , hose dry ?

There are some cruise control hoses / components in the area that are independent / isolated from the engine vacuum lines