When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
" I did the ISCV test Vee described. This did get the engine to run at 600RPM with the ISCV sensor unplugged, which was nice because it’s never been that low and smooth since I’ve had the car. But as soon as the sensor was plugged in again the revs were back up to 1600. Perhaps you could explain what that means Vee? I’m confused whether that means the ISCV is working, or if it’s broken. "
With the IAC unplugged it probably goes to a rest position which is a correct position for proper idle
once connector put back on it is receiving a command from the ECU for a bad position
Difference between a test command and a functioning command from the ECU
ECU connector check , look for missing 1 of 2 tang sockets , need 2 to grip the ECU side pins / blades
A slight twist of the pins / blades on the IAC connector , Hi Vee
But you probably don't need to do this since it behaved with a test
Don't buy a ECU yet , further research here at the Center for the Institute of Ponderance
But you have your spot / target
If you drive the car with the IAC disconnected , it will probably stall in traffic as you pop off the gas pedal
Your " not moving " ( idle position TPS signal ? ) functional output signal from the ECU to the IAC connector is 8 volts DC to the IAC valve on all 4 wires reference page 59
So, check all the relevant pins on the ECU and the sockets on the ECU plug? Is that just a visual inspection of them or can you use a multimeter to test their function, continuity for example?
TPS should be around 12.9% at idle. I think that's your problem.
The IACV test confirmed that it is ok.
The fact that you don't see the coolant temp is concerning. That coolant temp sensor plays a large role in how your car us fueled. It's a $20 part and 5 minutes of your time.
I think it's the TPS. Replacing it won't make a difference. It needs to be reset.
Only other Hail Mary I have for you is run the car til hot, while running, unplug the TPS....count to 10, slowly, and plug it back in. Tell me what happens.
Oil SENDER is different than than oil SENSOR. The sender just tells the dash needle what to do, the sensor is read by the ECU. You can unplug the sender and it won't affect anything...except that needle.
TPS should be around 12.9% at idle. I think that's your problem.
The IACV test confirmed that it is ok.
The fact that you don't see the coolant temp is concerning. That coolant temp sensor plays a large role in how your car us fueled. It's a $20 part and 5 minutes of your time.
I think it's the TPS. Replacing it won't make a difference. It needs to be reset.
Only other Hail Mary I have for you is run the car til hot, while running, unplug the TPS....count to 10, slowly, and plug it back in. Tell me what happens.
Thanks Vee, I'll try the TPS unplug later today. Hopefully a garage is able to help out with a TPS reset if that doesn't change anything.
Can the Coolant Temp Sensor be faulty even if the resistance reads ok both when hot and cold? Wouldn't that indicate a problem with the wiring rather than the sensor itself? I feel it's more likely that my OBD scanner is just incompetent, it's only a cheap one. It doesn't show me fuel trims or any other of the things that people on here mention are useful stats to read.
The temp gauge on the dash works, but I assume that is controlled by the little single wire sensor next to the CTS.
Last edited by tobywood13; Jun 5, 2024 at 05:27 AM.
Your assumption about the CTS is probably correct. There should be a chart regarding the expected readings from that coolant sensor somewhere around here.
Your assumption about the CTS is probably correct. There should be a chart regarding the expected readings from that coolant sensor somewhere around here.
I really think it's the TPS reset.
As my throttle position reading is low, at 9%, wouldn't increasing it to the ideal 12.9% cause the idle to increase even further? Or can a high idle be caused by TPS voltage being too high or too low?
see the point is the car might think it is at 9% when in actual fact it is much higher.....
my car was idling at about 1200rpm (in park) 800 (in gear) a while back.
now she is smooth as butter at about 620rpm (in gear)
i have been through this beating my head against a wall trying this and trying that and from my experience nothing you can do will fix it, and trying anything you are just wasting your time.
get the TPS reset and save yourself the headache (hopefully)
if it needs the reset (which it likely does) nothing else will fix it.
and if the reset doesn't fix it atleast you will know that you TPS is fine.
Parker, I haven't adjusted the TPS, not sure about the previous owners. I was thinking about trying the manual TPS adjustment before going to a Jag specialist. If I did this and moved the TPS a bit, would that mess up any future TPS reset? I don't want to get the adjustment wrong, give up, then find I have prevented a proper TPS reset from being done on a computer.
I never had an ECU / TPS orientation but what it entails is telling the ECU what is the idle mechanical stop TPS sensor reading ( a sort of null ) and adjust the ECU ( electronically ) only
There should be a within range the orientation can work with as not too far out
On paper the return wire ( Green / Yellow color ) to the ECU should read 0.60 + or - 0.02 volts dc with the connector installed , key on , engine not running
The valve cover is not a good ground , electrically isolated by plastic shouldered bushings on the 13 mount screws
Vee was working with someone that this 0.60 was way out ( 0.67 ) but somehow brought the idle back correctly
In reality the TPS mounting bolt holes ( tight dimension ) allow very little adjustment
You can nick the insulation as that wire goes over the fuel rail so not to have to remove anything , this will only give you information but as you can expect it should be out
So we come back to the idle screw going back to the original position
Or you can adjust the idle screw to get your TPS to read in the expected range of 0.60 + or _ 0.02
With the engine fully warmed up the 2 wire engine coolant thermistor sensor should read about 270 ohms to prove that the engine is going into closed loop ( efficient and emmisions ) , this will also prove your coolant thermostat / valve is not wide open
It's an easy cheap part and they do fail , by removing the connector it sees a cold coolant so back to open loop ( and there is some time delay )
"So we come back to the idle screw going back to the original position" - idle screw returned to original position as best as I could, although idle is now up to 1600 form 1400 before all this testing.
"With the engine fully warmed up the 2 wire engine coolant thermistor sensor should read about 270 ohms to prove that the engine is going into closed loop ( efficient and emmisions ) , this will also prove your coolant thermostat / valve is not wide open" - fully warmed up I got these readings from my ohmeter: .229 on the 2k setting, .21 on the 20k setting, 0 on the 200k setting. Does that prove everything is working as it should be?
I'll have a go at measuring the output of the return wire on the TPS now.
1. Nevermind the voltage reading from the TPS. It was 0.60v when new, and from the factory. Over time, as things get gunked up, the ECU learns and adjusts the expected base idle voltage, so blindly setting the TPS to a specified voltage is fruitless. The TPS reset is simply a program that tells the ECU to accept whatever voltage is currently being seen at idle. If it was ever done before, that 0.60v would be null. Everytime the TPS is reset, it basically creates a brand new baseline for the ECU, so you don't have to ever worry about messing up future TPS resets, whether you do it by a device, or manually.
You can either have the TPS reset by a device, or attempt to manually reposition the TPS so that the idle is somewhere between 700-720rpm, warm.
2. My throttle body does not have a idle screw, so I can't speak to this.
3. According to my XJS wiring diagrams, it shows Coolant Temp should read 1.2V at 72F, 0.65V at 190F
Hi Vee, I’ve just done the ISCV test out of curiosity, but now my car isn’t starting. Any ideas? Foolishly drove it a couple of miles to warm it up then did the test at the side of the road.
On that drive the idle was surging up to 2500 when coasting in neutral, and it felt sluggish accelerating.
Last edited by tobywood13; Jun 5, 2024 at 12:58 PM.
Your CTS values are mixed on the last meter setting , but working somewhat
Try to nick the return to the ECU wire ( color X ) and see a DC voltage reading and compare to the right side graph on page Y of the Jaguar 801s doc
The other sensor feed wire should be exactly 5.0 reference volts dc
Vee , by your's not having the idle screw explains why I never saw mine or I just missed it
To manually adjust the TPS to achieve the correct RPM takes some tool / patients dexterity upside down under there from the top , but can be done with TB remaining installed
If you suspect flooding in the future push the gas pedal all the way to the floor and the ECU knows not to give any fuel while you crank / clear it through
Coolant Temp Sensor Chart from a different source; basically, as long as your voltage reading is low when hot, and high when cool....I think it's ok. But if it's original, you may as well spend the $20...
