Hello all, my Jag has just failed its MoT on a variety of things, most concerning of all the emissions. It was nearly 20 times over the allowed limit for CO, and the Lambda reading was also coming up below the allowed margin. Additionally, I need to get the RPMS down from 1400 to 800 to pass.



As you can see, these results are absolutely abysmal. The Jag’s exhaust is practically a petrol hose at this point!


So, how do I resolve the idle issue? In drive, it is comfortable at around 800RPM. In neutral or park however it's around 1400 when warm, and closer to 2000 when cold. I've cleaned the throttle body and butterfly with a petrol-soaked rag, I've wound the adjuster screw on the throttle body all the way out so it no longer touches the rotating part of the throttle, I've adjusted the throttle cable to take off any pressure on the cable. I have checked that all 6 cylinders are firing. I've jumped the coolant temp sensor with wire when the engine was warm, it struggled to start but eventually sputtered in to life, no idea if it was running better with the sensor or the jumper wire. Interestingly, no temperature reading was displaying on my OBD scanner. The temp needle is just to the left of centre, and the fans rarely come on. Could it be a dodgy coolant temp sensor causing the rich running? Or a thermostat? Those are simple to replace so they're high on the list, but I don't want to start throwing new parts at the problem.

My manifolds are slightly cracked, and there's a bit of blow coming from the join between the downpipe and cat section. Would this cause the richness to go absolutely through the roof?

Any suggestions on what to do to test or resolve these issues will be greatly appreciated, I have two weeks to try and get the emissions and idle back where they should be. I didn't think I'd ever say this, but I miss carburetors for their two-screw tuning.

 

There is 1 fuse for all 4 / O2 sensors to read properly , must be heated

Fuse # 14 , right engine bay fuse box

The 2 wire engine coolant temp sensor must reach 88 C to be in closed loop engine regulation

The temp gauge is the 1 wire temp " bulb " thermocouple next to the 2 wire on top of the water pump housing

it can be checked against the chart on page 78 at engine cool / off temp

So at cool temp of 20 C you should see 2500 ohms resistance , 30 C / 1800 ohms

801S TITLE (jagrepair.com)

Surprisingly a crack in the exhaust manifold will make the engine run off scale rich , but you can check the cheap things first

Engine codes ?

In hindsight the adjustment screw on the TB butterfly stop should not have been adjusted , do you visually remember the original screw position ?

 

Excellent tips for me to be getting on with before I start buying things! Thanks Parker. No codes, my Jag has been very reluctant to show me anything unfortunately.

I’ll check the fuse tomorrow, and try and put the screw back where it was. I’ll also check the resistance of the coolant temp sensor. I did try it today when the engine was hot, it came back with a very low resistance so should be ok? As my OBD sensor wasn’t showing me coolant temp I couldn’t confirm if the thermostat was stuck or not, although I do have a hunch that the engine is running cool as the fans never kick in.

Unplugging the coolant temp sensor caused the engine to rev up steadily to 2000, is that a sign of anything? Jumping the connector with a wire caused a very hard and long start, seemed to be extremely rich from the smell and the smoke.

My last exploratory move will be to slather the manifolds in exhaust putty to see if that makes any difference.

I think I’ll also pick up some MAF cleaner and carb cleaner and have a go at taking the throttle body off for a more thorough clean. Perhaps the ISCV needs replacing also?

 

" I did try it today when the engine was hot, it came back with a very low resistance so should be ok?

The 2 wire thermistor ( feeds the ECU ) will read lower resistance the hotter it reads

88 C will read about 270 ohms

There is a simple test for the fans which run off a temp switch attached to the radiator

This independent system will have an effect on the 2 wire coolant temp thermistor reaching 88 C as well as the thermostat / valve in the coolant line

Do not remove the IAC valve yet as the 5.5 mm bolts break off , there is a test / reset

Keep asking questions

 

With the TSB done you will have a jumper plug on the 2 wire freon pressure switch on the top side of the engine

With this TSB done the fans will be on low mode with key in run position ( no engine running ) regardless of the engine coolant temp or freon pressure / A / C load

The fans test is........................

Remove the jumper plug on the freon pressure switch

Look on the radiator lower aft cars left for a 3 wire connector

with a paper clip jumper one white wire to the black wire , low mode

Remove paper clip and install to the other white wire to the same black wire , high mode

This is a low current section of the circuit so no arching hazard , just stay away from the running fans

 

i had previous high idle just like you.
went through everything,

turns out what it needed is a TPS reset.
find a mechanic with a ids system for the car and get a reset done.
5 min job.
worked for me.

 

Looks like the 2 charts on page 78 are backward from another

The sensor is fed a very precise reference 5.0 volt DC and the ECU return is a resisted / reduced voltage value

When I checked mine some time back as a cold engine I read about 1800 ohms resistance so good for me

 

Thanks Parker. Questions:

How do you test/reset the ISCV?

What is the TSB you mention?

Jumping the fans, what’s the aim of this? To see if they work which would suggest the engine is running cool?

 

" How do you test/reset the ISCV? "

Someone else had the test , will seek it out

" What is the TSB you mention? "

14-04 (jagrepair.com)

" Jumping the fans, what’s the aim of this? To see if they work which would suggest the engine is running cool? "

Just squares up yourself / engine confidence since you were unsure of the fans

 

Vee ,

Do you have the IAC valve reset / test ?

 

Thanks. So in normal healthy operation, are the fans supposed to run constantly once the car is warmed up, or is it just when you're sitting in traffic for example? If that's the case, the fact that they aren't coming on very often suggests that either my car is running cold (stuck thermostat), or thinks it is cold (dodgy coolant temp sensor), right?

I'll test the fans later, I know they do come on every now and then but it's not too frequent.

 

A manual reset of the IACV seems to just be a test to see if it is working, but give it a go:

1. Run engine until you reach normal operating temperature.
   
2. Switch ignition off.
   
3. Switch ignition on, wait 5 seconds, then disconnect the connector to the idle speed controller.
   
4. Switch ignition off.
   
5. Wait 15 seconds then reconnect idle speed controller.
   
6. Repeat two more times.
   
7. On last repeat, do not reconnect idle speed controller.
   
8. Start the engine.
   

Check the base idle speed, which should be at 550-600rpm. If it is, I believe the IACV is working. Once you shut the car off, it will resume behaving badly because something else is at fault.

Switch ignition off and reconnect the idle speed controller. Start engine and check to ensure that the idle speed is between 650-800rpm…it probably won’t be, but if you were successful in the previous step, I think the IACV can be ruled out.

 

It is 90% likely that a TPS reset will cure your issues. You can either head over to a Jag Dealer, or a shop that has a computer with Jag software to perform this reset.

Other suspects:
1. IACV
2. Coolant Temp Sensor
3. MAF Sensor
4. Oxygen sensors
5. TPS
6. Vacuum leak between intake manifold and MAF.

Do not worry about the exhaust manifold. It won't do this.

There is a way to adjust the TPS manually. You'll have to take the throttle body back off, loosen the screws holding the TPS in place to just the point where you can adjust it with some force. Then put the throttle body back on, turn the car on, adjust the TPS with the engine running until you get to 700-720rpm idle, warm. Then tighten the screws...all with the throttle body in place.

 

if you want to be 100% sure your idle is correct read your idle via a scanner connected to the OBD.
i know that my rpm gauge on the dash is off by about 50 or so rpm.
confirmed today when i had my mechanic connect it up and do a tps reset.
scanner showing it idle at just over 600 (probs 620) instrument cluster reading just under 700 (probs 680)

thought this was the case as my torque app (connected by bluetooth elm) was always showing slightly different to my cluster, thought it may have been the elm and app being wrong.
now i know it is a slight inaccuracy of the cluster.

 

Thanks Vee, I’ll have a go at that ISCV test. If I can get the idle down, then I can move on to tackling the insane richness level. I’ll give my local Jag specialist a ring as well to see if they have the right computer to reset TPS. Forcing it by hand seems a bit scary!

Spud, a good idea. Unfortunately I was getting 1400 on the tach and on my OBD scanner, so it is accurate and is very high.

 

Richness and high RPM are related , if you subtract the fuel RPMs should come down to an extent , less bang to move the crankshaft through

 

If you use a scanner tool / live data reader / ELM - 327 you should see 0.0 % short term fuel trim as it is related to combustion gasses / smog test

0 if perfect but the best expected is + or - 3 %

25.4 % is the limit of off scale

 

My short term numbers are constantly fluctuating.

 

This is the way. Shouldn't cost you more than 1 hour labor, even if you take it to a dealer.

 

Here are some updates from today’s testing.

OBD scanner is not displaying anything for engine temperature, or anything to do with fuel trims. Only reading that seems to change is throttle position, idle speed, and LOAD_PCT%, which is at 3.9 1400RPM, and 2.0 at 1600. I don’t know what this means. Throttle position is apparently at 9% when closed.

Cold coolant temp sensor resistance seemed ok, but I will have to confirm figure tomorrow as I forgot to write it down! Oops. When hot, coolant temp resistance showed the following: 0.229 on 2k, 0.21 on 20k, 0 on 200k.

Fuse for O2 sensor heaters was intact.

Fuses for both fan speeds ok. Fans failed the TBS checklist, although I assume that’s an AC issue so not important right now.

Fans come on at both slow and fast speeds when jumped at the lower left radiator connector.

I disassembled and cleaned the MAF, and the throttle body as best I could with contact cleaner and carburettor cleaner respectively. On the next drive, idle was down to 900RPM in park and neutral, but I had also accidentally unplugged the oil pressure sender so I quickly shut the car off when I noticed the oil gauge at zero. Does this affect anything to do with idle speed? My car does ALWAYS display maximum oil pressure on the dash, no matter the conditions, so perhaps that bumps up the idle a bit?

After reconnecting the oil pressure sender and turning car on again, the idle increased up to 1600, so it’s got worse from yesterday. Also, when revving in neutral (or in gear scarily) the revs will get stuck at 3500 and won’t come any lower without putting the car in a higher gear. I assume I reassembled throttle body incorrectly and something is physically getting stuck? All seems to be free on the top end.

On returning from my test drive the fans did come on as I came to a stop and ran for a few seconds with the engine off. I still suspect the temperature is a little bit too low. Either that or the car thinks it is a bit cooler than it really is? Will have to confirm the resistance of the coolant temp sensor when cold later today when the car has sat for a few hours.

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.

If anyone has anymore suggestions on how to diagnose this further, please fire away. Thank you all for your help so far!