katar83,

I think you're right - I grabbed the wrong part number. I have edited my previous post accordingly. Thanks!

Don

 

I believe the answer is yes. If the IACV is not closing properly because it is stuck or carboned up, excessive air can enter the intake, causing lean burning. This air was metered by the MAFS, but exceeds the amount the ECM expects in the combustion chambers based on throttle position. The O2 sensors report the lean condition to the ECM, which corrects by enriching the fuel mixture, potentially leading to rich running and reduced fuel economy. This would be consistent with your high positive STFT....

Cheers,

Don

 

Have you checked the exhaust manifold casting upstream of the O2 sensors for cracks or loose donut gaskets as fresh O2 is sucked in between exhaust pulses biasing the O2 sensor to command the ECU to go rich . It may be hunting once the RPM goes high above the target idle RPM . Just thinking out loud

 

I had cracked exhaust manifolds. I replaced them, (found new ones), but it didn't solve any problems.


I don't believe that cracked manifolds could cause such drastic idle issues...unless it was drastically cracked.

 

Hi All,

I've been giving this some thought this evening. This weekend I will, weather permitting:

1. Re-test car with original coils. 2 were faulty but I have sourced some 2 "working replacements".
2. Test IACV as described in this forum thread.
3. Disconnect MAF whilst car running to see what happens.

If that all delivers nothing new (which I suspect will be the case) I will take the car to a Jaguar specialist in West London that serviced previous cars of mine (Swallows in West Ruislip). I will let them do their own investigations - they are the specialists, but will suggest:

1. Check/recalibrate/change O2 sensors.
2. Re-clean throttle body and IACV and possibly replace IACV if needed.
3. Get them to check all engine earths
4. Check ECU connector (I probably will do this at the weekend too).

Cheers,
Nick

 

katar83,

I had missed your thread on the throttle body, but you included a photo that clearly shows the IACV stepper motor, and I am taking the liberty of copying it into this thread:


It does appear to be identical to the Renault Megane stepper motor that also works on the later XJ40s.

Cheers,

Don

 

I'd test the Rochester Valve. I never heard of it until few days back and someone mentioned it here that it can cause an air leak when faulty. I always had this random misfire when running on petrol in both current and previous x300 where every now and then rpm would drop by ~100 when idling, completely out of the blue and also cause a flat spot around 3k RPM when floored. It would then fix itself after steady driving for a couple of minutes. It always felt like an air leak and I chased it for a very long time, even changed the intake gasket but could never fully sort it out. Luckily my car runs on LPG and this was very rarely an issue while running on LPG hence it didn't bother me that much.

So I decided to test this 'air leak from faulty rochester valve theory' and pulled the vacuum line between first and second cylinder(counting from the windscreen), I assume this goes to the rochester valve as I cant see any other vacuum port in the intake that could be used for that purpose? I've taped the vacuum port and left the vacuum line venting under intake and misfire on petrol is gone and the petrol idle is perfect!

Also when switching between LPG and petrol when idling, RPMs would always drop for a second by 250RPM, I could never figure out why and this is also gone now! RPMs stay exactly the same when switching between LPG and petrol so clearly I had an air leak through that vacuum line and if it goes all the way to rochester valve this must be faulty too.

Try the same thing on your car, you might be surprised like me!

Since your problem, shows, when the car gets to temp and engine doesn't need that much air I suppose this could be it?
Now, where do I find a replacement in UK...

 

Installation note :

https://forums.jag-lovers.com/t/xj-s...cbc7714/348511

 

Also used on lots of General Motors cars. There is a Chrysler version too, that is mechanically identical, but the wiring is different for which pins make the pintle advance and retract.

GM part: http://www.rockauto.com/en/moreinfo....028854&jsn=465

Also be aware that the small bolts that hold it in place to the throttle body have loctite on them. Try and undo them and you'll shear off the bolts. Heat the bolts with a small torch and that will break the loctite bond, you only have to heat it to about 120C to break the bond.

 

Dear Mr. Katar83,

Very interesting... your description "every now and then rpm would drop by ~100 when idling, completely out of the blue and also cause a flat spot around 3k RPM when floored"

This is the perfect description of my car's issue!

I think you are on to something, I will test this over the weekend - first test i'll do. But you do pose a good question where do we source new ones from in the UK... Will do some internet research.

Cheers,
nickdabs

 

Quick internet research - couldn't find a new one, but secondhand from autoreserveparts on eBay

 

I've asked on xj40 forum, I'm sure there are used valves out there but I'm not sure I'd trust a used one. I somehow doubt that people that sell them can test them reliably and I suspect that if both my x300 had these faulty, then a replacement used valve wont last long. The ACDelco one from US eBay, cheapest one I found is £35 inc p&p and import tax so not the end of the world. Just need to remember that the ports are upside down on the Delco valve compared to Jag one so it needs to be fitted slightly differently or the tank might implode
If I find a spare 15 minutes over the weekend I might get under the car and follow the vacuum line, maybe its damaged somewhere, I don't even think I ever saw that valve although I have a rough idea where it is.

 

Hi,

On my car the supercharger gets in the way of the pipe from Katar83's photo so it is not easy for me to do the test without having the car in air, and unfortunately I don't have the adequate facilities for that.

I am going to take the car to the specialist on Monday and I have spoken to him and mentioned it as something to investigate. He was aware of the possibility of a failed Rochester valve and told me it does happen but is pretty rate.

Let's see what happens...

If I do need a new valve then a new one from America is definitely the wise option.

Cheers,
Nick

 

Not all X300's have the muscle air driven Rochester valve , mine does and is VIN # 755xxx . Easy way to tell is if you have 2 evaporative canisters one on the left side of the car in front of the rear wheel and a second on the right side as you look under the car without jacking . I only have one on the left . The canister close valve would be easier to go ahead and change then test as the wiring from the ECU gets taped off by other things and would only prove that it is electrically intact but still may be mechanically jammed . Same with the Rochester valve as they don't last forever and the test setup effort even though it has no wiring .

But the pic shows that if the EVAP valve ( located below the engine air filter ) is stuck open the you would have in effect a leak in the manifold vacuum . My vent line was off on this EVAP valve . Note that this is applicable to the 1 canister system and you would have to have the canister close valve open in the 2 canister system ( ? ) or canister rusted through and leaking . Epoxy patch might fix that . Canister close valve location pic in post # 13

https://www.jaguarclassicparts.com/u...-4-0-litre-usa Item # 15

Note on the previus post # 108 where someone found the pluming backwards on the Delco brand unit .

Simple test would be to remove the hose on the EVAP valve that goes to the intake manifold at the EVAP end and plug it with a bolt or tape and drive the car to see if she still has a spot of stumble under her bonnet .

Editing

 

As far as I know, the dual cannisters was USA market only. My Canadian market car only had a single cannister.

 

Makes sense as the 2 canister system is probably more effective , but I just double checked and mine only has 1 . Might have been a regulatory mandate or maybe a recall to address the early production models . I never could find a canister close valve on a 2 canister system as I electrically rang out the whole ECU connectors .

 

Thought this might be the case. If you'd like to do a quick test, pull it off the other end, that vacuum line attaches to a metal pipe near the oil and gearbox dipstick bottom. If you pull it off it will come off very easy and you can tape it off that end and therefore plug the possible air leak. Car will run fine with the rochester valve end venting there and you can either put it back after test(just enough space to put your hand in there) or ask your garage to do it for you when they will have it on the ramp.
That vacuum line is ~80cm so quite short.

The US eBay rochester valve is eBay item number:332552809025

 

Hi Katar83,

Thanks for this. I went and had a look but can't see the short pipe you describe. I have attached photo below. All I can see is a very long pipe that come from near the bulkhead and then goes past the bottom of the dipstick pipe.

 

My 1997 only has the single canister. I have the occasional stumble/miss at speed every once in a while too. I can't reproduce it on command, so maybe replacing these evaporative system valves are just a maintenance item after 20+ years and 100k miles.

However, ignoring for a moment the environmental/emissions requirements, if one just plugged the vacuum source on the intake, and left the evap line that runs back to the canister permanently disconnected, would that cause any running issues?

Also, there is lots of talk above about the rochester (pressure control valve) failing and being a low cost maintenance item. But what about the electrically operated purge evaporative valve? Would its malfunction cause any running issues, and thus too should be considered to be replaced if suspect?


BTW, regarding the IACV, there are units under $20 available. Not sure if they are OEM quality, but for $20 or less it may be worth trying if one thinks their IACV is also suspect.

https://www.ebay.com/itm/New-Idle-Ai...R/371555978063

 

I would say no on the engine performance if it is closed to leaks as the engine regulation is independent of the fuel tank off gassing but it shares the same ECU on the 2 canister system vs. the 1 canister system controlled by manifold pressure for muscle air for actuation and cycling . Nice pic Al that shows both sides , right being empty .

EVAP valve question ? If it stays open it would have an effect of having a leak in the intake manifold on the 1 canister system and would be the same if it included a stuck open canister close valve on the 2 canister system .

I'm off to get some bicycle spokes to fix my crack exhaust manifold