I am chasing a loping idle problem on my 95 XJS 4.0. After changing the plugs to iridium, replacing the Idle Control Valve, cleaning the Throttle Body, cleaning the EGR, cleaning the MAF and replacing the Crank Position Sensor, the problem has diminished but now will idle well and then drop to about 400 rpm, shudder, recover and lope for about 15 to 20 seconds before smoothing out again. It repeats this sequence like clock work while running. I am hoping the loping it does now is the Engine Computer trying to learn the new parts.
I have not replaced the MAF, but if I go there I need the correct part number for the 95 XJS 4.0 MAF. The MAF I have may be a replacement installed before I owned the car since it has a US Parts number on it but not a Lucas or Jag part number.

Can someone take a look at their MAF and let me know what the part number is? Knowing the correct part number will help me when shopping for a replacement unit and getting a competitive price. Thanks!

 

Hello;

Its LHE1620 AA for the MAF.

Your problem to me sounds like you have a bad coil or coils, or maybe water in the ECU connector.

 

Go to this site for all the jag part numbers.
Air Delivery System Components - Parts For XJS from (V)179737 to (V)226645 | Jaguar Classic Parts UK

Also, I wouldn't be too sure about the MAFS. Clean it with some MAFS cleaner by all means. Jag sent out this TSB as a lot of people thought they had faulty MAFS but they were actually fine.

https://www.jaguarforums.com/forum/a...lacing-maf-pdf

Be aware that if you have made a lot of changes it will take quite a bit of driving for all the adaptations to come back to the correct values unless you have the ECM reset.

Cheers,

Allan

 

Thanks for the quick replies. I will check the ECM for water contamination as suggested and then continue driving the car to see if the ECM learns the new components. Is the ECM under the passenger (US) kick panel (the panel under the dash, attached to the side of the car with the bottom touching the carpet)? I hope it is not one or more of the coils. I have unplugged each of them when the car is idling smoothly and they did cause the rpm to stumble until reconnected. That tells me they are all firing, but it could be intermittent.

 

First person I've heard going to iridium plugs and the engine liking them. Typically the hotter copper cheapies from Champion or NGK do the trick.

It's not like it's a pain in the butt to get to, or expensive.

Which plugs did you put in?

 

I put in the NGK BKR6EIX plugs. Might be a mistake. The reason I changed out the RC12YC4 plugs that were in the car was that 'out of the blue' the car started missing so bad I could not trust it to drive it. I could not locate the RC12YCC plugs locally, so not knowing any better, I thought the iridiums might be fine.

The closest Champion plug that I can find locally is the RC12YC. Do you know if the RC12YC is the same as the RC12YCC?

I just pulled the ECM to check for pin corrosion. All looks good there. The ECM also has the recall R493 PROM revision marked as being performed in 2002.

So, the problem lies elsewhere. I might have fixed the original problem and also introduced another by using the iridiums. I will gladly put in the copper core Champions if I can be advised on the equivalent to the RC12YCC plug.

 

Sorry. Correction. The plugs that were already in my XJS were Champion RC9YC4 plugs.

 

I had iridium plugs in mine when I purchased it and had misfire codes and set the check engine light several times before I went back to the champions. Problem solved… I was told by several Jag mechanics to stay with the champion plugs in my owner’s manual.

 

The ECU is on the passanger's footwell area.
The coils are a weak point in the XJS AJ16. They fade intermittently and can cause havoc. If this is your problem, make sure you buy Eurospare brand that have "Made In Japan" stickers in them. Anything else, is no good.

 

Found the ECM. Thanks for the guidance. The connectors are corrosion free and the ground connection is clean. I disconnected the battery to remove the ECM so, the ECM will have to relearn.

I will be glad to switch back to the Champion copper plugs. Is the Champion RC9YC correct for the 4.0? I have not found the RC12YCC plugs from the local suppliers. I am finding RC9YCs.

 

Champion RC12YC should be easy to find. RC12YCC are rare. Not much difference between the two. I forgot what the difference was, I think there was more copper on the YCC. In any case, you should have no problem finding it at $2 a pop.

 

Woo Hoo! The problem is on the way out. Just a little more work to be done by the ECM and I think all will be good. Not one to sit quietly after working on this problem for a couple of weeks, I bought Champion RC9YC copper plugs. The RC12YCC were no longer available. Just a memory except for the cross references. I removed the NGK Iridiums and put the RC9YCs in. I then checked the intake manifold bolts to see if any were loose. None were. I checked for vacuum leaks using starting fluid spray. None found. Since I had disconnected the battery earlier today so I could safely remove the ECM and check the connections for corrosion (none found), the ECM was going to have to learn to play with the new components.

Over this period I replaced the ISC, Crank Sensor, Plugs (twice), Coolant Temperature Sensor, cleaned the throttle body (twice), cleaned the MAF multiple times and cleaned the EGR valve. I have a new EGR valve on the way and will install that when it arrives.

I tried to start the engine. It would fire off immediately and then die unless I held the throttle open a bit with the foot pedal. I opened the air bleed/bypass screw located next to the ISC about three turns so the engine could get air. It started and idled. Not pretty, but it was running.............with a pronounced lope. The intensity of the lope varied. I suppose because the ECM was trying to make sense of it all. I played with the air bleed to minimize the lope. I then followed the instructions I found on how to adjust the idle on the 4.0. I don't remember where I found the instruction, but made a copy and kept it. I posted it at the bottom of this post. Once I completed the adjustment, the engine idle calmed down with just a hint of the ECM searching for the perfect idle. I took it for a drive and it did well, but still searching for the steady idle. When I got home, the idle was somewhat stable. The best it has been since it went nuts a couple of weeks ago. I think the ECM will continue to improve it. I don't know which of the shotgun approach components did the trick, but I am very happy to have my XJS running again. I suspect that the problem was the ISC all along and I may have introduced a wild card when I installed the iridium plugs. After the ECM finishes its job, I may put the iridiums back in to see if they in fact were a part of the problem.

Here is the IDLE ADJUST procedure that I followed.

IDLE ADJUST

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 (steps 3 – 5)
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 the idle speed is not within these limits, then adjust the air bypass screw to bring the idle speed within these limits.

Switch ignition off and reconnect the idle speed controller. Start engine and check to ensure that the idle speed is between 650-800rpm.

Thanks to everyone for your help!

 

I think I posted that for someone right out f my service manual, word for word.

That being said, I am almost convinced there is a TSB that changed the recommended spark plug for the AJ16 from the RC9YCC to the RC12YCC.

You may want to up the heat, as recommended by your friends on Browns Lane!

 

Vee,
Thank you for posting the idle adjustment procedure. I think it made all the difference in recovering my idle. The ECM is still working on it, but the periods of smooth idle are getting longer and the lope/surge/roughness is being minimized.
Can you advise me on where to source the Champion RC12YCC plug? Napa, O'Reilly, Advance and Autozone do not show it being available.

 

It does not seem to be available in the US. Even places that appear to advertise it, will send you an RC12YC instead and tell you it's the same thing. I've seen it advertised in a number of places in England, so perhaps going to your local Jaguar dealer might be worthwhile? I'm not sure of the specification difference between the two plugs or if it is just a nomenclature difference?

Cheers,

Allan

 

RC12YC is available at Autozone, but not Advance Auto. NAPA may sell them too, I'm not sure. Autozone typically stocks them, so at $2 a pop, they are easy to find and pay for.

The difference between the RC12YC and the RC12YCC is that the latter has a DOUBLE copper core electrode.

I have never seen a RC12YCC. I think you may be able to find them on eBay or from Europe, but I don't think it will make any difference. I would suspect that the double copper core might last longer, but at $2 and the ease of getting them in and out, why bother?

 

They do exist somewhere for sure

Here's one I recently pulled from my car.



Cheers,

Allan

 

The Autozone online site did not list the RC12YC so I did not visit them. I will drop by and ask them if I can secure them through them. This is a bit like looking for a unicorn.

If I understand the Champion naming convention correctly, the RC12YC is a hotter plug than the RC9YC. 12 > 9.

Another question. What is the purpose of the gasket used with the ignition coil? All my gaskets are defunct and I have ordered replacements, but just what is their purpose?

Thanks to all!

 

Champion Copper/Spark Plug (71) | AutoZone.com

Maybe they don't stock them in your store without ordering them. A generic search for RC12YC at AutoZone came up with this.

As for the gaskets, I don't believe they do anything. There's a thread on those things as well. I have been running my cars without them because I think they are way to pricey for what it is.

I was planning on making my own but I think the concensus is that most other cars which use coils, do not have a gasket.

 

I have not made it to Autozone just yet to try and get the RC12YC plugs, but was killing some time at Walmart this morning and stumbled upon a spark plug display in the automotive section. Low and behold, they had Champion RC12YC plugs for $1.93 each ( Do I hear an Amen?). The blister packaging said 'Small Engine'. What a hoot! Maybe Briggs and Stratton had a hand in the design of the 4.0.

I agree the $$ Jaguar charges for the coil gaskets is robbery. I ordered aftermarket copy gaskets in case (some how) it would make a difference in the ability to fire consistently.

I am still experiencing a cyclic lope in the idle, but it is diminishing in severity and in duration. It probably would not hurt to replace the original Lucas coils since they could have the full 133,000 miles of use on them and could be a factor. I am hoping the ECM will eventually learn it away completely. I will give the ECM some more time before jumping to replace the coils.

Thanks to everyone for their help and suggestions!