Im sorry, I put this whole topic in the INTRO section. I should have left the problem in this section only..wont happen again. Im just upset..my Jaguar has got me frustrated. (worse than my wife usually does )

My name is Randy and I have a jaguar xjs with the 12 cylinder motor. Beautiful car. I am the 2nd owner. It has 80K miles. The original owner had this car made especially for him. (very rich) It has every option- including moon roof and phone. Basically I let it sit in heated garage, do not drive in rain or snow...its under car cover and then I pull it out when the sun is shining so i can show it off with a spin around town- then wash and wax and back in garage she goes.
Now one thing I did notice is that some things do not match the 1988 model- there is supposed to be some diagnostics check on the dash??? but this car does not have that...The original owner took delivery of this car on Jan 2nd or 3rd..( I have all paperwork from everytime this car went in for service since birth) Is there anyway it could be more 1987 than 1988?
Anyway, now a problem has got my attention. THe car starts right up..sounds great. Then as soon as it gets warm it idles rough..and i mean pretty rough. When I have looked underneath the catalytic converter will be glowing red...any ideas?
Thanks
Randy

 

See my reply in the other section you posted in.

CHeers
DD

 

Hi
Thanks I did see other reply. This car has the Lucas ignition. Other things I forgot to mention...Car has had new plugs, wires, cap and rotor. Vacuume lines routing checked with diagram under hood, propane checked with any other vacuume leaks, compression is 135 to 140 on all cylinders. New temp sensor as well. Other work done not by me..new intake manifolds gaskets, fuel injectors, exhaust pipes back of catalytic converters replaced. I have disconnected the 3 wire O2 sensors each side and started car cold-this is supposed to keep the control in open loop. Problem still there. Suggestions? Like I said, wonder if makes a difference if car is 1987 or 88. Do u need vin?

 

First off, I can't think of anything significant that differs between the 1987 and 1988 V12; my recollection is they are virtually identical.

Second, Doug is right on, if this WAS a Marelli ignition, this would be a slam-dunk! Lost a coil or rotor, or both.

To light off a catalyst like that one of two things has to happen, seriously excessive fuel injection [less likely], or loss of ignition [more likely].
The first place I would look is the distributor, specifically the pick-up coil and its air gap. The reluctor air gap should be checked at all points. It should be .007in to .015in, and always checked with a non-metallic feeler gauge. If any of the points ever contact the pick-up itself, the field collapses and a misfire results. Cold there may be clearance, that goes away when it gets hot. Also the wires to the pick-up inside or outside the distributor can break inside the insulation and cause intermittent open circuits, same results. Actuate the vacuum advance and see if the base plate rotation is nice and circular, it can sort of slide sideways instead of rotating nicely if the pivot is stiff/bad.

On the fueling side I've run across injector power resistors that short to ground when they get hot. There are 4 in the module, each carries the load for 3 injectors. If one shorts to ground it fires 3 side by side injectors [sticking them open] until the key is turned off. Less likely though still possible, is a failure in the ECU that can cause a set of 3 injectors to stick open. Unfortunately injectors wired in packs like this are harder to fault trace, almost requires an oscilloscope to really see what's going on and catch it in the act.

Just a few things to think about, hope it helps!

 

Not that Steve's remarks need any any backup from me, but I agree with all above.

Cheers
DD

 

Thanks for the help..will check and get back here. I didnt think there was much difference, but when asked to troubleshoot from another guy in my area..this car didnt have something on dash that it was supposed to if it was a 1988.

 

Ok, after checking magnetic pick-up air gap & adjusting I reassembled and started car w/ vac advance disconnected. Vac gauge on open port, vac hand pump on vac advance-car ran for 10 min really good- 1000rpm's- 10" vac on gauge. When reconnected to vac advance-rough idle returned.
When disconnected back to gauge smooth idle, but fast.
Pumped up vac pump on vac advance as vac increased-idle increased till 15" then idle dropped to 500rpm and was smooth and nice. (all in open loop)
Installed "T" in vac advance line with gauge. Started car-fast rough idle. 10" vac on gauge, disconnected "T", put vac pump back on- pumped up to 15"- 500 rpm and nice and smooth.
Checked vac @ manifold before dump valve was 20". Experimenting: Manifold vac to vac advance-rough idle @1000rpm.
SO why does it idle smooth with vac pump and not the manifold pump?

 

Two things........
1. Are you absolutely SURE the pick-up base plate is rotating in a perfectly circular motion?
2. Have you fully tested the blue and orange pick-up coil wires in the distributor? Disconnect the pickup connector in front of the distributor. Connect an ohmmeter and watch the resistance. First use your vacuum pump and cycle the advance several times. Also use a pick or small screw driver and move the wires inside the distributor around, again while watching the ohmmeter. If at any time the resistance bobbles around, the pickup coil is suspect. My suspicion is there is a magic point in the rotation that the circuit is interrupted, and that point is hard for you to catch with the vacuum pump.

Hope that helps!

 

Ok- followed your procedure to checkout magnetic pickup. With ohmmeter across the wires in the plug I rotated the engine with a bar in direction of rotation and ohmmeter would move. This shows I had good connections in the plug. I twisted, tugged on the wires, moved the vacuum advance-ohmmeter did not move. Reassembled. My local shop provided me with a easier to read vacuum routing diagram than the one under the hood. Turned out that some of the routing was incorrect on the car. Corrected per diagram and no difference in the symptoms as before. I totally understand your theory about the broken wires in the pickup effecting the idle. Should I just replace the magnetic pickup even tho I cannot find a faulty wire?? Symptoms same.
I just don’t get how or why when I pump up the vacuum advance up to 15” I get a rock solid steady idle at 500 rpms. With the vacuum line disconnected from advance- idle is smooth, but fast.
Thanks for your time- too bad you didn’t live close by- I make my own beer and nothing goes better than a homebrewed English ale after you’ve worked on a Jaguar.

 

I'm trying to get my head around the results of your vacuum experiments. I might be missing something, or misunderstanding something, but I think it's a red herring. I think you're masking a different problem.

As you've discovered the vacuum advance control system is complicated. As I recall a couple key elements of the system is to ensure that there is no vacuum advance at idle and to ensure that you have 18º advance at 3000 rpm. There's much more to it but those are two benchmarks.

From my own experience I can assert that with no vacuum advance applied the V12 will idle properly and smoothly from 300 rpm on up.


(Aside: my own XJS came to me with all vacuum advance controls bypassed and the advance cannister connected to ported vacuum. Although it ran very well I tried multiple other combinations and never could make the engine run any better so I went back to direct connection to ported vacuum. I've heard from other owners who have doen the same, FWIW. And I must emphasize *ported* vacuum, not manifold vacuum.)


Anyhow, you've determined that the vacuum advance at the distributor does work the pick up coil seems OK. I would now go back to this statement from Steve.....


"To light off a catalyst like that one of two things has to happen, seriously excessive fuel injection [less likely], or loss of ignition [more likely]. "


....and continue looking for A) loss of ignition or B) overfueling.

Personally I would start looking for an overfueling issue at this point but we should wait to hear what else Steve has to say.

We've had another thread recently where artificially applying vacuum is a topic of discussion. There, too, I tend to think it's a red herring. I think overfueling (or some other problem) is causing low vaccum...and not the other way 'round....and applying vacuum is just masking the problem.

Cheers
DD

 

Now I'm beginning to worry you may have an incorrect distributor or vacuum capsule on the distributor. Look at the attached image and tell me what side of the vacuum advance saucer has the vacuum line fitting. Is it like the illustration, or where the red mark is that I added??

Cheers,

 

Which way am i supposed to be looking at it? My vacuum advance unit is on the passenger side.

 

What do you mean by "red herring"? Also, not quite sure what you mean by "ported" either. Can u tell me what fitting would be ported on the manifolds?

 

Which way you look at it doesn't really matter, what I'm after is which side of the "saucer" does the vacuum line attach to? Does it point more to the rear of then engine or to the front, the tube the vacuum line attaches to, that is.

 

Red herring = a false clue or distraction that diverts attention from the actual problem

Ported vacuum = a vacuum supply before the throttle plate so that vacuum is present only if the throttle is opened. There on some vacuum port on the throttle body, top and bottom. The ones that are ahead/before the throttle plates are "ported".

Follow up on Steve's angle. I *think* he's trying to determine is you have a "vacuum advance" or "vacuum retard" unti on the distributor.

Cheers
DD

 

Right-O, Doug. From Jaguar12's descriptions, I'm starting to think a vacuum retard distributor is in there where a vacuum advance type should be. I'm having a real problem coming up with another explanation for his findings.

Cheers,

 

It points to the rear of the motor. I am pretty sure it is a vacuum advance- its very similar to general motors types of vacuum advances.

 

It appears you have the correct advance then. The only other thing I can think of that might muck up the fueling in a such big way is the ECU or the integral MAP sensor. The next thing you should do is take the vacuum line of of the balance pipe at the rear of the engine. Connect your vacuum pump to it and pump it up. It should hold vacuum. That is a vacuum tube/hose affair that goes all the way to the boot where the ECU lives. Some time it corrodes, collapses, gets a hole in it, or gets mashed. If it holds vacuum, go back to the ECU and disconnect it. Then try the vacuum pump again, it should be free and open and hold no vacuum. You should also be able to blow through it relatively easily.

Now a good vacuum signal to the ECU is dependent on the throttles being balanced, I think Doug mentioned that. So do make sure that is checked.

Cheers,

 

Thanks a lot for all your help...I replaced the whole unit and she was purring like a kitten. I really learned quite a bit with the whole process of tracking down this problem.

 

Good news Jaguar12,

Just to clarify which unit did you change?