Hi Warren,
Now that you mention it, I do remember the 2 cars you mentioned. That's a long time ago.
Warren, you and Allan and others have been an invaluable help to me with some of the problems with my car, and again I am at a loss for words to express my appreciation to you for your help. Hope to be able to return the favor someday.
I'm satisfied that the ignition system is in order and the engine has plenty of power and is smooth except at idle intermitently. I suspect an injector problem and is there a way a do-it-yourselfer can check these? It seems to be dropping a cyl. at idle (750rpm) at times, mostly when the engine is fully warmed up. The miss goes away when you raise the rpm's to about 1200. I have run 3 doses of Techron in the gas tank but I know that this doesn;t work all the time. Have you had any injector problems?

Take care,
Bob (Spillwaybob)

 

I did have intermittent running but mine was fouled plugs - my car has been sat for 2 years as I have rebuilt things. Oh I also had a bad lead, bought these off eBay and they are now in the bin I ended up buying Top Gun leads no more problems.

 

Warren, did you ohm out the individual leads to find the bad one(s) or did you just replace the whole set? If the former, what kind of ohm readings should I get?
In the past on American iron, any ohm reading worked but the open circuit ones went to the bin.
Take care
Bob(Spillwaybob)

 

I ended up with a lead with no plug on it so it was pretty obvious. I had a set of 8mm Top Gun leads on my spare 6.0L engine so I just changed the whole set.

Resistance of the standard leads I would think would be in the 6000 Ohm range. I know the Top Gun leads are lower in resistance (gives better spark). The high resistance of the standard leads is to reduce induced ignition noise in the electrical system.

 

Warren,
All the leads passed the ohm test......readings varied between 4K and 8K depending on the length of the wire. No open circuits. It's beck together and running about the same. On Ford V-8's of this era, there was a minor problem which affected the idle.....carbon build up on the throttle plate which prevented the plate from closing completely. Do you know if the Jag 12s have this problem with the throttle plates? If so, easy to clean.
My problem with this idle thing is that I have never ridden or driven another like car. Mayble what I am experiencing is "normal" For instance, does this engine idle as smoothly as the 4 litre 32valve V8 in the XJ8's? I had one of those and that engine was smooth as glass idling at all times.
If you are getting tired of answering all of my stupid messages, let me know and I stop.

Take care,
Bob (Spillwaybob)

 

Dirty throttle bodies/throttle plates are not unique to any certain brand. it happens on Jag V12s.

As for idle quality.....

I've driven a number of V12s. Some were smooth as glass but most...including the one I actually owned.... had an an occasional slight roughness...more like a slight "tremble", really.

I'm never been particularly happy with the idle quality of any of my three Jags, to be honest, and complaints in this regard are fairly common. Seems like a "some do" and "some don't" situation and, as often as not, those with an idle quality issue defy all repair attempts. Or, perhaps more accurately, many of us give up before hitting paydirt. Some faults are too subtle to easily isolate.

Cheers
DD

 

I agree with Doug, mine is a little rough when cold but settles down and is smooth once warmed up.

I would expect it to be much smoother than a V8. Because the V12 has more bangs per revolution it does sound as though it is idling faster than it actually is.

 

To Doug and Warren, that's exactly the kind of info I wanted to hear about the idle situation. Unfortunately my expectations of the V12 were a little exagerated......like you, I expected a 12 cyl. engine to be much smoother than a V8. One tends to forget that the basic architecture of this engine goes back to the mid 60's and car engines did not idle smooth as they do today. As you say, these cars have many idiosycnrosies and this is one of them.
Thank you all for being so helpful.

Take care,
Bob (Spillwaybob)

 

Hi Bob,

As Doug and Warren have said, rough idle is a common complaint of both the V12 and I6 engines.

There can be many causes and quite often multiple causes, which makes it harder to solve. On large engines such as these, achieving smooth idle is difficult and even small changes in the performance of components can create an easily perceived problem.

In your case, the fact that it smooths out when you increase throttle may indicate you have a slight leak in one you intake manifold gaskets at the cylinder head. Because vacuum decreases as engine speed increases, the problem appears to go away as you rev the engine. Also when the car is cold, the fueling is in open loop mode and the fuel injection ECM uses a warm up strategy that enriches the A/F ratio. Again this might appear to smooth things out.

An easy way to detect this is to "spray" around each individual intake gasket with some propane. (You can use carb cleaner but be careful as there is a possibility of fire) If you have a leak, you hear the engine revs increase.

If no change, then here are my top reasons for rough idle in no particular order.

1. Incorrectly gapped or faulty spark plugs or lead sets. In the case of your engine there were TSBs about changing plugs and lead sets because of misfire issues.
https://www.jaguarforums.com/forum/a...cation-v12-pdf

https://www.jaguarforums.com/forum/a...plug-wires-pdf

2. Vacuum leaks

3. Clogged or leaking fuel injectors. Have them ultrasonically cleaned and flow tested. Techron will only do so much.

4. Dirty throttle body and/or incorrectly set throttle disc. Clean body with carb cleaner and measure closed throttle gap with a feeler gauge.

5. "Lazy" O2 sensor. Check output to ensure rapid cycling.

6. Purge valves not responding correctly to ECM signals. Try cleaning the connectors.

7. Coolant temperature sensor malfunctioning. Measure output resistance when engine warm and look for unusual fluctuations.

And then again, as Doug said, sometimes even the most dogged person may never find the answer

Cheers,

Allan

 

The idle quality issues on my Jags has always irked me a bit.

My clapped-out, hardly pampered, plain-as-mud old Hondas have always idled smooth as glass. My 1995 Ford F250 work truck with a ka-jillion miles....smooth as glass. In fact, now that I think about it, every car in my extended family idles a dead-smooth idle and nothing was ever done to make 'em that way. That's just the way they are, and many of them are 15-20 years old and very high miles.

Yet none of my three Jags has ever given a consistantly smooth idle! Despite my efforts (long abandoned, believe me) the best I've ever achieved with respect to idle quality is "Well, some days are better than others".


Grrrrr!

Cheers
DD

 

Allan,
You brought up a lot of good points, most of which I have not thought of.
Even though my leads passed the ohms test, there could be other problems with them that are not visible and they have been taken off the put back on 3 or 4 times so I have a new set ordered. At replacement time, I am going to remove the new NGK plugs and regap them to .035"
I have ruled out vacuum leaks because the problem is intermittent.
Cleaning the testing the injectors----will be done if all else fails.
Checked the throttle bodies and they had very little carbon deposits which I have cleaned.
Items 5 & 6 way over my head!!
Thanks for your interest and I will keep you all updated.
Bob (Spillwaybob)

 

Doug,
I understand and sympathize with your frustration about your Jags idle quality. My XJSV12 is only the third Jag that I have owned. The first was a "54 XK-120M with SU carbs and for it's day, it idled pretty good. 2nd. Jag was an '01 XJ8VDP that I bought on Ebay with 53K miles. I loved the engine/transmission in that car. Both were smooth as glass and in the 3 years that I owned the car, I did not have to do anything to the engine in that regard. I considered the 4-litre 32 Valve V-8 to be a very refined design.
Now for the V12, you have to realize that this engine was designed in the mid-sixties, before EPA emission standards and gov. regulations were in effect. So to be able to sell cars with this engine in the USA, Jag had to incorporate tuning changes so the engine could pass the EPA/Gov. regs. I believe this may have some bearing on the idle problem.
Engines designed post '71 had basic design changes favorable to emission regs. like variable valve timing, 4 valve/cyl. etc.
I'm not going to fight this much longer....see my reply to Allan below.
Bob (Spillwaybob)

 

Hi Guys,
I've had a few chances to drive the Jag since I installed new plugs and leads and inspected the distributor cap and rotor. The idle smoothness is improved about 50% and I am going to leave it alone for a while. And while on this subject, I bought a "Gear Head" swivel spark plug socket from Pep Boys and it worked perfectly and there was no problem getting 1A & 1B plugs installed. Perhaps some of the V12's use different brand A/C compressors which could make the job more difficult.
The next subject will be about noises. Since this is the only XJS I have ever driven, I have no way of determing what is normal or not normal. I would welcome the opportunity to drive/ride another like car for comparison.

Take Care,

Bob (Spillwaybob