Chasing Down Mild Idle Stumble

I’ve been chasing down a mild misfire on my 1988 V12 with 50K+ miles. I know it’s a common complaint here, but bear with me.

It’s not a total misfire, it’s the kind where the engine vibrates ever so slightly. The RPM, fuel pressure and vacuum (17 inHg) all hold perfectly steady at idle. It happens in Park at 900rpm or in Drive at 750rpm. Happens when cold or warm.

I’ve spent a lot of time ruling out ignition, fueling, and timing.

My guess is now valves because a few months ago, I did a compression test (engine was cold though). All cylinders were 180-195 dry. Wet compression test, all cylinders jumped to 225-300 except 3A and 6B remained at 180. They did not budge.

This leads me to believe either the exhaust or intake valves on those cylinders are not closing perfectly. The car did run rich for years, I believe, and original owner I believe didn’t put a lot of miles on it. So there could be a lot of carbon build up. How common are burnt exhaust valves on the V12? And is there anything I can do for now? Unfortunately, the car is not highway ready for an “italian tuneup”.

Also, if spark plugs were not seated properly due to bad threads, would that explain my wet compression test results? Or would I not even get 180?


Have you checked valve clearances?

Nope, I’ll have to read how to do that, not ready to pull cam cover yet.

But my gut instinct is telling me vacuum leak now. I’ve now determined the misfire is slightly worse when cold. When warm, still misfires, but I’ll sometimes get a high RPM in Park, especially with a hot engine. Rpm is perfect 750 in Drive, but I assume the O2s are working then.

I’ve replaced and checked all vacuum hoses, and AAV. I guess I should check every inch of intake manifold now. :frowning:

I’ll get this thing purring one day. I’ve come so far

I can see where two low cylinders, as you mentioned above, might cause a slight engine roughness.

If you have a vacuum leak, it couldn’t be much of one…given that your idle speeds seem OK. Typically a vacuum leak would increase idle speed…just as though the throttle was opened a teeeny bit. Can you adjust the idle speed lower with the AAV screw? If so, it suggests you don’t have air entering from elsewhere which would obviate the effectiveness of the AAV idle speed adjuster.

Anyhow, the ECU, not knowing how the air is getting in, simply adjusts fueling to match. So, if you’re thinking along the lines of a ‘lean misfire’ due to a vacuum leak weakening the mixture, I think you might be on the wrong track.

On all my Jags (and other cars, too) I’ve found that having the injectors professionally cleaned can do wonders for idle weirdness and roughness. The spray pattern and volume can degrade slowly over time.


Thanks Doug. I can lower idle too low by turning AAV screw. And I spent 30 minutes searching for vacuum leak with no luck. You’re prob right.

The engine is idling rather well, I just want the 100% smooth I hear about. But I do need to run the engine hard for a good carbon clean (it’s been 7 years I’m sure) and she’s 31 years old, so maybe I’m expecting too much.

Fuel injectors were profesionally cleaned and checked by Faircloth.

We have two V12 Jaguars (1990 XJ-S convertible and 1990 V12 Vanden Plas) and they both have that 100% very smooth idle most of the time (but not all the time). We also have an I6 E-Type and a Series III XJ6 and they are never as smooth as the V12s, never ever. When I drive one of the V12 cars after driving one of the I6 cars, I will occassionally panic at a stop light thinking that the engine died, only to look down and see RPMs, oil pressure and no warning lights. The V12 can be so smooth that I can’t feel the engine running at idle. But this is not always true for some reason. I know this is not a big deal for modern cars, but for cars that are almost 30 years old this is pretty amazing to me.

What spark plugs are you using, what gap did you set them to, and when was the last time you removed the spark plugs to inspect them?


Spark plugs are only few months old, I’m using NGK v-power tr5 2238. The simple copper ones. Gapped to .025.

I only inspected two plugs, they look good. A bit on the light side, kinda greyish white.

That sounds normal. Personally I would not spend much time on this issue until you can give it an Italian tune up. My .02.

The plugs I am using have a heat index of 5. A bit on the cold side. I’ve read some V12 owners here use NGK BR6EF or BR7EF, a slightly hotter plug at 6 or 7. Not sure what difference that would make.

I did get the engine nice and hot, and ran some water into the intake manifolds and went for a drive, got one little cloud of grey smoke. I swear it helped a wee bit for my idle.

I agree, I’ll leave it alone for now until I can give it a good run.

BR7EF is the specified plug for the 6.0L, just confined with my manual that this is the same plug for the 5.3L V12 as well (at least those with the Marelli ignition as mine)

Aside for the heat index, the TR5 plug is “projected” (BP5EF) while the other one is “non-projected”. I have no idea what that means in practice and will be the first to admit that in almost 10 years of ownership, I haven’t gotten my engine to the point where a coin can be balanced on top of it…

if i remember correctly, a hotter plug will help at high rpm under load, and a colder plug will help at idle?
or do I have it backwards?

I am not sure I follow – too hot or too cold is not good. Selection of plugs usually requires optimization of many parameters. There are “wars” in the archives which if you are curios yo can find – lots of info on indexing of plugs, electrod shape and material, brand, resistor vs non-resistor, etc.
IIRC, the high compression qualifies the Jaguar V12 as performance/race engine, thus colder plugs (faster heat transfer).
I remember I tried BR6EF awhile back, after I learned that they are specified for the sedans with the same engine as mine (same gap, and also non-projected) but the rolling idle condition never improved.

Yeah, I’m probably chasing a rabbit down a hole assuming plugs. On some of my turbo cars, I’ve had much smoother running engines switching from copper to platinum, not sure if it matters on the V12. That’s an expensive test with 12 spark plugs. I’ve read some of the wars about indexing, and plenty have backed it up in theory, but I never saw someone post back with a “eureka”. Maybe if I’m bored next winter…

You may try running through a tankful of “High” octane gas. Won’t cost you much and you may just solve your problem.
Attached are some various views on the subject.
Good Luck,

Good point, worth a try.
I’m currently running 92 super. I’d have to add octane booster. Maybe add seafoam to gas too?

I should also check motor mounts. I can feel the occasional vibration in my seat.

Here’s how I have been dealing with this:

No warning light’s or error codes=no (relevant) injector problems. Dripping injectors will show up as rich mixture on the O2 sensor readings and trigger an engine warning light. Also, just to make sure, check plugs for a dry, light grey or light brown surface. No soot: clean burn.

If all good, live with it. Or: Buy a Lexus.



Greg has a 1988 XJ-S V12. It doesn’t have any codes or check engine light. Those didn’t get introduced until the facelift cars I think late 1991/ 1992.


Thanks Paul,

I was somehow under the impression that the earlier injection cars with o2 sensors must have at least the CEL.

I have an 85XJS-HE. It always had a bit of a “stumble” at idle. I replaced the distributor cap,and the plugs but it was never as steady as I thought it would be. Years ago I borrowed a friends wide band oxygen sensor and put it in the tail pipe . From memory it was at stoichiometric at cruise speeds above 60kph and ran a bit rich at 50-60kph in D. I can’t remember the idle readings…
Now years later I have a refreshed engine with a megasquirt MS3 controlling sequential fuel injection with wide band O2 feedback and trigger wheel ignition. So full control over fuelling and ignition.
If I set the engine to run at stoichiometric on idle it has a “stumble”. That’s with about 23 deg advance. If I richen the mixture to around 13.5(ish) : 1 then I have a much smoother idle…at the same ignition timing. So my experience is that my engine is calmer at idle with a richer mixture…at the same ignition timing…and that is with the fuel injecting into the vicinity of each inlet valve just before the valve opens. Can you try to increase the richness of the mixture at idle to see if this makes any difference?? Maybe you could suitably modify the resistance of the coolant temp,sensor to fool the engine that it is running a bit cooler and needing a bit more fuel to stimulate the richer mixture…and see if this affects your idle…??Possibly the original designers trying to extract every last ounce of economy out of the HE