Rough idle and differing opinions by jaguar mechanics

Hi everyone, there is so many knowledgeable people on this forum so I was hoping I might get some help.

This is my first post so here is a little background. I have a 1982 xj6 which has a misfire and idles roughly. It has been like that since I purchased the vehicle.

In addition to the misfire, the car is mysteriously losing coolant. It is a moderate amount. It is not dripping on the ground, nor is it blowing white smoke out the exhaust.

Oil and coolant do not appear to be mixing together. Also, I have done a combustion leak detection test which supposedly checks for carbon monoxide in the cooling system and it proved negative. The liquid stayed blue which supposedly means it doesn’t have a leaking head gasket. It is a cheap Amazon Combustion Leak Tester so i’m not sure how accurate it is.

I have done both a wet and dry compression tests and there is a 30 psi difference between wet and dry compression tests. That is to say all cylinders were 150 psi with the dry compression test and 180 psi with the wet compression test. The engine was at operating temperature and throttle was open during the tests. Ignition and fuel disabled.

I am getting conflicting opinions by mechanics. One Jaguar mechanic with 50 years experience says the 30 psi difference is not the cause of the rough idle. He has inspected the spark plugs and believes it to be a head gasket leak in it’s embryonic stage. Furthermore he believes being in it’s beginning stage it can be repaired with a product similar to Head Gasket Repair Sealant.

Two other Jaguar mechanics (one of which specialising in rebuilding Jaguar engines) believes the cause of the rough idle is the worn piston rings which is causing the 30 psi difference between wet and dry compression tests.

I was advised if a cylinder had coolant entering it it would be cleaner that the other cylinders which is not the case. I have inspected all cylinders with a endoscopic camera and this does not appear to be the case. All cylinders appear to be similar.

Finally i did a leak down test and found that cylinders 1, 4, 5 and 6 had 15 % leak down. Cylinder 3 had 40 % leak down and cylinder 2 had 30 percent leak down.

If i could get your input it would be greatly appreciated as I’m very confused from the differing opinions by several Jaguar mechanics.

Thanks for your time and input.


Hi Nick.

These cars always have a rough idle if anything is just slightly misaligned.
For one thing, the difference between wet and dry doesn’t cause rough idle, the difference between the cylinders in the same test is what counts. Your values are good compression wise. Leakdown difference, not sure what is good and what’s not.

What can always be wrong is cam timing, injector clogged, a bent valve, mixture off. I assume you checked for air leaks with a propane torch or such; ignition tuneup needed, and give it a long tour on the highway, that almost always helps. Also, Frank will probably want to ask you about idle speed.
You can check for how strong the spark is, and pull injectors separately (just the plug) and try to isolate things if it is a single cylinder issue. Not very probable though.

The coolant could also disappear through a faint leak never to be seen, and I‘d stay away from these headgasket sealers. Your mechanic of course has better eyes than us as he’s seen the plugs already, and if he thinks that it definitely is a head gasket then, given some time passes and it worsens, it is time to do a proper job. However, you didn’t see anything with the endoscope and you certainly would have.
I have a slight coolant leak at a head stud, and had terrible idle that has gotten much better with a 150 mile run and back, and new spark plug wires. Oh, and by finally fixing an exhaust leak. We have your test values, feel free to send us pictures of your plugs if you want further evaluation. Ask ten people and you have twelve opinions, but those XKs hate idling for some reason.

Good luck.


Hi David, thanks for your input and help. I wanted to 100 % rule out a leaking head gasket, cracked block or cracked head before moving onto cam timing, fuel and spark related issues etc.

Unfortunately my attempts have been futile as there has been no definitive proof so far. This is either because the head gasket leak is at it’s embryonic stage or I’m barking up the wrong tree and it’s not a head gasket leak, cracked block or head and something you mentioned like off cam timing etc.

I thought the leak down test may have indicated a coolant breach or burnt valves on cylinder 3 and or 2 as these are the 2 cylinders with the most air escaping (leak down). I tried listening (with a stethoscope) for a hissing in the exhaust pipe, intake throttle body, dip stick holder, overflow coolant container etc for escaping air. I did this by putting 100 psi into the cylinders with the piston at top dead centre on compression stroke in each cylinder so both valves are closed.

The only hissing I heard was a little bit in the cooling expansion tank but saw no bubbles. The hissing was the same on all cylinders and not more profound on the cylinders with the most leakage (i.e. cylinders 3 and 2)

I’ll try and upload photos of the spark plugs. Also I’ve heard that if you put special fluorescent dye in the coolant a head gasket leak will eventually show if the dye coats the exhaust pipe tip or spark plug or plugs of the faulty cylinder . Apparently this is the preferred GM test for diagnosing a head gasket.



I don’t know about the tree you’re barking up being right or wrong but there should be something measurable happening when the head gasket or cracks are influencing idle that much. In fact I decided to never, ever worry about that possibility because then it’s engine out and do everything no matter what.
That’s why you should concentrate your efforts around the easier possibilities first, these being air, fuel and ignition in the simpler stages. Maybe you’re not seeing the forest because of the trees, if that saying works in English-
Try not to worry. Those engines are tough, and when they decide to go wrong they supposedly give notice. Mostly, at least, so given your current situation I‘d make a decision between driving it and having a little „fun“ with the easy things, or invest a hefty sum of time and money for lasting peace afterwards. I can assure you that depending on conditions it is always a great experience and something‘s always off anyways. It’s yours, enjoy it! Start at the simpler, cheaper things or it will bite back. Imagine doing bearings, reboring, valves, skimming and gaskets to find out your dizzy cap was cracked :slightly_smiling_face:

Have you checked valve clearances?

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I thought that might be okay (ish) given his compression results? And if too tight that would not change overly much or fail the test…

Caused low compression in my case

@Foggyoo That should be taken care of, no? Neither low nor uneven.
Comparing his values to my 9:1 engine he’s doing okay I think. See

Nick your sparkplugs look? Maybe use some platiums. Plugs are cheap has chips! I did have a rough idle on my 86 adjusting the throttle plate and that hard to get hex head adjustment screw. Also check your gap on your dizzy just because there are no points it could be a misaligned pickup there is a section in the Haynes manual on this procedure. Also could be your injectors and the seals for them. Could be anything. It’s hard to pinpoint. Your just going to have to find the culprit. But will be worth it. Good luck!


My xj6 is currently not drivable…but my Triumph Tr6 had a bit of a miss in the engine after a complete professional rebuild and run in time. It was enough that it annoyed me… I originally had it all spiffed up with all the plug wires routed and zip tied together. Looked good. I happened upon a aged mechanic friend who suggested I had bleed over across the plug wires of consecutive firing cylinders. The simple fix was to cut the zip ties and let the wires hang free. I don’t know if it applies to your scenario…but maybe food for thought.




How did you verify loss of coolant, Nick? The procedure is to check coolant levels over time, with the engine cold - and without adding coolant. Ie; if you fill the expansion tank full it will eject 1 - pints when run up to temp…

A common cause of coolant loss is simply loose hose clamps. As engine heats up pressure increases and a hose leaks; undetectable unless watched during warm-up. Or indeed pressurise the cooling system to 15+ psi while watching for leaks - and of course loss of pressure…

You seem to have checked, and eliminated. internal leaks - so the leak seems to be external. Check further for external leaks - including water pump seal and a heater core, and indeed externally around the head gasket. Locating external leaks is a patient and sometimes long procedure.

And I agree with David; 'stay away from head gasket sealer - they have side effects, and is basically sticking plaster on a broken leg…

Elaborate on the uneven idle, which is, as David says, an xk byword - a sort of cyclic misfire; 10 - 30 rpms at 800 rpms is not ‘abnormal’. Irritating - but few has managed to get the engine dead smooth, it takes some considerable effort…:slight_smile:

If the compression test shows ‘even’ between cylinders it does not cause uneven idle, and I fully concur with David; the difference between ‘dry’ and ‘wet’ pressures are immaterial in this respect.

The leak-down test supplements the compression tests - both are to detect valve and bore leaks. Explain how you arrived a the 15%, 40% and 30% leakdown - percentages does not ring that many bells.

Point here is that perceptible valve leaks, or indeed bore wear, should show up in the ‘dry’ and ‘wet’ compression test as different pressure in the cylinders. Obviously, with the ‘wet’ test reading considerably higher; there is noticeable bore/ring wear; - but it does not affect idle evenness, if compression is even between cylinders…

However, I’m a bit suspicious - it’s most unusual for pressures to be dead even between cylinders; a flat 150/180 psi on all cylinders is ‘unusual’ with a ‘used’ engine. Rerun the compression tests with the engine cold; it is just as revealing, and more ‘comfortable’ - paying attention to precise compression readings. And spin the engine with throttle wide open, of course…

Not that there is any indication that uneven compression is causing your problems. So other avenues should be explored as per David - the misfire/idle problem may well relate to ignition. Have you inspected/cleaned the dist lid for cracks and cleanliness - and checked the rotor for burn or axle wobble?

And indeed, uneven injection, clogged/malfunctioning injectors will fully explain misfiring and uneven idle.

So far; the engine itself seems innocent, but checking valve timing could be a next step…

xj6 85 Sov Europe (UK/NZ)

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I would do the dizzy cap and check the shaft and pickup, also have a good look ar the ignition wiring in the dark - they might be sparking or have crosstalk; new ones are very cheap - I paid £16 for a full set.
Then I’d search for vacuum leaks (injector seals are in fact a good idea, therefore go through the injectors), then go for a long drive.

The coolant loss is unusual if it is actually more than the „pint“, but maybe the dye would be a future step. But that’s not a issue for now, as it is just a slight leak. Over time external loss can be seen via the residue. Plugs are black magic, but the proper champions or ngks usually fo the job best.- cam timing and checking valve clearances can come later but have been indicated as problematic before. If you are comfortable with valve covers off and checking have a go. Supposedly the upper chain stretches enough so that the cams are aligned differently and that is most noticeable at idle (the reason why old racecars tend to have a rough idle is the same). If there is bleeding between cylinders 2&3 as you suggested, coolant shouldn’t be involved for now, but it could be (and should be) kept in mind. It will tell you when the time has come.


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Hi everyone, thanks all the input, it’s really appreciated. Here are the photos of the spark plugs.


sorry, I’m a bit late on this thread! If I understand you correctly your car is running fine except for that idle problem - well, that’s just great! You’re probably in a much better position than many other forum members …

Seriously, these cars don’t like not to be used. You don’t mention how much you drive it, but try to keep the cat in its natural habitat - go out on the highways and fast lanes and run through some two or three tanks of fresh gas and I bet it’ll get better each 100 miles. No Italian tune-up though, just slow warming up and then increasing output should do away with the carbon build up.

Of course, you’ll already have taken care to get fresh brake liquid, coolant and oil. In your case I’d put in new plugs as well and check the plugs after half an hour of peaceful running at 1500 to 2500 RPM. Your plugs # 2, 3 and 5 look like the cylinders are running lean. Maybe you also check whether there is an air leak at the intake. If one or more cylinders are leaning out they won’t contribute much to the running of the engine. Then it is no wonder it idles roughly. With higher RPMs the inline six will hide such faults.

If you say you’re losing a moderate amount of water, are you sure you’re losing any at all? If the water is not in the oil and you can’t see any white smoke (aka steam) at the exhaust, it is lost externally. There may be no puddles, but sometimes the water hose joints aren’t fully tight and the small amounts pressed out are evaporating right away. Or you might have a leak in the heating circuit with water lost inside the cabin. Any sweet smell of anti freeze?

Now, as David wrote - keep your cool and enjoy your wonderful car. At the stage you describe chances are you do more harm upsetting things by good intentions than really help. One thing you might keep on your to do list though is to measure valve clearances to avoid overly tight valves ($$$).

Good luck


75 XJ6L 4.2 auto (UK spec)

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Hi Frank, thanks for the wealth of information.

I topped up the coolant a couple of inches below the top of the metal overflow/expansion tank. After some time i.e. months (that’s how gradual the loss is) the coolant light goes on the dash and i top it up again at the expansion tank.

Interesting, I will use this method to check it over the next week. It’s such a slow coolant loss I’m not sure how much it’s going to go down.

Awhile ago i was at the petrol station and i noticed green coolant leaking out onto the concrete from a loose hose. Anyway i topped it up and drove it back to my place and replaced every coolant hose and clamps with new ones. Since then i haven’t noticed any coolant leaks on the ground or at the the heater core when i lifted the carpet up.

[quote=“Frank_Andersen, post:11, topic:369902”]
You seem to have checked, and eliminated. internal leaks - so the leak seems to be external. Check further for external leaks - including water pump seal and a heater core, and indeed externally around the head gasket. Locating external leaks is a patient and sometimes long procedure.

This is where i’m confused, the spark plugs shouldn’t have any corrosion on them is that correct ? I thought the slight rust on my spark plugs indicate internal cylinder leaks. However, if that is the case shouldn’t the compression test results be worse and different between cylinders. Mine are all uniform at 150 psi dry and 180 psi wet.

Good advice which i’ll follow. My worse case scenario is i try it and it doesn’t work but clogs up the radiator and heater core with the sealant and ruins the water pump etc. The Jaguar mechanic is very confident it will work but i’m not as optimistic.

The idle (misfire) will be a lot worse sometimes than other times. (but never is the idle smooth) It’s never consistently the same.

This is very promising as it means i might be able to get this thing running smoother. I’m confused as to why a independent jaguar specialist says this is the cause. The guy is the owner and actually talked me out of booking the car with him. He said the 30 psi difference between wet and dry meant it it had worn pistons rings and wasn’t possible to get it running smoother. He said the engine had a lot of blowby past the piston rings. The increase pressure caused by the worn piston rings was causing my cam cover gaskets to leak. This was the same diagnosis by a specialist jaguar engine rebuilder when i told him the engine ran better and smoother at higher freeway rpms than at idle. The differing opinions is baffling.

I’m going to have to get someone else to redo the test as i have may have done it incorrectly. For starters I had to do it on a cold engine where as some people call for it to be done on a warm engine. I have lower back problems so i can’t spend too long working on it. I definitely got the worst result on cylinder 3 with 40 % leakage and cylinder 2 with 30 percent leakage. The other cylinders were 15 percent leakage.

You may be right, i’m using an old compression tester so I’m not sure how accurate it is. I’ll buy a new one and repeat the test with the engine cold because i did it when it was at normal operating temperature the first time. (open throttle and spark and fuel disabled, strong battery, six engine revolutions etc)

I’ve checked the leads, rotor, cap, etc and they appear to be fine. I didn’t want to start throwing parts at it as the slight rust on the spark plugs makes me think that it’s the head gasket. The good uniform compression test results are confusing me. The compression tests results shouldn’t be so uniform between cylinders with spark plugs that look like that.

The vacuum gauge reading at idle is indicating that it could be wrong cam timing (or worn piston rings) so that will be the next step

Thanks for your detailed help Frank

You have done everything correctly. Just drive it, but do what Jochen wrote - just preventative maintenance with the clearances and see why it is slightly lean, which I second. And don’t worry, drive. The coolant loss doesn’t seem grave.
Piston rings, so what. Cam timing doesn’t hurt the engine if it plays any role; it’s just one of many suggestions. The further you take the car the better it’ll run, trust me. Maybe do an emissions test and set the CO and find vacuum leaks if present - propane torch or brake cleaner (temporary stains, can be wiped off) to find out. No need for further compression testing, you’ve done very well and close values per test indicate that your equipment can be trusted!

Hi Jochen, no worries, thanks for the help.

The engine operates fantastic at higher rpm’s at freeway/highway speeds. It just runs terrible at idle when i’m stopped at traffic lights. Some times it has a smoother idle than other times but it’s never has a great idle. The severity of the idle shake is not consistent.

I’ve done many long country trips as i love driving the car. It’s just the rough idle at lights is annoying. I drive it every day but on shorter trips during the weekday and on the weekends i take it on long country drives.

Changed everything except the brake fluid. I’ll do that in the coming weeks.

I get a Jaguar mechanic to do the adjustments and tune it up.

I haven’t checked yet for leaks, i plan on making a vacuum leak smoke tester and test it.

The engine runs great at higher freeway speeds so it might well be a vacuum leak which is more pronounced at lower rpm.

It’s baffling, no oil water mixture, no white exhaust smoke but rusty spark plugs.

This makes sense and would explain everything but why the rusty spark plugs ?

No i lifted the carpet and checked no heater core leak. No ant freeze smell.

If the slightly rusty spark plugs is normal then my next guess would be wrong cam timings. The car has had a lot of money spent on the mechanicals, suspension etc. My friend with an xj40 was shocked how clean it looks underneath. His xj40 is covered in oil leaks. Apart from a small cam cover leak it doesn’t have any

Thanks I’m going to do as you say and just drive it and get as much enjoyment as i can get out of it. If the head gasket gets worse hopefully it won’t be for a while and i can get more country trips out of it. Like you there is no way i’m going to be doing a head gasket once it goes, too $$ and the stop leak sealer isn’t a real solution .

As coolant heats up, it expands, Nick - and excess coolant is ejected. After a couple of cycles after a refill, the level stabilizes - and any coolant added will again be ejected…

The trick is to note the level over days, without adding coolant - to notice a gradual lowering of levels that will indicating a slow leak. Then, and only then, add coolant to the original level to get a measure of the loss…

The coolant light is not entirely reliable - but when it lights it is indeed pertinent to check coolant levels…:slight_smile:

Rust takes time to form - and rust on spark plugs is no indication that coolant is the culprit on an engine in use. And one must distinguish between rust and other deposits on the spark plugs.

The look of your spark plugs isn’t disturbing - clean and check plug gap…

Unless everything is perfect - any imperfection can be accused of whatever symptoms that appear…:slight_smile:

Finding a real culprit is difficult, many factors may be involved - and mechanics start with obvious faults, and rectifying them. Your engine is worn, but with even compression; even pressure is exerted when running - the engine is balanced…

The xk engine at higher revs will run perfectly, even with some major faults - unless there is an ign or injection problem. Your remark ‘engine ran better and smoother at higher freeway rpms then idle’ is suspicious. At highway speeds the xk should be dead smooth - if the idle is ‘s…’! Elaborate…

Compression testing when cold reveal as much as a hot test. The advantage with the cold test, apart from being easier, is that the temp is even throughout the test. Unless you maintain constant engine temp; a variant is introduced - and the point of the compression test is to ensure consistency throughout the test. It’s the difference between the cylinders that counts - not absolute pressures

Measuring manifold vacuum is a very relevant test, it will reveal a lot of faults - but require some interpretations. Generally, idle vacuum is an indication of engine power - ie, like state of wear.

A ‘new’ engine will show around 18" - 20" Hg, at some 800 rpms idle - as the engine wears; vacuum drops. Which may also be caused by incorrect ign timing. The vacuum should in any case be steady during the test; vacuum varying/oscillating requires some interpretations, but generally indicates valving, ignition or fuelling faults to be explored…

All this said; I agree with others just drive the car, a lot, with no worry. There are no indications that you have any serious problems - wear in itself is no deterrent.

The uneven idle, and loss of coolant can be pursued at some leisure - but report on highway driving anomalies. Any other lurking fault may eventually manifest itself, to be properly attended to. Needle in haystack work is seldom rewarding - unless you desperately need a needle…:slight_smile:

xj6 85 Sov Europe (UK/NZ)


I wonder if the idle is being exacerbated by a duff engine mount?