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The intake manifold gasket for the AJ16 engine should be metal (mine was).
Genuine gasket i have ordered long before is paper and green. I can't say for sure but it looks like nobody touched the intake manifold on my car before and i see the gasket there is green too. Also i've used two starting fluid cans spraying around the intake ports cold and hot and car doesn't react on it. Although it pretty much reacts with rpm raise if i spray it into the vacuum inlet on the intake manifold. So for now it looks for me like i don't have a problem with the intake manifold gasket but probably will replace anyway.
Also looking into TSB 05.1-36 i see "Due to individual driving patterns, along with the engine design characteristics, the occasional idle dip can occur. This is considered normal."
Also looking into TSB 05.1-36 i see "Due to individual driving patterns, along with the engine design characteristics, the occasional idle dip can occur. This is considered normal."
I had almost the same problem some years ago on my XJR. Stopped some times in idle speed. After changing throttle body, potmeter and the small valve, it was just the same. The solution, after a lot of different testings, it was bad connections and components in the ECU. After refurbishing that, it was great.
Some weeks ago my car started to go very rough in idle speed. Compression test shows that one cylinder had just 3 in compression. Caused by a valve who was not working properly. A workshop fixed it without removing the cylinderhead. Now i have a problem with vibrating engine when it idles. I will try to change the MAF-sensor, and hopefully it will work.
Last edited by NorwegianXJR; Oct 5, 2025 at 06:01 AM.
What can i say people... I was yesterday on a birthday party and the guy hosting it has a 96 XJ6 pretty much beaten but running well. It got nothing even close like rough idle and when i asked for a test drive i've found how much more his car responsive compare to mine. And he said mine feels like 10 people inside. So today I've decided to start from very beginning and did the compression test myself as it was done in a spring by a shop. They had gladly charged me said i have like 160 at every cylinder and its okay for the 75000 miles engine. Today at 77000 i've found it barely has 40 at cylinders 5 and 6. So... Looks like it's not a big deal to take the head out as it looks like a valve thing. And used "tested" head doesn't look expensive. Couldn't find my bore camera so order another one. Will try to have a look through plug holes.
Interesting that spark plugs look pretty much same with light brown color and i didn't receive any cylinder specific codes. Probably because of two in a bank low it was 0300 and 0314 for this and i was missing her scream about the engine failure.
hopefully can fix easy enough.
mine compression tested well on all cyl recently.
i don't have any other car i can compare with but would love to.
mine does only begin to idle rough once warm.
is yours the same?
here is a video.
it doesn't move too much and the needle makes it look smoother than what it feels.
you feel it being jerky and lumpy when it misses which does not appear on the video.
couple videos actually. one shows it slightly more than the other.
Since i am not driving it much i always thought it only runs rough when hot as every misses on cold i was counting as "it's just old and cold" and it was felt pretty much smooth on driving. Moreover it was underheated long time before i've tried to make an oxygen sensor orientation and found it doesn't heat enough to start the test. So replaced the sensor which didn't help. Then replaces the thermostat which helped and test passed. Since then in summer i've driven it much less and now i've found that it while it heats faster the rough idle comes sooner too and last couple days i feel it misses when idlind on D. Assuming it has good compression i've focused on other possible reason. Fixed vacuum leaks, manifolds to pipe exhaust leaks, balancer, plugs, coils, switched oxygen sensors, etc, etc. Anyway camera shows valves are round and undamaged, but 5 and 6 intakes have lots of deposits on seating surfaces. Decided to go with used head. Seller said it is "tested". At least looks clean.
With no EGR most likely you will need only to remove the intake elbow with the rubber joint. Everything else might be left untouched as there is enough slack to slide the complete assembly out to reach the gasket. You probably will need to somehow support or hold it while you will be cleaning the gasket residues but from what i've seen on the related topics and videos the old gasket peels off easy. And i still think there are not a lot of possibilities for intake gasket to leak unless the manifold is loose or mating surfaces are damaged.
With no EGR most likely you will need only to remove the intake elbow with the rubber joint. Everything else might be left untouched as there is enough slack to slide the complete assembly out to reach the gasket. You probably will need to somehow support or hold it while you will be cleaning the gasket residues but from what i've seen on the related topics and videos the old gasket peels off easy. And i still think there are not a lot of possibilities for intake gasket to leak unless the manifold is loose or mating surfaces are damaged.
if i do the gasket i will also do the ******* hose.
so will be a little more difficult.
aside from internal damage the only option i think i have left is exhaust leak and i will be replacing headers in a couple months
made really good progress with this but just can’t get it right.
it was much much worse running before. Being really rich
Last edited by Spud Maat; Oct 7, 2025 at 09:21 PM.
so looks like maybe coils may have been the very last piece of the puzzle.
so i had swapped my coils from a known running car.
however they were unbranded coils.
at the time they made no difference to how the car ran so i thought nothing of it and i left them.
but after scratching my brain over and over trying to work out this instability in idle when hot i thought, "its gotta be when the coils are heating up"
so today i swapped them back.
the ones i put in are the original from the car, yellowish resin with black bar and brass where the bolts go. labelled as "made in japan"
the ones i took out are yelowish resin, no black bar, silver where the bolts go and no branding or info.
car was running very very well and idling smooth.
so i tested the coils.
all 6 of the unbranded (assumed made in china) measure around 1.4ohm on primary resistance (across each connector) (multimeter on 200ohm setting) and they read a lower than expected resistance on secondary resistance of about 1.1 megahohm (one connector to coil electrode) (multimeter on 2M ohm setting)
when i tested the made in japan coils i got the same about 1.3 ohm on primary resistance and then on secondary resistace 5 of them gave me the expected 7.5 MegaOhm whilst one of them gave me a higher reading of 11.5 mega ohms.
all working for now but it look likely one of these is on the way out.
the cheaper alternatives with lower resistance are terrible.
they must drop down to very negligible resistance when hot and causing inconsistencies with the spark.
this might be a good thing to look for when testing out coils.
Yes, the coils are difficult to bench test. There is a specific machine that was available at one time, but I think it went the way of the dinosaur.
People seem to report when they've had success with aftermarket coils, but for the most part what I've found is when you take off the boot, the cheapies have a small contact area for that spring with the rod, whereas the better ones have a much larger area.
If you people don't mind i'll post what i've found on the coils i have. Since it's the most alive thread on misfire now it might be somewhat useful here.
Car has the recent receipt on coils replacement because of misfire and bad start. They had found the oil flood in the plug dwells on cylinders 5 and 6 (sic! i now have low compression there) so replaced two coils, six plugs and valve cover gaskets.
When i was in a very beginning in my studies on this car i've taken some knowledge on coils like the resin color, bar or no bar in it, springs and rods in the plug caps, Japan made and non-Japan made, etc, etc. Inspected mine and found i have two factory (dated 15th week of 1995), two different Jaguar replacements, and two aftermarket with black resin one with the bar and yellow screw bushings and one without the bar and white bushing. Assuming i probably need a coil kit i've bought the used kit taken off from the car with more or less confirmed 45k-ish mileage (at least by VIN there is an auction record with actual mileage still aveilable). They all like mine factory but the date is in the end of 1994 (they are now installed). Misfire still there so recently when i got back to my resolving trial i've bought the aftermarket kit from Welsh (they say these ones are rather good) which are black resin, have bar and white bushings.
So here is the brief
Left to right.
1) Aftermarket installed before. Black resin, no bar, white bushings. Has FS0300 mark on the side. Primary coil is 0.8-0.9 Ohm resistance and 19.26M at secondary. Measure was done with positive into plug contact and negative on primary. Reversed is infinite resistance as most COP units have a suppression diode inside
2) Another one aftermarket installed before. Black resin, have bar. Yellow bushings. Marked as D 20230225 on the side. Primary 0.8-0.9, secondary infinite both directions (probably more than 20M as my Klein MM600 looks like have this limit despite shown as 40M in the specs but not sure). The only one that has spring loose on the rod.
3) Welsh Enterprises supplied aftermarket. Have 6 of them new cut checked only one. Black resin, have bar, white bushings. 2505-34675 number on the resin. 0.8-0.9 primary and 17.05M on the plug. Plug contact is smaller in diameter than others
4) Jaguar replacement
5) Jaguar original
and another one Jaguar replacement below
As you can see, the replacement Jaguar are different by the rods and springs from factory installed. And they are different by stickers
4 and 5 have same black sticker
and different whites - replacement has it with Jaguar logo and JG/42533 number and i was able to find only one offered for sale by this number and nothing about it in the parts catalogs
Factory has a big Lucas sticker same as many factory used takeoffs still being offered for sale
and finally other Jaguar replacement have no black "Made in Japan" sticker and the Jaguar sticker is also different with Jaguar logo and regular LHE 1510 AB part number.
Some like this are also offered on eBay and all of them don't have black sticker too. So not sure on country of origin. And the resin color is not yellowish as other Jaguar's i have.
As for the measurements all Jaguars have same 0.8-0.9 as aftermarket ones on primary and No Continuity on secondary both directions.
For me it was no difference between mixed set of coils i've had on purchase, used factory set or new aftermarket. It misfires on hot idle same on any set. Plugs are new Jaguar EBC8143 (actually are French made Champion RC12YC).
Anyway as i said above i ****ed up with compression test. Should make it myself right from the beginning. Now i have a replacement head on hands. Passed the kerosene test, flat on surface, clean and ready for replacement. Hopefully on Wednesday i will do it.
Last edited by Ya Tigor; Oct 12, 2025 at 06:50 PM.
You might have to eventually address the coils. I know they're expensive, but it may need to be done. There is no way to bench test these coils. Wait until your other work is done.
#1 and #2 look to be the worst. I'd suspect failures there. What you can do is buy a pack of cheap ones on Amazon (UF384). They might last six months, the could last a year, who knows, maybe longer. In any case, you can use 5 of them and start mixing in each of your old coils one at a time, over a day, couple of days or a week, and see which coils throw off the engine.
Primary resistances on those look good.
secondary resistances look high.
from what i can gather 7.5 Mohm is what you want.
once again bench testing is hard. You can test when cold and try en test when hot to get the best idea possible.
mine now in car all have the same made in japan as your 4 and 5 and all working well with primary at 1.3 ohm and secondary at 7.5 Mohm on 5 and 11.5 Mohm on one of them.
seems from what i can tell is too high causes failure and too low causes instability.
with the ones i had previous that tested at 1.1Mohm only on secondary i had a lot of visible interference in my reverse camera.
which is gone now i have the higher resistance coils.
so they clearly were playing havok with voltages
i will look to find some spare oem as i can when i can afford. Just to have. As i can’t trust anything aftermarket now
i've already have fresh more or less cheap aftermarket set from Welsh Enterprises (#3). They stand these are good. Also Autozone has two aftermarkets available on next day delivery or pickup in store. One is Duralast (looks pretty same to #2 but with marking numbers on resin like #3) and somewhat expensive at $130 and Uro for $40+ which looks different of anything i've seen - black resin and the bar in the center not touching the outside housing. Comes with 24-month unlimited mileage warranty BTW. And the factory set i bought from ebay is also looks working good. So i have 18 coils to swap around :-) But for now i'm 100% sure low compression rear cylinders is a main issue to address now :-)
Primary resistances on those look good.
secondary resistances look high.
from what i can gather 7.5 Mohm is what you want.
It is said that many over plug coils are used to have some switching or suppression schematics inside so primary is not constantly tapped to secondary with one end thus these coils are not supposed to have any continuity to secondary coil to measure, so there is useless and "known good replacement" is the only test here unless the primary is broken and ECU brings a code for this.
Other type is similar to common coil and the primary tapped to secondary. In that case secondary must be in Kohm range like 5 to 15 usually.
Looks like we have coils with the schematics inside and those rather big Mohm values are just a deviation in the manufacturing process although 1M i think is low here and a probable fault sign...
