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I had problems with rough idle on my 95 XJR last summer, and after different attempt fixing it I suddenly got the idea that this can be caused by some cracks in the exhaust manifolds.
So my question is: Can cracks in the manifolds cause rough idle?
A crack in the manifold or loose donut gaskets can draw in fresh O2 rich outside air as the exhaust pulses going down the pipes suck in air , this diminishes at higher RPMs
The fresh air bleeding in will bias the mixture command ( STFT ) to add fuel / go rich to consume the O2 , this to the extent the rich mixture will go off scale , + 25 4 % spitting black fuel specks out exhaust pipes on wall
Easiest thing is to ensure the 4 downpipe stud nuts are tight as some have been found
loose , 15 mm an extensions from under only , one has very low wall clearance hence just slightly place the square drive in the socket as will need about a 15 degree angle
In some cases may need to replace donut gasket that Vee had a number for , 60 MM downpipe OD
Rich can also come from vacuum leaks as mine had a hose missing attached to the bottom of intake manifold cylinder 2 or 3 , this will also diminish at higher RPMs
The XJR does have a tough time honing in the idle that those who have would know the XJR details
Last edited by Parker 7; Feb 11, 2026 at 03:45 PM.
If removing exhaust manifolds , on installation I found myself damaging the flair ( hitting manifolds down pointing studs ) on end of the downpipe ( donut gasket seat ) as you draw it upwards to clamp down So would have to keep close observation as things draw together and adjust accordingly
I cut the web off the downpipes as this probably only serves the purpose of not being damaged ( 2 pipes spread ) as they move around factory
A built a lever to pull up downpipes for better ease out of 3 foot X 1 inch X 1 inch angle iron
Last edited by Parker 7; Feb 13, 2026 at 09:27 PM.
The cracks are a bit bad. You can hear it easily when the engine runs. This is what I have changed: 2 fuelpumps, 2 relays for fuelpumps, fuelfilter, fuel pressure valve, 6 fuelinjectors, 6 coils, 6 sparkplugs, 2 oxygensensors, MAF-sensor. And now the ECU is in for service. So it's not much more to change. Then I got the idea that the cracks can give the oxygensensors wrong signals. I was going to change the coolant sensor, but got one with a wrong contact. I am going to give up the car if it's impossible to get it right. I don't want to, because I have welded all the rust, and changed the sagging roof lining, restored the rims with brand new tires + a lot more, but it's no fun to drive a car who is not working properly.
Thank's for the answer anyway.
Coolant Temp Sensor is a critical part of the Jaguar fueling map. Do not underestimate it's importance. If you had an OBD2 reader, you would be able to see what temperature the ECU thinks the engine is at. It is also a very cheap part, and very easy to replace.
Another item that used to be reported often as causing havoc is the throttle position sensor. Even when bench testing successfully.
Lastly, I wonder if a flakey crank position sensor could cause this. Again, it's a cheap part and easy to replace. OBD2 reader won't help diagnose this.
You seem to have covered everything else. After that, it's just cleaning up ground points.
I think I am going to buy a coolant temp sensor. I also changed the throttle position sensor and EGR-valve a few years ago, and I changed the crank position sensor last summer. The throttle body is a nightmare to remove from a XJR. Is the X300 compitable with OBD2? I have tried my OBD2 reader, but no car was found. I heard that those cars requires OBD1, but I am not shure. I think the last thing I didn't try is the cracked exhaust manifolds, som maybe I will search for some. It's a good idea to change them eaven if it not will solve my main problem. Thanks for helpful tips.
The MAF sensor for the supercharged vs. the normally aspirated is a different part number but looks the same . Vee had a suggestion to slightly twist the connector pins / blades to give better contact , At correct idle the return wire from the MAF will be around 1.2 volts DC climbing toward 5 on throttle up
The X300 is OBD 2 in the earliest design / integration / try so not the most reliable
The correct titania based O2 sensors , clean installation with no anti - seize on sensor tip , cleaned grounds serving the O2 sensors , external ground wire on the ECU mount bolt ?
There is 1 fuse for all 2 or 4 sensors to heat them up to read correctly ( sensor physics ) right engine bay fuse box either fuse # 10 , 12 , 14 , or 16 but only 1
The manifolds are not sized different but have different holes for items of smog pump and EGR pipes for different regions of world
New coils have been found bad as a rule so maybe swap around 1 and watch codes
Last edited by Parker 7; Feb 13, 2026 at 09:33 PM.
Reading this thread made me cringe a little. It all sounds so familiar. I had a '95 XJR as my daily driver for many years. I nicknamed it "The tormenter"
Originally Posted by NorwegianXJR
I think I am going to buy a coolant temp sensor. I also changed the throttle position sensor and EGR-valve a few years ago, and I changed the crank position sensor last summer. The throttle body is a nightmare to remove from a XJR. Is the X300 compitable with OBD2? I have tried my OBD2 reader, but no car was found. I heard that those cars requires OBD1, but I am not shure. I think the last thing I didn't try is the cracked exhaust manifolds, som maybe I will search for some. It's a good idea to change them eaven if it not will solve my main problem. Thanks for helpful tips.
Not sure about ROW cars but the USA-market 1995 cars were an early iteration of OBDII which wasn't fully developed, as mentioned. Non-sensical live data, phantom codes, reluctance to set codes....all common complaints and all adding to the confusion.
I replaced cracked manifolds on mine (twice, I think) but doing so never solved any of the problems I was having. Back then they were easier to find. But, yes, replace them on general principles if you can find good ones.
Any corrosion at the ECU pins? Look very carefully.
Agreed on revisiting the ignition coils. I carried spares at all times and remember doing by-the-roadside swap-outs.
"Rough Idle". Is it rough at all times? And how rough is it? Mine was never 100% glass smooth at all times, even when I bought the car with just 30k miles. An occasional slight tremble was something I just learned to live with.
if I understand you correct, was this problem caused by the routing of the manifolds towards the exhaust pipes? And is it important to route the manifold towards the pipes correct to avoid damage? I am norwegian, so I have to ask to get it right.
The idle is almost constant, but when the engine is cold, it starts after about one minute, You can see and feel the engine shaking irregulary. When the throttle is open, it stops, but I think I could feel a bit uneaven enginge when I drowe it. The car is now stored for the winter, so it's not easy to check things right now. I don't have any pictures of the cracks, but they goes all the way from top to bottom in the middle of the manifolds. You can easy here the blowing noise when engine is running, and typical loud engine, as in a broken exhaust pipe. It can also wery welI be a broken doughnutseal. I have some manifolds without cracks in order from ebay right now, so I will change them anyway. Is there anything that i have to be careful about when I replace the manifolds?
On installation the manifolds themselves have to be bolted to head first and then the downpipes must be drawn upward to connect to the manifolds , the side clearance makes this the only way . The damage I made is drawing the downpipes upwards
Checking the 4 studs pointing down that the nuts are tight easiest first check
Exhaust manifold crack. Rough idle
Is the X300 compitable with OBD2? I have tried my OBD2 reader, but no car was found. I heard that those cars requires OBD1, but I am not shure. I think the last thing I didn't try is the cracked exhaust manifolds, som maybe I will search for some. It's a good idea to change them eaven if it not will solve my main problem. Thanks for helpful tips. 
