ok so i 94 xj12 and i fix some things on it ,when it started ,it ran fine but when i give it gas it takes a second or two to get up in rpm's , so i found out only one side of the motor was firing ,exhaust was only coming out one side ,the side that did fire, i took off the exuast at the cat and it was pluged bad you could tell it had heated to a point that it wraped the metal but i un pulged it and its still wouldn't fire on that side , so did a few things with the vacum line that gos to the fuel rail and got the other side to fire but it still is lacking power and dosen't rev like it should ????? is the motor f**ked from the cat being pluged that bad
,it runs fine on the one and when i get the other side to fire but acceleration not there

 

I would remove the cat from the exhaust manifold on the plugged side. The cat is probbably garbage if it was built up that bad. Or at least unscrew it from the manifold so you have a large leak. Take the car for a spin and see if you have returning power and firing.

If you are still having power loss I am guessing the heat build up probably burned valve(s). I would remove the cat and drive the car and see if it improves. Otherwise a compression test will tell you if you are having mechanical problems.

I am not sure if I understand that you were sure none of the cyls on that side were firing. Sounds like the clogged cat was your problem.

 

i punched out the cat and it is firing at times seems to be getting better i'll hope for the best when i get it back to gether
and the cat was so bad i punched up melted pieces

 

yeah there is some wrong on that side of the motor ,i can get it to fire only by holding the throttle above idle, then it cuts out when i rev it, but will at time fire agian when holding it at higher rpms , and after i get it to fire and put it back at idle it misses or only some cylinders fire at times ,
then last it had back fired once after revving it ,but i think thats from the gas not burning in the cylinders then one fires and burns all the gas in the exhaust
so you think this sounds like burned valves ????and can i dive it tell the motor gives out then repair it ???

 

Well I am sure there is someone else here that can chime in with better advice but I think it is probably a valve issue.

I think this because you get intermittent firing that you describe as erratic and acts better at higher rpms. I would guess burned valves/valve seats because of the obvious extreme heat that was in the area. Have you taken the plugs out and looked at them? Do they look oily or very white?

You can still drive the car but I have read that for long periods it can lead to a 'glossing of the piston walls' which will cause poor compression and require a whole lot more work. If you drive it untill the engine dies then you may risk needing to at least have your block remachined and have new piston rings installed.

If I where you I would check e-bay, there are often refurbished complete heads for sale. Even a complete used one from e-bay or craigs would work.

But first I would spend a few hours and do a compression test, check lifter clearances, make sure you are getting good clean spark, all injectors are working, then go from there.

 

yeah the spark plugs were black and gummed up so i changed them out ,
1)so do i have to take out the motor to change the head?
2)and do i have to change both of the heads ?
3)how do you check valve clearance ?
4)and check injectors ?
thanks for your help

 

one more thing its a 94 xj 12 do i have to get a head from the same year and model of car ?

 

ok i cant find any heads out there so know any where that can rebuild it ?i cant find any thing

 

I am unfamiliar with the v12 engine but I do not think the engine needs to be removed to remove the head(s).

You might not need to replace either of the heads, I am sorry if I sounded like you had to. If the head did infact warp then a machine shop can usually machine it out if it is not an extreme case. If the head cracked then you would probably want to find a replacement. You should not need to replace both heads if only one were to be bad.

However I would not assume I had a warped head, it is just something that can happen due to high heat. I would guess it is not your problem. My guess is you have one or more burned valves or valve seats.

However it could just be that some of the valves are out of adjustment.

To check clearances you need to first find the firing order of your engine. (jag-lovers probably has a guide for this as well.) Then you will set the number one cylinder to TDC.

After that you can take the valve covers off to expose the cam shafts. [You should replace the gaskets for the valve cover upon reinstallation or you may leak oil]. There is one camshaft per side. You will notice the camshaft lobes. There will be 2 lobes for each cylinder.

One of the lobes opens the intake valve and one of them the exhaust. You will notice that the lobes on the number 1 cylinder at TDC will both have their elongated parts sticking outward opposite of eachother. In other words neither lobe is pressing on the valve tappet so both valves are closed. You will use a feeler guage to measure the distance between the camshaft lobe and the tappet while that cylinder is at TDC. Then you will rotate the engine(by hand) to bring cylinder number 2 to TDC and you will then measure its lobes with the feeler guage. Continue for all 12...

If anything looked scored or otherwise deformed that is a sign of a problem.

Although I do not think there is a Haynes manual for the v12 model I recomend you pick up the Haynes for the xj40 anyways before you do any real work. You can gain a lot of the technical ideas from the book and then apply them to your car.

Injectors can be listened to with a mechanics stethoscope or by putting a bar up against the injector and listening to the bar. The idea is to hear the injector clicking as it opens and closes to inject fuel. The bar will help amplify the sound comming from each injector. You should hear a fairly fast paced ticking if the injector is operating.


If the plugs were black and gummed up then there may be oil entering the cylinder. Is there any noted oil loss? What do the new plugs look like?

Did you do a compression test? This is an easy test. If you have good compression then your problem may not be in the valvetrain or head at all.

It might be worth your wile and time to take it to a shop and have them look at it. They will be able to perform a leak down test quite quickly and note other signs that will point to the probable cause as well.

Again I don't know what the specific numbers are for valve clearance, compression pressures, etc. However they can probably be found here without much difficulty. ImageShack® - Online Photo and Video Hosting is a photo of the camshaft assembly for the xj40 v12 to help you get an idea.

 

i dont know about oil loss haven't ran it enough my self ,the new plugs were a slight clear brown ,and i haven't done a compression test yet thats the next thing on my list to do

 

the side of the motor that was giving me all the problems has no spark or very little at times and when i switch one of the wires from the side of the motor that runs good it gets better but its dosen't spark like it should looks really weak compared to the other side

??

 

There could be other problems in the engine, but the engine is most likley to be running on one side only because of a damaged/worn rotor arm in the distributor only firing one bank of cylinders. This is sometimes called "Marelli burn" and is an inherent design flaw in the Marelli ignition of the V12 cars.

The rotor arm has two sides on the V12; one fires bank 1, the other fires bank 2.

If one side of the rotor arm becomes worn out and stops producing a spark at the plugs on that bank of cylinders, the other can still continue to fire the cylinders on the other bank as normal, thus one half of the engine continues to run. As the engine runs, the other non-running bank dumps the fuel down into the exhaust causing a fire. It goes without saying, but don't run the engine like this!!

The rotor can fail and cause this problem in the absence of any other issues, so it could be that there are no other engine problems in your car. If the fire has not damaged anything important under the bonnet (hood) a new distributor cap and rotor arm could solve the problem, along with a new catalyst, at least on that side.

David

 

db in uk is correct that the rotors can fail.
Replace the rotor arm and see if it runs normally.
If it still runs only on one bank, swap the ignition coils and see if it runs on the other now.

I have a box full of burnt out coils and rotor arms for the V12 engines. It is a common problem.

good luck

bob gauff

 

thanks
ok well just bought a cap , rotor , and wires
now all i can do is hope for the best

 

today i got my cap rotor and wires i installed them and when i went to start the car same thing only half the motor was running so i thought i should try and switch the coils sense it the last thing i can try before i give up now all switched i start the car and the side that didnt run was running and the side that was now it wasnt so the A coil was bad now im off to get a new one

what a good day thanks every one for your help couldn't have done it with out you :icon_chickendance ::ico n_chickendance::ic on_banana:

 

Wow glad it was just a simple electrical issue. And we were talking about removing heads....

I is uually something like that. Simple but elusive problem. Have fun in that v12

 

fun oh i will !!!
i still cant believe how easy it is causes of the damage it did to the exhaust and cat
thanks for the help

 

so i installed the new coil and the motor fired on all 12 cylinders what a great sound but it has a high idle now

 

To try to cure the high idle try cleaning the mass airflow sensor and check your idle adjustment on the throttle. It may have been previously adjusted to compensate for the failing ignition.

 

where is the mass air flow sensor and the idle screw ?