so a 4 months ago i bought a 97 xk8 with 79k on it.i got it for a good price because it has a very rough idle.and when i mean rough it idles and loaps more than my mustang track car with a big cam in it...of awhile now i have been on the hunt for and vacume leaks i can find.nothing has helped..changed the fuel filter and check the coils,nothing..i get no codes but today i started using my scanner to monitor it..my short term trim bank 1 is 19.5%.long term trim bank 1 is 14.8. short term bank 2 19.5 and long term bank 2 12.5 to 18%..i ohm the injectors and they all are within good spec..i dont have any vacume leaks,i have basically replaced every hose under the hood trying to eliminate vacume leaks..cleaned and had the maf tested and its good..what do you guy recommend? 2 things are kinda making me think though.when i start the car cold or hot it revs up to anywhere from 1500 to 1900 rpm for 3 seconds only then drops to 700 rpms and idles rough.it doesnt stay high like every other vehicle i have then slowly drop..its up then 3 seconds on the dot it drops like a rock and idles rough as all hell.just something that doesnt seem right...car has great power and drives great and smooth but idles like it has a cam..any thing i should be checking???

 

Driver side valve cover, remove that hose on front of it, unclog the inside of the hole with a drill bit or paper clip on the nipple on valve cover that is blocked with black carbon.

Then clean and refresh the throttle body,and check the ground wire on the passenger side cat up front.If its green its toast-poor chassis ground.

 

+1 on above.

If this does not resolve the problem, your timing chain may have skipped a tooth due to a broken secondary tensioner. Even if it has not, the secondary tensioners need to be upgraded ASAP to avoid engine damage. Search for the many discussions here on this.

 

i did the part throttle orafice when i got the car but i will check the ground..i also replaced the tensioners with gen 3 right away and have checked the chain hasnt jumped a few times to rule that out...i searched and took care of anything and everything i could find on this site for the first 4 months before i posted anything...i think i have when through all stickys and taken car of most of the regularly talked about problems within the first few months..i search extensively before i post so i dont make threads that have been

 

Er, those are quite big LTFTs. What are they parked at (say) 2500rpm? If they head much lower then go air leak hunting.

 

ive been monitoring the o2 sensors and bank 1 pre cat is only showing .045 volts and fluctuats alot..bank 2 pre cat is .900 volts and steady and seems to be the only one working..bank 1 post cat around .040 volts and bank 2 post cat o2 is about .050 volts and goes to 0 some times...do you think that means there bad and only one pre cat is working???that could mess with my trims and cause a rough idle i would think???im changing both pre cat 4 wire o2 with OE replacments tomorrow and we will see what happens...are the post cat 2 wire o2s really critical to fuel trims???

 

Do the readings I posted rather than part-swapping, which gets costly very fast. Suppose it turns out you DO have an air leak, changing O2s isn't going to be a fix.

 

O2s changed and huge improvment.idle still a little lumpy.i noticed the vacume hose that goes to the can looking thing on top of the throttle body was missing.i found peices of it down by the cats and melted.i cant find where it even comes from.can someone tell me where that vacume line comes from so I can trace it.

 

The vacuum line goes through the bulkhead where the ECU lives and into the right side fender well. If you can splice it before the fender well that would be handy, otherwise you will have to remove the liner. It goes to the vacuum reservoirs for the cruise control.
The other line coming out of the bulkhead connects to a tee below the right bank head and is the source of vacuum.
This hose being missing means you have had a air leak and no cruise control.

 

No wonder it ran badly with such a vacuum leak.

 

After I get home today and repair it I will update you guys

 

ok fixed all the vacume lines and it didnt help one bit..i know im getting unmetered air somewhere so i went about a smoke test..smoke test showed a leak where the plastic upper plenum bolts the the metal base intake manifold...so i smoke tested everywhere there was a gasket and all that turns up is the front passenger side runner is leaking sucking in smoke at idle..now my question is i see gaskets for sale that are individual gaskets that go inbetween the plastic and metal..and gaskets that are one piece with all four runners...so do i need the one piece gaskets,1 per side or the individual gaskets requireing 8 total to do the job????where almost there guys!!!!!

 

Look the manifold over real good i found that i had a cracked manifold and had to replace it. This was after i replaced the gaskets.

 

i will go over it with an eagle eye...thanks for the heads up

 

This thread has a good number of pictures of the tear down and repair of an intake leak.

https://www.jaguarforums.com/forum/x...ir-leak-87618/