hey guys and gals first of all i like to apologize for posting so much lately; just trying to gather more information.

as you know from my other thread my car had a coolant leak (most likely a valley heater hose [under the supercharger]). also that my car ran at higher temperatures 228F. therefore i want to test the head gaskets before proceeding to repair the hose.

i would like to know what is the proper way to perform a compression test on a 01 XJR...

i have never performed one before; but from my understanding are these the correct steps??:

1)disable fuel pumps from the trunk fuses correct?
2)do i remove all coil packs & spark plugs at once ....or do i remove one at a time and test each one at a time while others are still attached.
3)check the readings for each piston.
repeat for every piston and ill post my results here.


am i missing any steps? thank you

 

Step one...correct
Then unplug and remove coil packs and plugs from all cylinders so the motor moves freely. Insert and thread the tool into cyl 1 and preferably (but not required) have your assistant roll the motor over 5 or 6 times. You'll get a reading, write it down, then go to cylinder 2 and repeat. Once your done with all 8, compare your findings.

 

. . . and stick the handle end of a screw driver into the throttle body to hold the throttle plate open. With the throttle plate closed there will be a vacuum preventing the cylinders from reaching their max compression reading.

The engine should be warm. Run it up to operating temperature and then start removing the coil packs and plugs. By the time you get all off it will be just right for the compression test.

. . . and do it at least twice. You do not have to reheat the engine.

 

Oh, and when I say cylinder 1,2 etc....I mean the forward bank, which is 1,3,5,7 and the backset bank 2,4,6,8. This could be important if you need to ask us or someone local about those cylinders.

Hey test point, do you run your car up the Dragons Tail on the Blue Ridge? I have a friend who has a cabin on the mountain above Ellijay and another buddy who lives in Ball Ground.

 

His cylinders are numbered 1 front, 2, 3, 4 rear right bank; 5 front, 6, 7, 8 rear left bank.

Seems obvious, but buy or borrow a compression gauge with sufficient reach for deep plug wells.

 

Weeeell, slap me silly, thanks Jhartz, and here's the cylinder orders for V8's over the years, http://www.jagrepair.com/images/Trai..._numbering.pdf

 

You are welcome!

 

Been through the Dragons Tail several times on the motorcycle but not the XK8. This is the greatest place in the world to live if you like twisty roads. The road to Lake Nantahala is even better than the Dragon. Many have been through the Nantahala Gorge but few have seen the lake a couple thousand feet above.

 

okay guys something is wrong....currently in the process of doing a compression check; okay steps i have taken....

1) took out the fuel pump relays in the trunk (two of them since its an xjr; checked the manual correct ones removed)

2)took off the coil covers to remove ALL of the coil packs and spark plugs

3) removed MAF sensor and intake assembly.

4) attached the compression gauge to the front cylinder correctly, and had my assistant sit in; i re attached the battery

5) assistant turned the key to the cranking position and i got a "restricted performance" message and no crank at all...

what did we do wrong? should i re attach everything and do one at a time? thanks

 

i figured it out; it was a ground sensor on the passenger side (US) above the coil over...the grey connector with one wire...engine is cranking over with all the coils and spark plugs out ....i will post results here!....doing a cold engine crank 5 revolutions per cylinder.

 

Aggressively waiting . . . .

 

results!!!!: sorry for the wait loll

this test was done on a cold engine (no pre-warmup). 5 crank overs per cylinder and held psi for 1 min on each cylinder.

drivers bank: front to back
cyl 1) 118psi (slightly less than 120 mark)
cyl 2) 118psi (same as above)
cyl 3) 120psi on the mark
cyl 4) 120psi on the mark

passengers bank: front to back
cyl 1)120psi on the mark
cyl 2)120psi on the mark
cyl 3)117psi slightly more than 115mark
cyl 4)118psi somewhat less than 120mark


engine was cold. 5 cracks per cylinder test. made sure it held psi for at least a min. no drops during that time....

so im assuming the engine is good; safe to remove the supercharger and repair the hose.

 

Looks good....its even for the most part, I'd say your good to go there.
The only other thing I might try, and that's only if you haven't put it back together, is a little oil in each cylinder, before testing each one, to so see if you get better compression. Not saying it will, but to see if it balances better since she was cold.

 

phew good to know thanks highhorse; only reason i did the test cold was because it already has a coolant leak so i did not want to risk it. i already put everything back on. bought new spark plugs too since i thought might as well. they all had even burn; they looked old and dated. they were NGK IRs local auto store didnt have enough so i bought NGKs platinums.

now its time to remove everything and get to the hoses. i will need help with that.

 

Good move on the new plugs, iridium would have been better for the S/C, but get what you can. They are an easy fix for an engines woes, because they most likely are a symptom or the cause.
The hoses aren't as hard as you think (follow the links I gave you in the other thread), but as I stated in another thread from someone else doing those hoses. The new ones are a little thicker (if yours are original) that I got from Welsh and you may need a slightly larger clamp? I wound up using my original ones, but man they were a bear getting on. The hardest part were the connections behind the bypass neck. Putting the bypass back on and having such a rigid new short neck hose was kind of tough. Be sure to clean up the necks, you won't regret it. I wound up using some lithium to help get a couple on even as clean as I got them. We can discuss that in your other thread though.