Quick question

[xjluvr]

91 XJ 4.0
When I pull off #2 plug wire there is no change in rpm , pull all others there is.
I am about to replace the rod bearings in this thing, noticable rod knock, Is there another issue I should also be looking into here? It does have a semi rough idle in drive.
need to add, I do have spark to the plug on #2, plug was just changed yesterday, Today it was black a sooty.
TIA

 

[MoFo]

Is the plug in question oily? If so, you probably have no compression in that hole.

You could have a bad plug wire with very high resistance. If that is the case, you will see spark at the plug when you lay it out on the block, but it will not fire under compression. If you have an ohmmeter, check the resistance of the wire. Resistance wire should be between 5,000 and 10,000 per foot from one connector to the other.

Black sounds like too much fuel, likely an injector stuck open. If that were the only problem though it should have been firing and producing some power. How is the gas mileage?

I would check the wire, then do a compression test. If you don't find anything, you could try another injector. Just don't pay the dealer price for it. Do a search about injectors. Where to buy and what to buy has been discussed countless times here in the past.

Find the cause of this problem before you put in the rod bearings. If it is a compression problem, you could replace the piston with the rod bearings.
MoFo

 

[red91]

or you could just pick another wire from the pack,and alternate them. If it follows the wire...BINGO. If not the search continues. You could also try it with the plug as well...swapping out for another in the chain.

 

[jneary]

before buying a new injector you can switch it with one from another cylinder to rule out that problem. if you are planning to replace rod bearings because of a rod knock, you need to replace the connecting rod also. the bore of the rod gets elongated from the knocking around the crankshaft . bearings wont fix the noise. i would do a compression test on the cylinder that is mifiring. you may have a burned valve. if there is enough problems mechanically with this motor, it may be time for a replacement.

 

[xjluvr]

before buying a new injector you can switch it with one from another cylinder to rule out that problem. if you are planning to replace rod bearings because of a rod knock, you need to replace the connecting rod also. the bore of the rod gets elongated from the knocking around the crankshaft . bearings wont fix the noise. i would do a compression test on the cylinder that is mifiring. you may have a burned valve. if there is enough problems mechanically with this motor, it may be time for a replacement.

This seems to be the route I will be taking, replacement that is, #2 cylinder is dead, engine has a number of other problems as well, but only paid $500 for the rig with a perfect body and interior, so it wont be too awful bad.
Thanx for the responses guys.