I recently posted a thread on a mystery vibration in my '93 ZJ with the 4.0 I-6 at low idle. The only thing I can find that might be a problem is the cracked exhaust manifold. My question is:
CAN THIS CAUSE THE I-6 TO RUN ROUGHLY BY MESSING UP THE READING TO THE OXYGEN SENSOR?
The vibration becomes tolerable if I take it out of drive and put it in neutral and turn off the AC, thus effectively raising the rpm's by a few and allowing the engine to run a bit smoother. It seems like the rpm's increase and decrease slightly every few seconds while idling also. Could this be the computer picking up conflicting info from the O2 sensor and other EFI sensors?
mine ran like crap - cracked manifold - it causes the O2 sensor to "see" more O2, then pours the gas in. you end up with a folded out spark plug. that's what the bad idle is from. just fixed it all. ebay has a stainless steel exhaust manifold for $150 (or so). you could go with header(s), if you have the cash..... check the plugs...
let me know how it turns out for you.
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If the exhaust manifold is cracked, get it fixed very soon. Its also a safety concern. An exhaust leak in the engine compartment can get into the cabin from the fresh air intake below the windshield. And you can't smell carbon monxide...bad to breath.
If the manifold is cracked bad enough, it will cause a rough idle. I would also check the ignition system for problems.
When you have your manifold replaced, use either a new one from the Jeep dealer or install a Banks or Borla header. The aftermarket "Cheep" manifold are just that ....Cheep. I have installed a number of them, They usuall come back .. cracked.
[FONT=Trebuchet MS]01' Jeep TJ (Jeep #5)
3.5 RE Custom SuperFlex, Raingler, Kilby, Currie, Banks, Borla, American Racing, Custom mods, Always more in the works.