Car Talk!

RedFury · January 21st, 2009, 08:08 PM

didn't those have a problem with a crossover pipe of some sort that went over the top of the valve cover Solo?

SoloCamo · January 21st, 2009, 08:16 PM

Quote:

Originally Posted by RedFury

didn't those have a problem with a crossover pipe of some sort that went over the top of the valve cover Solo?

Not from what I can recall.

Gomer · January 21st, 2009, 08:46 PM

Quote:

Originally Posted by Whir

I replaced the spark plug wires and plugs just now. Didn't make a bit of difference.

It does it all the time. Cold, warm, wet, dry, whatever. Does it much harder when the motor is under load, especially in 4x4.

Did the plugs look "normal", did any of them look different than the others?

RedFury · January 21st, 2009, 08:46 PM

I thought the inline six had some issue with something melting or deforming due to heat. it was revised later IIRC. Perhaps it was earlier models than his and yours? There was a service bulletin from what I remember when I was dealing with cars at the shop I worked at...someone mentioned it to me...hell, it was 12 years ago...leave the old man alone!

**Whir** · January 21st, 2009, 09:19 PM

The plugs are brand new, I just replaced them today.

No, that's definitely not the motor you're thinking of, Red. The I6 has the intake right over the exhaust, both on the same side of the motor.

I'm really starting to think it is something mechanical. It knocks pretty bad under load. Low fuel make that happen? Doesn't seem like it should.

Gomer · January 21st, 2009, 09:26 PM

The old plugs... did they look normal? uniform?

RedFury · January 21st, 2009, 09:33 PM

Knocking under load usually means predetination ( pinging ) which can be pretty loud. If it were something mechanical, the noise wouldn't suddenly stop if it were mechanical at a higher RPM.

Everything in my gut says "lean condition". A lean condition would leave the plugs looking clean, especially if it only started happening a short while ago. Long term effects would show on the plugs more apparent than yours did. There will be no carbon build up on them as they will be running hotter and burning off any deposits. Long term exposure to a lean condition would cause the plugs to erode from the excessive combustion temperatures. But if the lean condition is just fringe enough to run lean and keep them clean, but run better sometimes, you won't necessarily be able to use the plugs as a gauge to engine operating conditions.

I'm wondering if you maybe don't have a pocket of air in your cooling system and it is messing with the temp sensor until you get to a certain RPM which gets liquid coolant to the sensor and trips the right reading to the computer to compensate the air/fuel ratio.

**Whir** · January 21st, 2009, 09:42 PM

I don't understand what you just said. ;_;

The plugs looked pretty normal to me. A little bit of orange-ish deposit on them. None seemed overly carboned or eroded. No streaks up the porcelain to indicate leakage.

I have run several bottles of injector cleaner though it in the past two months. Usually with only a quarter or half tank.

**RobRich** · January 22nd, 2009, 08:43 AM

I thought maybe the coolant temp sensor, but his issue occurs regardless of temperature, but instead does occur based upon RPM. That is telling me the problem is somewhere else.

SoloCamo · January 22nd, 2009, 09:10 AM

Quote:

Originally Posted by RedFury

I thought the inline six had some issue with something melting or deforming due to heat. it was revised later IIRC. Perhaps it was earlier models than his and yours? There was a service bulletin from what I remember when I was dealing with cars at the shop I worked at...someone mentioned it to me...hell, it was 12 years ago...leave the old man alone!

Can't get much earlier than my 88...The 4.0L first came out in '87...and my old '87 didn't have that issue either. Then again, I was the second owner of the '87...and third(?) owner of the '88. So who knows what may have been done. However, I don't recall reading anything similar over at jeepforum.com.

Anyways....I'm really stumped as to what is causing that noise on your 4.0L, whir. First time I've seen a problem like that on the 4.0L... (sorry, not much help

)

EDIT: Ahh...one thing comes to mind. Have you checked your motor mounts? I've heard of pretty horrible vibrations from broken ones...odd that it would go away at high rpm's though. However, it's something I'd definitely at least check over.