4.0 knock from #3 gets worse with no load

[setchmo]

Hey guys I have a 1990 xj automatic with the 4.0 motor with 193,000 miles on it.when I bought it I noticed the motor had a knock to it but it goes when in the higher rpms. It makes a kind of a metal clanging noise for 1/2 a second when I start and shut it off, I figured it was a cracked flex plate. Today I was adjusting my tps and decided to pull the spark plug wires one by one to see if the knock went away. When I got to the #3 cylinder and I pulled the wire it got a lot worse( sounded like a midget was hitting the motor with a hammer) when I plug it back in it goes back to its normal knock. All of the info I've found said its a rod bearing if it quiets down when you take the load off it. No one says what it could be if it gets worse. I'm not a professional wrencher so any ideas would be appreciated.

 

[old_man]

The only way to know for sure is to pull the pan and rod cap.

 

[setchmo]

Dang I guess I'll be pulling the pan this week. Thank you for the reply!

 

[Digger87xj]

wrist pin

 

[setchmo]

Stupid question, since I would need a full rebuild to fix a wrist pin. What's the likely outcome if I don't fix it. I'm sure it could lead to engine failure (I'm guessing) but how long night that take?

 

[Digger87xj]

IF it's a wrist pin, usually they last a long time, if ever failing completely.

 

[Ecomike]

Digger is usually spot on when he makes a claim like he did here, but I would do a compression test and if OK, then run some seafoam or equal to remove carbon buildup and then some Restore oil additive to see if that helps, short of a rebuild, internal inspection. It does not sound good. But I have had my 87 mage god awe-full sounds at times the last 10 years, now at 287,600 miles. MMO plus Lucus Oil additive (thickener), and Restore plus running mine at higher rpms at times, around 4000 rpm freed up stuck lifters and she purrs like a kitten now. Only thing I have done to the engine was new valve stem seals for two that were leaking.