1992 4.0 auto runs great down the road, at an idle it is rough.

Just bought this 92 with 110K miles from Nevada and shipped to me.

Rough very rough idle cold and warm although a bit better when warm.
Cleaned my IAC valve thinking that was it, but no change.

Bought and Replaced Cap, rotor, wires, and plugs (plugs 1/2/5/6 looked the same black and old. Plugs 3/4 were dryer and had a reddish tint.)
No change on idle but smooth power when you get on it going down the road.

So I bought a new IAC valve and still nothing.

Cant find any vac leaks but when I do open the vac line at the T for the cruise control (dont have cruise) the motor runs the same, does not rev up or down and still has bad idle and its about 800rpms.

once you rev it up to 1200 it smooths out.

Never had a jeep that I could not pull codes on so kinda at a loss here.
any insight to where to go next?


thinking about a full bottle of seafoam in the oil, run it a few days and change oil.

thanks

 

 

no not at all. dont have those tools and honestly never thought of those since it drives so nice down the road and with 110K original miles.

can grab loaner tools and check compression this week.

 

 

Good to know, will check compression asap.
Out of 13 4.0 jeeps this is the first to have to check compreeion.
Although all the others were 96 and up so OBDII scan