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4.0 Barely runs

jeepsx3

NAXJA Forum User
Location
Ky
91 4.0 aw4 This is my son's truck but I try to keep it running for him. The darn thing starts but goes to 2500 rpm max and spits and sputters. I retrieved code 23 and 51 and looked them up. (MAT and o2 sensors) I pulled the ECM fuse and cleared the codes. I wanted to drive it to see if it set them again, but it isn't driveable and don't think it will idle long enough to go to open loop mode.
I also changed the plugs thinking that might make it run smooth until it warmed uo enough to possibly set the codes again. The sensors have not been tested yet or changed just thinking that it might run better with the plugs changed as the old ones looked fouled. Didn't help tho. Also Fuel pressure is at specs. Any ideas?
TIA
 
My bet is on a clogged CAT. That kind of mileage and symptoms leads me to believe that. A vacuum guage would show over all low vacuum if the CAT is plugged. You also get a much quieter exhaust with a whistling sound sometimes. Unbolt the fitting before the CAT and let it run open with no CAT for a few minutes. If that is it, the power change will be quite dramatic.

Since you have problems even at an idle, pull the O2 sensor. Just that much more of an opening can yield a significant difference for test purposes.
 
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