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Recently picked up a 1991 D150 2wd which was originally a 3.9 V6 truck, but now sports a 1990 TBI 5.9 V8. When cold, runs great, however, when warmed up after just a few miles, runs like a bag of marbles! Hesitating and leaning out badly. I noticed the TB temp sensor is disconnected, as well as the O2 sensor, since a bung was not welded into the collector downstream of the header to accomodate it. I know this sounds obvious, but would not having the O2 sensor plugged in cause it to run this poorly?? I would say so, but just want to hear some feedback from you guys. Apparently, the previous owner thought it was fine to not have it connected! Thanks very much!!
 

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AirFuelEddie
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It WILL run dis-connected but with both the Throttle Body Temp Sensor and O2 sensors dis-connected,,but then the poor 'Computer' has only the Map sensor& TPS and ECT to aquire it's 'look up' fuel and ignition info from. The problem with this is the computer never gets to 'closed' loop and keeps it in open loop. This will cause momentary enrichment then lean then rich....but it's not programmed to do this in a fast rate of adjustment as the O2 can give it the info much quicker as far as how lean/rich the burn is and how much spark lead should be used.. I would get those connected as soon as possible.
 
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