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350 running rich


67malibu
03-07-2012, 01:24 PM
'88 350 TBI K1500 211K The truck has been increasing running rich (smell it) and the mileage has gone from 17 to 12 over several weeks. She cranks easy cold, hard warm but runs completely normal at speed with no loss of power or acceleration. At idle ( in gear and warmed up) it chokes and will die unless I keep the rpms up a little as I come to a stop light. There's no hint of ignition issues (no skipping, timing looks good.) This TBI unit has been on it for about 40K and has had no issues before. No codes in the computer. No apparent vacuum leaks or hose issues. The O2 sensor is old so I'm replacing it today. Any ideas?

777stickman
03-07-2012, 04:54 PM
Possibly the Coolant Temp Sensor located on the drivers side by the thermostat housing.

67malibu
03-08-2012, 10:30 AM
Changed the O2 sensor (looked fairly messed up) but no help in performance. I'm not sure about the coolant temp sensor causing this but I'll check it.

777stickman
03-09-2012, 09:45 AM
There are 2 sensors. One in the drivers side head for the temp gauge. The one in the intake is for the ECM. If the ECM does not see that the coolant is up to operating temp, it will keep the fuel flow in the "enriched" mode and cause the symptoms you have described. Kind of like a choke stuck closed. Hope this helps.

67malibu
03-12-2012, 06:46 PM
Finally got around to changing the sensor, the engine idle came down a little but it still stumbles and dies sometimes when you come to a stop in gear after having driven for a while. It's also difficult to crank while hot, sort like it is still flooding.

j cAT
03-13-2012, 08:47 AM
Finally got around to changing the sensor, the engine idle came down a little but it still stumbles and dies sometimes when you come to a stop in gear after having driven for a while. It's also difficult to crank while hot, sort like it is still flooding.

rebuild the TBI. likely fuel leak regulator and gaskets. could be injector[s]

67malibu
03-16-2012, 10:29 AM
Well, I forgot that these older ECMs sometimes need some time to adapt. After the second day of driving the truck is back to normal. I remember a few years ago it took a full day of driving for the ECM to adapt to a TBI adjustment.

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