My truck ran great last year, and I was getting 18mpg on the freeway. Then it started running bad. I've replaced EVERY sensor (and the whole EGR valve) on my engine and have taken it to 4 different mechanics. Everyone says "it runs great". However, I took an old ECT sensor and plugged it into the ECT lead for the computer and let it hang off the front of the engine (so it would read a much lower engine temp). As I expected, the thing ran great and I've been getting better mileage. However, the problem is that I don't know what else to check/replace to fix this? Any ideas. I may replace the computer as a last resort, but I'm hoping the problem may be elsewhere... My dad has suggested the intake manifold may have a bad gasget..? Anyone have any ideas??
Thanks,
- "Nealio"
= Aaron K. Neal - 1991 F150 4X4 4.9L I6 =


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#- KTMNealio '91 F150 4X4 4.9L I-6 -#

 

It sounds like your truck is happiest in the computers open loop mode. On cold start up, the EEC runs in open loop. It does this because until the O2 sensor gets good and hot, its output is feeble and unreliable as to what mixture it thinks it sees. In open loop, the EEC doesn't look at the O2 sensor, and does not try to fine tune mixture. It sets the injector ON duration using pre-programmed values depending on the Throttle Position Sensor, RPM, maybe the Manifold Absolute Pressure senser too. Once the engine coolant temp gets up to some value, sensed by the ECT, it figures the engine has been on long enough that the O2 should be hot and reporting good. So it goes into closed loop, where the EEC adjusts injector duration by O2 sensor, in addition to the others. You probably have a HEGO, the heated type of O2 sensor. Has 4 wires, two are for a heater circuit. Heater circuit gets 12 volts through a fuse in RUN, through the heater, to ground.
If the EEC is really unhappy with the O2 output, its surprising it didn't set a code.
By hanging your ECT in use out in the open, I expect you are keeping the EEC in open loop mode. So need to figure out WHAT is wrong that appears in closed loop mode only. At least that is where I would start.

 

Granpa, thanks for the info. I was thinking that I was looking at a too lean condition (which I still may be), but you have pointed me in the right direction. Your help is appreciated.

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#- Aaron K. Neal '91 F150 4X4 4.9L I-6 -#