Posted by jimg.allentown on March 26, 2020 at 09:22:11 from (24.115.193.250):
In Reply to: 91 Ford F150 trouble posted by 55 50 Ron on March 25, 2020 at 18:41:06:
Your truck has the older EEC-IV system. This system does have built in diagnostics, but not to the level of the OBD-II systems.
When you first start it cold, it is in open loop operation. This gives it a rich mixture to start, then leans it out according to readings from the B/MAP sensor (or it may have a MAF sensor instead) and the IAT and ECT sensors. This is pre-calculated and programmed into the PCM. Since this is pre-calculated, things like vacuum leaks or bad sensor readings can upset the balance of the system. Ford began using MAF sensors in place of the B/MAP sensors some time in the early 1990s. Your system could have either one.
Contrary to what has been posted, the oxygen sensor on Fords has NO INFLUENCE on mixture unlike GM and Chrysler systems. Ford engineers consider oxygen sensors to be "after the fact" sensors. They are only there to monitor the overall functioning of the system. If the system is not working properly, the O2 sensor "sees" that and causes the PCM to set a code and light the MIL light (Check engine).
Your problem most likely is a vacuum leak given the symptoms, although a bad reading from a temperature sensor could make the PCM be fooled into going into closed loop before the engine is ready. Another possibility is if the throttle body has been cleaned, it raises heck with the PCM. Sometime in this era, Ford redesigned the throttle bodies to eliminate the need for periodic cleaning. Along with that they added a sticker indicating that the throttle body should not be cleaned. If your truck has that sticker, cleaning the throttle body can cause drivability problems, but if it does not have that sticker, it may need cleaning. 1991 was a transition year. Mater of fact, all through the 1990s there were a lot of transitions. From OBD1 to OBD2, from R-12 refrigerant to R-134a, from B/MAP sensors to MAF sensors, and the list goes on.
There really isn't a short answer to your question. You need to do some diagnosis before you take it on a ride through the parts department.
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