1988 f-150 4x4 302efi when I start truck it runs like it is suppose to, but after 10 – 15 min. the check eng light comes on and the idle goes south and starts running very rich. If I turn off truck and restart it the same thing happens, runs great for 10-15 min. I cleared the codes and with koeo I get 11 (good) I run truck for 10-15 min, when light comes on I do a koer test and get code 42 (system rich) I then turn truck off and do another koeo test and get a memory code 41 (System was lean for 15 seconds or more (no HO2S switching). Parts replaced, HO2S (twice just to make sure I did not get a bad one from store) tps (and adjusted to 1.0) idle air control, heck I even changed muffler just in case it was half plugged up. I cleaned throttle body, Checked for vac. Leaks, none found, tested map sensor and most of the others per my Haynes book, When check eng light is on it runs rough and idles up and down between 500 and 1000 rpms. Tail pipe is black and sutty. What do you guys think? Computer maybe? I cleaned the contacts on it, plugged it back in. same problem. I think the computer is suppose to run in a loop mode when first started to allow truck to reach operating temp, but not sure. It would explane why it takes 10-15 min. for problems to start though..Pulling whats left of my thin hair out…sorry for long thread.

 

Have you checked the wiring harness going to the O2 sensor? I'd check that to make sure there is continuity between there and the computer pin. Also check the O2 ground wire from the computer to the bracket on the back of the engine right above the transmission. Check for power for the O2 sensor, the ground, and the return to the computer on the O2 harness. I'd also check for vacuum leaks and for leaky fuel injectors.

 

ok, i just checked ho2s again. its sending .96 volts. checked for grounds all good. getting 11.9 volts to ho2s koeo. checked between sensor and ecm all good. checked for vac. leaks none found. checked vac. at map sensor, getting 19-20 in hg. With koeo voltage at map vref wire is 4.90 (good) , between the map bp signal wire I get 2.94 w/o vac. (bad. Book says 5.0) applied 20 inhg vac bp wire still reads 2.94 (bad. Book says 1.0) on my way to get new map sensor.

crap, and a few other words. that was an expensive shot in the dark..... the new one has the same readings..... and truck still has same problem!!!!!!!!!!

 

found it!!!!!!!!! fuel pressure regulator!!! will be adding fuel pressure tester to my list of tools... thanks for the reply 2006. it lead me to check a few things that i had overlooked.