I have a 1988 f150 4x4 with a M5OD and a 302.
My problem is that when you lug the engine you can hear a pop pop pop from the air box. no codes, 80,000 on reman engine. Timing chain maybe?

please help, she's a good horse.

 

Check the ign. timing though I would have thought that would have thrown up a code if out. Also check spark plug wires and replace if in any doubt and check the distributor cap for any cracks or traces on the inside. If replacing plug wires don`t forget the coil wire.

 

All the igniton parts are new, and timing is spot on. Could it be running to rich/lean? In warmer weather(40 degrees+) if the truck sits for 5-6 hours it would start then die within 4 or 5 seconds, but the 2nd times the charm. Now that it hit the freezing mark the truck starts right away but still backfires under load. the way it starts and dies in warmer weather would tells me it may be to rich. and now in the colder weather it needs more fuel when it's cold so it runs fine, this problem leads me to the coolant temp sensor, but the backfire is making me furious. I work on other peoples junk and diagnose just fine, but when it comes time for my junk I can't figure it out.

I think it's timing chain related, so I will start there.

 

I agree with you that your truck must be over fuelling at startup when weather is warm and that it is probably a sensor fault. If all new ign. parts then valve timing would be worth a look as you propose. Another possibility might be a burnt valve-a compression check may show this. Good luck, I hope you solve it.