Hoping someone can point me where to go next on the issues I am having with a rebuilt install on my 2002 f150 4.6. The previous motor locked up at 97,000 miles due to oil pump failure, was running fine before that. I picked up a rebuilt block and swapped over all of my sensors, intake, throttle body, etc. Just got in in last night and the idle jumps from about 1000 rpms to 500 rpms and almost dies. It also has no power whatsoever. Only code I pulled is P1504. Chevked the IAC valve and it gets 12.5 volts from the pcm and resistance across the terminals in 9 ohms. Resistance from terminals to case ground reads open so I assume that is ok and it is not grounding out. In reading the forum the P1504 code is exclusive to an electrical issue, but I am unsure as to where I should look for problems next. On another note, I was checking for vacuum leaks and seems to almost have no vacuum pressure from the throttle body. I was thingking might need to check compression on the cylinders next. ANY IDEAS????? Thanks

 

Also wanted to add that the truck runs exactly the same whether the IAC sensor is plugged in or unplugged??

 

Replace the IAC...They are very finnicky and make sure it is a Motorcraft replacement. I had an IAC do the EXACT same thing (vehicle up and died right in the middle of the road) so I replaced it and all is well. GL

 

Its not the IAC, swapped with another known good one from my other truck.

 

Update: Found the problem had a massive crack in the lower plastic intake. Runs great now.