I have a 99 F250 super duty with the v10 that will not start if it's below 55 degrees. Replaced battery, fuel pump, filters, all kinds of things. But if I put a space heater in the truck on the passenger side floor board and aim it towards the dash for about 10 minutes it fires right up. Without the space heater just cranks and cranks but won't start. Someone told me to replace inertia switch. Any thoughts? It's done this for the 3 years I've owned it.

 

There is a good chance you may be looking for a PCM.

 

Would plugging in an ODBII "reader" into the OBD port help identify a failed PCM or other component?

 

I've had the truck scanned numerous times and nothing about PCM or anything has ever come up.

 

During an event,, I'd have a fuel pressure tester installed, injector noid light (or meter etc) , and spark tester as 'life support' ready to see which, if any, are missing during the cooler start temps..

A good diagnostic tool could display PIDS for an active PCM and for injector pulse, cranking/RPM values and others 'needed' for a start. Spark is easily checked, and the Inertia can just be bridged or jumped to bypass as a test to rule it out so not to replace a good one. Generally the inertia is good, or bad, not usually temp failed..
Although a cranking fuel pressure test would diagnose the fuel pump run, and PCM control of,, and a good/closed Inertia and active fuel delivery if it's suspected, checking for a hot/active spark, fuel delivery, and the PCM injector pulse for a starting fuel drop (spray) from the injectors may point you in a direction.

 

If you want to you might consider trying the following,
Take a hair dryer and try warming the PCM itself when the temp is below starting temp. If the vehicle starts fine when the PCM is warmed up , you will probably need to replace it.

 

thanks I will give all that a try