I have a 2003 Ford Expedition with the 4.6. It was missing on 1 and 3 so I changed coils and plugs on it. When I went to start it, it didn't even crank. Battery is fine, plenty of juice. I discovered that as soon as I turned the key to the on position I was blowing fuse 34 which is for Mass Air Flow sensor, powertrain control module, fuel injectors, idle air control valve and the fuel pump relay.

I have no clue why that fuse keeps blowing or where to start looking. Any input would be appreciated.

 

Two possible trains of thought here,
1. If the fuse didn't blow before you worked in it you might want to recheck your work and see if you possibly pinched a wire in the process

2. There is evidently a direct short in the circuit. Taking some of the components out of the circuit should help pin down which , component or wiring is the cause.
You might try the following,
a. Unplug the Maf sensor
b. Remove the fuel pump relay,
c. unplug the coils and injectors
Now when you turn the key on , does the fuse still blow ?
If it does, we either didn't disconnect all the components from the circuit or there are more we don't know about or a wiring problem.
If it doesn't blow, reconnect or reinstall one at a time and check the fuse key on ,you will know which one is the problem the minute you install/reconnect the last one that caused the fuse to blow.
You will put some codes in the computer, that a scan tool or code reader will pick up . That is what it is supposed to do , and after you drive the vehicle a few trips and all is well they will be automatically removed.

 

I'll give it a try tomorrow and let you know. Thanks for the input.

 

I disconnected everything and turned the key on and the fuse didnt blow. Then I put in the fuel pump relay and it blew when I turned the key to the on position. I am going to run to the auto parts store and get a new relay. Hopefully that fixes the problem.

 

I replaced connected everything back up except the fuel pump relay. The fuse did not blow. I put a new relay in and the fuse blew right away. How do I trouble shoot this electrical issue not that I have narrowed it down?

Correction: I pulled the PCM power relay and then the fuse did not blow. Where do I go from here?

 

How did you determine the relay in question is for the fuel pump?

 

That was my mistake. I misspoke. The relay I pulled was the PCM power relay. When the relay is out and i turn on the key the fuse is fine. But when I put the PCM power relay back in, it blows the fuse each time. I've been cleaning terminals and grounds, rechecking connections and still have the same problem.

 

Is this what you did?
Disconnected all the listed items and installed the relay last?
If so, what you should do is;
After you disconnect all the listed items, install the relay. Does the fuse still blow? Yes-No?
If yes , either we left out a shorted item/component or there is a wiring problem. No need to go any further unless that is the case.
If no, relay installed, now reconnect the disconnected items one at a time watching to see if the fuse blows. Keep connecting the items until the fuse blows.That is the shorted item/system. Should not be difficult to do. Let us know what you find, thanks.
I still am curious if you didn't have the problem before switching coils and now it comes up, there has to be something that was pinched or installed incorrectly.