I just bought a 1999 with a 5.4L that runs terrible. I had to trailer it home, it was deal so I can not lose any $$ if I do not fix it but I would like to get it running right.

I have been searching the forums on COP's. The previous owner says he replaced all 8 with low mileage junkyard parts as well as injectors and spark plugs.
I tried to use my temp gum ot read what cylinders were misfiring but it is too hard to access and get accurate information.

My question is, can i just unplug a COP and try to determine what cylinders are missing? I do not want to damage any electronics.
There are no codes but it is missing on at least 3 if not 4 cylinders and will not idle at all.

 

Another thing I thought of is whether or not I can just disconnect the power to each injector without causing harm?
I want to diagnose what cylinders are missing then go from there.

 

I cannot help you on the COP question as I have never tried it, I just changed mine out because the computer said misfire on Cyl 4. But, I have unplugged the injectors on a previous truck (1986 F150 302 EFI) I had while trying to diagnose it. Nothing as far as damage resulted from this type of diagnosis. But, the old truck was not OBDII either, so not sure. If it were mine, I'd try the fuel injectors...but that is just me

 

Try taking your COP's out and ohmming them. There is a thread with the values. Make sure that you don't have any moisture in the sparkplug wells. Inspect the boots for cracks, and replace as needed. Apply dielectric grease to the bottom of the COP spring, and reinstall. See if this might help.

You can unplug a COP one at a time to see if that will help.

Good luck!

 

Thanks for the info. I read the post on OHMing and that is my intention when I figure out what or if one or any of them are bad. I was trying to avoid just pulling all of them but I also do not want to hurt the PCM or anything else by unplugging the wires. Can this be done while it is running?

 

yes you can do that, start the truck, and disconnect one if motor starts to run worse then thats not it, plug that one back in and move on to the next if you unplug it and nothing changes then thats your culprit i did that on mine because i was not getting a CEL. The coilpacks are pretty hard headed, they wont throw a code unless they are dead dead.