idles and runs good but.. at WOT it misses at lower rpms somethimes mostly when hot..

[F 150Cobra]

idles and runs great normal trottle but give it a wot and it misses lower rpms till 3800 and then smoothens out back again.. or somethimes at wot it misses you let go and floor it again its ok back.. is it ignition or injectors?

all the ignition system was replaced last year this month.. even has an msd box... and plugs was replaced 2 months ago.

any ideas?

 

[NeXtras]

didnt you put a new PCM in it? or what ever its called on these years?

 

[Skandocious]

Sounds like it might have to do with engine load... Maybe a vacuum issue? Just thinking out loud...

 

[NeXtras]

bad egr? that would give you ping/knock.. possibly a bad plug? sometimes there dropped and you dont know if the porcelin is good or not til its in the truck... did you check the dist cap for cross fire?

 

[F 150Cobra]

jepp i did change the ECU.. but i tried both and both missed .. how to chek for crossfire? in the dist cap?

 

[smokey]

yep look for carbon tracing inside the cap. If you have one of the cheap caps with the aluminum posts get a good one and chuck that one in the trash can.

 

[mrfixit]

Yes, check what Randy said, and some other things you want to look at:

Double-check ALL the spark plug boots. It's entirely possible with the 5.8 for a boot to have slipped off the tip of the plug but still run fine until you load it.
Ya gotta make sure you firmly press ALL the boots to make sure each one is well-seated on the plug.
on the left (drivers) side of the engine, to the rght of the coil, is a vacuum manifold. Make sure none of those hoses have slipped loose. A vacuum leak can cause your MAP sensor to not function correctly.
ALSO, check the connections on the coil... the Solder connections in the moly plugs have been known to break, and would fire fine til you torque the engine, which pulls at the harness and creates a misfire. (took me forever to find that out... and was a danged head-scratcher!!!)

 

[F 150Cobra]

i think i solved the problem.. the MSD wasnt getting good ground from the pole of the battery they where dirty... i cleaned it up now it runs better.. havent got time to properly test it yet but its better till now

 

[mrfixit]

Great to hear! Hope that is all it was, and you have trouble-free months ahead!

 

[F 150Cobra]

Great to hear! Hope that is all it was, and you have trouble-free months ahead!

yepp thanks trouble free for the new owner.. im selling her