2001 F150, 5.4 Triton, 143,705 miles.

Truck exhibited a dead misfire on cylinders 3 and 6. Suspecting wet coils, I removed all the coils and found no evidence of moisture. 4 coils had been replaced at 132k miles with Bosch coils and 4 were OEM. Replaced OEM coils with Bosch coils. Misfire persisted on 3 and 6.

Took it to a local shop who swapped the coils around and the misfire persisted on 3 and 6. Shop indicates the injectors are "clicking and stop when unplugged like they're supposed to". Noid light also verifies that the PCM is firing the coils as necessary. Shop then did a compression check and got 160 to 170 psi on all cylinders. Shop now claims there may be a broken valve spring and they want to tear into it to verify.

Shouldn't a broken valve spring manifest itself as reduced compression? Wouldn't it be unusual to break a valve spring on 3 and 6 at the same time?

They've had the truck for a week with no answers. Any ideas as to what the problem could be?

Thanks in advance for any help!

 

How did the spark plugs look?

 

Spark plugs looked good, consistent from cylinder to cylinder and showed no signs of wetness from fuel. These plugs have 11k miles on them.