This is regarding the truck in my signature.

I changed spark plugs in the truck at 90K miles, recently. Removed Motorcraft, replaced with Motorcraft. Don't know how long the old plugs were in there, but they didn't look too rough. However, all of them were gapped at .35, whereas the standard for this engine is .42-.46. I gapped all of the new spark plugs at .44.

After doing one fill up of fuel since, I've found that I've lost 3 mpg. 10 mpg dropped to 7 mpg. All in town driving with front hubs locked most of the time since it's winter. Only other things changed on the truck around the time of the spark plugs were as follows: Bosch O2 sensor, fuel filter, exhaust (removed 2nd converter and installed pipe with turbo muffler), distributor cap and rotor, 9mm plug wires, battery, oil change, belt.

Even with the growly turbo muffler, I don't think that I was driving the truck any more aggressively since the change of parts. I baby this truck like you wouldn't believe. The truck idles and runs smoothly, much better than before. The weather has been nasty cold and the truck does get a fair amount of idle time in, but no more than before.

I'm open to all relevant suggestions on this. Thanks.

 

I would do three things. First, pull the codes to look for any problems, if you find something fix it. Second, check all vacuum lines and wires around where you were working, make sure nothing got unplugged or broken when you were working on the truck. Look real close at the vacuum lines, on these older trucks they get real brittle and break really easy. It seems like every time I get under my hood I break some rotted out vacuum line. Third, check your mileage a couple more times, to be sure. I just don't see any reason you mileage would have dropped. Good luck.

 

Pulled codes tonight:

KOEO: Code 332 - Insufficient EGR flow detected

KOER: Code 538 - Insufficient RPM change during Dynamic Response Test -or- Invalid cylinder balance test (throttle position movement) -or- Invalid cylinder balance test (cylinder identification problems).

Anyone care to toss me some advice on this? I don't quite understand the Code 538. All info is appreciated.

 

During the KOER test you are supposed to spike the throttle at one point. If you hit it too quickly, it will throw this code.