I recently did a tune up on my 1999 Honda Accord LX 4 cylinder last weekend, changed spark plugs (NGK G-Power pre gapped and checked at 0.4in) changed spark plug wires (NGK), distributor cap and rotor, and my gas mileage has gone to hell since. I can't seem to figure out why. The only things I could find from an inspection is my breather hose from the valve cover has a small tear, the pcv valve is a little loose, and my belts need to be changed. All of that was present before the drastic drop. It didn't get great gas mileage before, something like 23-24mpg, but now I'd say it's around 10-15mpg. I did change my front brake pads, calipers, and rotors at the same time, but I highly doubt it is the cause of the problem. There is no check engine light on at the moment. Other things I have done before that are change the thermostat, valve cover gaskets, alternator, battery, downstream O2 sensor, coolant, oil, oil filter, and brake fluid. Your help would be greatly appreciated. I tried to be as thorough as I could. Any suggestions?

 

It doesn't sound like you put the OEM spec plugs in; you might want to check your Owner's Manual and buy the exact plugs called for.

 

Downstream 02 sensor could be the issue if it is not OEM.

I know I was having fuel consumption problems way before my CEL came on. Than I had CEL come on... I originally put in Bosch O2 Sensors.... I replaced both upstream and downstream. Also replaced the Catalytic Converter... still had fuel consumption problems... and think the CEL light in process would come on and go off... I ended up having one of the new aftermarket Bosch 02 sensor go faulty. Replaced with OEM. My fuel consumption came back normal. I even put a new gas cap on. I recently turned 316,000 miles on car.

 

Most folks say make sure you don't use Bosch O2 sensors, pretty much all of the rest work well.

 

O.4 inches is not the correct gap for your vehicle

also, is the engine running smooth? if not, check the plug wires for good connections and routing.

 

NGK ZFR5F-11
The -11 at the end means the gap is 1.1 mm or 0.043 inch. He probably just mis-typed.

 

sucks, but don't figure gas mpg just based on one week's travel. Just pay the tab and check it again once you have more total miles on the tune-up - and the weather warms up...