Gas mileage

[xvr843]

Today i did a tune up, i bough the most expensive spark plugs for my car. Now im getting terrible gas mileage. Any ideas??

[Jrs3800]

What kind of plugs?

[nos4blood70]

What all was in your tune up?

[Krenzy]

Did you check the gap? Are the wires seated onto the plug fully, as well as the coil?

[human]

Just because a spark plug is expensive doesn't mean it's necessarily the best one for a particular application. Also, check the condition of your wires and coils? Any rust on the terminals? Wires are pretty inexpensive; a decent set of NAPA/Belden wires will probably run you maybe $30. Perhaps start with wires and see how it goes. I had to do a progressive tune-up on my Aurora (I know, different engine but the principle is the same) and managed to improve fuel economy by a good 5 mpg.

I was having some pretty severe misfires, so I started with the coils. When I replaced them, I found several of the terminals were rusty, as were the contacts inside the corresponding wire boots. It was a week or so before I could put the wires on, but with the coils alone, my mpg went up from about 13 in town to 15 and the misfires were significantly reduced but not entirely eliminated. Replacing the wires got the mpg up to between 16 and 17 in town and the performance issues were reduced to an occasional shudder when the car shifted into overdrive. Replacing the plugs eliminated that issue and further increased mpg in town to a little over 18, which is about as good as it gets in a '95 Aurora. During that process, I also added Lucas oil to a couple of tanks of gas to help clean out the injectors.

I had a similar issue a few years ago when I first got my '95 Bonneville. Replacing plugs and wires worked a miracle on that car. I just used the basic ACDelco copper plugs, same as the ones that went in it at the factory, and a set of NAPA/Belden wires. Performance and MPG both improved significantly.

[Jfridge92]

And, if those plugs or wires say Bosche, likely there's your problem.

Did you change the O2 sensor in that tune up? Same thing, if it's bosche, your car is hating you right now.