Changed Plugs...Now horrible gas mileage!

My car '99 996C4, just hit 45K miles so I decided to change the plugs on it since I just bought it last July and am not sure if the previous owner had them changed at 30K. I got the plugs from Sunset Porsche and they were the direct replacement Beru four tip plugs (999 170 207 91). I changed the plugs with the car on a lift so the process was pretty straight forward. My car used to get about 16.5-18.1mpg combined city/highway buy now it gets 10.1-11.0mpg. Is it possible that one of the coils isn't seated properly onto the tips of one of the plugs? I checked that all of the bosche type connectors were snapped onto each coil and didn't have any problems threading the plugs into the cylinders. I did notice the car sputtered the first time that I started it but that hasn't happened since. The car idles perfect and pulls strong it is just the horrible mileage that concerns me.

are the plugs properly gapped? Also, did you put the proper wires w/proper cylinders?

Your 16-18 mpg sounds really bad, my '99 Cab C2 averages over 23 combined, about 20 when driven fairly aggressively, and 29 when driven conservatively on the highway. When I bought it the previous owners average was 27. If your problem was with a coil, wire, defective plug, you would certainly notice a significan't drop in power as that cylinder would not be firing and would be dead. Also you would get a CEL showing which cylindering was misfiring. Your milage could very well be a computer error. Verify your milage by filling your tank, driving a specified distance and refilling.

is the car misfiring? you may have a cracked spark plug, if the spark plugs were dropped at any time by the parts guys or you they are done

I will put the car back up on the lift this week and make sure the coils are connected up correctly; doesn't seem like there was enough slack to reach the wrong coil but I will check. The plugs were all in great shape and the came in a multi-pack when Sunset Porsche shipped them to me. Lastly, I don't think that the gap can be changed on these plugs since they are 4-pronged. I still want to change out my fuel filter; I did the oil change last month with Castrol Synthetic. I wonder if my MAF is going bad? Oh yeah, and no misfires at all. The car starts right up and idles great.

Sounds strange indeed. I would think anything that bad would trigger a CEL. Good luck...

For each spark plug, I'd pull up the corresponding wire's connector hood and make sure that the connectors are in right. I changed mine awhile back and found that they were not proper in place even though it sounded as if the connectors snapped in correctly. Also, did you torque the plugs down to the proper setting?

David, On another note how difficult was it to change them? Loren spoke about a special tool that helps out..any comments..tips?

spark p;ug socket?

The hardest part of changing the plugs was getting the top and bottom hex bolts off. Some of the coils were obstructed by the exhaust system. The easiest thing to do is: Break each bolt free with a hex wrench and then use a hex socket w/o the wrench to spin the bolt while wiggling the bolt to make it spin easier. Some of the plugs were hard to get to so I had to use (in order) spark plug socket, 3" extension, universal joint, 8" extension, socket wrench. Remember to turn the new plugs counter-clockwise to make sure that they seat properly before screwing them in. Auto-xr, How did the engine run when one of the connectors was not snapped on correctly. Because my engine runs strong and idles perfectly; I just get worse gas mileage.