Decreased fuel economy mpg after spark plug change

Bought a used 2008 Cayman S 2 months ago (50K miles) and didn't have good maintenance records so I performed a major service on it myself. Good news is everything looked great and clearly had been well maintained. I replaced the spark plugs (it had the Bosch FGR5KQE0) with NGK BKR7EQUP. The previous spark plugs looked great, but I went ahead and replaced them anyway. Now I'm getting about 3 mpg less in both city and hwy driving according to the on board computer. However, the car runs great with no misses and same power by butt dyno. Has anyone else experienced this or know of any reason why a spark plug change could make this difference? Would a durametric be able to diagnose any problems with the plugs and/or coils? I thought about pulling the current plugs to look for fouling or other issues but don't really want to because the job is a bit of a PITA and my lift has my old BMW on it at the moment and won't be coming off for a while.

interesting. me thinks a bad plug/coil would throw a code. incorrect (too small) a spark plug gap perhaps, and incomplete fuel burn??? total guess. Google it?? If you find out, let us know.

I would manually calculate your MPG instead of trusting the display. I have a 2.7 with TTP oil pump that hurts mileage a bit and get 20-21 MPG around town, high 20s on highway. Also I forgot the impact of lower temperatures where you are located. As Jake Raby says: the engine should not make noise.......

Would winter gasoline (with the added ethanol) or just simply winter driving alone have anything to do with it - I think cars run full rich while they are warming up?

I thought about the cold weather and the additional warm up time and that may be part of it. But the computer is registering lower mpg even after warmed up and during hwy driving. This evening I listened to the engine closely, and heard a ticking that sounded like the fuel injectors and I figure is normal, but now I'm worried about a loose plug. I torqued them all to spec (21 ft-lb), but it may be worth double checking them. Not sure if a loose plug would cause this or not. Is a ticking sound normal from the engine (can't hear from inside the car, only on the side near the air intake vents)?

Look at the old plug tip compared to the new plug tip. The Bosch plug has an exposed center electrode. The NGK plug has its center electrode hidden in the ceramic tip. What the heck is that all about? I know the cross references -- at least one I tried -- matches the NGK to the Bosch but based on appearance there's no comparison other than the NGK is a spark plug . You won't like this but my advise is to go out and buy Bosch just like you removed and install those and give the NGKs to the trash can.

If the plugs weren't such a pain to change, I would just put the old Bosch plugs in for a few weeks and see if there is a difference, then put new ones in. But I think I'm going to do what you advise and just put in new Bosch plugs so I don't have to do it again. I didn't notice the difference in the electrodes until you pointed it out. Will be curious to see what the plugs look like when I pull them and see if there is any fouling. Was hoping someone else may have used the NGK plugs and noticed anything similar.

Bosch also makes a plug with the center electrode hidden in the ceramic tip it's called Bosch platinum plug. The electrode is a platinum wire smaller than a human hair with ceramic encasing it. The spark plug gap won't ever change because the ceramic doesn't erode away. Ion's ago I put Bosch Platinums in a 1988 Chrysler Fifth Avenue. The original Champion plugs had only 8000 miles on them. I changed the original Champions with new Champions with no change. With Champion plugs the car pinged even on premiun and got 14 mpg highway. With the Bosch plugs all ping went away and mileage went to 19 mpg highway. Both brands of plugs were normal heat range. I was able go 100 miles more on fuel tank with the Bosch plugs. I sold the car 90,000 miles later and the plugs still looked good but I replaced them just because of their age. Performance was the same with the new Bosch plugs. Over the years I changed out plugs in other vehicles from AC Delco and Autolite to Bosch Platinums but was never able to get near the mpg increase like I had with the Chrysler. I always had poor performance with Champions overheated, cracked electrodes, poor fuel economy and ping. Bottom line go back to the Bosch plugs that Porsche recommends for the Cayman . You can't go wrong with Bosch plugs in any vehicle IMHO. Bye the way I use only Bosch plugs in all my vehicles.

U supposed to use OEM for plugs in most cases, pre-gaped for the car. I ran Bosch in my Nissan and it ran horrible, switched NGK and everything was fine

did you ever swap out the plugs? and if so did your mileage go back to normal? I had the dealer do an oil change, new spark plugs and air filter last month. Since then I have also noticed an average of 3-4 mpg less. Took it back to the dealer and they didn't see any faults or issues. Tire pressure and alignment is normal. Car drives fine. :-(

it was time for my 60k maintenance so I did it myself although the car (except for an error code something with o2sensor) ran extremely well. (2007 2.7l Cayman ) I did an oil/filter change, cabin and engine air filters and bosche spark plugs . Once installed, I started it up and it ran horribly with miss fires and flashing check engine light. Immediately turned it off and proceeded to check my work. I meticulously checked each plug, coil pack and associated wiring to the coil pack making sure the wiring connection to the coil pack was 'snapped' in place. I connected durametric prior to restarting to erase all codes for a 'clean' start. I then restarted the engine and still ran poorly with flashing check engine light and same codes I had after first changing the plugs. The original plugs were bosche (correct for the model) as were the new ones. I decided to let it rest for a few days since I was starting to get frustrated. I came back and removed a plug to study it compared with the old ones. After a few hours, it dawned on me that the 'spacing' between the metal ceramic covered lead (where the spark originates) and the (4) arms that come close to that lead are miles apart whereby the arms were situated down the ceramic sleeve. I thought there is no way a spark can jump that. So I returned the bosche plugs whihc were replaced by the NGK plugs correct for this model. I reconnected durametric erased all the codes from this issue, installed the new NGK plugs and voila....it ran like a top ...like butta. Here is a pic of the new Bosche plug (top) and the newest NGK plug (bottom)...big difference in configuration.

Same thing happened to me on my Subaru and nobody knows anything even Subaru customer service. The answer from dealer and Subaru.. DUH!