2008 5-speed, 110k miles, new plugs. I'm driving along and it feels like I'm losing power (I think) for an instant. It's very quick, so I'm not sure if I'm gaining power or losing it. When I feel it, it's within seconds after shifting to a higher gear while accelerating (but not every time) or going up a reasonably steep hill while accelerating or constant speed. I never feel it going downhill or at constant velocity on a flat or slightly uphill road. Does anyone have a clue why this is happening?

 

Sounds like bad coilpacks or you need a valve adjustment.

Search for either one (Google works better than the forum's search), they're both common around your mileage for 07-08 Fits, and there are several threads documenting individual cases.

If you haven't had a valve adjustment done, you were due at ~100k miles, so you might as well get it done.

 

People have used the terms "hiccup," "stutter/stuttering," and "stall/stalling" in previous threads. That might be useful for your search.

 

At that mileage its also possible you are just starting to get fuel starvation from a partially clogged fuel filter. These, unfortunately, are probably part of the pump assembly in the tank. Its hard to imagine why valve adjustment would cause an intermittent problem. If this only started right after changing the plugs, then you might suspect something with the coilpacks. Did you carefully inspect the connectors and boots while you had them out? Hopefully, you used stock-type plugs and did not cheap out or use some goofy aftermarket design like E3.

 

I did a valve adjustment and replaced plugs at 107k miles. The plugs are NGK iridiums ($12 a piece). The 'hiccup' problem started at 110k.

I have not tried fuel injection cleaner yet. I do wonder about the coilpacks. When I replaced the plugs and reinstalled the packs, I expected the packs to snap in, which they didn't. It didn't feel right, but there wasn't much you can do to screw it up. You install the packs and secure it with the bolt. All 4 packs felt the same way and the car ran fine for 3,000 miles. From reading the coilpacks threads here, there doesn't seem to be a way of testing them. You just buy a new one and replace using the trial-and-error method.

 

Since it wasnt coincidental to the plug replacement, and I assume no CEL (no coodes), I would again start to question fuel delivery under load.

I have not changed plugs yet, so dont know how Honda coilpacks fit, but Ive done the BMW many times and they dont snap in, just push on, like you described. I inspect them closely and put a little dielectric grease on the boot part.

 

The fuel injection is on my list (I just bought a bottle of Techron). There was another thread here mentioning that the coilpacks could be cleaned. After investigating this further, I removed the boot from the headpiece and put some dielectric grease on all of the contacts I could find. There seemed to be some corrosion on the springs. After cleaning and lubing, the car ran fine this morning but I need to drive awhile to know for sure.