I've been seeing a gradual decline in my mileage long term on my '09 base Fit, A/T. I use both Shell top-tier and c-store regular, whatever's cheaper. No diff between them that I can tell. City has dropped from a usual 33-34 avg to only 29, and hwy might get up to 31 cruising @ 70, used to go as high as 38. There have been no starting/driveability issues or CEL's. Valves were first done 3-4 yrs ago @ 17k when the LMS recall was done soon after I bought her. I'm now at 71k. Tailpipe is clean & shiny with just a hint of very light brown if you wipe with a paper towel.
I have considered doing the adj myself, but being as it's my only car, I'd better let the dealer service do it. I routinely do oil change, brakes and such, but digging out the intake manifold and getting the clearances perfect is a bit over my skill level.
It isn't noisy, but I'm betting the exh are getting tight (hence quiet) while the intakes may be on the loose side? Also, should the plugs be done? I know they're supposed to be good for 100k, but while they are accessible, do them? Kind of like replacing the water pump while doing a timing belt, just makes good sense at that time. What would you do?

 

Tough call, since they were checked @ 17K. It seems that most clearance issues occur at the first check.

Your plugs should be OK (they are Iridium) and they are not that difficult to replace. They don't need to be touched @ the valve adjustment.


So, the timing belt/water pump (or serpentine belt/tensioner bearing) scenario is less applicable.


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Regards your declining mileage, do you perform a couple runs to red line every month? This will serve to keep the injectors clean.

Remember, Top Tier gas is a preventative measure. You should not expect to see a difference one tank to another.

 

I have done that sometimes on getting on the highway, but have heard what sounds like a loud metallic clatter if I take it above 5500-6000. Scared to wind it any higher, this might cause unknown damage. Valvetrain/spring issue? No problems or noise at normal revs, no smoke or burning oil. I run Pennzoil Platinum 0W-20 and Honda filter, change at 10-15% on the MM. K&N flat filter in the airbox. Main reason I'm having valves serviced and new plugs is declining fuel mileage. Something is amiss.

 

5000 RPM is enough revs to open the injectors long enough for the fuel detergent to keep them clean. Keep up the good work.



Metallic clatter may be the all-too-common rusted catalytic converter heat shield. Look under there, and see if the shield is rusted/broken at the bolts. Years ago, our 20K mile '07 Fit had a buzz from a loose shield grommet. It was NOT caused by rust...


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Causes of declining MPG can be difficult to pinpoint. My wife's van is suffering from that as well. Sometimes, a 'lazy' O2 sensor is to blame, but don't spend the $140 USD cost of the sensor unless you get it checked out by a shop. (And DON'T be tempted to buy a cheapie generic O2 sensor.)


Heck, maybe it's as simple as having the heat controls on one of the two DEFROST positions, which always runs the A/C without turning on the indicator lamp.

Or, if you have changed your commute (more stop & go) that could do it also...

 

If you do not live in an EPA non-attainment area and you are not burning "top tier" fuel, then you might want to throw in a couple of cans or injector cleaner once a year. EPA non Attainment areas require 10% ethanol in the fuel and "Top tier" requires 9%. Around here we do our best not to burn either of those fuels for several reasons. The primary reason being poorer fuel mileage and higher RVP numbers.

Forget my suggestion for injector cleaner if either of the two above apply.

 

Pretty sure it isn't from the exhaust, sounded very sharply from under-hood. Very rapid, sharp repeating knock as if metal being hit sharply with a hammer. Not pre-ignition, that'd show up under hard load at low rpm's. I have started using a dose of Lucas Upper Cyl Lube/Injector Cleaner for the past few tanks, no change noted. A/C cycles off under heavy accel as it should, then comes back on when you let up. Hopefully there's no broken springs or burnt exhaust from this. Got a svc appt tomorrow AM to do valves + plugs. Tampa Honda, $388 with shop fee & tax. Will mention to check the 02 sensor as well.

 

Got the service visit done yesterday. Took f-o-r-e-v-e-r for it to cool completely below 100F, since ambient was around 90F in the service bays. Waited over 6 hours for everything to get done. At least they did it right by making sure it was cold for the adjustment. Tech noted they were off and did need adjusting. Plugs were also changed, then ECU was scanned and road-tested while monitoring it. No deficiencies found, everything in spec. On receiving the car back after service, seems to run just a tad smoother, but I never really noticed it before I brought it in. Just seems that way. Valvetrain sounds a bit quieter. I recorded a before and after sound clip for comparison, have a listen here:

https://www.youtube.com/watch?v=2mxT...ature=youtu.be