Auto Mechanics, does this sound right?

[Band]

I took my wife's car (2004 Honda CRV) in for a valve adjustment because it was due and because the guys from Car Talk specifically called out the need for that service on this vehicle recently.

For highway driving this car for years consistently had 27 mpg but it has dropped to 24 consistently in the last year or so.  The first tank after the valve adjustment it dropped to 20, an unprescented low, and I can see that the second tank is producing similar results.

I asked the mechanic what he thought about that and he said lower mpg is to be expected right after a valve adjustment and "in about a month" it will improve and get better mpg than before the service.  I always have my BS meter on and this one made the meter glow a little.  What say you, is the mechanic right or is this a line of BS?

[timberghost72]

BS. If your valve adjustment was out of spec and then adjusting them to spec, it would improve milage by some. Either he did not adjust them correctly or there is another issue. Or you figured your fuel mileage wrong.

I would ask him to explain that reasoning and how that is physically possible in detail.

[jackelope]

Are you actually calculating your gas mileage or are you going off what the car is telling you(if it has that feature..?)

I call BS too, but the last valve adjustment I had anything to do with was on an older Cummins.

[Band]

I calculate gas mileage myself.  It is very simple to do, and trust me, my calculations are correct.  Unless someone has a better idea I'll wait until I figure the mpg on the 3rd tank and if it's still down I'll be looking at the service place to make it right.  Can't imagine an unrelated problem that makes such an obvious difference in mpg would hit exactly coinciding with the service.

I'll withhold the name of the place for now but so far they have a couple of strikes against them apart from this issue.

[coachcw]

id try a computer reset first and idle learn . remember winter fuels cause a 10-15 percent drop as well.

[Band]

remember winter fuels cause a 10-15 percent drop as well.

I measure mpg on every tank and I've never seen a drop in the winter.

[jackelope]

remember winter fuels cause a 10-15 percent drop as well.

I measure mpg on every tank and I've never seen a drop in the winter.

That is completely abnormal FWIW.

[HUNTINCOUPLE]

Maybe the squiral under the hood needs a friend?

[HUNTINCOUPLE]

No that would not be correct. Check your rice level and calculate how many grains per mile you get. Then tell the mechanic he owes you a couple bushels of rice.

[timberghost72]

Was that the only thing done at the shop? Any other recent repairs even if you think unrelated?

[Boss .300 winmag]

I have a 1998 civic that has never had the valves adjusted after 300,000 plus miles and still gets 32-34 miles a gallon. 

[Jonathan_S]

No that would not be correct. Check your rice level and calculate how many grains per mile you get. Then tell the mechanic he owes you a couple bushels of rice.

  yeah buddy

[LEN]

There COULD be a logical thing here. The compression may have gone up with the valve adjustment. And if the plugs and wires are on the down hill swing, then plug firing COULD drop mileage.

LEN

[timberghost72]

There COULD be a logical thing here. The compression may have gone up with the valve adjustment. And if the plugs and wires are on the down hill swing, then plug firing COULD drop mileage.

LEN

How would compression go up? IF that was possible, that would mean the valves were so far out of adjustment (WAY too tight) before hand and the car would have been running terrible along with very bad MPG before the valve adjustment. This is not even a possiblity IMO.

[Seahawk12]

Sounds like he (the mechanic) adjusted them wrong and they are now too tight. He probably thinks that in a few months they will have loosened up a bit.
Personally, I would ask a different shop what they think and then confront the original mechanic with some more information.