ok, what is going on with my car?
 

My gas mileage is 30mpg now. Before it used to be mid 30's

The thing is I took the car for major service and they even adjusted my valves( honda civic). My map display on my Scangauge 2 is at 4.2-4.7

Before it used to be 3.8-3.9 at idle. I know that the less the number the better the mpg. My car idles at 650~rpm.

Anyway if I take my car in they will say that there is no problem and the car should get low 30's. My engine light is not on, so I need to find the problem. What would make my MAP display jump up and stay there? I think that's what's causing it. The car runs fine, and it's definetly not the cat because I had it replaced last year.

Did they do timing belt?

What other service did they perform? And, what year is your Civic?

Since they adjusted the valves, that alone could affect how much air/fuel the engine takes in (I think). They may have been set too low, so there wasn't as much air/fuel getting into the engine as Honda recommends. I don't really know how MAP sensors work, that's just what it sounds like to me. It could definitely be something else that they adjusted though.

And, many newer cars have a 'learning' procedure - whenever the battery is reset, the ECU changes how certain systems react in the car based on your driving. Your ECU might just still be trying to find the right settings.

Another thing to check is tire pressure, if you normally keep your pressure higher than the car recommends. Many shops just change the tire pressure to whatever the car says, even if it's already higher than it.

They changed the water pump and the timing belt and all the other belts 2 years ago. The tire pressure is fine, that's the first thing I checked.

I don't even know where to start. They changed all the fluids (engine, transmission, radiator, clutch, brake), they adjusted valves, checked the timing, replaced the spark plugs, did compression check, replaced the air filter, cleaned the engine bay. Since they did a lot of things I don't where to look. I checked for any hoses having holes and came up with nothing. Checked my pcv valve it's good, I replaced it half a year ago. I think they adjusted my idle because it sometimes it idles at 615-630rpm, but that could also be the valves. right?

Car is a 99 honda civic

How long ago did you take it in for service? My car takes about 3-4 days after losing battery power to get back to normal (idle goes way off).

I took in for service at the end of april. That's when it started but I'm just really lazy when it comes to doing something.

changed air filter. it is getting more air, the ecu gives it more fuel to optimize. thrash it around in some dirt..or wait. a 99 should be quite fast after ecu reset. Waitng for it should not be happening. I would guess it knows it has more air to play with with new filter :)

They changed the air filter too, I looked if there was something there. Didn't find anything not in it's place. Maybe the ecu is going WTF because a lot of things were changed at the same time?

How long will it take the ecu to adjust to everything?

It really shouldn't take very long.. a few start ups and drives.

Your MAP sensor could be going out. I don't know how much one is, but you could test it with a multimeter.

They replaced the O2 sensor right?

Also, maybe it so happens that they DIDN'T disconnect the battery. Highly unlikely, but, there is many cases where say, you had a big hole in your exhaust and drove on it like that for a long time, and then replaced it, but didn't reset your ECU/PCM It will continue to run on the old settings. Resetting the Computer has fixed problems like this before.

So maybe you could try disconnecting the battery for about 15 min, with about 100 miles before the next fill up and drive until you need to, well, fill up and see how it fares next tank.

You know I had a crack in my header and they replaced it last year. I doubt they reset the ecu.

I'll reset the ecu and see what happens.