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Alright guys i have a 2001 civic ex coupe and cant figure out why im getting 27 mpg. Gonna list everything that has been done to the car so you guys know what im working with. D17A2 engine, rebuilt head, every seal in the head is new, head has been shaved, new rear main seal, new head gasket, new alt, new water pump/timing belt, new belts all around, tranny over hauled (every bearing in tranny is new), new excedy clutch kit, 10lb flywheel, aem cold air intake, sittin on 17" rims with new tires, 0 gauge big 3 done in engine bay, single 12" sub (sub and box weigh about 60lbs together), 2 amps pulling about 1000 watts, new ngk iridium plugs, fresh oil change, and new filter on the air intake. Thats about it. All of these parts have about 4-5k miles on them. Before rebuilding everything i took a beach trip and got about 41 mpg (all highway). Didnt really check much other than that trip. Now im getting about 27 mpg weekly. I drove the car like a saint this past week shifting at 3k for 1st and 2nd and at 2.5k for third and fourth, didnt down gear at all, didnt rev ever and got the same exact gas mileage as i did when i beat on it every once in a while during the week. Any ideas why this motor, made for fuel efficiency, is getting such low gas mileage?

 
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Is it cold now where you live? I'm losing 3-5 mpg in the winter as compared to the summer. The 17's probably aren't helping, and new tires can hurt a little too. Are the new plugs gapped correctly? Is your weekly drive mostly in city or highway? There should be something else though to make it THAT low

 
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It fluctuates during the week here, 60 one day 30 the next. Weekly drive is about 75% highway 25% in town.

 
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Something i forgot, whenever im going down the interstate doing about 75 mph im sittin at 3.5k rpm, at 80 its at 4k, is this right? In my girlfriends 93 civic and my old 87 accord, i never went above 3k on the interstate unless i was doing over 80. Maybe this could be it?

 
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That is a little low. The temperature can effect your mpg. Have you replaced air filter and are your tire pressures good? How are you calculating your mpg? I took mine down to the Carolina's and got 40 mpg but that was cruise control locked and over 8-10hrs. For my daily commute I get around 31 mpg. A bunch of little things can add up. Ill check my rpm's with my 01 5 speed. That sounds about right but I believe ex's are geared slightly different then an Lx.

 
Rep Power: 209

is the engine heating up properly?

does the temp guage read "normal" near the middle?

an engine that runs too cool will burn more fuel

 
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Air filter was changed 2 weeks ago, tire pressures are all good. Well this past week like i said i drove like a saint, every time on the interstate i set cruise at 70 mph and didnt beat on it once and got less than 28 mpg. Thats with 200+ of the 280 miles being interstate.

 
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Yeah engine warms properly, stays a touch under the middle when at operating temp.

 
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check O2 sensor

clogged cat?

any CEL?

remove the plugs, see if they look normal

if your not sure post a pic here

 
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No CELs however, the car likes to "rev" by itself sometimes, usually when the car is cool or hasnt been at operating temp very long. By rev i mean when i turn the steering wheel while backing out of the driveway it'll rev to about 1.5k and stay there till i stop, does the same thing when im doing down the road and shift into neutral, it stays at 1.5k rpms until i stop. This normal? And on very rare occasions it was rev madly by itself. Ill stop and it'll just rev from 300-2k rpm about 3 or 4 times and quit, that only happens rarely. I think someone told me this could be an o2 sensor so maybe thats the prob?

 
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Btw i really appreciate all the help thus far man

 
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aaahhhh....see....it helps to tell the whole story


that sounds like a problem somewhere in your intake....an intake manifold leak, IAC valve, TPS, clogged PCV valve, etc etc

if your a handy guy and enjoy working on your car i would remove the entire intake and clean everything out, replace the PCV valve and intake manifold gasket

 
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Heh tried to tell everything from the start but its a lotta **** and i forgot some stuff . Well, do i enjoy it? Hell yeah. Do i trust myself to do it? Hellllllll no. Lol. Luckily my folks are all master mechanics and i learn from them. So its looks like i got a project for someday this weekend. Pretty sure the instake manifold gasket was replaced when the head was rebuilt, but im not 100% sure. But im like 99% sure none of the other stuff was replaced. So ill take a look at all of it and we'll see what happens, thanks man.

 
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Consider your self lucky. I get 23-34 MPG, 03 LX 5spd. All city, rush hour winter driving in NJ.

200 or so of highway miles thrown into the mix and its 27 MPG.

100% highway and it's in the 40's, but that was in the summer last time I checked.

The spread between winter-summer city and highway is truly amazing.

 
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That could be entirely possible. It's possible to crack the plastic manifold when putting it back on after you had the head work. Before you take the time to remove it though, try cleaning the throttle body and check for vacuum leaks.