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?

 

Have you inspected your fuel injectors? Have you ever tried running SeaFoam through the system?

 

Havent looked at the injectors and no i havent ran seaFoam before

 

well, i would suggest that you might want to wait until your new clutch kit and flywheel have been broken in. being that the two surfaces have not been worn in together, you cant be too confident in getting that 40mpg so soon. also, you have that extra load of weight in the rear that puts a load on the new clutch and flywheel... though, i dont think that should effect it THAT much. another thing - is your alternator a honda OEM alternator? if not, the pulley/mass of the alternator itself might be heavier than the stock one. this can put a significant load on the motor when you're driving around. i have felt the differences between that of Honda OEM and aftermarket brand alternators. one last thing... what size are your tires? if the size of your tires does not equal the equivalent to that of stock tire/wheel size, then you may notice a huge difference in mpg's. that's IF you didnt have them PRIOR to the time you were getting 41mpg.

 

Clutch kit and flywheel have about 4-5k miles on them now, still think it needs more time to break in? The new alt i believe is a remey? cant remember 100% sure. It came from Autozone i believe and is a oem replacement. When i got the 41 mpg going on the road trip i had a system in the trunk that was close to 3 times heavier than my current one. Tires are 215 45 17s, this is as close to stock as i can get with 17s. Also, the tires on the road trip were about done, bought new tires shortly after rebuilding everything. I talked to a honda tech last night and he says he thinks its just a tight motor, with everything being new it all is still breaking in and once it does it should be back where it was. Any idea if there is truth to this? He also said go down the interstate, leave it in second and drive at 5k rpm for 2 miles. Never heard anything like that so i havent tried it yet, lol. Oh lastly, 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

 

Also, 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?

 

well the revving is your power steering putting a load on the motor so that is normal. the tech could be right, and the motor might just need to loosen up a bit with all those new belts and such. one thing is for sure, your alternator is probably heavier than stock and that could be adding to your junky horsepower.it wouldnt hurt to check/replace your O2 sensor. try that out and see is anything changes. one last thing, you will notice a little bit of a difference with that tire size. but it shouldnt be that huge of a difference.

 

Ditto on the 02 sensor. They can fail or operate incorrectly without triggering CEL sometimes......If over 65K I would change it regardless.....

 

Well i think ive solved the first half of my problem. Took the car to advanced auto to check and see if maybe it threw an engine code and the guy before me had it turned off, and voila, there is a leak in the emissions system. Im thinking this is the cause? Guy said to check the pcv valve, iac valve, tps, and check for intake manifold leaks? Think this could be the culprit?

 

I think you've got a situation here of death by a thousand cuts...

Moving from stock wheels/tires to larger wheels will likely have a large increase in tire/wheel weight (lightweight wheels would mean only a small increase). Increasing wheel weight will decrease mileage, especially in stop-n-go/traffic lights.

Used tires have less rolling resistance. New tires take a while to break in and will get worse mileage than your old tires, with all else being equal.
http://www.tirerack.com/tires/tirete...jsp?techid=177

Is your tire pressure the same now as before?

Unless your recent driving is all highway, you will likely get less mileage than your 41mpg trip.

Cold Air Intakes typically give a small boost in power but a slight decrease in mileage. At the risk of oversimplifying things, cool air is more dense, your computer will compensate by adding a little more fuel.

 

Yeah tire pressure is the same as before, same tires, just newer. And yeah i know i wont get 41 mpg, but im driving about 220 miles all interstate and 60 city, should be a little higher than 26-28 from what ive seen.

 

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