civic with bad mpg help
 

Hey everybody I am glad I found this site! Now on to my problem. I had a 99 civic ex a while back. I kept it stock and in good running condition. It would give me 34mpg average. I work as a courier and have to pay my own gas so I love civics. An accident took that car away. Now I have a 00 DX hatchback. But it's only getting me 31mpg. It has all the stuff from the ex in it. Like power locks, windows, mirrors, and even the motor and tranny. It had a short ram intake and DC 4-2-1 header along with a hi-flo cat and 2.25 pipping with a RS-R muffler at the end. It is stock quiet I'm not into the loud fart can noise. I found the stock airbox at the junkyard. It didn't come with the tube that goes under the air filter and loop to the other side of the fender. But its better like this because it will suck air from the engine bay now right? I've been reading warmer air is better for mpg's. Do you think the bigger exhaust pipping could be hurting my mileage? I mainly drive on highway from San diego to Los Angeles every night. I already did the the normal tune up stuff. Cap & rotor, wires & spark plugs, 5w-20 synthetic oil, PCV valve replacement, first o2 sensor, valve adjustment. Should I change the second o2 sensor? I was told it didn't affect air fuel ratios just the first one is this true? I would think I would get better average mpg's than my old civic since its the same drive train in a lighter chassis. Maybe I'm missing something. I appreciate your help

the hatchback doesn't weight much, if any, less than the coupe or even the 4 door acording to the msn.autos but the worse fuel economy is probably from the performance mods your new civic has...

does it have bigger wheels/tires than your old one? any other mods?

The cold air intake is definetely not helping and who knows what other engine mods are done to the car

Well like i said i took off the after market intake and put the stock one back on. But it's missing the tube that goes down from the filter box and loops to the outside of the fender. So its sucking air from the engine bay. I actually got some 14" enkei rims on it that only weigh 12lbs. So its the same size as stock and lighter. Could it be the second o2 sensor or exhaust making me loose mpg's?

i'm not sure...the intake sucking air behind the engine will probably do better than by the fender because it's getting warmer air than it would be by the fender

I thought a lot more guys would know about civics of this forum. But everybody seems real quiet.

I am by no means an expert, but from what I understand the larger exhaust is likely hurting your MPG, however it's probably not a huge amount. The lightweight rims are always a bonus. I wouldn't worry about taking in intake air from the engine bay, as GRU said, having a simple warm air intake should be working in your favor.

How is everything else mechanically? Brakes dragging, running rough, etc?

Do you have basically the same driving that you did with your EX? Simply a different route or type of driving can have a pretty serious effect on your fuel economy.

Since your car is a 2000, I would recommend getting an UltraGauge or Scangauge II. If you search the forums here they have been discussed a lot, and everyone I know who has gotten one has seen instant improvements in fuel efficiency. :thumbup:

Tire pressures? Alignment? Dragging brakes?

-soD

Possibly it has a si trans in it? And just do a compression and vacuum test on the car. Check vAlves.

Don't look for one solution; look for several. It could be the exhaust, or it could be a bad O2 sensor (if it's really bad), or several other things contributing. When my 1998 DX upstream o2 sensor failed, the results were really bad for FE. But it's also worth thinking about the car shape. The sixth gen coupes have a coefficient of drag of 0.32 and the hatch (yours) has a 0.36 because the back creates so much turbulence. That difference is costing MPG for certain. Beyond that, in general, performance mods like the CAT and the headers and the exhaust you have will result in decreased FE, or at least that's the consensus wisdom here. It makes sense too when you realize that their purpose is to increase flow to increase power by increasing fuel burned, right? Or wrong? Could also be tires. Are you running sticky performance tires or new tires? Low rolling resistance, or even good solid used tires with a modest amount of tread wear would be better on the fuel.

Now that you mention the drag co-efficiency, I noticed the hatch didn't feel as stable on the highway as my coupe did. The tranny is definitely an EX. It has the same gear ratios as my coupe. 00 si trannies are for B series motors so it wouldn't bolt up to my engine and they have super short gears. The tires are new. Nothing fancy about them just the plain stock size. I replaced the second o2 sensor(down stream) this morning. Now I'm off to the junkyard to find a stock exhaust system. I researched it a little last night. I found an article out of an import performance magazine that had a 99 ex with 60mm exhaust. The car actually lost power & torque between 2000-3500 rpm. The dyno sheet showed it didn't get the power back until after 4000rpm and then made more hp at the top of the rpm range. So you guys are right again. Thanks for your input. A can scan gauge is now on my christmas list. Please feel free to make more suggestions. Wish me luck at the junkyard.

I found a stock exhaust and put it on. Then I heard the exhaust leak I had missed. So on monday as I drove around the car actually felt really weak. It struggles to get up hills and no improvement on mpg's, maybe a little less if anything. I'm guessing it because of the leak. On tuesday I put the 2.25" exhaust back on and the car is back to running normal. I did notice that if I use 89 octane instead of 87 the fuel economy goes up from 31mpg to 33mpg. I know the EX motors have a slight increase in compression. So has anybody else experienced this or any ideas on whats going on

The second oxygen sensor has nothing to do with your mileage. It just checks to see if the first one is doing it's job properly. Big wheels, lower final drive ratio, larger exhaust will all cost you mileage. So will your driving habits and your driving environment.

regards
Mech

Not sure I agree with Old Mech about taller tires necessarily costing in FE, since Sentra-SER seemed to find somewhat different in this pretty thorough test: http://ecomodder.com/forum/showthrea...tml#post246084

As for the differences in mileage you describe driving around, I think you have to consider something else Old Mech said: that driving style (and conditions) will affect FE significantly. I would keep the stock exhaust on the car... that's "normal." The EX is only rated for like 127 HP, so it's not surprising it feels slower than what you might be used to. My DX in 106 HP, and it does scale hills a little slowly. But it does all it needs to do and then some.

The large primaries on the dc header are hurting your low end torque. Couple that with a small exhaust and you have a recipe for abysmal economy. The stock manifold is where the economy is to be had. The exhaust after the cat converter isn't of any importance as far as economy is concerned.

I've also found that mid grade fuel and 1 to 2 degrees of advanced timing can help torque under 3k rpms, where it's most beneficial when trying to hypermile.

With narrow tires at sidewall max, proper shifting, and optimum engine tuning it should be easy to crest the 38mpg mixed driving mark.