So as everyone probably knows, our Civics are naturally talented machines which can go long distances without refueling, sometimes challenging hybrid territories. A 30+ MPG rating is normal, even after years and years of abuse.

However, my CX hatch has been dropping mileage, sucking up precious gas like it's SI brother. Right now, I'm only getting a disappointing average of 28 mpg...

Mind you, this hatch has seen no racing or modification. All bone stock and driven like the old man next door...any ideas to what may be going on? Maybe it's the fuel? (Shell/regular)

I can't put a finger on it. One day it shoots a 36, and then for a few weeks high 20's.

 

Last full tune up? Crappy gas?

 

Check your electrical grounds, every single one of them with an ohm/multimeter to the battery ground cable. If they're showing anything above 0 ohms, they need to be cleaned. ESPECIALLY check the ground ohms for the body of the o2 sensor to the battery ground. If the o2 sensor's body ground (screws into manifold) is above 2 ohms, the signal becomes so weak that the ECU for the most part will ignore output and default the car to idle at 750rpm instead of the stock 670rpm. Next thing I suggest is replace the PCV valve since I can guarantee it's clogged on that car. Clogged PCV valve will decrease fuel economy and performance. If you find your acceleration isn't smooth or you can't easily maintain speed on the highway, then the next thing up is cleaning the IACV. Let me know if you have any other questions as I've got this same engine in my civic.

 

1) Plugs, wires, cap, rotor, ignition timing = tune up.

2) CEL codes?

 

True he could give it a tuneup but I think he should do the stuff I mentioned since it's the cheapest thing he can do for the car.

 

Those are all worthwhile maintenance items, but I don't think they will solve the problem. For example, a clogged PCV valve may cause idle and emissions problems, but is unlikely to have a large impact on gas mileage.

 

True, at the moment spending the cash to fix the problems will be real tough.
I don't have all the know-how to fix everything myself, so I'll have to refer back to H-T or dig up some other useful information. But for now, I'll go and take a look via your methods fleabag. Gracias! But keep throwing ideas out if you can, maybe there's something that we may have overlooked.

Here's the little rascal

 

The fact of the matter is that it costs money to own and drive a car. I too first look for $0 car fixes but in this case, I think you'll need to cough up some cash. Good luck.

 

I went from 28 mpg back up to 33....thanks to Gum Fuel Cleaner!

I thought the 110 heat was the source of the problem after getting a tune up...
my high is 37+

 

I think it's time you sold it to me.

 

Driving habits, routes taken and traffic can cause major fuel economy fluctuations. Plus many other variables impact FE as well.

Drunk post?

 

Nope just trollin'

I've noticed as much as 6 MPG difference between Sheetz (28 mpg) and Exxon (34 mpg)

 

our car's use a 4 wire oxygen sensor pretty sure one of them is a ground. and wouldn't the dvom show infinite resistance ?

 

there should be little to no resistance through the ground side of a circuit. ideally less than .2 ohms.

 

Wouldn't you like to know! My fuel economy plummeted 20% when my PCV valve got clogged, replaced it, fuel economy went back up to higher than where it was prior. Even better, not only did my fuel economy go up, but so did the performance.

 

They may be 4 wire o2 sensors but o2 sensors still all work the same in that they use the exhaust manifold as a ground. 5 wire (7 wire) or 1 wire, they all utilize the body of the sensor which is screwed into the exhaust manifold as a ground and if it's poorly grounded, the signal won't get to the ecu.

 

Given the pure emissions function of the PCV valve, what's your theory on how replacing the PCV would have such a large impact on mileage.

 

try all the easy things first. like the fuel treatment and filter. those are not expensive. im new to h-t and don't know how to quote. but they were right on the tune up, but also on cleaning your connections.

 

PCV valves are not 'pure emission devices'. They're crankcase ventilation systems that reduce the pressure inside of the crank case. It's important to replace the PCV valve not just for emission purposes, but because if you don't, you can cause the piston rings to go bad and to burn oil thanks to the buildup of crankcase pressure that isn't being released. In the 1960s, hot rodders discovered this phenomenon and would vent the crankcase pressure out to the atmosphere. Around the same time, the governments were trying to combat pollution problems in their cities and one of the cheap, easy solutions automakers discovered was to have PCV valves installed into their cars but instead of venting into the atmosphere, they would reroute those gasses back into the intake manifold.

I'm surprised I'm being asked this question since I thought it was pretty well established that clogged PCV valves not only hurt performance and fuel economy but can also damage your engine since the crankcase pressure builds up and forces its way through unintended passages.

http://www.aa1car.com/library/pcv.htm

 

Clogged fuel filter? Filthy air filter? Low tire pressure? Junk in the trunk? Driving with AC alot? Aerodynamics and weight reduction are things to look at when wanting more MPG..Perhaps rolling the windows up 75% while on the freeway and a good dump before the cruise!