94 Civic VX - Not great MPG

[paulhale]

I have been popping in here for a while, but this is the first time I'm actually posting. Thanks for all the great info.

I got my VX a little over a year ago with about 137k on it. It had been lowered dramatically (read "dropped on its nuts"), had super wide 17" rims, had a "customized" air breather, and had a non-standard exhaust (read "super loud muffler" and "no cat"). I had it raised up as close to spec as I could get it, replaced the wheels with the lightest aluminum 13s I could find, and fixed all of the exhaust leaks. Also replaced plugs, wires, dist cap & rotor, brakes, air filter, valve cover gasket, fuel filter, fuel pump, steering column, cleaned the O2 sensor, and had the stock 5spd torn apart and rebuilt with all new seals and bearings.

After some serious commuting, it now has 167k on it. For most of that time, I was getting 43 MPG pretty consistently (80% highway driving @ 65MPH). In the past month or so, I have noticed a drop to about 36-38 MPG. Where do you guys thing I should start looking for issues? I have no CELs and the car doesn't run any different than it has been. The car runs Mobile1 10w30 High Mileage.

[add|ct]

Drive only 55 MPH on the highway for as much of your next trip as possible and report back your new MPG.

Check your tire pressure, EGR valve/ports clean? Good oil choice, IMO, and that won't affect MPG much so no problems there just keep the intervals sane for your vehicles conditions seen.

Otherwise, might be a good idea to look at the CVs from my recent experience. Check the condition of the boots at all 4 locations(2 wheels, 2 by the tranny/engine interior CV). Just a few quick thoughts. Some others may offer more advice. It's possible you could be having exhaust leak re-surface, since you are apparently away from stock and since there were leaks present that you had fixed, without a CEL.

[VXman]

It may help to hook up a Lean Burn monitor. here is the link http://www.gassavers.org/showthread....ghlight=vx+laf
to see what the O2 sensor is reading. I know mine is getting in the high 30s mpg driving normal. But with the monitor i was able to see when i was in lean mode and kept it there as much as possible n was able to get 55 mpg.

Now with the monitor i was able to see that the O2 was not reading as it should. I got readings I think were to wide. also it went from very lean to very rich like 1.1 to -1.2 with nothing between so in other when it wasn't in lean then it was in full rich. there should be a in between and it should be more like -.6 to +.4 i belive.

So, in other words it may be your O2 that has gone bad. I can let you know how mine works after I get the O2 and post the new readings and also the new mpg's...the mpg's my be a while though due to I am doing body work and paint to my car this month but I should have the readings up....i hope this helps

[benfrogg]

If the exhaust isn't stock and the factory manifold was removed, the 02 is pretty much going to be aftermarket. If that is the case, it's probably 3 or 4 wire, which makes the lean burn capabilities of the car not work at all. That would put you in the 30's or 40's depending on other issues.

If you haven't already, read this ENTIRE thread, originally posted by me:
http://www.gassavers.org/showthread.php?t=12045
There's tons of info about all the things that you'll find wrong on the vx and what to do about it.
B

[paulhale]

The O2 sensor is the stock 5-wire.

I know there is one small exhaust leak that has developed recently that I have just not had time to fix. This header is a 4-2-1 performance header and for some reason, there is a disconnect half-way down (between the manifold and the part where the header connects to the rest of the exhaust). The leak is at this disconnect point. I am planning on fixing this soon, just haven't taken the time.

I know the front CV boots are fine because I looked them over this weekend during an oil change. I will check the rear ones.

What tire pressures do you guys run? I have heard of people over-inflating tires for improved performance, but this just seems dangerous to me.

Ok, I'm going to go read the two threads that you recommended.

[benfrogg]

55psi rear (20psi above sidewall) and 65psi front (13psi above sidewall).
B

[shag wagon]

You have a leak at 2. You probably wont fix this until you have the flange ground or milled flat. This will probably piss your o2 sensor off. You either need to get a stock manifold on there or get a name brand header. Some retard put a header on my CRX HF and I guess they thought they were going to get some serious speed. I mean I dont understand why people install aftermarket performance accessories on a car meant strictly for gas mileage.

So definitely fix that leak. If all else fails, its a Civic, you can weld the flange together and you won't see much of a problem with it going back on. But I like being able to disconnect it at the 2 so I won't catch hell with the whole thing having to come out.

[paulhale]

Ok, after reading through that 6 page thread, I wanted to clarify a few more things. The timing and valve lash have been checked. The clutch assembly has been replaced. I haven't done anything with the EGR ports, intake manifold, IAC, of PCV valve. I have sprayed cleaner into the throttle body, but haven't taken it apart to clean it.

Another thing I remembered while reading all of that is that I put some Water Wetter into the radiator. It wasn't to fix anything, I just had it in my garage and thought it couldn't hurt. This was relatively recent. Does artificially lowering the temp of the coolant cause MPG issues? I saw something in that thread about issues using a thermostat that was rated for the wrong temp.

[benfrogg]

Cleaning the IACV/ intake manifold/ egr ports will help if they are bad. Mine were fine and had never been cleaned at 187k. They were dirty but not plugged. A friend's crx with 150k (?) or so was completely clogged. It's hard to say why some are and some aren't but it's worth the time either way.
Maintaining lean burn as much as possible was a big one for me. Down shift on inclines if it means keeping it in lean burn. As far as I can tell, lean burn works in all forward gears.

I've been meaning to do a writeup on specifics about the lean burn driving qualities. I've found a couple of different "modes" while driving on mostly flat roads and mostly mountain type roads. The flat road type is always better for FE, but it's hard to maintain all the time. It requires an EOC or key off/restart to get back to it.
B

[bowtieguy]

Originally Posted by paulhale

Does artificially lowering the temp of the coolant cause MPG issues? I saw something in that thread about issues using a thermostat that was rated for the wrong temp.

yes. if you have a 170 or 180* thermostat, it would be beneficial to install a 195 one.