Hello all. I'm working on a 99 Ex manual trans with 80k miles. The car has no drivability issues, idles smoothly at normal speeds, and pulls strongly as it should. The CEL is not on, nor has it been yet. The problem is that the fuel mileage is currently under 20 mpg in the city. It is running rich enough to get the oil smelling like gasoline. I am changing the oil frequently in hopes of not killing the bearings.

I put in a new O2 sensor after I first noticed the fuel mileage, and it did not make a huge change in performance, though the engine did feel a little sharper. The mileage temporarily increased, but is now back under 20. This leads me to believe that the O2 was okay all along and we are dealing with an intermittent problem being caused by something else. I checked throttle position sensor, intake air temperature sensor, and map sensor. All are in spec. I did plug wires, cap and rotor, and regapped plugs to .040. There was no significant change in drivability. The tires are inflated correctly. The fuel pressure is in spec and does not leak down significantly with engine off, as tested several times, even overnight once.

I believe the results of the fuel pressure test rule out a leaking injector or a leak in any line between tank and fuel rail. I could be wrong about this. If anyone knows any other check relating to this, please chime in. Otherwise, fuel pressure seems to respond normally to changes in engine load and unplugging and restoring vacuum to the fuel pressure regulator, which I believe indicates correct function of the regulator. I suppose there could be a leak on the return fuel line from regulator to fuel tank. I will check on that tomorrow.
I checked as well as I could for exhaust leaks, and could not find any before the O2 sensor. I put a timing light on it at one point. I did not attempt to disable the advance, nor could I find any timing specs, so my measurement is largely irrelevant to me. The timing did seem to be in a normal range, and to respond as it should to zipping the throttle with the car stationary.

I installed a test pipe in place of the cat, and used the bung for the second O2 to install a wideband O2 sensor. Fuel mixture was good enough under all conditions to convince me that the car was running fine and I had just been chasing my tail. I never saw fatter than 12.5:1 at wot and it was normally closer to 13:1. It was normally 14.5:1 to 15:1 at idle, and leaner than 14:1 while cruising at light throttle. That all seems about right to me. Compression and leakdown are in spec as well.

We drove the car about 120 miles after checking the fuel mixture, and found the fuel mileage to be under 19mpg. I took the oil cap off this afternoon and found a large volume of gas to be coming from the valve cover. This would be indicative of a lot of blowby to me, except that the compression and leakdown test okay. I'm putting a Honda pcv in it tomorrow, but I am pessimistic about how much of an effect it will have.

I apologize for the long post, but I have done nearly all I can think of to diagnose this thing, all without finding any definitive issue with the car. I appreciate any advice anyone can give.

 

Does the CEL work?

It's hard to reconcile the air-fuel ratios and the oil smelling like gas. You might want to double check for an exhaust leak.

 

The CEL does work. I replaced the pcv valve today and saw no change in the amount of gas coming from the valve cover with the oil cap off. I''m going to check the compression and leakdown again. I can't remember what the results were last time, except that they seemed okay to me. I don't know why I don't write these things down. Something has to be wrong with this engine though. There is a lot of gas coming out the oil cap, especially once the engine is warm. I unplugged the pcv hose at the manifold, and there is definitely vacuum at the port. There is also gas coming out of the hose with the oil cap on. This leads me to believe that the pcv is functioning correctly. There is no way a normally functioning engine should have this much blowby. I must have done something wrong checking the compression and leakdown. I'll have time to give it another go tomorrow.

 

try to throw a fuel carb cleaner see how that goes. that what help my mpg go back up after i have low mpg

 

i had the same problem 19 mpg and i fixed it and now it around 38/39 with ac on. here the a check list so i can help you.
1. what is the fuel pressure at when the fuel reg. is plugged and unplugged. make sure the ac is not on, radator fans not on when you check the psi on the fuel.
2. check every spark plug, closely examine the tip, check if one plug has more carbon deposits. you may have a faulty injector.
3. is this the orignal motor
4. and most important what is yout tire SIZE? p185 65 r14 this is what made 26mpgs to 38mpgs seriouly

 

Well, I'm still going around with this thing. I retested the compression and leakdown, and they are in fact in a normal range. There seems to be nothing wrong with the engine. Also, while driving the car, I noticed that with a full tank the gas mileage appeared at first to be okay. Then I left the car for about an hour while I was in a store, and when I restarted the motor I noticed the fuel gauge had moved significantly.

I pulled the tank yesterday, and found that the undercoat on the bottom of it had turned sticky like tar. The undercoat on the rest of the car was not like this, except along the driver side of the tank. It would appear that a significant amount of gasoline has leaked from somewhere and wet the undercoat where it is sticky. I can not find any evidence that gasoline has been on top of the tank. It does not appear to me, after a fairly thorough examination, that any high pressure or return lines have leaked.

The charcoal canister is stained and appears to have been wet, though I can't be sure it was gasoline. It could just be spilled oil. I cleaned it up and will check it again. Anyway, I can't find any obvious source of a fuel leak. The filler neck and all connected tubing appear in decent shape and do not seem to be leaking. I suspect that something is amiss with the evap system which is allowing gasoline to escape somewhere. There is a small rubber line going into the framerail which appears, judging by the pattern of the sticky undercoat, that it could be the source of the leak. I disconnected the hose from the framerail, and wiretied it in a position that will allow me to see any puddle of gasoline it makes if it leaks. I will keep an eye on it after I drive the car again.

I will be too busy to look at the car again for a few days, so for now the car is reassembled for my father to drive. I told him to keep a fire extinguisher handy, which is wise to do anyway. I have made a habit of it since I witnessed a friend's 90 Civic burn on account of a faulty wiring harness. We are certainly asking for trouble driving around leaking fuel. It has been a while since I worked on newer cars, and I don't remember much about OBDII evap systems. I will have to do some research about what can go wrong and how to test it. I did mange to plug all the lines on the tank and put some air pressure in it. It held pressure, which tells me the tank itself is not leaking. If anyone can tell me anything about what can go wrong with our evap systems without turning on the CEL, what could cause the leak I am seeing, or any related tests I can use to check operation of individual components in the system, I much appreciate if you could help me out. I feel like I have a lead as to what may be causing my issues here, but only that. I have no idea how to proceed in diagnosing this thing without knowing more about it's evap system. If no one can give me any direction here, I may try to track down a good shop manual for the car. Any suggestions on that? Sorry for making another long post. Hopefully I get this thing straight soon. At this point, I don't think there is much left to be an issue other than the evap.
On a separate note, I replaced the cat-back with a stock replacement system and can not believe how much quieter the car is. I had not realized how loud it had gotten, as it happened gradually. I almost miss the sound of the leaking exhaust. It really sounded good.
Tunerboy, the tires on the car currently are 185-60-15. They are bigger than stock and will give lower speed and odometer readings for a given speed and distance traveled. This will also make fuel mileage look better than with the stock tires.

 

I put a small bottle on the end of the disconnected hose, and after driving the car several times, have not found any fuel in it. I have to do some research on this before I know what to look at next. Does anyone know if a Helm book #61S0307-link -would give me some details on the the evap system, or should I look at a different book?

 

Seems like most threads with poor gas mileage for Civics it's always the EX model. My 98 EX has 20-22 city with 120K on it... not acceptable. Hope you figure it out so I can try your remedies.

 

Douche Baggins I won't ask what made you come up with that name. That is an interesting thought on the Ex thing. I have looked at many threads detailing attempts to diagnose poor fuel mileage. I had not paid much attention to the specific model of the cars with the issues though. I need to do some more research to know what differences there are between Ex and other model Civics, and between those of different years. I am still not sure what to do on the Helm book. Any suggestions are appreciated. If no one tells me otherwise, I am buying the one I mentioned in my previous post. I certainly hope it will be helpful to me in some capacity, more so than the likes of Haynes at least.