25 mpg highway and dieseling is neither fine nor normal.

Code 90 indicates a problem with the EVAP emissions system. You might be able to get more information about the problem by pulling the code with a scan tool at AutoZone. Start by checking the gas cap rubber seal for damage and for leaking vacuum lines under the hood.

 

Gosh, well that's an embarrassing problem. The gentleman who filled my gas over the weekend neglected to tell me my gas cap broke.

 

In a sub-forum this seems like such a trivial thread. Anyways, I went under my car to check for any header leaks and I couldn't find any, though I have a very novice eye. However, I did check the entire exhaust line and I have a massive hole near my muffler. There's the main pipe that runs under the car, followed by a connecting joint, followed by the muffler... the hole is right after the joint (between the joint and the muffler). It's probably large enough to put my pinky into.

So, to start, this could be the light noise I'm hearing when I turn off my engine... I definitely know a rattle is coming from this area, but I don't know if the hole is merely leading me to believe a light amount of dieseling is going on. I have a couple questions at this point.

1. Could the hole in my muffler be causing any type of problems whatsoever, besides a mere noise inconvenience? Ie, lower gas mileage?

2. Is the muffler a fairly easy part to remove / replace? It looks like I'd just undo the bolts at the joint and unclip the rubber holding it in place, uninstall, and install new in the reverse order.

3. While I didn't see any leak near the header area, would it make sense to pull out the O2 sensor and remove the cover surrounding the exhaust manifold area so I can inspect it from above?

Thanks a million...

 

Long Thread here. Anyway, just like to share that I have a 2004 Honda Civic SI coupe (EX in US) Manual. I average between 720 and 810 kms to full tank of gas. City and Highway. My old 2003 Honda Civic LX coupe auto was about the same.

Now, when I first got the car. I was lucky to get 550 kms to a full tank. I did my own maintenence check up:

Oil Change, got rid of that shop cheap crap and went full synthetic Castrol 5W-20.
Changed spark plugs, figured what the hell, went with NGK Iri.
Changed fuel to Supreme. Regular just burns too quick for my liking.

Also put injection cleaner in it.

Within 2 weeks I went from 550 kms to 700+kms to a full tank. Throttle response is awesome. Got lots of get up and go. Though the Falkien **** tires I have SUCKS!!! less then 5k and the two front tires are almost buld. Considering they were new, they sucked even more on wet pavement.

 

I've pretty much tried all of the stuff you mentioned, and more (with the exception of switching to 91 octane per RonJ's advice):

- Mobil1 fully synthetic 5w20
- K&N oil filter
- NGK spark plugs
- new plug wires
- new rotor / distributor cap
- seafoam treatment (in vacuum tube, crankcase and gas tank)
- new PCV valve
- new air filter
- cleaned IACV (which is still acting a little lazy, maybe needs replacement)
- set timing (while CEL was triggered)
- checked tire pressure
- pulled spark plugs after leaving ignition at II; could not see any excess fuel
- checked all tubes I could see from above and below the car
- checked as much of the exhaust line as I could and found a hole in the muffler pipe (before it reaches the muffler, about the diameter or a dime)

So now I'm planning on replacing the muffler... it looks easy enough, and sounds like hell, though I doubt that'll fix anything since every thread I find on back-pressure tends to lead to it being a myth. Beyond that, I'm not sure if I should remove the cover from the intake and inspect the whole intake manifold / header region. For what it's worth, I still haven't replaced the fuel filter either.

Anyways, sorry for the long thread. I'm just trying to save a little money while in college. The guy who sold me this car was a shady bastard who aparently fakes his identity and gave me a messed up title that I had to battle my way through to clear up, so at this point, I still feel like I could be driving a time-bomb. :/

 

Shouldn't of had to go through all that stuff man. The only other thing that I did other then what I listed was put a CAI AEM V2. That's about it. If I had your car, I would just drive it until it drops, and save the money. Seriously, this much trouble, just isn't worth the time. My opinion.

 

No offense, but I'm not interested in drive-it-till it drops logic. I depleted my whole line of credit for the car and it's still in excellent condition. The body and paint and interior are spotless. The engine compartment is very clean, all the tubes are in excellent shape, the engine sounds good, there are no leaks. Even spending 2-3k on replacing the whole engine (which is obviously not necessary) is cheaper than spending another $4-5,000 on another car to get me through college.

I have been a computer tech for a long time now, and sometimes a lot of troubleshooting steps are required to find a seemlingly simple issue. While I may have a complex issue with my car, for the benefit of the doubt, I will continue simple troubleshooting until I've exhausted my resources, at which point I will consider more drastic measures, or as you said, drive it till it drops.

Again, no offense, but I've had the car for 3 months and I'm not ready to throw it out. If anyone (RonJ maybe) has advice for me based on my post 2-3 posts prior to this one, please let me know...

Thanks a ton.

 

The small hole in the exhaust pipe will cause some excess noise but is an unlikely cause of the poor mileage. Nonetheless, fix the leak as it could cause noxious exhaust gases to enter the cabin.

Based on what you have done thus far, I would recommend that you next measure the fuel pressure with and without the vacuum hose of the fuel pressure regulator connected, if this has not yet been done. Since replacing the ignition system components, have you checked whether any of the new plugs looks fouled? Were the plugs properly gapped?

 

Cool, thanks RonJ, you are awesome. Out of curiousity, would you say that $40 is a sound investment for a fuel pressure gauge? Doesn't look like they can be rented / borrowed from any of the local stores here in Oregon (Autozone usually loans that type of thing).

 

Not sure -- I haven't priced fuel pressure gauges. I have a Harbor Freight Tool store in my neighborhood that generally has very good prices. If you have one in your area of Oregon, have a look. You will also need a special adapter that allows you to connect the gauge to the Honda fuel filter.