Unexplained 100mi/tank drop! '03 civic HX

[FastFebur]

Hello everyone!
I've joined this site in hopes of figuring out what has happened to the
100-150miles i'm loosing per tank of gas. I've owned my 2003 Civic HX (5speed)
since 2007. It had 60k miles when I bought it.
I've always gotten just shy of 500 (sometimes 10+) miles per tank of gas which would always equate to 40-44mpg depending on my driving style. She now has 250k+ miles. I've never had any issue with her but since about 210k I've noticed a steady decline in mpg. Averaging 34-36mpg all highway.

Basic maintenance has always been performed by myself aside from last timing belt + water pump @ 200k which was done by Honda.
I have no CEL, oil leaks, smoke or anything abnormal that'd indicate an issue to me. No oil is burned (cx every 10k) nor is the coolant ever low. I keep my tires (fairly new) at around 40psi. I first thought that I'd maybe have a fuel leak so I kept it in my garage for a few days several times with varying levels of fuel in the tank and I wasn't able to sniff anything out. Last smog (California) passed.
Plugs were done at about 220k
Air Filter @ 240k
I conducted a cylinder pressure test not too long ago which all came up within spec.
She rolls freely when in neutral so there's no indication that theres something holding it back.

Im starting to commute 180 miles/day 2-3 times a week now and a drop in mpg like I'm experiencing is a determining factor of getting gas in a safe place (Sacramento) or a bad place (Oakland).

What should I turn to next? Maybe a cylinder leak down test??

[Daschicken]

Ever cleaned out the EGR valve and ports? I'll bet they are completely clogged at your mileage if previously neglected. Cleaned out the intake? It was worth a few mpg for my accord v6.

[M_a_t_t]

Another possibility is that it only leaks when the fuel pump is on. So you might not notice anything in the garage. Have you tried letting it idle and watching for leaks then? Does it smell like gas after you get out right after shutting it off?

I have also read (about my van's engine, it might be applicable to yours) that it is possible for gas to leak into the intake and use the extra gas there, but this usually comes with a poor running issue.

I think if it is running fine, then it must be leaking before the engine somewhere.

Just some suggestions,
Matt

[Xist]

That is a huge decrease. I cannot imagine it solely being due to EGR, but I would definitely clean it. It was easy. Everyone says they reuse their gaskets, but my plate gasket had carbon I could not remove. When I finally started getting it off, I was scratching it, and I figured carbon would just deposit there, but the EGR valve gasket is metal with a metallic paste. The ones mechanics showed on YouTube were just sheet metal. I was not confident my gasket would seal afterward. It was pretty cheap.

This was my experience: http://ecomodder.com/forum/showthrea...ges-33469.html

Many Civic owners have EGR problems around 200,000 miles, at least for my generation.

In that thread I compared my first tank after cleaning the passages to the previous ten, but I just figured out the first ten after the cleaning.

Before: 3,697.3 miles on 90.147 gallons: 41.01 MPG
After: 4,906.9 miles on 104.872 gallons: 46.8 MPG

That was a 14.1% improvement. Unfortunately, my fuel economy has fallen back down, but I have low compression.

Regarding the leakdown test, I cannot find anything explaining when you need to do one. I thought it was if the compression test indicated a problem and you were trying to track it down.

If you have not cleaned the EGR, that should definitely help. I am not sure I would do the leakdown test, except you have not localized the problem, so you might as well start testing less likely culprits.

[JockoT]

I recently came across a thread on a Jazz/Fit web site (struggling to find it or I would have posted a link) where someone had a similar issue. After doing all that you have done they tried a leak down test and found a slightly bent intake valve. Their compression test had not revealed the problem.
I am not saying you have a bent valve, just pointing out that a leak down test often reveals issues that a compression test doesn't show up.

[jcp123]

Aren't HXs also fairly sensitive to the condition of their O2 sensors? I don't know HXs very well but from the reading I have done, I do recall that they can be finicky with regards to lean burn and thus the inputs it reads to go into lean burn.

[Xist]

Oxygen sensors are the kryptonite of lean burners. From Honda they are supposed to be $400 parts.

Wait, what?

$516.15 shipped from Majestic.

It says $680.88 list price.

Since when?

$260 from Amazon

But it is made by NGK\NTK, and that is $144 from Amazon

Just saying they are expensive, but not necessarily as expensive as it might seem.

[FastFebur]

Oh yea, I forgot to add that I had replaced both o2 sensors about 15k miles ago. Not because i needed to but because i felt like they should be after 200k. Still no improvement

[Xist]

What brand sensors?

[FastFebur]

Both are Denso