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Hello all, I'm new to this forum. I wanted to survey members here about my car's issues. Lately I've noticed my 2004 Honda Civic EX Coupe with 258,000+ miles has been suffering from poor fuel efficiency and slow acceleration. I have been regular with the oil changes. I used to get 29-30 mpg, now it's around 23-25. I used to get around 200-210 miles at half tank, now it has dwindled to 160-170. I also noticed acceleration problems. For example, coming from a stop or making a turn, the car struggles with accelerating. Top speed is not a problem though; I can still cruise it at freeway speed. Just the acceleration and the poor fuel efficiency is disturbing.

Any ideas where the problem might lie and what I can do?

 
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on the engine side, air filter and spark plugs could attribute to lowered fuel economy. Additionally, due to the acceleration issue, pesky vacuum leaks can cause a world of harder to diagnose acceleration issues. Timing could possibly be a factor, too, and considering you're at 258k, do you know the last time the timing belt and water pump were replaced? I think the manual calls for every 80-100k or so? I can't remember off the top of my head.

outside of the engine bay, tires, wheel bearings, and brake caliper lubrication (e.g. sticking brakes) could attribute as well.

Worst case scenario, head gasket leakage can have a hand in it, too, especially considering the D17 was prone to head gasket failure.

 
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I just replaced timing belt and water pump about a year ago. I think I'll start with the easier ones: air filter (I know I replaced this a year ago too) and spark plugs (not sure when the last time was). Tires and wheel bearings are new (1 year old) as well.

I'm concerned about what you said about vacuum leaks or head gasket failure. I don't recall any work being done on the engine itself, for 258k miles. All I've done were just maintenance stuff (oil changes, etc.).

The car still runs well, just really awful fuel economy and acceleration.

 
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Cat might be clogged

 
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when last o2 sensor was changed?

 
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I don't think it has been replaced. Not that I know of...

 
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sadly yours is a wide band sensor (up to 2002, they are cheaper narrow band ones), but a failing o2 sensor can make mileage drop down considerably while not triggering a CEL
used to be like 250 for the primary sensor

 
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Might be worth using a scanner to check long and short fuel trim values. This is how much less/more fuel the ECU is adding based on the air/fuel ratio coming from the sensor.

 
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Hello everyone, I'm resurrecting this old thread I started. I realize I have left some posts unanswered.

Anyway, since then, I've put on another 5,000-6,000 miles. I'm now at 264,000 as of today.

I recently had EGR valve and another valve that goes along with it (starts with an "I", but I forgot what it is). I haven't replaced the O2 sensor, not that I remember. This might be worth looking into.

The mechanic I brought the Civic to noticed my brakes have seized and did an overhaul. It's been running fine since then, however, I still have lost around 20%-25% of my MPG (now averaging around 25-27 when it used to be 30-33). I think it slightly improved, but nowhere near the 30 MPG I used to get.

I wonder if age, mileage, wear and tear are the cause.

To the experts on this group, should I just accept this fate?

 
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Do you have a suggestion where to get this and what brands are good?

 
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any cheap bluetooth dongle will do, paired with smartphone to view live data.

And make sure to check fuel pump housing cracks. My civic went from 27mpg to 30 -- for every full tank 1 gallon was spilled before that.

 
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IACV valve.
I still think it could be the sensor...
After 2 years, you might have used fuel enough to pay for the sensor...
Always use OEM electronics, Denso or NTK, Hondas no like aftermarket electonics like snsors, alternators, etc

 
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I just checked my notes, it was the EGR valve and PCV valve. My mechanic said both go hand in hand.

I think I will follow your suggestion on the oxygen sensor and will replace both. To my knowledge, they haven't been replaced yet after 21 years / 264,000 miles. Just checked, it will cost around $300 for Denso, both upstream (I think it's called air/fuel ratio sensor) and downstream (the one by the catalytic converter).

Many thanks for your suggestion!