2000 Honda Accord
260k miles
4cyl veetack
white

I've seen a drastic drop in mpg in the last few weeks. I'm losing about 80-100 miles per tank, or ~7-8mpg. Nothing has been changed mechanically, and my driving habits have been the same. Temperature is steady, just under the halfway mark. So I don't think the thermostat is stuck open. The car is running fine, acceleration is normal, idle is normal. I can't find any leaks. Ideas?

9/26/2014 1:12:00 AM

O2 sensor

9/26/2014 8:01:38 AM

Possibly a clogged catalytic converter. Have you ever had it changed? 260k miles is definitely enough to clog it.

9/26/2014 8:03:56 AM

^I'm wondering the same thing.

9/26/2014 8:34:28 AM

The cat was replaced at around 100k. I would assume that if an o2 sensor were bad enough to make the mpg drop this much it would throw a CEL. Up and downstream o2 sensors have been replaced in the last 50k anyway.

9/26/2014 9:20:01 AM

Guys, come on. The diagnosis is pretty simple...



He's low on VTEC fluid. You're welcome

9/26/2014 9:25:16 AM

This is actually a type r veetack, not backwards compatible with VTEC fluid.

9/26/2014 9:50:39 AM

this is probably a dumb question, but you just never know with some car owners...

how is your tire pressure?

9/26/2014 9:55:44 AM

the problem might be that your car is white. Maybe paint it another color?


How are you tracking your mileage? Curious of your methods. Do you always fill up at the same station?

9/26/2014 9:57:44 AM

also, even though your habits are the same, are you driving te same routes?

my 91 civic got 36 avg on my weekend trips home from NCSU, however around Raleigh it got ~24

9/26/2014 10:25:47 AM

Mileage tracking: Fill up, reset odometer, divide mileage by total gallons of next fill up
Different stations all the time.

Tire pressure is good.

I'm all over the place with routes, but I've been driving this car for 10 years, so I know about what mileage I should be getting at any given time. I did notice that when I have a tank that's almost strictly highway miles the mpg doesn't seem to suffer as much.

9/26/2014 10:43:20 AM

Quote :

"The cat was replaced at around 100k. I would assume that if an o2 sensor were bad enough to make the mpg drop this much it would throw a CEL. Up and downstream o2 sensors have been replaced in the last 50k anyway."

Both of the O2 sensor failures on my truck were preceded by a drop in MPG in the last few tanks of gas before the CEL came on. Both times I saw the drop in MPG and new something was wrong weeks before the light actually came on. It takes a lot of repeated faults before the computer decides to kick on the CEL.

If you're due for a tune up (plugs, check plug wires, fuel filter, air filter, etc.) it's not a bad idea to go ahead and rule them out. You can check resistance of the O2 sensors with a multimeter to determine a possible fault in the meantime.

9/26/2014 10:59:24 AM

What fuel do you fill up with? I'd imagine 87 octane? Sometimes older engines prefer higher octane fuel due to carbon build up within the engine. If you have some build up, there's a chance the 87 octane could make the engine knock more often, which would prompt the ECU to retard timing and reduce engine efficiency.

That's really a bit of a stretch, but it wouldn't take much effort to see what results you get with a different octane.

9/26/2014 12:29:15 PM

Quote :

"Temperature is steady, just under the halfway mark. "

I think O2 sensor is a good guess, but it could also possibly be the thermostat? Different car ECUs are different but many recognize that the car isn't quite warmed up and enrich the air/fuel ratio. I've had a stuck thermostat before and my temp needle would climb all the way to the half mark but never quite make it all the way. Mine threw the CEL eventually though if I remember correctly.

9/26/2014 5:13:08 PM

I was losing 2-3 mpg with a bad fuel pump o-ring

any signs of gas vapor leakage? smell, blackened fuel door, etc?

9/29/2014 4:33:56 PM

Quote :

"I was losing 2-3 mpg with a bad fuel pump o-ring any signs of gas vapor leakage? smell, blackened fuel door, etc?"

for about 20k miles before i sold my saturn at 200k, i started getting a blackened fuel door...it had dropped from low 30s (all city) to upper 20s and i never really thought about it (just figured it was getting old)

tmyk

9/30/2014 1:56:10 PM

slip a dryer sheet over the exhaust outlet and secure it with a rubber band. drive around with it on for a day. when you remove it there shouldn't be a black circle. if there is you know you are running rich. that will let you know you do have a a/f ratio error and not just shitty gas.

[Edited on October 1, 2014 at 8:29 AM. Reason : -]

10/1/2014 8:28:16 AM

Quote :

"slip a dryer sheet over the exhaust outlet and secure it with a rubber band."

dumb question...by "outlet" you mean pipe, right? i guess i figured that got too hot for a rubber band

also, if you have two pipes, i assume do one on each?

10/1/2014 9:33:07 AM

yes. the end of the exhaust pipe. shouldnt melt the rubber band. and only one side will be sufficient unless one side flows a lot more than the other.

10/1/2014 7:01:12 PM