So I have a '08 TL-S 5AT, I' ve had it for 4 years, NYC driven (Queens), it has 86k miles on it and recently the avg MPG dropped 3 MPG (from 17 MPG city to 14 and 31 mpg highway down to 24) since late last year and over this spring and summer I've done the following:
-Replaced the EGR cannister
-Replaced the semi dirty OEM air filter w/ K&N AF
-Replaced the OEM summer tires all 4 tires with All season tires
-Replaced all 6 spark plugs as well as the Ignition coil (Don't know. the brand of the spark plugs)
-Took it a friend who has a code reader and didn't pull up anything
-Used 1 bottle of seafoam in the tank and it seemed to fix it temporarily but it eventually went back to 14 mpg.
-Changed the engine coolant, Tranny fluid, and brake fluid just recently
-Took it to the Acura dealership for a simple diagnosis (the service guy was nice enough to do it for free), and all they found was a lot of build up in the EGR valve but quoted me at 1200 to take the engine apart to clean it and even warned that it might not fix the problem
-Oil life is at 60%
I've never put anything below 93 octane in the tank. Ive checked and did the math myself by resetting the trip odometer after every full tank of gas and then dividing the gallons (Usually 12.7 gallons) by the number of miles driven. Normally I was able to drive 216 city miles on a full tank of gas and now it's a good day if it goes up to 190. I haven't changed my driving habits but the sudden drop in MPG is annoying. I'm a little uneasy about shelling out 1200 just to clean the engine, and I don't know how comfortable or experienced are these local mechanics around me are with taking apart an entire engine and putting it back together.

Before anyone tells me that there's a 'million of these theads' I've checked most of them and most of them are dead end threads with no solution with no indication if their problem was resolved or not.
If anyone has any idea if clearing up the EGR valve worked out great please let me know, im at wit's end.

 

This.

 

I changed those in response to the bad gas mileage it couldn't be those because it hasn't gotten worse or any better

 

Or if anyone has any tips or suggestions on what to try next is highly appreciated.

 

Plug in a reader and check short and long term fuel trims/O2 sensor activity.

 

Change both of your upstream O2 sensors.

 

Post your MPG and your average MPH on the MID. Judging by miles driven gives no indication of how it was driven.

When were the tires changed....before or after the MPG drop?

 

On the MID it shows an avg mpg of 14 and 14 mph and most of this is stop and go traffic, but the thing is that it used to hover around the 16 to 17 mpg range in the same traffic
And the tires were changed in response to the mpg drop.

 

When I brought it over to my friend's a while back, it seemed normal but I'll try again.
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I'll try to see if it has anything to do with the O2 system and if it's not it's definitely the EGR system. But could it still be the O2 sensors even if it doesn't pull up any codes?

 

Yes, O2 sensors have a tendency to get lazy without throwing any codes.

 

Thanks I'll definitely look at the O2 sensors more closely

 

Btw the way, does anyone know what the part number is for the upstream O2 sensors and how many of them should I buy?

 

I would find out what spark plugs you put in. It is highly recommended that you stick with the OEM plugs, which are NGK Iridium plugs. I'm not 100% sure whether lower quality plugs will impact MPG like you have noticed, but I know the Iridium plugs are recommended because of the high compression ratio of our engines.

It could be an O2 sensor, but I had one actually throw a code and didn't notice any significant impact in my MPGs. Is the car running rich?

 

The shop were the ones to install the spark plugs and if i go and ask they're probably going to remember, and I think it is running rich. But the strange thing is that it's throwing any codes, and like somebody said on here it could be just that the O2 sensors are just lazy.

 

You could pull one of the spark plugs out and check. The front 3 are pretty each to get to.

My old car was running rich for a long time before it threw a code for a bad O2 sensor. It didn't read out MPG's and I never tracked them, so I'm not sure how significantly MPG's were affected during that time.

I also had the cat converter go bad shortly after replacing both the O2 sensors on that car. Someone else may be able to offer more insight, but I'm not sure what, outside of O2 or bad cat, would cause the car to run rich. Maybe a faulty injector? I know I've seen a few threads of people having faulty fuel injectors. I think there is a relatively easy way to trouble shoot them as well.

I don't want to start pointing you down the wrong path though. You could potentially throw a bunch of parts at the car and still not fix anything. I know I have done that with mine on an issue I was having.

 

*Not going to remember
*Not throwing any codes

 

Grab a ratchet, spark plug socket and a 6 inch extension and pull those suckers out to see what they were.

 

I would check the plugs before you spend any money on anything else.

I am not sure if the plugs would cause the significant MPG loss you are experiencing, but it's worth looking into. A set of 6 NGK Iridium's can be had for about $50.

Tires can also cause MPG loss, so potentially this is a two-fold issue. What tires are you running?

 

The tires are hankook ventus s1 noble2 all season tires and they're new tires but like I said before the tires were changed after the MPG started to drop and saw almost no difference in the MPG. One bit of information I almost left out; the mpg actually dropped to 8 on the MID initially and gradually rose to 14 after the ignition coil and spark plugs were changed but I will give updates in the coming weeks on what's going on.

 

So it turns out that the spark plugs were Densos and I am now replacing them with the NGK laser iridiums. Will it make much of a difference?
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Yes.

 

I have an 08TL and I never got the mileage that people reported. I just found out that my front brakes rotors is really hot after I drive the car on the freeway or driving 30 minutes or more. Apparently my front pads are dragging on the rotors which I account for my lower than average MPG. This is happening on my 2002 Honda Odyessy also. This might be happening to the OP too.

What can I do to keep my pads from dragging? Bleed my brakes? Rebuild my calipers? Any ideas appreciated.

 

Rebuild your calipers. They're seized

 

Replacing the spark plug w/ NGKs is doing the trick so far. It's currently a little under the 1/4 tank mark and its only gone 30 miles since the last fill up.

 

I'm not sure how you can draw any conclusions from 30 miles of driving.

 

I too have seen a drop in mpg with my 07 TL-S... tho not to the extent you did.. I am seriously considering checking to see what brand spark plugs I currently have.. make sure they are NGK Iridiums.. plugs were changed a while ago but I have no idea what was put in... so at the same time if I swap out the plugs I figure I may as well put new ignition coils in too... My question is regarding the coils.. is it imperative to replace with OEM coils (which cost significantly more) or can I go with an off brand? OEMs run about $50/.. where a set of 6 off brand may cost $60 total... I don't always go cheap but that is a significant difference..

thanks

 

A few comments:

• The consensus is, if you're going to change ignition coils, use OEM; that said, I believe I've read reports of good luck with Denso coils as well.
• Coils don't gradually go bad, either they work or they don't.
• As a general rule I recommend against changing coils as a prophylactic measure; replace them individually when they fail. Why? Because I've seen Honda OEM coils go way beyond a quarter of a million miles, and others fail in less than 100,000 miles. If you replace them early the odds of having a single coil failure in next 100,000 miles is virtually identical to not changing them at all.

 

Before I changed the spark plugs, at 30 miles, it would be near the 1/2 tank mark and that was with some highway miles mixed into that, so that looks like a good sign if its all city driving and its just now reaching the 1/4 tank mark.

 

Thanks for this.. I'll just check the plugs and replace if needed

 

To be quite honest, you cannot make any determination until you've driven at least three full tanks and then averaged them together. The only other option is if you routinely reset the trip computer at every fill-up, then you could go by any relative difference.

 

I do. That's why I'm saying it lol