R18 MPG Issues: Please See and Discuss

sk8nava

sk8nava

Well-Known Member
393
343
Slugger City, KY
Vehicle Model
LX
Body Style
Sedan
Ok guys, so recently, in December, I installed my HPS SRI with my P2R TBS. I have noticed some things after the install aside from the "Tighten Fuel Cap" error message. After installing them both, I noticed my MGPs went down to about 29-31mpg from 34-35mpg; not too bad. While messing with the Fuel Cap issue with taking the TBS on and off to rid of the error message, it finally went off. I decided to take the TBS off semi-perminantly to see if the error mesage will come back on....BUT...I've now noticed with just the Short Ram Intake on and no TBS, my MPGs we down to about 19mpgs city and about 23-24mpgs highway and that is with the ECON button on with cruise control...

DISCUSS:
Are there any other R18'ers, or Si Owners, out there with an intake/TBS installed that have notice a dramatic change in their MPGs?
Why would I lose around 15mpgs with just an intake compared to the intake WITH the TBS?

Any insight is appreciated from you guys!
 
winter gas additives would hurt mileage some, but not 15mpg. Did you pull the negative off the battery after the removal? You can let it idle for like 10 min or so and try to get the ecu to relearn the idle air/fuel mix?
 
I was going to say perhaps there is a leak somewhere and your stoich (A/F) is way off. A leak after a sensor or something. That is a horrendous loss.

I dropped 3ish when I played with my intake just because the stoich was altered but it corrected after an ECU reset.
 
Have you tried to put the stock intake back on and check your mileage then?
 
If the problem is worse with just the intake its maybe a problem with the intake functioning properly after the mass air sensor. The TBS perhaps was sealing a point better than the intake alone?

( on another note... have you tried switching to a wheel that fits properly? Hahahaha im jk man.)
 
Well, like i said I lost about 5mpgs after the install of both the SRI and TBS...I rode my stock air box for a day so I could mess with the intake a little to make sure the rubber housings were still good in the filter and that the TBS gasket was still good. My mpgs then only went up a few, but when I installed everything back on with the TBS I was still getting the random on and off "Tighten Fuel Cap" message where it would be on sometimes then be off sometimes. I uninstalled the TBS and after i was done checking everything to make sure of proper install, I turned the car on to check that it worked, error light still on, then turned the car off and disconnected the neg terminal for about 15 minutes then turned the car on and let it idle for a bit before i went and drove it around. After I turned it on, the light was off and still hasnt come back on since which is good, but now I'm fighting to get good gas milage. I'm creeping at 23.6 right now and that was on the way to school this morning going 65 cruise control with the econ button on..
 
I don't think single trips would be a good indicator of mileage. I would be basing it on a full tank (iMid and manually). Also, are you located somewhere cold and letting the car warm up before leaving?
 
Its been in the teens here for a couple of days, its 17 degrees right now and its mid day...but i always warm my car up in the morning
 
sk8nava said:
Its been in the teens here for a couple of days, its 17 degrees right now and its mid day...but i always warm my car up in the morning
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Depending on the length of your trip to work and how long you let it warm up, could affect your mileage quite a bit.
 
Hmm...ok well I'll ride on it for a couple more days then see how everything is going on it then i guess i check back on here. Today I just figured that it would be different since i was on the highway for about 25 minutes to school at a steady pace.
 
Hate to be a stick-in-the-mud, but I just don't see enough advantage to change from stock........... Just my 2 cents.
 
Monk said:
Hate to be a stick-in-the-mud, but I just don't see enough advantage to change from stock........... Just my 2 cents.
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Not sure about the 9th, but when I swapped out my stock intake in my 7th, for an AEM V2, it opened up the engine compartment A LOT. Definitely made working on it sooo much easier. Of course, that is not exactly worth a $300ish dollar investment.
 
My winter fuel economy is better than that, including 80% stop and go in the city 5 days a week, letting it warm up for at least 15 minutes a day before heading to work. Winter additives kill economy, as do engines that never get hot. Last night it hit -39C here.
 
If you are referring to a issue with your average MPG going down and not just overall , MAS could be a good reason your ecu would send excess fuel but also if your warming up your car and you sit in it you can literally watch your avg MPG's go down. Your cars ecu when at idea will register 0MPG'
Don't know if this will help or not but good luck with coming up with a solution !
 
Oh is your mileage being taken off the iMid screen or is it when you fill up? miles driven/gallons in?
 
When I get in my car and go, my mpg on the iMid used to be at 35-36 driving highway and 34-35 city/stop&go
When I installed the intake and TBS, the best mpgs I've gotten were around 31. A few days later I got the tighten fuel cap message.
I took off the TBS and now I'm getting between 19-20 city and just hit 24.1 today on the way to school.

Its when I'm driving, but when I get off the highway and hit the lights and come to a stop, it jumps down back to around 20mpgs. I filled up yesterday and it only said I had 355 miles to go on that take, but when I used to fill up before with the intake and TBS, It would say 450-460 and with the stock air box i would get around 480...so from the stock 480 miles to a take down to 355 kinda hurts ya know
 
I put in an SRI and mine has been getting around 390 to the tank since then (I havent had the Eco mode on since i drove back from the dealership this summer). I got a tighten fuel cap message a few days later and i took it off and put it back, ever since then it hasent been back on. I get 31.9 stop and go driving with 3 or 4 30 min highway trips back home from school every month. And i dont drive very fuel efficienty so I think that there has to be a different problem not caused by the intake. just my thoughts on it
 
sk8nava said:
Its been in the teens here for a couple of days, its 17 degrees right now and its mid day...but i always warm my car up in the morning
Click to expand...
And i never drive unless the blue temp light goes off either
 
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