In the past month @ 100k miles, I changed out the oem spark plugs to the NGK Iridiums. At first I thought I was getting great gas mileage because I was showing 170 miles at half a take. It used to be 150 miles. When I'd fill the empty tank up and do the calculation, its slightly lower than before.
Last week, I bought a the K&N drop in filter. I notice an increase in horsepower but I think the mpgs are slightly lower because I'm at a half tank at 140 miles.
The only mods I've done are removing both resonators, and I installed a diy ram kit - tube running from the stock intake box to the fake vents where the fog lights would be. I made a few cuts in the fake vent and used a dremel to make the opening cleaner/better.
Does anyone have any ideas on why I'm losing mpg's? I don't really have a heavy foot.

 

Probably the DIY ram kit.....personally if I were you I'd just get a real CAI

 

what kind of brand of gas are you filling up with? did you do the 100k service? and seafoam it

 

I dont think dropin filter decrease mpg.

 

i bet u got less gas mileage is bc u were flooring it to hear the sound or see if the k&n gave u any hp. which is slight to know and dont know how u can tell. i get around 175-190 miles at half tank, on a completely stock tl-p

 

thats what im thinkin. if he noticed an increase in hp... he will also notice the decrease in mpg.

 

I used seafoam at about 92,000 in all three spots. I really don't drive fast or started driving fast after I put the filter in. I'm 29 and have a kid, so I tend to drive conservatively
Before I put the filter in, the car seemed sluggish off the start (which is probably from the original filter getting dirty from the ram air tube). After the filter change, the "sluggishness" was gone which caused me to not press the gas pedal as hard when the light turns green, etc.
I was thinking about doing the seafoam treatment again today.

 

honestly there is no reason to lose any MPG with a new air filter (or gain really any IMO). I would not look at the gas guage to judge your mpg. do a few full tank runs and calculate it manually.

 

This past weekend I filled it up and it was 21mpg (city), normally it is 22-23

 

was it the exact same driving? either way an air filter will not make much of a difference other than on the highway - may increase the mpg very slightly. I have had a K&N filter for over 50k miles and my mileage is the same unless I alter my driving or when winter gas first comes. 22.5 (70 city 30 highway) and 28 - 29 all highway. I would see if the mpg starts to go back to normal. maybe your ECU needs to get used to all that you have done to the engine. confirm your tire pressure has not changed also

 

It may not have been the exact same driving, but my mpgs have been pretty much the same for the last 10k miles until I added the plugs and filter. I did the seamfoam treatment last night and things already seem better. My idle use to be slightly above 750, now it is slightly below. My CEL is on, but I can take care of that if it stays on.

What all is done for the 100k miles service?

 

One thing that I forgot to mention is that my radar detector would go off every time the rpms would go over 5k. I guess the cigarette lighter sends a signal out when rpms get too high....

 

well if ur RPM's are goin over 5K then ur definitely flooring it dude....

 

That was during the seafoam treatment

 

seafoam treatment really does wonders to smooth out the idle for sure. you should look at the DIY section to find other tips. EGR port cleaning is a must and a very easy DIY that you should try to get done.

check your manual for the 105k service, but mainly you need to get the timing belt changed - at the same time they usually flush the coolent, change the H2O pump and the other 2 belts. this will cost around $650 total give or take a few hundred, so shop around.

also getting the valves adjusted is recommended, I have not done this but the mechanic that did change my timing belt said it is usually needed around 100k also - about $300

other than that if everything is running smooth it sounds like you are good to go. I have 154k on my '99TL and the only "issue" is the engine mounts are bad - original transmission still

 

hey i just got a 99 a few days ago with 133k on it. i think i might need motor mounts also. when it turns over every time, itll throw a small vibration thru the car like a hit a pot hole. do you think it is the motor mounts?

 

i just removed my intake resonator today, what model is the K&N filter that fits our cars? i have 02-tls

 

dont know exact part number. if you go to a auto part store theyll look it up for you. i went to advanced auto and it was about $48, but they would have to ship it in