logix7

was searching through all the gas/fuel mileage related threads looking for some suggestions. my 92 is getting like 10-12mpg here in the chicago winter. i know these cares werent made with fuel economy in mind but hell, my v8's get better mileage. I came across the thread concerning leaking caps in the ECU but it didnt specify if the 92 model year was included for the sc300.. i believe it said 93-96 or something like that. so I havent checked there yet. only mod my car has is the BFI, otherwise its totally stock with 209k on it. I had a stuck brake caliper and took care of that. All wheels spin freely. regular oil changes and diff fluid is fresh. have not changed trans fluid since i bought with 204k on the car.

besides the basics (plugs, wires, cap, rotor, pcv, fuel filter) what else should i look for? o2 sensors? clean the throttlebody/intake? i have new plugs to put in and a pcv for when i seafoam. and will follow with a fuel filter down the line. planned on checking the valve clearances as well in the spring.

suggestions would be great. thanks

logix7

alignment is good. done it a couple times at my shop. i keep the air pressure at 35. 17" wheel. 245/5017 rear 235/45/17 front. ive had to let it warm up for at least 10-15 the past couple of weeks because we've been below zero most mornings (this morning was -8)

Zerodrag

iTrader: (5)

I don't know your driving habits, but that is something that affects your overall mileage the most. Consider slower acceleration and lower cruising speeds (if you are prone to speeding on the highways). What weight of motor oil are you using?

logix7

ive tried driving it like my old 4cyl honda.. nice n slow. made no noticeable diff if i did that or hot-rod'd it. using 5w30 syn blend oil

1997Soarer

iTrader: (1)

In weather as cold as we're currently seeing in Chicago, your mileage is actually expected. Highs in the single digits / low teens with lows below zero. Automobiles in general aren't made for this sort of weather. You might want to consider blocking part of the radiator with cardboard just to keep the engine from overcooling itself. At least until this frigid spell passes and temps rise back up into the upper 20s.

Also, when it's this cold out, cars experience more drag due to the air density being higher. That's also the same reason things sound so different during this extreme cold. Ever noticed how you can hear cars driving farther away? Or a jet landing at O'Hare or Midway sounds so much closer flying overhead than they do normally? That's because of the density.

1997Soarer

iTrader: (1)

You listed everything for a tune up, that is definitely something you should do. Also do a drain and fill of the trans fluid when it gets warmer. It should be only 4 quarts of fluid. Draining and filling doesn't hurt transmissions regardless of how many miles are on them. Flushing, that is definitely something I wouldn't recommend you do. I say swap out the 02s. Those things are probably originals and even my old 1990 integra with a one wire O2 performed better with new sensors than the old ones.
Cleaning the TB probably won't help your MPG but it would clear up any idling issues you might have. I'd wait to take it off and clean it after you seafoam the intake manifold.

REMEMBER to SEAFOAM BEFORE you change the spark plugs. Seafoaming usually fouls plugs, so its best to do it on the set of old plugs then swap in the new ones

logix7

car runs great. which shocked me (it was sitting 6-7 years). i know we always get worse mpg's in the winter around here. i havent done those maint. items yet. i've just been accumulating the parts and waiting for it to warm a bit. i did plan on seafoaming prior to changing the plugs. I read on a couple threads that I should really just focus on replacing the 2 primary o2's and dont worry too much about the rear secondary?

1997Soarer

iTrader: (1)

Correct. No reason to change out the secondary O2 unless you get a CEL for it. It's just there to tell the ecu the cats are working

BiGEZ

iTrader: (24)

Same here, my 99 SC400 fully tuned up with all new plugs, o2 sensors and such late summer. Was getting 22 mpg in the summer and fall. Now with the very cold temps I am getting about 18 mpg. Then I let it warm up quite a bit before driving though. Car runs great otherwise.

logix7

i wish i could get 18 lol. is there anyway i can check if my car is running rich? i do plan on doing the o2's and the other stuff soon but its just killing me,

mrmj2u

iTrader: (2)

You can smell the exhaust fumes and tell if the car is running rich, it should smell like gas if its rich. If it is running SUPER rich, you will be able to see black crap coming out of the tailpipe.

The high tech way to do it would be to purchase and install a wideband gauge.

logix7

update: warmer weather- getting 15-16mpg avg

replaced front o2's with densos, spark plugs with NGKs, pcv valve and seafoamed.
plug wires dont look too old, same with the dist. cap

still getting 15-16mpg



whats next???

Zerodrag

iTrader: (5)

Well, based on the fueleconomy.gov site, you are averaging close to the listed. I could imagine wider tires and overall age of the vehicle to make up for the difference in MPG...

https://www.fueleconomy.gov/feg/Powe...SC&srchtyp=ymm

KahnBB6

iTrader: (5)

It sounds like your SC300 is running well again. An average of 15-16mpg around town or in the city is normal for these cars. Highway fuel economy can get up to 22mpg average.

Aside and FYI the issue with old leaking ECU capacitors does affect 1992's as well. Yours does not seem to have this problem right now based on the vehicle's current health, however it is something you should have taken care of at some point for the long term health of the car. You could also locate an ECU with the same part number and have that reconditioned as your backup ECU.

1997Soarer

iTrader: (1)

Your numbers are normal for an OBD1 SC300 in city driving. The MPG average is around 19-21 because your car would get good mileage in steady highway driving (23 MPG+ if you were driving from CHI to INDY or STL).

My advice would be to change out the wires and dizzy cap + rotor. You say they don't "look too old" which tells me they don't look new either. Wires need to be replaced every 60k, caps and rotors every 15k. That's just routine maintenance for distributor based ignition systems.

With gas prices continuing to rise, I would seriously consider ECO-tuning. Lighten the car; don't carry excess junk in it. Take everything out your don't need/use. Have 19in wheels on it? Try downsizing to 17s. They weigh less and lighter wheels make a huge difference in MPG especially in acceleration. Also set your tire pressure at 35psi.

Are all your wheel bearings good? Do all 4 wheels spin freely if they're in the air? Did you ever change your trans fluid?