hi i have a sc400 year 1993, 78000 miles.

i filled about 11.75 gallons, and used up about 9-10 of it already and im sitting at 130 miles, so im guessing by the time im down with the next two gallons i'll be at 145-150 ish, comes out to 12.5-13 ish mpg... pretty bad...

how and where i drive: a good mix of sprint driving, easing the gas lightly, hard breaking, slow braking, some highway, mostly local

i have ECT power off (i believe when u turn it on it makes car sporty but lower mpg)
i also have O/D on (o/d doesn't show on dashboard and button is pressed in), i believe this unlocks 4th gear and improves mpg on highway due to lower rpms

1. having o/d on during local driving, does it improve mpg?
2. does having o/d on mess with transmission?
3. does having ect power on mess with transmission?
4. when i change from park to reverse or drive (flat surface) the whole car does one hard shake? tranny problem? and i heard new tranny fluids is bad cuz it makes it too slippery? and friend suggested i pull emergency brake and then shift then pull down emergency brake...
5. when i brake while reversing, my brake lever stutters like 2-3 times, what kind of problem is this?
4. -main question: whats a good way to improve my mpg?- my friend suggested changing the fuel filter (whats a good fuel filter??) and i have a k&n air filter on the way (improves mpg by how much?)


edit: i seafoamed as well, the engine and the gas tank, all the white smoke is gone.

thank you for reading this !!

 

Well first of all welcome to CL! Alot of helpful tips are already laying around here just waiting for you to use the search button! (Upper right corner.) And that car is actually a good find, '93 78,000mi. But that mpg is so wrong, I have a '95 SC400 and I'm running 20-25mpg. Now, next time you make a thread, post a pic of your car!

To answer your question;

1-3; anything that is not for normal driving may mess up your transmission, may cause premature wear and tear, like the ECT, I only use it when my car is heavy as hell, or going up a hill and you can feel that it is having a hard time. I'm not really sure what the O/D is for...(maybe someone can shed a light for me as well)
4; people on here said it might be carrier bearing for the propeller shaft/drive shaft. I have the same problem, I haven't looked into that as I'm fixing alot more under the hood and it's not really a problem. But, what I've noticed, when I warm up my engine before driving it or shifting it it doesn't make that clanking sound.

I'm not sure about your other question...

 

sounds like time for a tune up! you should start with that fuel filter i go with oem but k&n is good, new spark plugs, rotor and air filter if you haven't done that in awhile. try to run some of that seaform or other type of injector cleaner.

and yes OD is your 4th gear leave it on for mpg

 

for mostly local getting 12.5-13mpg doesnt seem rediculously bad. yea its not great but its a 18 year old v8. my old 94 Ls400 got 15-16mpg with 50/50 highway and city driving.

but keep the ect off and o/d on. you should see improvement in mpg with the new k&n fileter if the stock filter is dirty and needs to be replaced.

has any maitenece been to the car at all or is it all original stuff like the air filter, spark plugs and such.

 

yes first things first tune up time.

Change plugs , wires, air filter, fuel filter and go from there.

 

o/d just makes your engine work a bit harder and keeps rpms up higher which uses more gas in exchange for power. Turn that on if you don't need the power.

 

when the "O/D off" light is on, that means O/D is off?

cuz whats confusing me is that when i turn on the ECT POWER it lights up

so if "O/D off" is not on = OD is on?

 

I'm not really familiar with the ECT, what is the difference between that and O/D?

 

From what I understand so far, O/D unlocks your 4th gear, so if you do alot of city driving you can turn it off so it wouldn't keep on going to 4th gear and keeps your RPM higher so you have better acceleration.

The ECT is like a sport mode. I actually notice better performance when on ECT. The gas pedal seems to be more sensitive. And this would also give you bad MPG. LOL

 

overdrive is your 4th gear. ect is a sport mode. all it does is changes the shift points in the transmission to make it feel sportier, it doesnt really hurt milage i dont think. i have a 95 sc400 with 210,000 miles on original motor and trans getting 17-19mpg around town with a low rpm misfire that i havent been able to fix yet

p.s. thats with ect on all the time

 

So if i turn od off and ect on. I get super power?

 

Yeah it's time for a tune up! I have a 92 SC400 with 250K on the clock now, I'm getting roughly 16 in town (with A/C on ALL THE TIME) and last week I drove to Dallas and back and logged roughly 22 MPG's entirely highway (minus putting around the airport).

Have you tried resetting the ecu yet? These cars are notorious for running rich, so I actually reset my ecu every quarter of a tank. (There's an EFI fuse in the engine bay that if you remove will reset the ecu)

Also the O/D light should be off, shouldn't be able to see it in the cluster. That's probably what is really killing your mileage, considering that it's constantly in third gear.

 

i reset the ECU, still getting about 12mpg local, dont get on highway much ~~

if i remove the efi fuse, put it back in, how long does it take for the ecu to read it again? my friend suggested i keep revs low for 30 mins