SC400 Gas Mileage Issue
 

Hey guys, i got a problem with my sc400. I have a 1998 Sc400 114k on the dash , it is giving me some issues. I'm getting 16 mpg and that's me driving calmly. I have a couple friends who are getting 18 mpg same year without an issue and they beat on their cars more than i do. I brought my car to 10 different mechanics and none of them had the answer to the problem. I change my battery, alternator, air filter , fuel filter , spark plugs, and used techron this was all done about 5,000 miles ago. I do my oil changes every 3k miles Mobil 1 fully synthetic 5w-30 and i fill up my car with BP, Exxon, Hess and Sunoco gas super all the time. Its weird i cant get to the bottom of this and find out why. Any comments, ideas , or help would be greatly appreciated .
Thanks for reading

The only two other things I can suggest is check your brake caliper pins to make sure they're not frozen and check the wheel alignment. But to be honest, 16-18mpg in the city is pretty normal. I'd only worry about it if you were getting this kinda mpg with a lot of freeway driving.

^This. Also buy new o2 sensors. I bought both my OEM densos on ebay for 85$ shipped. Try a new pcv valve, new gas cap seal, check tire psi, etc.

What would frozen brake caliper pins have to do with bad gas mileage? Wouldn't dragging brake pads be extremely obvious? I feel like I'm missing something here.

i'm just throwing an opinion out there, not a fact, but couldn't leaky capacitors potentially cause the same problem too?

Nope, unless both the upper and lower pins are frozen, it won't be "extremely" obvious with just one of the pins being stuck. Brakes might heat up a bit more than usual, but won't be burning hot since the brakes always have a small amount of drag by design, even when the pins are greased up and working properly. I had a frozen pin on one side and didn't notice till it came time to change my pads out.

Okay, now I need to go check my caliper pins...

^ This.

PCV valve, and raise your tires to 32 psi.
If that doesn't change anything, then replace the O2s (only the 2 primary's). They're due to be replaced anyway because of the age, but because they can be a real nightmare to change, I always tell people to replace them last

Also, inspect all your factory ground wires. Follow the - wire from the battery to the chassis and the engine and see its condition.

If your using a MPG dash counter, Reset it and start over. It could be just an estimated average of you and a bunch of other factors. ALSO switch up gas stations and run with one specific brand of fuel. I used to get from a local Citgo and half tank 93 premium driving around town would last 2 days (no more than 40 miles) but I switched up to Shell and I drove to CT 60 miles each way (120 total) and still have a quarter tank from a half

I had a caliper drag it sucks, Jack car up ,and spin the wheel for resistance. I was in the process of doing O2 sensors do i have to do all 4? or just the front?

You may want to change the capacitors in your ECU
 

I had this same exact issue...I was getting 10-12 MPG on my SC400 1992.

I bought it @ 111k and it was super clean but it had terrible gas mileage so I started searching CL.

Here's what I found:

https://www.clublexus.com/forums/ls4...capacitor.html

If you haven't already, change out the caps since they probably need to be replaced.

Good Luck!

p.s. I saw that you're in NJ...if you send me the ECU I can help you with this.

Thanks I actually just dropped my car off at my mechanic yesterday and im replacing two o2 sensors, pcv valve, and a coolant temp sensor. If it doesn't do good after that then ill look into this. Also what county are you located in?

I live in bergen county but my buddy has his equipment in Carteret right off the turnpike. So if it doesn't work just shoot me a message.

We were able to replace everything in about 2 hours.

good luck!

Hey just an update...

I did a full tank test and I went up from around 225 miles per tank to around 378 or about 19MPG

WOOT WOOT!

NICE! I wish that was me lol But are you going to the meet april 12 in jersey city at liberty state park?