93 SC300 200,000 miles. No CEL.

So far Ive done spark plugs, wires, cap, rotor, cleaned out the Throttle body, Seafoam through oil, gas tank, brake booster, and changed the fuel filter. I sprayed around for vacuum leaks but havent found any. Air filter is still white.

All that and the car still gets 14mpg. I only floor it when I get onto the freeway. Anything else Im missing? Can a O2 sensor go bad without a check engine light?

Ive searched through many threads similar to mine with no real leads to whats causing the bad mileage.

Car idles @ 1250-1500 in Park and 1000 in Drive

 

No real help here but for comparison, I have a 93 SC 5 speed with 170K+ and I get 22mpg average and that's with some pretty aggressive driving.

 

Yep, i got a bad O2 sensor and the CEL comes and goes. Did you check your timing; change oil after seafoam? you've done just about everything to increase mpg

 

Damn. I kind of think my ecu is busted. Before I did my tune up it was running very rough. I think it wasnt firing in one of the cyl. The car never threw a cel when even though I knew the car was misfiring. Is there anyway to lower the idle?

 

If you have a high idle, you may have a vacuum leak somewhere that the ecu's trying to compensate for by dumping gas...Bad O2 sensors will kill your fuel mileage as well....You should start with that and see how she does.

 

I'm not sure but.. your car idles high...

i remmeber mine to be around 600 ~ 700 parked.

 

You Should do a compression test. I have a 92 5 speed SC300. Car ran fine under normal load, but you couldn't smash in it. It also got 15 MPG on a good day, bad day, Up hill, down hill, tail wind... No one could figure it out, checked my brand new plugs, they looked like poo. So I did a compression test while they were out, turns out I had 2 cylinders only making 45PSI... No check engine light. No sensors showing signs of a Bad Engine.

 

Im putting my money on bad or dirty MAF>

 

Damn what was causing your lack of compression. Did you end up swapping out?

Car seems to accelerate well. Was your car sluggish? Is a compression test hard to do. Ill be doing a search.

 

I thought 93 scs had a solid state MAF that cant be cleaned.

I drove the car to palm springs and I was getting 20ish mpg cruising 70 and 40ish through the canyons.

 

^ i believe thats correct ..

 

I get about 18 mpg if I'm lucky. My car is in great condition except for my rear tires, but the gas mileage sucked even with new tires.

 

14 mpg? there is def something wrong, i get that in my sc4

 

Dude I get about the same in my 93 sc300 5spd in the city and about 20mpg on the highway. I also just changed out my sparkplugs and wires. I dont know how people here get 20+ mpg.

 

My 97 SC300 averaged 22mpg and got as high as 28mpg on the highway on long trips.

Try to figure out the idle issue, its supposed to be 600-800. My SC only idled that high on a cold start but went down after it was warm so maybe you have a temp sensor thats not telling the ECU that its warmed up??