Hello All,

New to the forum, did plenty of searching and have not come up with a solution, which is why i'm posting.


I recently purchased (8 months ago) a pristine 92 Lexus SC300 from an original owner with 70k miles on it. When purchased I was obtaining 18mpg, now, i'm down to 10mpg.

Thus far, I've replaced the plugs/wires/cap/rotor with all OEM equipment and thoroughly cleaned out the throttle body. Nothing.
3 days ago, I changed out the two upstream oxygen sensors after a friend hooked up a diagnostic machine to the ecu and found the car wasn't switching from rich to lean fast enough. Once again, nothing.

No lights, not problems with the car other than that. The speedometer doesn't work, but that is unrelated. I'm tired of dumping money into this and both my mechanic friends are unsure what it could be. Any help is greatly appreciated!

Thank you all.
Nate

 

That's a huge drop for mpg, how do you know it's that much since your speedo don't work?

 

Just the speedometer doesn't work (from what i've been searching on this forum, it is a common problem, but not correlated to this issue) all other guages and lights work; tach, gas, temp, odometer, check engine light etc....

Need some help on this guys!!!

 

after you did this what did the diagnostic say...
I changed out the two upstream oxygen sensors after a friend hooked up a diagnostic machine to the ecu and found the car wasn't switching from rich to lean fast enough. Once again, nothing.
change the downstream 02
change the fuel filter
you did not mention the air filter but i suppose you have already changed it.

 

Never had a chance to re-read the diagnostics. Presumably, it may be the same as there is no difference in MPG, possibly a MAF problem?

-Air filter replaced with a K&N...
-Downstream 02 shouldn't affect fuel consumption.
-Fuel filter was disregarded by my mechanics (I do not know why)... they think the problem is more significant with the drop... maybe injectors? Compression issues, dont know.. I will look into the Fuel Filter again though, just hate to keep guessing.

Anyone else experience this type of significant drop in MPG??? If so, solutions!?!

Thanks for the help thus far, more ideas the merrier!


PS: I should mention, the car runs fantastically... Literally, I have people in shock to think its two decades old so i'm not sure what else the problem could be!!

 

go for the seafoam man blow all the crap out

 

Forgot to mention that. That was the first thing I did about a month ago Cant win this.

 

What about egr delete? I was having similar problems. Blocked it off and its night and day

 

where in NJ are you? i have a 96 and im getting 30 highway 25 city something is def off

 

Hmm, what does this entail?



Majority of the day in Philadelphia live close to Cherry Hill.

 

Have you weighed your right foot lately? :-) just kidding!

Have you noticed any changes in the overall performance of the vehicle since the MPG drop? Also, I'm just curious as to what "diagnostic tool" you used on the car since it's OBD-I. Do you have a CEL? Have you had the MAF tested? Are you injectors opening and closing properly? Your injectors might be stuck open and just dumping fuel into the engine when it's not necessary.

 

Hey Tweak,

No change, by all accounts, the car runs phenomenally.

My buddy used his Snapon Modis.. Said the "pulse width is high but is pulsing" so we started with the 02 plugs. He hasn't ruled out the MAF yet, he wanted to do the cheaper one first. Can you actually test the MAF for proper functionality? Also, How about the valves, can you test them as well?

Assuming CEL is Check Engine, no, I have none. He stated it wouldn't show on OBD I even with bad 02s so I changed them out.

What a pain this is turning into.

 

At this point, the next thing I would do is clean your injectors and make sure there is nothing holding them open, such as a gunk buildup. You mentioned that the car is a 92 with 70k on it. 70k in 20 years isn't ideal to keep the functionality of parts up to performing standards. Things dry out, and rot when they sit for long periods of time and gunk can build up on other parts. I wish I had some more intelligent advice to give you....

 

I have a maf off a 93 sc300 if u want pm me

 

1992OBD1's will not switch from lean to rich very quickly on a logger, but the engine itself is doing it faster. You have to hook up a sniffer to the exhaust pipe to get accurate readings man.