I have had a significant decrease in gas mileage for a long time. I had a really difficult time passing smog a few months ago:

-new plugs
-new cat
-new muffler
-new 02 sensor
-new air filter
-new MAF sensor

After all that work it passed smog.... barely.

15 mph - RPM 1550 CO2 14.7 HC 50 CO 0.59 NO 0
25mph - RPM 2435 CO2 14.8 HC 30 CO 0.49 NO 1

I am concerned it will not pass smog next time. I would like to get my fuel mileage back and figure out why HC is running so high. Also, I know the timing is spot on. No CEL or codes on the OBD scanner.

The move to a different state comments on smog are not helpful. Just looking for some advice to get my gas mileage back.

 

i have felt your pain. used to live in cali...
have you thought about injector cleaner.
maybe a scan tool to check engine tune.

 

did injector cleaner, seafoam and a OBD scanner. No luck.

 

seafoam gotcha.
the scanner, my question is, can you check air fuel ratio afr to make sure ecm is not in limp mode, or in open loop with the scantool?
iirc supposed to be in open loop not the default (rich) closed loop safety mode.

 

not sure what you mean by open loop and closed loop?

i do not know how to check the air fuel ratio?????

 

Try resetting the ECU, pull the EFI fuse and then turn the key to the on position a few times to drain the residual charge in the ECU.

If you use the scan tool it will have a a lot of stuff to look at, just make sure everything seems normal.

Coolant temp should be around ~180-200
Long term and short term fuel tirm is also important

It will also tell you if it is in closed loop or open loop mode (open loop is for cold starts and WOT pulls, Closed loop is for everything else. It runs a lot richer in open loop)

 

Just the opposite is correct. A warmed up, well tuned engine should operate in closed loop mode i.e. the O2 sensors are reporting data to the ECU and the ECU is making air/fuel corrections based on conditions. When warming up and under certain other conditions i.e. WOT, the engine will run in open loop. Simply put, open loop is when the engine is least efficient in operation.

Closed loop described on page 15 and open loop on page 16:
http://www.autoshop101.com/forms/h25.pdf

Also useful:
http://www.4x4wire.com/toyota/4Runner/tech/OBDII_ECU/

 

OP, do you have any vacuum leaks? A small crack in a hose or the intake hose causes unmetered air to enter the system and can do strange things.

New PCV valve?
What plugs were used when changed?
Have the plug wires ever been changed? How many miles on them?
Are you using any coolant?
Fuel injector issue i.e. stuck open?

High HC troubleshooting, page 2:
http://www.ncttora.com/fsm/1996/SIL/.../cohc/insp.pdf