4-runner is running too rich?

Most likely throttle positioning sencer

check your crank and cam sensors...see if one is not plugged tight in

More than 1 reason. Each may take an independent corrective action. The cold start problem can be a failure of the "Cold start cycle" which uses a temperature sensor to set a rich fuel mix and speed up the idle.
You would need to replace one of 2 sensors. The second sensor operates the dash gauge or light.

Air leak- The side having a rich mix is compensating for excess air entering the engine on that bank of cylinders. With the engine running, use a spray water bottle and wet the seams of the intake and vacuum lines for that bank of cylinders. The rpm will change or stall the engine.

Manifold runners. Some models have manifold runners for the intake. These change airflow and they can stick in the wrong position. This requires removal of the intake, so try the easiest reasons first.

Please rate my info.

Josh, same problem with my suburban, very frustrating. Two issues to check.
Underneath your distributor cap and ECM module there is the pickup coil. The reluctor is most likely corroded really bad. It is about one hour work but you would be surprised how dirty and corroded that area gets. The reluctor is a fixed part of the distributor shaft and the little nubs break magnetic field which is
processed by your onboard computer. If the pulses are not correct the computer is telling your TBI injectors to spray which will flood out your engine. Ten of fifteen minutes later, it will start up again and then die shortly after. Have a new pick up coil on hand too ($10.00) since your in there. My reluctor looked like it was in Vavy Jones locker it was so crusty. Second- the fuel relay. Nororious for developing little cracks on the mini circuit breaker which get wider as voltage and amps increase due to resistance (dirt,moisture). The relay will shut off the fuel pump if the oil pressure signal gets cut, and if your breaker/circuit board is
bad the board heats up and trips off your fuel. When the board cools off in ten or fifteen minutes, starts up and then dies when engine gets warm/hot. It really had me stumped for quite some time. Hope ths cuts your wrench time down considerably.

Vent valve has failed closed or cannister is clogged up. Have the evap system checked from the tank to the charcoal canister. If you need to drive to a mechanic, try driving with the gas cap loose.

images for 1998 toyota 4 runner fuel pump cut-off switch

Hello: Will try to give you instructions: Attempt to start car, leave key on (security light flashing) let set for 10 minutes, After 10 minutes security light out. Turn key off for ten seconds, turn key on, let set for 10 minutes, turn key off for 10 seconds. turn key back on let set 10 minutes, turn key off for ten seconds, the engine should now start and stay running.

CHARGE BATTERY / TURN KEY TO RUN POSITION FOR TEN MINUTES
SECURITY LIGHT SHOULD GO OUT / TURN KEY OFF FOR 3-5 SECONDS
TURN BACK TO RUN FOR TEN MINUTES / REPETE / USALLY TAKES
3 TEN MINUTE CYCLES / THAN TURN OFF / TURN ON SECURITY LIGHT
SHOULD GO OUT AND ENGINE START AND RUN

i advice you to change the spark plugs,this is cause because at cold start the engine needs a rich mixture in order to maintain a smooth idle,before driving the car,when starting from cold ,what you have to do is to accelerate up to 2000 rpm for 3 minutes then you drive it,then it should be ok

ok here you go
make sure the battery is fully charged befor starting
turn ignition to run
attempt to start once then release the key in the run position
let vehicle sit with the key on for ten minutes
the security light should turn off
turn igniton switch off
wait 5 seconds
turn key to run position
let vehicle sit for 10 minutes
turn key off
wait 5 seconds
turn key to run position
let vehicle sit for ten minutes
turn key off
wait more than 6 seconds
and start engine