77 is a test code so unless you were preforming the test ignore it.
on a 1991 3.0l the 85 code means the solenoid for the EVAP system purge canister is not working.
This will not cause your engine issues, the canister is probably located near the rad support and the solenoid will be attached to a hose on the canister.
EVAP system prevents gas fumes in the fuel tank from polluting the air.
Since it is an "emission" feature it will will turn on CEL instantly.
Because of your idle issues I would clean the IAC(idle air control) valve first.
It is on the upper intake by the throttle plate it has a 3 wire connector.
remove and clean.
Computer uses IAC Valve to set idle.
Once IAC is clean, warm up engine and let it idle, unplug IAC Valve wires, RPMs should drop to 500 or engine may even stall, either is fine, if idle doesn't drop then you have a vacuum leak, which can cause all sorts of issues including bogging and surges in idle.
Normal engine idle should be
1,500+ at first start cold, computer opens IAC valve all the way
Then idle should drop to 1,000-1,200 based on outside temperature, colder = higher idle
As engine/coolant warms up the idle should go down, target idle is 700(manual trans), 800(automatic)
Since idle is changing I would think the ECT sensor is working.
ECT(engine coolant temp) sensor is used, ONLY by the computer, to set choke mode or warmed up mode, these rarely fail, but not never.
ECT has TWO wires
Dashboard gauges temp SENDER has 1 wire, they look similar but are different.
I would also clean the MAF sensor, it is the main sensor for computer to use to gauge the air/fuel mix.
Google: clean MAF sensor
Dirty MAF causes bogging and rough running
Last edited by RonD; 10-06-2014 at 01:45 PM.