These cars/engines do run rich from the factory, it is for engine safety (rich is safer than lean) and is not rich enough to cause any driveabilty issues, there should not be any fouling of plugs.
If you are hearing a backfire through the exhaust on decel you are rich.
On decel the system goes "Lean" since engine vacuum increases and there is absolutely no load, excess fuel is not needed.
If you are rich on decel the combustion is taking place in the hot exhaust, hence the backfire.
Rich can be caused by numerous things...
1)Ignition System
I do not think ignition is your issue since the problem only takes place during a no load condition, if the ignition system were causing the issue it would more than likely happen under load when KV requirements increase.(Kilovolts, Voltage needed to have amperage flow from the coil to the spark plugs and jump the gap to ground, as load increases KV requirements increase, 10-15 KV @ idle, 25-30 KV @ WOT/Full Boost)
2)Fuel Pressure
a)If Fuel Pressure is high or the Fuel Pressure Regulator is leaking you will be rich.
b)Remove the vacuum/pressure hose from the FPR, look for raw gasoline in the hose.
c)Checking Fuel Pressure
*The wiring may be different at the pump and the ASD Relay will be different year to year(84,85-87,88-90,91-up) but the test is basically the same. *MPFI (multi-port fuel injection) works on pressure differential across the injectors. *Since the injectors are exposed to manifold vacuum total...
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3)Leaking Fuel Injector(s)
a)If you have a quick leakdown of fuel pressure (See "Checking Fuel Pressure in step #2) with the engine off this will need to be checked, this will require removal of the fuel rail and injectors to check.
4)MAP Sensor Calibration
a)The MAP Sensor (Manifold Absolute Pressure) is the "Dominant" Fuel Control sensor on the vehicle, if its calibration is not correct or it is not seeing the proper vacuum/pressure fuel control will be off.
I am posting this so I do not have to keep typing this over and over and will make it easier for members to find help on their own. Fault Code 13/14... No Change In Map Voltage From Key On To Engine Start Map Voltage Does Not Equal Throttle Position. Map Voltage Low Map Voltage High Map...
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5)EVAP System (Rollover Valve In Fuel Tank, Vapor Canister, Purge Solenoid)
a)If there is an issue with the EVAP (Evaporative Emissions Control System) and fuel vapors are being allowed to enter the engine on decel you will be rich.
6)Oxygen Sensor Switching.
a)This will allow you to see exactly what is happening with the fuel control system.
b)The O2 Sensor is a fuel trimmer, it is used to bring fuel control during "closed loop" operation (Idle and no load cruise) into a tighter range for fuel economy and to lower CO emissions.
c)The engine controller attempts to keep the A/F ration at 14.7/1 (Stochiometric) during closed loop operation.
d)When exposed to oxygen in the exhaust system the O2 Sensor through a chemical process produces a small voltage, .1-.9 volts with .450 volts being 14.7/1. The engine controller through small fuel corrections attempts to keep the A/F at 14.7/1. If the voltage is below .450 (lean) the controller will add fuel, as this happens the voltage will go above .450 volts (rich) and this constant adding/subtracting fuel causes the voltage to constantly cross above/below .450 volts.
e)If the voltage is latched below .450 volts (lean) a Code 51 (lean exhaust indication) may set in memory.
f)If the voltage is latched above .450 volts (rich) a Code 52 (rich exhaust indication) may set in memory.
g)To view O2 Sensor voltage/switching you need to tap into the O2 Sensor Signal Line using a Digital Multi-Meter, then you can watch O2 voltage to see if you are rich or lean.
h)The O2 signal wire is BK/DG (Black/Dark Green) on the "Harness Side".
(Do not confuse this with the DG/BK (Dark Green/Black) wire, this is a 12 volt power circuit for the O2 heater circuit)
