OK … This ongoing problem is affecting my automotive mental health.
As I’ve previously posted I’m getting a “Check Engine Light” as regular as clockwork. The code is #4 which on my '89 pertains to the O2 sensor circuit. It comes on as soon as the O2 sensor warms up to it’s operating temperature (600deg F) and signals the engine computer to shift from the open loop (rich fuel trim signal permanently stored in the computer used during engine warmup) to the closed loop (fuel trim based on actual data input from various other sensor and the O2 sensor).
Everything that I can now determine tells me that when the system goes to closed loop the O2 sensor detects an over rich condition that is too great for the computer to compensate for and it signals the CEL #4.
I had my exhaust analyzed and it showed that it failed the emissions test.
HC (standard 120) actual 157 … failed
CO (standard 0.67) actual 1.56 … failed
So I looked up possible causes of a rich condition and I …
- Plugged air filter replaced air filter
- Faulty mass air flow meter replaced MAF
- Faulty fuel injectors replaced injectors
- Faulty fuel pressure regulator replaced FPR
- Spark plugs replaced spark plugs
- Faulty ECU replaced engine computer
- Faulty ignition wires replaced ignition wires
- Coolant temp sensor replaced engine coolant temperature sensor
- Faulty idle control valve recently removed cleaned and adjusted control valve
- Faulty O2 sensor replaced O2 sensor … TWICE
After all of the above I still get the CEL !! This has really left me scratching my head.