Why is my engine running rich?

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 …

  1. Plugged air filter replaced air filter
  2. Faulty mass air flow meter replaced MAF
  3. Faulty fuel injectors replaced injectors
  4. Faulty fuel pressure regulator replaced FPR
  5. Spark plugs replaced spark plugs
  6. Faulty ECU replaced engine computer
  7. Faulty ignition wires replaced ignition wires
  8. Coolant temp sensor replaced engine coolant temperature sensor
  9. Faulty idle control valve recently removed cleaned and adjusted control valve
  10. 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.

Connect a pointer voltmeter to the probe signal and see if it oscillates properly

The oxygen sensor often cheats on how the exhaust is leaking, measure the signal with a meter as it looks.

I know I’m just parroting my earlier responses, but has to be a vacuum leak …unmetered air detected, ecu richens up to compensate, compensation beyond allowable 10%, CEL

Don’t know what else it could be? It would drive me crazy too, I feel for ya!

Intake manifold gasket pinched? Exhaust gasket NFG?

Maybe it will help you something.

Hi Grooveman, looking at the positive side at least now you know it is not an erroneous warning light and you actually have a rich running engine and your engine management system has been doing it’s job correctly all along. In my mind searching for an actual fault is much easier than finding something causing a 'false ’ warning light.
Did you remove the spacer piece you fitted to the Lambda sensor and position it back into the gas flow yet ?
The most common cause of a rise in HC is actually a WEAK fuel/air mixture. Unmetered air enters the combustion process, the lambda sensor detects this extra oxygen in the exhaust stream and then instructs the engine management to increase fuel to air ratio to counteract it.
The only logical way for this to happen is an unmetered air leak somewhere on the vacuum side of the induction system. It may be just a split in a little rubber hose somewhere or the tail stub its pushed on to has come loose around the base, or one of the gaskets on the manifold, or even the manifold to engine gasket itself.
You could try spraying carb cleaner around any suspect places, if you manage to hit the right spot it will be sucked into the engine through the leak and you will hear the exhaust note change or the engine splutter.
Good luck.