1986 2M4

Hi, I am trying to understand how this works. When the car is first started cold the computer looks to the engine coolant sensor for readings after it warms up it looks to the Oxygen sensor...correct. If so the engine coolant sensor is not being used as such after the car warms up?

The ECM has "open loop" operation when the car is cold, and uses some default numbers to run the car (it is set up to be rich, similar to the choke on a carburetor). When the car gets hot, it uses feedback from multiple sensors to run the car more efficiently, ...they call this "closed loop". It samples these numbers about once a second (OBD1).

So what keeps the car from running rich all the time? Or that is what is on the oxygen sensor when I use a voltmeter.

In "closed loop" operation, the ECM reads the CTS, MAP, O2 sensor, and the TPS.....thinks about it, and decides how much fuel the injector should shoot into the manifold. The voltage you read off the O2 sensor is the number the computer uses in it's equation.

[This message has been edited by Gall757 (edited 11-17-2014).]

SO with my O2 sensor disconnected and the car idling, its sending a signal from the sensor that it's a rich mixture.
Is that the proper of way of testing the O2 sensor?

Infinite resistance (open circuit) on the CTS results in a rich fuel mixture. I don't recall if the O2 sensor is the same way, but it could be.... O2 sensors are inexpensive, so most people just replace them if they look funny.

Actually if memory serves me correct it was showing about 0.75 volts or so with it disconnected and idling.

[This message has been edited by newfiejeff (edited 11-17-2014).]

https://www.fiero.nl/forum/A...010418-2-005413.html

Now that was a mouthful!!
So what it is saying is that my 0.75 is a good value? It doesn't really fluctuate or it doesn't show it on my digital meter.

Sounds like a good value.....O2 sensor reading should not change quickly.

[This message has been edited by Gall757 (edited 11-17-2014).]

Got it!
How can I tell if my car is operating in a closed or open loop?

I know it might be a stupid question.

I don't know for sure, but a safe bet would be when the idle goes down after warm-up.

You need a scan tool to check close mode is enabled or not.
Don't trust many posts here or anywhere saying close loop is enable at engine temp above x° etc.

Depending on ECM PROM Programing...
ECM can run Close Loop as soon as O2 is warm enough to operate. (Scanner reports Oxygen Sensor = Ready.)
Can be in < 1-2 minutes of engine run time.

87 and up DIS L4 goes Close Loop very fast even when ECT and MAT says Engine is cold.
V6 could take longer to heat O2 sensor because is farther down the exhaust.

And Don't confuse Close Loop and Learn Control... Very, very short story... Is Not same thing.
Learn Control on/off depend on condition of BLM and Integrator.

Testing... Still Need a scan tool. Even if Sensor test good using Volt meter or just replacing the sensor, bad ground(s) or O2 wire can cause ECM problem to read the sensor.

See my Cave, Scan Tool Help and rest of section.

Most times, running rich is cause by iffy injector(s), bad seals, or bad plumbing/regulator causing high fuel pressure.
High fuel pressure can kill Fuel Pump. Fuel Flow is Coolant for the pump. High pressure often restricts flow...

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