MAF Used at Idle?

[2000StreetRod]

Does anyone know if the PCM utilizes the MAF voltage when the engine is idling?

I disconnected the MAF output to the PCM and replaced it with a variable voltage supply. I set the voltage to 0.8 volts - the same as my stock MAF output at idle. Even though the engine was cold it started normally and ran smoothly. I then slowly increased engine speed to 2000 rpm and then read the OBD codes. None were present. I then varied the voltage between 0.5 volts and 1.5 volts and the idle speed did not change and no codes were present. I shut off the engine.

Next I turned off the voltage supply (0 volts output) and restarted the engine. It idled normally and no codes were set. I slowly increased engine speed to 3000 rpm and then back to idle. This prompted a MAF voltage too low code.

The older Bosch injection systems utilized an idle contact switch on the throttle position sensor (TPS). There isn't one on my Sport TPS. The PCM must use the minimum TPS resistance to determine closed throttle. The PCM must rely on the Idle Air Control (IAC) to maintain idle speed at closed throttle but what does it use to adjust the richness if not the MAF?

How long does it take for the O2 sensors to heat up and the PCM to switch from open loop to closed loop? After switching to closed loop the PCM could rely strictly on the O2 sensors to adjust the richness at idle but it would be erratic and nonresponsive.

 

[Gus Gorilla]

I would imagine that the idle mixture would be a preset amount, the same as idle speed. The codes didn't set originally because usually a code needs a certain parameter to set. One drive cycle, 5 drive cycles, one key cycle, 30 seconds, etc. Once the conditions are met it will set. Really, until the O2 sensors come online the PCM will run a richer mixture. As for how long until the O2 sensors come online that is going to be determined by ambient temp, exhaust conditions, drive style etc. I believe they heat up to 600*F for operating temperature. A way to know if they're on or not is if your vehicle will lockup the torque converter. When in open loop status the TCC will not lock. This is easier to see in the winter of course, but without a scantool that will show all the sensor readings and what the vehicle is really doing there's not much way to know open or closed loop.

 

[2000StreetRod]

I would imagine that the idle mixture would be a preset amount, the same as idle speed. . . . Really, until the O2 sensors come online the PCM will run a richer mixture. As for how long until the O2 sensors come online that is going to be determined by ambient temp, exhaust conditions, drive style etc. . . . A way to know if they're on or not is if your vehicle will lockup the torque converter. When in open loop status the TCC will not lock. This is easier to see in the winter of course, but without a scantool that will show all the sensor readings and what the vehicle is really doing there's not much way to know open or closed loop.

I agree that the idle mixture is probably preset at start up since changing the MAF voltage has no effect. There is probably a lookup table based on Intake Air Temperature (IAT). Otherwise, the richness would be way off for cold winter and hot summer. I may substitute a 100K potentiometer for the IAT and determine its impact on richness.

I believe the reason for switching to heated O2 sensors from the old unheated ones was to shorten the open loop time. I would think that 5 minutes would be long enough but maybe not. I didn't know about the torque converter not locking until closed loop. I've ordered a wideband air/fuel ratio meter. For constant engine conditions I should be able to see the ratio change when the PCM transitions from open to closed loop.

 

[2000StreetRod]

Idle Speed & Simulated IAT

I replaced the IAT sensor connections to the PCM with a variable resistance that varies from 10K ohms to 60K ohms - the equivalent of 125 deg F to 50 deg F respectively. I adjusted the potentiometer to 35K ohms (70 deg F) to match the ambient temperature. I adjusted the MAF voltage to the PCM to 0.8 volts to simulate idle. The engine was cold and when started ran smoothly at normal idle (550 rpm). No codes were set. (As an aside I should mention that my engine starts better and idles smoother from a cold start with simulated inputs than when the MAF/IAT is actually connected. Before driving the vehicle again I'll clean the MAF/IAT and measure the resistance of the IAT.)

Within the first few minutes I began varying the simulated IAT from one extreme to the other. There was no noticeable effect on the idle speed and I could not detect any change in the exhaust color (no black smoke).

After several minutes rapidly changing the simulated IAT from one extreme to the other changed the idle speed slightly but only for a second or two. The effect was consistent until ignition off at 20 minutes. No codes were set.

Also during the first 20 minutes of engine running, I varied the MAF voltage from 0.25 volts to 2.0 volts. There was no corresponding change in idle speed or smoothness and no codes were set.

I conclude that the heated O2 sensors close the loop with the PCM in just a few minutes and that the PCM places higher priority on the O2 feedback than the IAT sensor. Also, the PCM places higher priority on idle speed than richness.