Idle Speed and Air Temp/Humidity Options

[StarBear]

As getting ready for the 2022 Northeast Gathering, I'm encountering a semi-recurring idle issue on my 1974 1.8L LJet FI. When outside air is cool and dry, idles just fine at 950-1000 rpm. When outside air is warm and humid, after warmup the idle falls off to barely running. I've just been adjusting the air flow screw (maybe 1.5-2 full turns) to open it. Of course, the reverse happens - if I preadjust the screw it idles near 1800 then settles down to 1100 after the engine gets totally warm (15 mins or so).
I presume (?) there SHOULD be some regulation, perhaps with the ECU or AFM, to monitor air temp and humidity and make adjustments automatically? I didn't have to manually do this adjustment in the past.
The AAR works and tests out fine. Just annoying to have to pull over after running a while and adjust the screw. (IMG:style_emoticons/default/dry.gif)
Your thoughts @Van @Van_B and @Wonkipop ?

[ClayPerrine]

In reference to the air flow meter, the air temp sensor is in both the 74 and the 75, but the wiring is different. In 74, it shared a common wire with the wiper contacts. In 75, they separated it out because of electrical interference.

Double check the ECU number is correct for the year of the car. A 75 ECU on a 74 car will fit, start and run, but the air temp sensor will not work at all. Same goes for a 74 ECU on a 75 car.

The comment about the throttle body being worn is also very good. I had fits with the idle on Betty's car for a while, until we got a 2.1 Bus throttle body. Once I did that, the idle seemed to get much more stable.


Clay

[StarBear]

In reference to the air flow meter, the air temp sensor is in both the 74 and the 75, but the wiring is different. In 74, it shared a common wire with the wiper contacts. In 75, they separated it out because of electrical interference.

Double check the ECU number is correct for the year of the car. A 75 ECU on a 74 car will fit, start and run, but the air temp sensor will not work at all. Same goes for a 74 ECU on a 75 car.

The comment about the throttle body being worn is also very good. I had fits with the idle on Betty's car for a while, until we got a 2.1 Bus throttle body. Once I did that, the idle seemed to get much more stable.


Clay

@clayperrine Thanks, Clay. It is the correct ECU, as I'm the original owner and swapped a matching number ECU several years ago. This issue just started when the weather got warmer.
Can the sensor in a 6-pin 74 AFM be replaced (even if difficult)?
Will try the throttle body test, as Van suggested.