Need help with ESP8266 analogue and 4051 multiplexer

Projects General Guidance
1

I have an Node MCU ESP8266 checking a wind vane value and battery level using a multiplexer. I can read get consistent analogue values when there is no multiplexer, but when the multiplexer is in use I get a weird voltage drop to the analogue pin across both channels.

This is the result, each value has a 1 second delay after switching the multiplexer input. Without touching anything the analogue value slowly drops and then returns to normal over about 30 seconds. This repeats after about another 30 seconds...
Battery 563 windvane 393
Battery 563 windvane 393
Battery 563 windvane 393
Battery 563 windvane 393
Battery 563 windvane 393
Battery 563 windvane 392
Battery 563 windvane 393
Battery 563 windvane 392
Battery 563 windvane 392
Battery 563 windvane 392
Battery 380 windvane 334
Battery 272 windvane 232
Battery 113 windvane 78
Battery 21 windvane 11
Battery 6 windvane 6
Battery 6 windvane 7
Battery 7 windvane 8
Battery 8 windvane 9
Battery 10 windvane 12
Battery 15 windvane 17
Battery 18 windvane 20
Battery 21 windvane 24
Battery 27 windvane 30
Battery 38 windvane 45
Battery 59 windvane 80
Battery 126 windvane 187
Battery 285 windvane 389
Battery 560 windvane 393
Battery 563 windvane 393
Battery 563 windvane 393
Battery 563 windvane 393

Code below

void setup() {
Serial.begin(9600);

pinMode(D5,OUTPUT);
}


void loop() {

digitalWrite(D5,HIGH);
delay(1000);
 int battery = analogRead(0);
  Serial.print("Battery ");
  Serial.print(battery);
  Serial.print("    ");

  digitalWrite(D5,LOW);
   delay(1000);
  Serial.print("windvane ");
  int vane = analogRead(0);
  Serial.println(vane);
}

Any ideas why??

2

Schematic?
If it works without the multiplexer then the problem is not in the software, apart from the fact that you have no output signals controlling the multiplexer's channel selection.

I think you have inputs on the multiplexer floating and you have no decoupling capacitors on it, but without a schematic who knows?