diff --git a/python/uds.py b/python/uds.py
index 9e496e9f81..5bf35088ea 100644
--- a/python/uds.py
+++ b/python/uds.py
@@ -376,13 +376,30 @@ def send(self, msgs: List[bytes], delay: float = 0) -> None:
         self._recv_buffer()
 
 class IsoTpMessage():
-  def __init__(self, can_client: CanClient, timeout: float = 1, single_frame_mode: bool = False, debug: bool = False, max_len: int = 8):
+  def __init__(self, can_client: CanClient, timeout: float = 1, single_frame_mode: bool = False, separation_time: float = 0,
+               debug: bool = False, max_len: int = 8):
     self._can_client = can_client
     self.timeout = timeout
     self.single_frame_mode = single_frame_mode
     self.debug = debug
     self.max_len = max_len
 
+    # <= 127, separation time in milliseconds
+    # 0xF1 to 0xF9 UF, 100 to 900 microseconds
+    if 1e-4 <= separation_time <= 9e-4:
+      offset = int(round(separation_time, 4) * 1e4) - 1
+      separation_time = 0xF1 + offset
+    elif 0 <= separation_time <= 0.127:
+      separation_time = round(separation_time * 1000)
+    else:
+      raise Exception("Separation time not in range")
+
+    self.flow_control_msg = bytes([
+      0x30,  # flow control
+      0x01 if self.single_frame_mode else 0x00,  # block size
+      separation_time,
+    ]).ljust(self.max_len, b"\x00")
+
   def send(self, dat: bytes, setup_only: bool = False) -> None:
     # throw away any stale data
     self._can_client.recv(drain=True)
@@ -461,12 +478,7 @@ def _isotp_rx_next(self, rx_data: bytes) -> None:
       if self.debug:
         print(f"ISO-TP: TX - flow control continue - {hex(self._can_client.tx_addr)}")
       # send flow control message
-      msg = bytes([
-        0x30,  # flow control
-        0x01 if self.single_frame_mode else 0x00,  # block size
-        0x0a,  # 10 ms separation time
-      ]).ljust(self.max_len, b"\x00")
-      self._can_client.send([msg])
+      self._can_client.send([self.flow_control_msg])
       return
 
     # consecutive rx frame
@@ -480,8 +492,7 @@ def _isotp_rx_next(self, rx_data: bytes) -> None:
         self.rx_done = True
       elif self.single_frame_mode:
         # notify ECU to send next frame
-        msg = b"\x30\x01\x0a".ljust(self.max_len, b"\x00")
-        self._can_client.send([msg])
+        self._can_client.send([self.flow_control_msg])
       if self.debug:
         print(f"ISO-TP: RX - consecutive frame - {hex(self._can_client.rx_addr)} idx={self.rx_idx} done={self.rx_done}")
       return