diff --git a/History.md b/History.md
index 159b889dca..9c9a4dd182 100644
--- a/History.md
+++ b/History.md
@@ -2,6 +2,7 @@
 
 * Features
   * Your feature goes here <Most recent on the top, like GitHub> (#Github Number)
+  * Uses stdio `flush` to avoiding mutating $stdout and $stderr `with sync=true` ([#2486])
   * Prints the loaded configuration if the environment variable `PUMA_LOG_CONFIG` is present ([#2472])
   * Integrate with systemd's watchdog and notification features ([#2438])
   * Adds max_fast_inline as a configuration option for the Server object ([#2406])
diff --git a/lib/puma/error_logger.rb b/lib/puma/error_logger.rb
index 7a2bb462b0..4b1f183add 100644
--- a/lib/puma/error_logger.rb
+++ b/lib/puma/error_logger.rb
@@ -15,7 +15,6 @@ class ErrorLogger
 
     def initialize(ioerr)
       @ioerr = ioerr
-      @ioerr.sync = true
 
       @debug = ENV.key? 'PUMA_DEBUG'
     end
@@ -32,7 +31,7 @@ def self.stdio
     #   and before all remaining info.
     #
     def info(options={})
-      ioerr.puts title(options)
+      log title(options)
     end
 
     # Print occured error details only if
@@ -54,7 +53,7 @@ def debug(options={})
       string_block << request_dump(req) if request_parsed?(req)
       string_block << error.backtrace if error
 
-      ioerr.puts string_block.join("\n")
+      log string_block.join("\n")
     end
 
     def title(options={})
@@ -93,5 +92,13 @@ def request_headers(req)
     def request_parsed?(req)
       req && req.env[REQUEST_METHOD]
     end
+
+    private
+
+    def log(str)
+      ioerr.puts str
+
+      ioerr.flush unless ioerr.sync
+    end
   end
 end
diff --git a/lib/puma/events.rb b/lib/puma/events.rb
index 29280075d2..f96d553845 100644
--- a/lib/puma/events.rb
+++ b/lib/puma/events.rb
@@ -30,9 +30,6 @@ def initialize(stdout, stderr)
       @stdout = stdout
       @stderr = stderr
 
-      @stdout.sync = true
-      @stderr.sync = true
-
       @debug = ENV.key? 'PUMA_DEBUG'
       @error_logger = ErrorLogger.new(@stderr)
 
@@ -66,6 +63,8 @@ def register(hook, obj=nil, &blk)
     #
     def log(str)
       @stdout.puts format(str) if @stdout.respond_to? :puts
+
+      @stdout.flush unless @stdout.sync
     rescue Errno::EPIPE
     end
 
diff --git a/lib/puma/runner.rb b/lib/puma/runner.rb
index e3e8cfdfbf..af3996d958 100644
--- a/lib/puma/runner.rb
+++ b/lib/puma/runner.rb
@@ -106,8 +106,8 @@ def redirect_io
         end
 
         STDOUT.reopen stdout, (append ? "a" : "w")
-        STDOUT.sync = true
         STDOUT.puts "=== puma startup: #{Time.now} ==="
+        STDOUT.flush unless STDOUT.sync
       end
 
       if stderr
@@ -116,8 +116,8 @@ def redirect_io
         end
 
         STDERR.reopen stderr, (append ? "a" : "w")
-        STDERR.sync = true
         STDERR.puts "=== puma startup: #{Time.now} ==="
+        STDERR.flush unless STDERR.sync
       end
     end
 
diff --git a/test/test_error_logger.rb b/test/test_error_logger.rb
index 38aba15522..08be915803 100644
--- a/test/test_error_logger.rb
+++ b/test/test_error_logger.rb
@@ -10,6 +10,14 @@ def test_stdio
     assert_equal STDERR, error_logger.ioerr
   end
 
+
+  def test_stdio_respects_sync
+    error_logger = Puma::ErrorLogger.stdio
+
+    assert_equal STDERR.sync, error_logger.ioerr.sync
+    assert_equal STDERR, error_logger.ioerr
+  end
+
   def test_info_with_only_error
     _, err = capture_io do
       Puma::ErrorLogger.stdio.info(error: StandardError.new('ready'))
diff --git a/test/test_events.rb b/test/test_events.rb
index d0da584cfb..ddd9659cdf 100644
--- a/test/test_events.rb
+++ b/test/test_events.rb
@@ -24,6 +24,15 @@ def test_stdio
     assert_equal STDERR, events.stderr
   end
 
+  def test_stdio_respects_sync
+    events = Puma::Events.stdio
+
+    assert_equal STDOUT.sync, events.stdout.sync
+    assert_equal STDERR.sync, events.stderr.sync
+    assert_equal STDOUT, events.stdout
+    assert_equal STDERR, events.stderr
+  end
+
   def test_register_callback_with_block
     res = false