diff --git a/engine/engine.c b/engine/engine.c
index 4ede77a..1a02bbe 100644
--- a/engine/engine.c
+++ b/engine/engine.c
@@ -146,13 +146,40 @@ int register_callback(void *func)
     return 0;
 }
 
+#define SENTINEL_VALUE  0x5AFEC0DE
+/* These values should hang around if IGR in OPL is activated */
+extern u32 Uninitialized;
+static u32 *pOldSetupThread = &Uninitialized;
+static u32 *pSentinel = &Uninitialized + 1;
+
+void InstallHook()
+{
+    *pOldSetupThread = GetSyscallHandler(__NR_SetupThread);
+    SetSyscall(__NR_SetupThread, KSEG0(HookSetupThread));
+    *pSentinel = SENTINEL_VALUE;
+}
+
+void RemoveHook()
+{
+    SetSyscall(__NR_SetupThread, *pOldSetupThread);
+}
+
 int __attribute__((section(".init"))) _init(void)
 {
 #ifdef _HOOK_9
     /* hook syscall */
-    OldSetupThread = GetSyscallHandler(__NR_SetupThread);
-    j_SetupThread = MAKE_J(OldSetupThread);
-    SetSyscall(__NR_SetupThread, KSEG0(HookSetupThread));
+    if(*pSentinel == SENTINEL_VALUE && *pOldSetupThread)
+    {
+        /* When the engine is installed it hooks SetupThread and keeps the
+         * address to the original callback in an uninitialized area that should
+         * be retained if IGR is activated in OPL. The sentinel value signals
+         * that the engine has already been installed, so we need to temporarily
+         * restore the original SetupThread callback before "installing" it
+         * again.
+         */
+        RemoveHook();
+    }
+    InstallHook();
 #endif
     return 0;
 }
@@ -160,8 +187,7 @@ int __attribute__((section(".init"))) _init(void)
 int __attribute__((section(".fini"))) _fini(void)
 {
 #ifdef _HOOK_9
-    /* unhook syscall */
-    SetSyscall(__NR_SetupThread, OldSetupThread);
+    RemoveHook();
 #endif
     return 0;
 }
diff --git a/engine/engine_asm.S b/engine/engine_asm.S
index 44c299e..08c5624 100644
--- a/engine/engine_asm.S
+++ b/engine/engine_asm.S
@@ -57,6 +57,7 @@
 	.globl	eh_backup
 	.globl	CodeHandler
 	.globl	BootCodeHandler
+	.globl	Uninitialized
 
 #ifdef _HOOK_9
 	.globl	HookSetupThread
@@ -798,8 +799,8 @@ HookSetupThread:
 	addiu	$sp, 0x10
 
 	/* jump to original SetupThread() */
-j_SetupThread:
-	j	0
+	lw	$t1, Uninitialized
+	j	$t1
 	nop
 	.end	HookSetupThread
 #endif
@@ -860,6 +861,9 @@ numcodes:
 codelist:
 	.space	(MAX_CODES*8)
 
+Uninitialized:
+	.space /* Anything after this address should be retained when activating IGR in OPL. */
+
 	.set	pop
 
 /* EOF */