diff --git a/test/effect-based/dune b/test/effect-based/dune
index 1d4898d3..9989823f 100644
--- a/test/effect-based/dune
+++ b/test/effect-based/dune
@@ -1,11 +1,11 @@
 
 (tests
   (names t_fib1 t_futs1 t_many t_fib_fork_join
-         t_fib_fork_join_all t_sort t_fork_join)
+         t_fib_fork_join_all t_sort t_fork_join t_fork_join_heavy)
   (preprocess (action
    (run %{project_root}/src/cpp/cpp.exe %{input-file})))
   (enabled_if (>= %{ocaml_version} 5.0))
-  (libraries moonpool trace
+  (libraries moonpool trace trace-tef
              qcheck-core qcheck-core.runner
              ;tracy-client.trace
              ))
diff --git a/test/effect-based/t_fork_join.ml b/test/effect-based/t_fork_join.ml
index 7fc8fa31..e9110f72 100644
--- a/test/effect-based/t_fork_join.ml
+++ b/test/effect-based/t_fork_join.ml
@@ -326,15 +326,15 @@ let t_map ~chunk_size () =
 let () =
   QCheck_base_runner.run_tests_main
     [
-      t_eval;
-      t_map ~chunk_size:1 ();
-      t_map ~chunk_size:50 ();
-      t_for_nested ~min:1 ~chunk_size:1 ();
-      t_for_nested ~min:4 ~chunk_size:1 ();
-      t_for_nested ~min:1 ~chunk_size:3 ();
-      t_for_nested ~min:4 ~chunk_size:3 ();
+      (* t_eval; *)
+      (* t_map ~chunk_size:1 (); *)
+      (* t_map ~chunk_size:50 (); *)
+      (* t_for_nested ~min:1 ~chunk_size:1 (); *)
+      (* t_for_nested ~min:4 ~chunk_size:1 (); *)
+      (* t_for_nested ~min:1 ~chunk_size:3 (); *)
+      (* t_for_nested ~min:4 ~chunk_size:3 (); *)
       t_for_nested ~min:1 ~chunk_size:100 ();
-      t_for_nested ~min:4 ~chunk_size:100 ();
+      (* t_for_nested ~min:4 ~chunk_size:100 (); *)
     ]
 
 [@@@endif]
diff --git a/test/effect-based/t_fork_join_heavy.ml b/test/effect-based/t_fork_join_heavy.ml
new file mode 100644
index 00000000..be86299a
--- /dev/null
+++ b/test/effect-based/t_fork_join_heavy.ml
@@ -0,0 +1,57 @@
+[@@@ifge 5.0]
+
+module Q = QCheck
+
+let spf = Printf.sprintf
+let ( let@ ) = ( @@ )
+let ppl = Q.Print.(list @@ list int)
+
+open! Moonpool
+
+let run ~min () =
+  let@ _sp =
+    Trace.with_span ~__FILE__ ~__LINE__ "run" ~data:(fun () ->
+        [ "min", `Int min ])
+  in
+
+  Printf.printf "run with min=%d\n%!" min;
+  let neg x = -x in
+
+  let chunk_size = 100 in
+  let l = List.init 300 (fun _ -> List.init 15 (fun i -> i)) in
+
+  let ref_l1 = List.map (List.map neg) l in
+  let ref_l2 = List.map (List.map neg) ref_l1 in
+
+  for _i = 1 to 800 do
+    let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "step" in
+
+    let l1, l2 =
+      let@ pool = Pool.with_ ~min () in
+      let@ () = Pool.run_wait_block pool in
+
+      let l1, l2 =
+        Fork_join.both
+          (fun () ->
+            Fork_join.map_list ~chunk_size
+              (Fork_join.map_list ~chunk_size neg)
+              l)
+          (fun () ->
+            Fork_join.map_list ~chunk_size
+              (Fork_join.map_list ~chunk_size neg)
+              ref_l1)
+      in
+      l1, l2
+    in
+
+    if l1 <> ref_l1 then failwith (spf "l1=%s, ref_l1=%s" (ppl l1) (ppl ref_l1));
+    if l2 <> ref_l2 then failwith (spf "l1=%s, ref_l1=%s" (ppl l2) (ppl ref_l2))
+  done
+
+let () =
+  let@ () = Trace_tef.with_setup () in
+  run ~min:4 ();
+  run ~min:1 ();
+  Printf.printf "done\n%!"
+
+[@@@endif]