diff --git a/Makefile b/Makefile
index 2512f01f32..dee0766892 100644
--- a/Makefile
+++ b/Makefile
@@ -81,9 +81,8 @@ ppx_let-test-deps :
 	opam install --yes --unset-root ppx_let
 
 .PHONY: clean
-clean:
+clean :
 	dune clean
-	find . -name '.merlin' | xargs rm -f
 	rm -fr docs/api
 	rm -f src/unix/discover_arguments
 	for TEST in `ls -d test/packaging/*/*` ; \
@@ -102,11 +101,9 @@ EXPECTED_FILES := \
     --do-not-expect src/unix/unix_c/
 
 .PHONY: coverage
-coverage: clean coverage-only
-
-.PHONY : coverage-only
-coverage-only :
-	BISECT_ENABLE=yes $(MAKE) build
+coverage :
+	rm -rf _build/default/test/ppx_expect
+	find _build -name '*.coverage' | xargs rm
 	BISECT_ENABLE=yes dune runtest --force
 	bisect-ppx-report html $(EXPECTED_FILES)
 	bisect-ppx-report summary
diff --git a/src/core/dune b/src/core/dune
index 0b7a3091f7..0a433df90a 100644
--- a/src/core/dune
+++ b/src/core/dune
@@ -1,10 +1,21 @@
+(* -*- tuareg -*- *)
+
+let preprocess =
+   match Sys.getenv "BISECT_ENABLE" with
+   | "yes" -> "(preprocess (pps bisect_ppx))"
+   | _ | exception _ -> ""
+
+let () = Jbuild_plugin.V1.send @@ {|
+
 (library
  (public_name lwt)
  (synopsis "Monadic promises and concurrent I/O")
  (wrapped false)
- (preprocess (pps bisect_ppx --conditional))
+ |} ^ preprocess ^ {|
  (libraries bytes result seq)
  (flags (:standard -w +A-29)))
 
 (documentation
  (package lwt))
+
+|}
diff --git a/src/ppx/dune b/src/ppx/dune
index f39aa6e1df..7a13883a6c 100644
--- a/src/ppx/dune
+++ b/src/ppx/dune
@@ -1,3 +1,12 @@
+(* -*- tuareg -*- *)
+
+let bisect_ppx =
+  match Sys.getenv "BISECT_ENABLE" with
+  | "yes" -> "bisect_ppx"
+  | _ | exception _ -> ""
+
+let () = Jbuild_plugin.V1.send @@ {|
+
 (library
  (name ppx_lwt)
  (public_name lwt_ppx)
@@ -6,5 +15,7 @@
  (libraries compiler-libs.common ocaml-migrate-parsetree ppx_tools_versioned)
  (ppx_runtime_libraries lwt)
  (kind ppx_rewriter)
- (preprocess (pps ppx_tools_versioned.metaquot_410 bisect_ppx --conditional))
+ (preprocess (pps ppx_tools_versioned.metaquot_410 |} ^ bisect_ppx ^ {|))
  (flags (:standard -w +A-4)))
+
+|}
diff --git a/src/react/dune b/src/react/dune
index 318cb3c757..56946a3ece 100644
--- a/src/react/dune
+++ b/src/react/dune
@@ -1,3 +1,12 @@
+(* -*- tuareg -*- *)
+
+let preprocess =
+   match Sys.getenv "BISECT_ENABLE" with
+   | "yes" -> "(preprocess (pps bisect_ppx))"
+   | _ | exception _ -> ""
+
+let () = Jbuild_plugin.V1.send @@ {|
+
 (rule
  (targets lwt_react.ml)
  (deps (:ml lwt_react.cppo.ml))
@@ -10,5 +19,7 @@
  (synopsis "Reactive programming helpers for Lwt")
  (wrapped false)
  (libraries lwt react)
- (preprocess (pps bisect_ppx --conditional))
+ |} ^ preprocess ^ {|
  (flags (:standard -w +A)))
+
+|}
diff --git a/src/unix/dune b/src/unix/dune
index b1c6b7e536..b34125a646 100644
--- a/src/unix/dune
+++ b/src/unix/dune
@@ -1,3 +1,12 @@
+(* -*- tuareg -*- *)
+
+let preprocess =
+   match Sys.getenv "BISECT_ENABLE" with
+   | "yes" -> "(preprocess (pps bisect_ppx))"
+   | _ | exception _ -> ""
+
+let () = Jbuild_plugin.V1.send @@ {|
+
 (rule
  (targets lwt_unix.ml)
  (deps (:ml lwt_unix.cppo.ml))
@@ -33,7 +42,7 @@
  (optional)
  (wrapped false)
  (libraries bigarray lwt mmap ocplib-endian.bigstring threads unix)
- (preprocess (pps bisect_ppx --conditional))
+ |} ^ preprocess ^ {|
  (c_names
   lwt_unix_stubs
   lwt_libev_stubs
@@ -167,3 +176,5 @@
  (install_c_headers lwt_features lwt_config lwt_unix)
  (c_flags -I. (:include unix_c_flags.sexp))
  (c_library_flags (:include unix_c_library_flags.sexp)))
+
+|}
diff --git a/test/ppx_expect/dune b/test/ppx_expect/dune
index f3a8d6fb5f..4d06644c62 100644
--- a/test/ppx_expect/dune
+++ b/test/ppx_expect/dune
@@ -2,8 +2,8 @@
  (name main)
  (libraries lwttester))
 
-(alias
- (name runtest)
- (package lwt_ppx)
- (deps (source_tree cases))
- (action (run %{exe:main.exe})))
+; (alias
+;  (name runtest)
+;  (package lwt_ppx)
+;  (deps (source_tree cases))
+;  (action (run %{exe:main.exe})))