diff --git a/scrapy/core/engine.py b/scrapy/core/engine.py
index 8ea50a73d..100f519ef 100644
--- a/scrapy/core/engine.py
+++ b/scrapy/core/engine.py
@@ -23,7 +23,6 @@ from scrapy.item import ScrapedItem
 from scrapy.item.pipeline import ItemPipelineManager
 from scrapy.spider import spiders
 from scrapy.spider.middleware import SpiderMiddlewareManager
-from scrapy.utils.defer import chain_deferred, deferred_imap
 from scrapy.utils.misc import load_object
 from scrapy.utils.defer import mustbe_deferred
 
@@ -219,7 +218,6 @@ class ExecutionEngine(object):
                             signals.send_catch_log(signal=signals.item_dropped, sender=self.__class__, item=item, spider=spider, response=response, exception=pipe_result.value)
                         else:
                             signals.send_catch_log(signal=signals.item_passed, sender=self.__class__, item=item, spider=spider, response=response, pipe_output=pipe_result)
-                        self.next_request(spider)
 
                     if domain in self.closing:
                         return
@@ -234,9 +232,10 @@ class ExecutionEngine(object):
                     elif output is None:
                         pass # may be next time.
                     else:
-                        log.msg("Spider must return Request, ScrapedItem or None, got '%s' while processing %s" % (type(output).__name__, request), log.WARNING, domain=domain)
+                        log.msg("Spider must return Request, ScrapedItem or None, got '%s' while processing %s" \
+                                % (type(output).__name__, request), log.WARNING, domain=domain)
 
-                return deferred_imap(cb_spider_output, spmw_result)
+                return task.coiterate(cb_spider_output, spmw_result)
 
             def eb_user(_failure):
                 if not isinstance(_failure.value, IgnoreRequest):
@@ -248,7 +247,7 @@ class ExecutionEngine(object):
                 log.msg('FRAMEWORK BUG processing %s: %s' % (request, _failure), log.ERROR, domain=domain)
 
 
-            scd = self.spidermiddleware.scrape(request, response, spider)
+            scd = mustbe_deferred(self.spidermiddleware.scrape, request, response, spider)
             scd.addCallbacks(cb_spidermiddleware_output, eb_user)
             scd.addErrback(eb_framework)
 
@@ -263,7 +262,7 @@ class ExecutionEngine(object):
             request.deferred.addErrback(lambda _:None)
             request.deferred.errback(_failure) # TODO: merge into spider middleware.
 
-        schd = self.schedule(request, spider)
+        schd = mustbe_deferred(self.schedule, request, spider)
         schd.addCallbacks(_process_response, _cleanfailure)
         return schd
 
@@ -304,9 +303,9 @@ class ExecutionEngine(object):
                 return response
             elif isinstance(response, Request):
                 newrequest = response
-                schd = self.schedule(newrequest, spider)
-                chain_deferred(schd, newrequest.deferred)
-                return schd
+                schd = mustbe_deferred(self.schedule, newrequest, spider)
+                schd.chainDeferred(newrequest.deferred)
+                return newrequest.deferred
 
         def _on_error(_failure):
             """handle an error processing a page"""
@@ -317,13 +316,12 @@ class ExecutionEngine(object):
 
         def _on_complete(_):
             self.next_request(spider)
+            return _
 
-        dwld = self.downloader.fetch(request, spider)
+        dwld = mustbe_deferred(self.downloader.fetch, request, spider)
         dwld.addCallbacks(_on_success, _on_error)
-        deferred = defer.Deferred()
-        chain_deferred(dwld, deferred)
         dwld.addBoth(_on_complete)
-        return deferred
+        return dwld
 
     def open_domain(self, domain):
         log.msg("Domain opened", domain=domain)