View Full Version : Flat spot off idle - NA Fairlane

24-02-2010, 05:47 PM

Engine (3.9 MPFI in NA Fairlane) been playing up - running on 4 cylinders - traced to bad distribitor cap and leads - back to running on 6 cylinders but now has slight flat spot just past idle then power comes back in but driveability not good at low speeds and has to crank over for a while before engine starts to run - new plugs, leads, dissy cap and cleaned injectors and all new filters fitted. Compression good so probably not head gasket related etc. A few stains around inlet manifold branches where meets head so might tighten manifold just in case of leak.

My current thinking may be that (my probably worn) timing chain has slipped a cog so will remove rocker cover to check valve timing also will check ignition base timing but this shouldn't have changed although I understand dissy drive gear can wear badly too and affect ignition timing.

Any thoughts that might provide a clue before I rip further into it - will extract fault codes from computer too but hoping someone may know exactly what problem may be.

Thanks in advance for any suggestions.

25-02-2010, 03:25 PM
Were your spark plugs correctly gapped? Most come pre gapped these days but worth a look. Should be NGK BRE527Y-11 spark plugs ( if using NGK)

25-02-2010, 03:52 PM
Thanks for your reply - I only ever use NGK plugs now and I checked gaps before fitting. Ignition problem was furry corroded distributor cap terminals and 2 faulty leads. Previous owner may have heard of concept of maintenance but never did any. Probably because dissy and coil so hard to get to - would like to meet the dude\s who designed that.

Incidentally got set of 8.8 mm Topgun Performance (low resistance) plug leads and the spark seems noticeably stronger at the plug than the old Motorcraft leads when compared to one another side by side.

Just how common is a slipped timing chain - anyone else ran into this? Seems to come good at higher revs just dead past idle. Have lots more investigation to do and may be something else but hoping to pick up your experiences.


28-02-2010, 08:01 PM
Still would like to know if these engines have a known issue of slipping timing chains when they get worn - one EFI expert's site on the web suggests this can occur with these Falcon motors.