Engine 1989 5.0 Cylinder 4 Misfire Help!!

Discussion in 'Fox 5.0 Mustang Tech' started by burns201, Apr 11, 2014.

  1. My 1975 Maverick has a 89 5.0. Cylinder 4 is dead (rear cylinder passenger side). "Sorry it's not a Mustang but you guys should know more about this than anyone else." It is Not firing and blowing lots of gas out of the exhaust. It has a street avenger carb, weiand intake, MSD 6al, msd coil, msd wires, msd cap and rotor and stock distributor. It has spark at the cylinder. I even swapped out the MSD box, cap and rotor, plug wires and got new plugs(same problem). Took off carb, cleaned and reinstalled(still same problem). Tried a friends edelbrock carb(same problem). Compression tested all cylinders and all are good. Checked valves and they are opening as they should. Looked in cylinder with camera and it still looks new. Roller rockers rock as they should. I am at a loss. I am thinking either the Distributor (even though it has spark possibly just not firing at the correct time), or could it be a intake manifold or head gasket leak. Any help would be greatly appreciated. I have been trying to figure this out since September. Thanks
  2. sounds to me like you have a vacuum leak.
  3. Couple of questions.

    What does the plug look like.
    What are the plugs gapped to
    What are the results of the compression test
    Are you sure you are getting spark to that plug
    Bypass the POS MSD box and see if if runs
    Check the spark plug wire on the cap to see if you have them wrong
    Hook up a vacuum gauge and check for leaks
  4. Plug looks brand new (never fired) just wet with gas. No oil or coolant on plug. All plugs have the correct gap. I have checked the plug for spark. Plug sparks when out and spark jumps from ens of wire to plug while in cylinder. Already bypassed MSD and it didn't fix it. Wire are correct on the cap. All cylinders read 145-150 while the dead cylinder reads 230-240. Both mechanics told me it is from the extra gas taking up space in the combustion chamber. When car runs all the gas blows straight out the exhaust.
    I am now thinking the spark may be too weak when the cylinder is under pressure so I am going to try a new module in the distributor. Also wondering if it could be a bad intake gasket or head gasket.
  5. Only thing you didnt check was the vacuum reading. Pull the distributor and have it tested.