Expert: Sean Halliday - 10/11/2010

Question
blew head gasket and it cracked the head. bought new head and installed. truck started up but runs rough at idle speed and when I put it in gear it runs worse. Changed plugs and wires, getting spark on all cylinders and now I did a compression check and the number 4 and 6 cylinders the compression is 145- 150 and the number 5 cylinder is 120. checked the other side which still has the original head and the compression is 120. Did I just get a bad head and need to return it. Will this make it run rough like it is missing.  

Answer
Check each cylinder to see if all are firing as this sounds like a misfire.Remove each plug wire in turn for a few seconds and see if the rpm drops.If the rpm stays the same with the wire removed,that is the faulty one.The readings are ok for compression as 120 is good enough to run.Im thinking you have crossed wires or a bad injector.If all cylinders have spark and good compression,it must be the injector.It is easy to cross the wires so check the firing order to distributor is correct.