I have an 88 with a 4.9 I6. It misses when it wants to. It has the mechanic confused too. here is what it's doing:
It missed pretty bad from time to time, and has been doing so for over a month and a half now. I do not have any idea what it is. It misses on 2 cylinders that are side by side. When it misses the compression on these 2 cylinders is way low 55 on one 75 on the other. When it is running fine the 2 cylinders have upward s of 125 psi. It has been suggested that it may be a head gasket in the initial stages of blowing. It has also been said that it could be a cracked head.
What confuses everyone is that it doesnt do it all the time, and that it has low compression on 2 cylinders when it is missing and normal when it is running right.
What the he!! is wrong with my truck??

 

im gonna venture a guess that its a valve or valves sticking. what makes me think this is that the pressure is ok, then all of the sudden- bam- low pressure.
drop me a line at edenroadmaster(No Email Addresses In Posts!) if i can help more
eden