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Truck started missing all of sudden.

Getting spark, but plugs are not firing. Suspected head gasket has a bad spot between 3 & 5; compresson was low. Now its good, but still doing same thing.

Any help would be appreciated

check for moisture in the cap. 3&5 are right next to each other. start with the basics before you condem the head gasket. was this vehicle overheated just before this problem?

cap is good, he was driving the truck and all of sudden started missing. compression was less than 55 on 3&5

Injector balance is good; plugs & wires are good.

we just did a 02 explorer who said they spent over 1k for tune up stuff about 6 months ago. it just started running rough and checkengine light came on. it is a state law enforcement vehicle. with the hacks that installed the sat, radio, computer, lights,sirens, etc. the wiring under dash looks like spaghetti with butt connectors and scotch blocks. scanner would not comm. it turned out to be #3 &4 coils.

autoxperts wrote:cap is good, he was driving the truck and all of sudden started missing. compression was less than 55 on 3&5

Perform a leakdown test on the affected cylinders. That'll tell you what failed mechanically. Also, a running compression test would be in order along with a vacuum test. How many miles on this thing?

If it has low compression then it definitly has an internal problems.

compression is 80 on 3, 90 on 5 after head gasket replacment

so if i understand this correctly you replaced the head gasket and the compression increased but is still low. i hope that you looked long and hard at the head because most of these are junk when you take them off. a straight edge may have shown a deviation in the head between 3 and 5. a new head gasket would compensate a bit. this may be why you have an increase in compression. another is that the valves are adjusted improperly. i would take the plugs out of 3 and 5 back off on the lifters and do a leak down test. i wouldn't be shocked if there was a leak between cylinders. if not the do a compression test on them with the lifters backed off and see if there is an increase. try it with a squirt of oil in the cylinder and see if the pressure jumps. if yes yank the pistons. also when you have the heads off check the piston travel. it's a long shot but i've seen bent push rods do this.

had heads milled and value job done on them

Even so, I'd still do what Steven said. Just because you had the heads milled and valve job done on them doesn't mean it was done right. Had a vehicle once that I sent the heads out and had them reworked during a head gasket replacement. Put it all back together just to find out that a valve guide was installed wrong...had to take it all back apart. It was a machine shop I have used for years...never had a problem. Come to find out that he had a new helper that he did not supervise well that day...happens to all of us...and it happened to me...and it could have happened to you. I'd recheck everything.