Just got a 1979 F150 4x4 with the 300 strait 6. Around town it did fine but if I go out on highway at 65 mph or above I'm getting a huge amount of bubbling coming out of radiator into the overflow container.

So I pulled the head and checked the block and head for flatness and installed a new Fel-Pro severe duty head gasket. Same problem afterwards.

So before I pull the engine any other ideas? I'm guessing cracked cylinder which can be re-sleeved.

Could it be overheating? Or compression getting into cooling system?

Thanks Mike in St Louis

 

Yes, it is compression getting into your cooling system. If you put in a new head gasket it is likely the head is cracked - or - less likely, a cracked cylinder (unless the engine has been overbored, which increases its chances of a cracked cylinder). Do all of the plugs look the same? If coolant is leaking into the engine you may see a plug that looks leaner than the rest. Good luck.

 

All clean plugs and no coolant mixing with oil. Head is new or at least looks like it just came out of box. My guess is block but I guess I could take just the head in to have it checked. I'm not the one that swapped it on.

 

that's a bummer to hear. whomever swapped the head / gasket should have noticed any visible damage to cylinder walls, etc. when doing the work...knowing the original symptoms you were dealing with. although, not all issues are easily spotted unfortunately.
do your engine temp gauge work in the cab? what does it read at idle versus cruising on the hwy? might want to use an IR temp gun and take a reading as well. may give a more accurate reading that way. be sure that the entire cooling system is properly serviced and filled up. be sure to "burp" it a couple times to eliminate air in there too.
so you have clean plugs, and no coolant in the oil at all? any oil in the coolant? could always pressurize each cylinder with compressed air, and listen around for the seepage in the radiator, oil cap, and dipstick tube. leakdown test and compression test would also be beneficial.
keep us posted on your findings as time allows. it may seem like a mystery, but it sounds like you're nearing some answers. don't forget to take plenty of beer breaks...it allows ample time to meditate on what to do next haha!