You are sure, you are not losing coolant?

You do not replace a head gasket, without testing first.....

You could have an air pocket......anytime you open up a coolant system, you can create one yourself, if the system is not properly "burped", and 3.4'* are notorious for this...

I would use a Lisle funnel setup at the radiator cap.....you make sure there is coolant at the bottom of the funnel......start the vehicle....coolant will start to rise in the funnel, a little, as the coolant expands from heat buildup and pressure.....then when the therm opens, the coolant in the funnel will drop dramatically, verifying that the therm has opened and that coolant is flowing through the radiator(if it doesn't drop, the therm isn't opening, the radiator is plugged, the pump is bad, etc.)....while this is happening, any air pocket will come out of the funnel....if the level in the radiator drops a lot when this happens(due to the air pocket), now fill it to the neck(engine still running), remove the Lisle funnel, and install the cap.......now run the vehicle and see if you still have wild swings.....

Several ways to check a head gasket......tool you attach to the radiator neck that will check for exhaust gases in the coolant......or on a warm engine, pressurize the system overnight with a coolant pressure tester.......next day, remove the plugs and disable fuel and spark....have someone crank over the vehicle....if coolant comes out of any of the plug holes, especially cyl #1, it'* a head gasket.....

Martinnhs12, I reread the original post, and since the previous owner used stop leak, it'* also possible that radiator is plugged, as you suggested....

Another thing to check is the heater core......the heater core is the bypass for the system.....if it is plugged, you will have poor heat flow out of it, and it will cause overheating problems too....and air pocket will cause similar symptoms of a plugged heater core(the air pocket will restrict flow through the core).....

Note: One thing I have seen a lot of, is misdiagnosis, where the lower intake is blamed....it could be leaking/seeping, but there could also be a head gasket problem....if you don't do proper diagnosis of the head, and just do a lower intake gasket, you will be doing the same job twice(500-800 bucks for a lower intake job)....

I'm pretty sure. I guess I'm not a 100% sure.

I wouldn't think so. I have a compression tester and a leak down tester (but my old air compressor finally gave up) I'll get to the compression test today or tomorrow.

This is what I'm hoping. Considering my temps seem to be all over the place. In fact, I drove home last night and it never reached the middle mark.

Ordered that funnel today, Amazon.com says it'll be here on or before the 30th.

I ran the pressure tester but I did it on a cool engine. Taking the radiator cap off after I've run even for a few minutes sprays coolant everywhere. Even when the coolant isn't hot. Which is another reason why suspect a clog. I set the pressure at 22 psi and let it sit for 5-10 minutes and the pressure never went down. Now, I understand that the metal in the heads and block change when it heats up and could hide a problem while cold but it seemed, at that pressure, I'd have at least seen a little drop. But maybe wrong about that considering I'm use to working with old cast iron heads.

This was my plan for today but I'm going to wait for the funnel and try to burp the system first. No need to waste all that coolant just because of an air pocket. I think I'll pull out my old vacuum gauge and check vacuum consistency today. Maybe run a compression test if I'm up for the heat.

If I end up going in to change one, I'll spend the extra and do both at the same time.

Thanks for the help so far. I'm sure I'll have more questions in a few days. Like, how to get to those rear plugs, haha.

Outside temps were in the low 90s today so not to bad with the breeze coming in.
Decided to check compression.
Engine rotated forward. (Really?) Wires unplugged, two bolts and one nut taken off the coil pack cluster. That last nut, drivers side, under the EGR valve (I think that'* what it is), dropped six 13mm sockets and three 1/2 inch sockets back there. I can't seem to reach that nut or any of my missing sockets. Really not sure what to do now. I'm out of 13mm/ half inch sockets. I've been working on getting that one nut for an hour and 45 minutes. Any hints?

OK. I removed the EGR valve (?). Once i got that off, I was able to reach my sockets. I had to remove the hard vacuum lines; then get a long flat head screw driver down between plug wire bracket and pry that off and out of the way. Then I was able to get a 13mm on a six inch extension and loosen that last bolt. (Tip for anyone searching this thread at a later date, the easier way would have been to remove the EGR valve from it'* bracket, a stud and nut coming up through the bracket just to the left of the EGR. That info would have been nice a couple hours ago.)

Pulled the plug wires and plugs. The ones to the right and middle are pretty easy but the one under the alternator was tricky. Coming at it from under the EGR tube seemed the easiest. Although I dropped the plug once i got it loose. And i lost light so I haven't had a chance to look at it yet.

Couple things I noticed. These engines run lean.

MIDDLE PLUG MIGHT HAVE SIGNS OF A BLOWN HEAD GASKET.

It'* stained yellowish orange, maybe the color of burned coolant.

Ran out of light so I'll do a compression test tomorrow.

Actually, i know better than that. Coolant cleans plugs, right? Makes them look new.

Alright. Didn't get much done today, had a lot of running around to do but I got 5 of the 6 cylinders compression tested. The 3 up front, the passenger side back under the alternator and the middle one in the back all came out with readings between 145psi and 150psi. I still have the back drivers side to test but I ran out of light.

But so far so good. Also, Amazon.com says the Lisle Funnel will be delivered tomorrow.

Checked the final cylinder. It'* also at 150psi.

I'm going to go with it NOT being a blown head gasket. Not sure if this also checks for a blown intake gasket.

How do i test for a blown intake gasket? I would have coolant in my oil, right? I don't, so does this mean I have a clog or bubble somewhere?

I really want to start putting the engine components back together. But I'd hate to get it all back together and have someone tell me, I should have checked something else while I had the plugs out. So I'm holding off for a day or two before I start putting it back together. Then I'll flush the system and burp it. Hopefully I'll have an answer to this overheating issue then.

I have to side with tech2 ideas. These engines are very bad about headgasket. Bleeding the system on this engine is very critical and can be a pain. Fyi- A compression test sometimes wont show a bad headgaskets. My advice would be to put the engine back together and get a kit that detects exhaust gases in the engine coolant.

OK. I'll keep checking. Suppose to be around 108f today so I'll probably wait till tomorrow.
But I'd really be surprised at this point. I have only one symptom of a blown head gasket. Overheating. Erratic overheating at that. No smoke, no CEL, good compression, no oil loss, no coolant loss. But everyone keeps saying head gasket. Seems weird, but I'll give it one more look.