Where to go next- overheating problem
 

First, I am new to posting here. I love reading this forum to try to find little pieces of information for my husband when he has to work on my Blazer.

My Blazer seems to be falling apart lately. The biggest issue right now is it's overheating. It's a 98 4x4 v6. We have replaced thermostat, radiator cap, filled it with coolant, and bled the air out. Had the head gasket checked, it's fine. Can't seem to find a coolant leak anywhere, no puddles on passenger side floor. It will heat up to just under the red on the temperature gauge, then drop back down to normal after driving a few more minutes. The heat in the car is usually warm, but will sometimes blast cold air out of nowhere.

Another problem it's having is when driving 50-55 mph, the engine seems to lag a little then rev back up. You can see the rpm gauge drop then spike, then return to normal. I have no idea where to even start on that one.

Just hoping some of you guys might have some ideas on where to go next on the overheating issue? I love my Blazer and want to get her fixed back up!

Maybe the radiator is clogged up, and it's not getting any flow through it?

Make sure it is bled completely. I have started drilling a small hole(1/16") in thermostats when I replace them and positioning the hole at the top. That allows air to bleed by as well as allow a small amount of flow so hot coolant will reach the thermostat sooner. That is probably not the problem though. If you have changed just about everything you are down to either the water pump or radiator as the problem. I suppose a lean condition could heat it up faster. Also do a pressure test on the system and make sure the radiator cap is sealing good. A dirty seal will lose pressure and overheat once it reaches temperature.

has any radiator sealer been added ??
a slow flowing radiator can easily be clogged with sealer

I have not personally added any sealer. I've owned this Blazer for three years, who knows what the last owner may have done with it. I checked the radiator today and had to add about 1/3 gal of antifreeze, which means it has to be leaking somewhere, or it didn't get filled all the way. I'm thinking it's leaking and I just can't find a leak. Probably going to change water pump next.

Also, I had a pressure test done and the result was that it's holding pressure in a normal range.

Check your oil, maybe leaking gasket

Even if it's leaking, as long as you keep the radiator filled, it shouldn't overheat. I drove for a month or two in the hot Texas summer with a radiator that leaked a gallon every two days or so. I just carried around a couple gallon jugs of water and checked a couple times a day, until I finally got a chance to replace the radiator. Even without a fan shroud, the hottest it ever got was maybe 200*.

But, if you have to keep adding to it, you got a leak somewhere, and you need to find it. Like newguy said, check the oil. If it looks like chocolate milk, you found where the coolant is going!

I hadn't checked my oil because I had a mechanic test the pressure and he said the head gasket was fine. Decided today to check oil just in case, and sure enough, the underside of the oil fill cap had a substance that looked like butter all over it and all down in the oil fill neck. Now we have to figure out which part is leaking.

The water pump been replaced? A busted impeller will cause these probs.