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.

I'll take Lower intake manifold gasket for the win

Hehehe nice. If we can't laugh at this stuff we'd lose our minds I guess. Haven't replaced water pump yet. Will have to wait till this weekend to work on it.

Not superhard, just time consuming

Heat and then no heat indicates that the coolant is low. Pull the cap off the radiator when the engine is cool. It should be full to the top. Top it off with 50/50 antifreeze if it is low. Next check the coolant level in the overflow reservoir. It should be filled to a noted line level or about half full if you cannot find the line.

I meant to update this and totally forgot. We ended up changing the intake manifold gasket and I wanted to drive it for a few weeks to make sure we got it fixed before I updated. So, we changed that, and up until yesterday, it seemed to fix the overheating problem. Then yesterday, it started overheating again. Came home, let it cool down, checked the coolant in the overflow and the radiator and they were both low. Looked for visible leaks, can't find one. I checked the oil, it looks normal and is at a normal level. Last time the oil fill neck and cap had junk all over it from water getting in the oil, and the level on the dipstick was extremely high from all the water. So we are back to square one. Either the intake gasket wasn't done properly and was just good enough to get me by until now, or now the head gasket has gone out.

I would pull the radiator out and check it for good flow. It has probably gotten clogged up and just cant keep up with the heat transfer rate needed and coolant is flashing off through the overflow vent. As a matter of fact the entire coolant system could probably use a good flushing , block included.

That would be a good place to start before we take the whole thing apart again trying to repair gaskets.

The coolant system is just that, a system. Each and every part in the chain needs to function properly in order for it to work. also check to see if you have flow with the engine running, are there any noises coming from the water pump.?Iv'e seen pumps lose the impeller and you cant tell at all from the out side. just some strange noise and no flow at the radiator filler neck when up to temp.

Thanks Gregg64 for the info. Whatever has gone bad, it's something that is leaking out the coolant somewhere where I can't see it, because coolant is definitely disappearing. Couldn't have picked a worse time of year for this kind of problem! It may have to just sit in the driveway until spring.

I would change the oil again just to be sure no coolant is leaking into the pan from a gasket.