98 Jimmy, heat is hot then cold

I have a 98 jimmy. I have had a problem for a few years. When it is cold out and I turn the heat on, it will warm up. But unlike when you get to hot and turn it down or off, I will find myself chilly and feeling the cold air out of the vent. It is kind of cyclical (it blows hot for a half of a minute, then cold for half of a minute).
Any ideas? It's getting cold out

I have a 98 jimmy. I have had a problem for a few years. When it is cold out and I turn the heat on, it will warm up. But unlike when you get to hot and turn it down or off, I will find myself chilly and feeling the cold air out of the vent. It is kind of cyclical (it blows hot for a half of a minute, then cold for half of a minute).
Any ideas? It's getting cold out
are the lower intake gaskets leaking, such a common problem on these. but another thing to check which is also a big deal on these, when its at operating temp, grab the heater core hoses, on the firewall, pass side, 2 hoses coming out just above and beside the distributor cap. if one is hot and one cold, or one is warm and the other not, then there is a blockage in the heater core, and it needs to be backflushed. if so unhook lines at the front, one on water pump and one on pass side of intake, water hose, flush one way till water comes out other hose, then flush other hose till water comes out the other one.

I will try the hoses today. here is an update though. I changed the thermostat to no avail. I then added some coolant, and this seemed to help the situation. The temp went back down, and the heater was working better (it was not burning me out of the car). That lasted about 200 miles. I was on the expressway yesterday and I looked down and the engine temperature was pegged. At this point I noticed that the heater duct was blowing out cold air even though I had the heater cranked full blast. As I tried to limp it down the road, I would notice that the heater would start blowing out some warm air and then the engine temp would go down a little. It kept cycling like this. When I finally stopped, it was overheating, but more from the engine than the radiator. I added 2 20 oz bottles of water to the reservoir and it kept the engine temp down low enough to get me home.

Just checked the hoses and one was hot and the other was cold. I am officially leaking coolant now. It seems to be leaking from the engine somewhere.

Just checked the hoses and one was hot and the other was cold. I am officially leaking coolant now. It seems to be leaking from the engine somewhere.
big chances are the intake is leaking. although water pumps are known for going on these as well as frost plugs and block heaters.

if coolant is leaking, then there is now air in the system, hence the blowing cold air, so one hose will end up being hot and the other cold. if you hadn't of said you were adding coolant/water i would say blockage, but since you are.

if its the water pump, thats easy enough to do yourself, but given the common problem for intake gaskets on these, its better to take it in due to having to take the distributor out and set timing.

have the lower intake gaskets ever been done? original gaskets were like cardboard, first updated ones were plastic, next update was metal/plastic, last update, best by far are metal with rubber coating. but there is still corrosion on the heads and intake surfaces from the coolant, sometimes its easily sanded off, other times not. another guy in this section had to replace his intake manifold do to the corrosion causing a hole.

and the block heaters are common for leaking on these, drivers side above oil filter adapter and front driveshaft. or just a frost plug if you don't have a block heater. but it is also a common problem for the front frost plug on the drivers side ahead of the other one behind the motor mount to corrode and start leaking. only solution for this i've heard of is to stick a block heater in the hole, with no cord of course.

not trying to scare you, but i've fixed lots no heat/coolant leaks on jimmies and blazers. its the no 4x4 i have problems fixing on them. i'm licensed and was at a gm dealer for 6 years, my specialty areas are engine/tcase and diffs.

I've had it for about 9 years. I don't think the lower intake gaskets have been done. That does sound expensive, not what I was hoping to hear. I am not sure, but the water pump may have been done a few years ago, (but not in the recent past) Thank you for the help, I will get in touch with a garage tomorrow

I've had it for about 9 years. I don't think the lower intake gaskets have been done. That does sound expensive, not what I was hoping to hear. I am not sure, but the water pump may have been done a few years ago, (but not in the recent past) Thank you for the help, I will get in touch with a garage tomorrow

don't know the cost in us dollars, but the lower gaskets in canada, depending on location are around $800 plus.

I took it in and they said the lower intake gaskets were leaking. They fixed that and also found a blockage in the heater core. They changed the gaskets, flushed the system and changed the oil for $465. It was a pretty good price. I now have heat and all seems well. He said that I may need another flush in a month or so.

I took it in and they said the lower intake gaskets were leaking. They fixed that and also found a blockage in the heater core. They changed the gaskets, flushed the system and changed the oil for $465. It was a pretty good price. I now have heat and all seems well. He said that I may need another flush in a month or so.

normal procedure is to replace the thermostat when doing an intake, as well as flush the heater core, so i suspect you should be good, but its never a bad idea to flush the heater core both ways like i mentioned before every so often.

The nice warm heat lasted about a week, and now it is back to blowing cold. They had said I would probably need to flush it again in about a month, but now I am worried that this is going to be a monthly procedure. If there was just a little filter they could put in just before the heater core.

The nice warm heat lasted about a week, and now it is back to blowing cold. They had said I would probably need to flush it again in about a month, but now I am worried that this is going to be a monthly procedure. If there was just a little filter they could put in just before the heater core.

if it only lasted a week, i would be checking for something else leaking. if it gets any air in the system, it will blow cold. will it blow hot when you rev it up and blow cold at idle?