I have a 99 LS with 3.1 purchased new by me in 99. All original and I try to repair/replace with genuine gm oem to keep the authentic. Has 103k and never have had issue till recently. Had a problem with a knock sensor and replaced but still have the trouble code.
Anyway, the big issue heat and or cooling. The engine does not get up to correct operating temp. I leave in Iowa where the current temp will cause brittle things to break off. I can start the car 1/2 hour before I leave for work and the thing is barely throwing any heat. Once I start driving, the engine will warm to the point where the thermostat opens and then the heat goes away that had built up. Replaced the thermostat and have bleed air from the system, so I think. To the point I only get antifreeze from the bleeder. Tried the Lisle no-spill funnel, still no luck. I'm lost. Thanks in advance.

 

Mine is similar and my son had a 96 LS too and I don't recall any such issues.
When is the last time you had the Radiator flushed? then that is done it will usually get the heater core too. but when the heat core goes bad they usually leak into the floor of the passenger side of the the car. I live in Nebraska so the same type of issues would come about for me. There is a possible problem with the climate control unit. those 3 dials up on the dash above the radio. make sure the slide control on the last one is all the way up.
what you could do is see if you have a U-pull salvage yard in your area and if so find a Monte of the 5th gen and take that part out of the wreck and replace it in yours all 5th gens are the same so you wouldn't have to distinguish between the Ls and the Z.

 

It's not that I don't get heat in the car, what little heat is built up in the motor and cooling system, I do get interior heat. The problem is the engine does not reach the correct temp. The engine is slow to build heat (temp) and does till thermostat opens and then the temp drops and never builds back up. Today while sitting with engine running for a period of time, it got close to operating temp, but not there completely. I'm going to jack the front of the car this weekend and try air bleeding. If that doesn't work, going to suffer through till warmer weather and then try a flush.

 

I have the same problem in my 05 and I gave up. I assume its either air in the heater core or the heater core is just lined with gunk. However, why would coolant read so low? Itll get up to 180 then when I turn the heat on, it goes to 160ish and never come back up. Even jacked up the front and back a bazillion times trying to bleed air out. LOL theres no air in there and im now certain I have no leak.

I tried a little experiment.. Blocked the radiator with cardboard and went for a test drive. Got the coolant up to 220 and had decent heat but still nowhere near what it should be. The trans fluid also went up to 220 so thats not a permanent solution haha.

If you find anything out with yours, I would appreciate any suggestions. Ill do the same but I think ive done everything short of changing the radiator and heater core.

 

I did them same thing with cardboard leaving just very little opening. Didn’t change a thing. Weather permitting, I may change the temp sensor although I don’t think that is the issue. I honestly don’t think that the engine temp is getting to operating temp. Dazed and confused in Iowa.

 

Be careful with the cardboard thing. Im not sure about the 99s but in the 05s the trans cooler is built into the radiator.

Same thing with me about operating temp, mine usually stays around 160*.

 

I had pulled the cardboard shortly after swapping the thermostat. I was really surprised when the cardboard didn't raise the temp. When weather gets a little warmer will put more effort into it. Gotta be something so simple.

 

Curious about what it does in warmer weather? My temp gauge gets up close to 200 but in the summer time is seems like the engine compartment is usually hotter that I think it should be after I stop. Mine I only go to Carshows with anymore and I still have the hood insulation in it.
Where in Iowa?
I used to live in Denison back in the 70's I was just back there a couple of weeks ago. Visit some friends.

 

For me it never really goes above 178* even on the hottest of days. Right now at -2* it wants to stay at 160.

Something to add tho, I have an aftermarket radiator that came with the car. Its a super cheap and super thin one but WOW does it work incredibly well.

 

Live in a small town of 600 35 miles north of Cedar Rapids. I never the car in winter, but due to the grandson taking over my work, I reluctantly started driving the 99. I had not driven it since the spring of 21. The temperature if I recall correctly never got up to what it should have. I always got decent mileage out of it, but with this cold weather and lowered operating temp, it is always running rich. This cold weather and a 6 day work week doesn't give much time to work on it. Come warmer weather, will do the flush and whole nine yards, see I can get it straightened out.