Forgot to mention that the temp stays in it's normal operating range
Check your low coolant sensor and wire leads at the coolant tank. As for the low heat, make sure you have the right mixture of 50/50 coolant/water. If your still having issues with your heat, you may have to flush out your your coolant system. There's actually a TSB issued on the DexCool by GM. The sludge is really internal rust in the cooling system due to air pockets.
Pulled up a rather GM generic TSB....you may be able to get an idea of whats up.
Rust in Cooling System, Heater Inoperative, Blows Cold Air, Engine Overheats (Flush Cooling System) #99-06-02-012D
Rust in Cooling System, Heater Inoperative, Blows Cold Air, Engine Overheats (Flush Cooling System)
1996-2000
Chevrolet and GMC S/T Models (Blazer, Jimmy, Sonoma, S10 Pickup)
1998-2000 GMC Envoy
1996-2000 Oldsmobile Bravada
with 4.3L V6 Engine (VINs W, X -- RPOs L35, LF6)
This bulletin is being revised to include additional model years. Please discard Corporate Bulletin Number 99-06-02-012C (Section 06 -- Engine).
Condition
Some customers may comment that the heater is inoperative, blows cold air, engine may exhibit an overheat condition or the coolant reservoir has rust in it.
Cause
Vehicles equipped with DEX-COOL® coolant, which may have been operated for extended periods of time with a low coolant level, usually in excess of 32,000 km (20,000 mi), may be susceptible to the formation of a rust-like material in the cooling system.
Correction
Flush the cooling system using the repair procedures that follow.
Parts and equipment required/suggested for this correction are listed below.
1 each *Prestone® Flush and Fill Kit #AF-KITP U.S. (#00050 in Canada). This kit can be obtained from your local parts supplier and can be used on all vehicles repaired under this procedure. Save it after performing the repair, as it is reusable.
1 each *Prestone® Heavy Duty Cooling System Cleaner, GM P/N 12346500 - 1 per vehicle. This cleaner is in powder form and contains a neutralizer. Do not substitute other cleaners. The diluted cleaners that are available in liquid form are not recommended for this repair.
5.7 L (6 qt) of 100% (non-diluted) DEX-COOL® coolant (GM Spec 6277M).
1 each new thermostat, GM P/N 12563335.
2 each new radiator cap, GM P/N 15075565.
1 each *Prestone® yellow funnel. This funnel can also be obtained from your local parts supplier. It fits snugly into the radiator opening for an easy fill and is reusable.
*We believe this source and their products to be reliable. There may be additional manufacturers of such products. General Motors does not endorse, indicate any preference for or assume any responsibility for the products from this firm or for any such items which may be available from other sources.
Important
Verify the fuel level in the fuel tank. The fuel level should be above ¼ tank before this procedure is started.
As a precaution, verify the oil level in the crankcase. Add the amount required to bring the oil level into the normal range.
It is recommended the vehicle be placed in a position so that the following are available:
A sanitary sewer -- not a storm drain, etc
A monoxivent system (if the outside temperature is less than 10°C (50°F)).
Hot running water
Notice
Due to the complexity of these procedures, the following repair steps must be strictly adhered to in order to achieve the intended results. Any deviation or substitution may result in sub-standard cleaning/flushing results or system damage.
Diagnostic Procedure
Install the Scan Tool. This will allow monitoring of the coolant temperature throughout the procedure.
Caution
As long as there is pressure in the cooling system, the temperature can be considerably higher than the boiling temperature of the solution in the radiator without causing the solution to boil. Removal of the radiator cap while the engine is hot and pressure is high will cause the solution to boil instantaneously - possibly with explosive force - spewing the solution over the engine, fenders and the person removing the cap. Under some conditions, the engine coolant is combustible.
Important
The installation of a NEW radiator cap at the beginning of this procedure and the second new cap at the end of the repair procedure is necessary in order for the correct pressure to be achieved and for the repair to be completed successfully.
The radiator cap is NOT a good indicator of the general condition of the cooling system. Typically the underside of the radiator cap will exhibit a greater amount of contamination than the rest of the system. It is important to evaluate the condition of the cooling system by checking the following before beginning the repair procedure:
Remove the radiator cap. Install a new radiator cap, GM P/N 15075565. A properly operating radiator cap is essential for this procedure to work properly. Be sure to wipe off all radiator cap sealing surfaces prior to installation.
Start the engine.
Using a flashlight or lead light, look into the radiator. From the right side of the vehicle, peer into the radiator toward the left side of the vehicle. Observe the top three rows (or tubes) of the radiator. (It may be necessary to drain off some of the coolant to see the top three rows of the radiator). If it appears the coolant is able to flow through the third row down from the top, the vehicle should be repaired using Procedure A. If it appears the coolant is NOT able to flow through the third row down from the top, then the vehicle should be repaired using Procedure B.
Stop the engine.
Repair Procedure A & B
Important
If available, use coolant exchanger Pro-Fill 42-75100-KM, Power Flush 211-07601/DEX (available through the GM Dealer Equipment program), or equivalent to evacuate the system of coolant. Follow the manufacturer's instructions for exchanging used coolant with new coolant. However, for this procedure, use water in place of new coolant. Be sure to exchange 1½ times the cooling system capacity of the vehicle with water. This will ensure that only water remains within the cooling system. Once the coolant has been displaced with water, drain the water from the radiator.
If you are located within an area which regulates the disposal of used coolant (check with your local water treatment facility), all coolant and coolant/water mixture must be captured until the water runs clear of coolant. Use a clean 208 L (55 gal) drum or similar to contain the waste coolant. Refer to Corporate Bulletin Number 00-06-02-006 for information on coolant recycling and removal services.
Drain and properly dispose of the coolant.
Caution
Any obstructions in the hoses to or from the coolant recovery bottle may cause over-pressurization of the cooling system. This over-pressurization could result in serious personal injury.
Inspect the following hoses in order to assure there are no obstructions and the coolant is able to flow freely:
The hose to the coolant recovery bottle.
The relief hose from the coolant recovery bottle.
Disconnect both coolant hoses to the heater core from the engine.
Back flush the heater core for 5 minutes after clear water is present. Pressurized water not to exceed 345 Kpa (50 psi), may be used.
Reinstall the heater return hose to the engine.
Install the radiator cap and properly tighten.
Install the "T" from the flush and fill kit to a 25.4 cm (10 in) piece of 19 mm (¾ in) heater hose. Tighten the clamp.
Install the hose to the engine. Tighten the clamp.
Install the other end of the "T" in the heater core inlet hose. Tighten the clamp.
Disconnect the upper radiator hose from the radiator. Properly position the hose, so the accessory drive belt or any pulley/belt cannot touch the hose.
If you used coolant exchange equipment, disconnect it.
Attach a garden hose to the "T" of the flush and fill kit. Turn on the water and back flush the cooling system for at least two minutes after the water runs clear.
Start the engine and continue back flushing the cooling system for five additional minutes.
Turn the engine "OFF".
Turn the water "OFF".
Re-attach the upper radiator hose to the radiator.
Open the radiator drain and drain all water from the radiator.
Remove the lower left water pump crossover bolt. This allows lowering the water level in the left side of the engine.
After the water stops running from the water pump bolt hole, reinstall the bolt.
Close the radiator drain.
Mix the Prestone® Heavy Duty Cooling System Cleaner, GM P/N 12346500, in approximately 3 L (3 qt) of warm water. Pour the mixture into the radiator. Top off the system with plain water and reinstall the radiator cap.
Run the engine at 1600 to 1800 RPM with the air conditioning in the maximum cool position. To achieve the 1600 to 1800 RPM, place a dime between the closed throttle stop and the screw. This screw should never be adjusted, under any circumstance. Its only purpose is to keep the throttle plate from sticking in the bore when the throttle is slammed shut. It is not a minimum idle (RPM) control adjustment screw.
Important
It is imperative that all air pockets have been removed from the cooling system in order for the cleaning procedure to be effective.
