Hot heat at first, but quickly cools off!

Ok guys, I've searched and cannot find an answer to this question. I've got a 94 RMW which I purchased 1 month ago. The PO replaced the heater core last year. When I turn the heat on at 90, I get HOT heat for about 30 seconds, and then it becomes lukewarm. This simply will not do for the cold Iowa winter we are about to embark on. Car was low on antifreeze when I bought it, filled it and seemed to help a little, at least with prolonging the amount of time I get heat. Any ideas on what to check first, or has anyone encountered the same situation?

Sounds like a clogged heater core. Very common.

Link to flush procedure.

More than likely some crud will come out and once done the heat will be hot again.

Thank you, I will give that a try. I wasn't thinking a flush would be necessary since the core is only a year or so old, but after reading that other members have to do it yearly it sounds like a solid place to start!

With LT1 cars, plan on flushing it every fall. They plug up that quickly.

95BRMW wrote:

With LT1 cars, plan on flushing it every fall.  They plug up that quickly.

Really?

Damn i,m way behind as usual,.

I am with Nick.

I don't know about every fall. I did flush the LAB car a couple of years ago and have not had an issue since. Might do it when I tune her up this weekend. The coolant is looking a bit old. (3 years). But the Radiator is new.

NOTE: This is a reprint of a pictorial I posted on the other forum some time ago. The pictures may be helpful for some:

There have been a couple of recent questions about lack of heat, and some of the answers (my own included) suggest flushing the heater core without really explaining to the uninitiated what that entails. Since it's such a quick job, I took a half hour to snap some photos. Note that this is of an LT1 model, the earlier TBI cars are piped a bit different and I'll mention the differences but don't have one at hand to show photos.

The first thing you will need is an adapter to connect a garden hose to the heater hose. You can pick up one of these (or similar) at any auto parts supply shop:



(I added the brass quick-disconnect to match my garden hose). There's some plastic "tee" fittings around too, but they are inserted permanently and I'm not a big fan. The one pictured is aluminium and features a Schrader valve fitting for those really stubborn clogs.

Unless you are draining and refilling the entire cooling system, you can pinch off the engine side of the hoses to minimize coolant loss. You can buy special clamps for this purpose, but Vise-Grips work well too. Three are required for the LT1 -- the two from the engine and the one under the reservoir.



Separate the supply hose by releasing the hose clamp down stream of the black plastic restrictor and gently twisting the hose a bit while pulling it straight back. You shouldn't use a lot of force and you should pull straight back to avoid cracking the plastic nipple. The restrictor is labelled with an arrow showing the normal flow direction.



If you look inside, you can see the 1/4" restriction. It's there to control coolant flow at high rpm. Obviously it would also minimize flow during flushing, so we want to be down stream of it:



The return hose is similarly separated from the tee to the reservoir, and the hose line adapter inserted in the line. Note that we are flushing in the opposite direction from normal flow. Turn on the water and run until you get a nice clear stream. In extreme cases, small shots of shop air through the Schrader valve create a turbulent flow that helps scour the piping and blast out the crud. The air bubbles will come out somewhat explosively though, so make sure the discharge is pointed away from you! Don't introduce shop air unless the water is flowing -- you don't want to overpressurize and damage the heater core!.



When you are happy with the results, shut off the water, disconnect the adapter, drain the heater core (you can gently blow into the return hose to facilitate this) reconnect the two hoses, release the clamps and add about 1 cup of pre-mixed coolant to the reservoir. Crack the bleeder valve on top of the thermostat housing (pliers are probably required) to bleed any air bubbles and run the engine for a couple of minutes before shutting off the bleed valve (CAUTION: if your car is a tow-pack model, your hand will be perilously close to the mechanical fan. DO NOT ATTEMPT TO OPEN/CLOSE THE BLEEDER VALVE WITH THE ENGINE RUNNING.) Close the bleeder valve securely, but don't crank hard on it -- it's pretty tiny and easily damaged by too much force.





The TBI models are similar in principle, however the supply to the heater core comes from the manifold and the restrictor is built in to the hose. The easiest way is to separate the quick connect fitting of the supply hose right at the firewall connection to the heater core. If I Recall Correctly, it's the hose to the passenger side of the car, but you can quickly trace the hose. The nice thing about the TBI is that you don't have to clamp off the hose -- just keep it up above the level of the engine.

The heater core return hose goes straight to the radiator near the fill cap and it has a traditional hose clamp fitting. Simply disconnect the return line at the radiator and insert your flush adapter there. Then flush until clear as described above.

Another possibility is a partially stuck open thermostat. You can easily check this with OBD data, or, the symptoms are it will heat when sitting, but the heat will cool off as you drive.

What is normal operating temperature for these motors? Mine will get up to 170, no warmer with mixed highway/city driving. I drove 30 miles in town tonight, lots of stop and go. I do have tow package car, so wasn't sure if the mechanical fan played a role in the car staying so cool?

Hotter than that. I would guess your stat is hung slightly open. On one of mine, it wouldn't close the last 1/8", and the difference was amazing.

Mine was running 177-178° on the highway this morning. That's pretty typical according to the dash gauge.