heat only at high RPM problem fix

this is mainly to help others that have had this issue

ok I was having moderate heat only at 2000 plus RPM on my LT1 94 rmw, first thing I did was remove the restrictor on the inlet and that helped a little but at idle I would still get very little heat from the vent, the airflow though was fine.

So the other day I back flushed just the heater core. This was made easier for me as I had already spliced one of the lines when I had removed the restrictor in the intake hose, so on the engine side of the splice I clamped off the hose and on the engine side of the tee going to the reservoir I clamped off that hose so that I wouldn't be draining the entire coolant system.

I had bought a garden hose end splice, (used if you need to replace the end of your garden hose) , and put that in the outlet hose which is the upper hose, and then hooked a garden hose up to it and just barely cracked the valve to let water start to flow.

Once I saw clear water coming out I shut off the hose and disconnected it, then I made a mixture of CLR and hot water, using a funnel I poured the CLR mix in until it was coming out the other end and then clamped both hoses off to let the CLR sit in the heater core. I let it sit for 1 hour then repeated the garden hose flush and then hooked everything back up, started the car and checked to make sure there where no leaks after I burped the system.


Drove the car to work today when we had frost this morning, and it's a night and day difference from what it was to what it is now. Before I would have the temp set to 90 and I only get decent heat when I was doing 55mph+, now I could NOT leave it set to 90 with how hot it was coming out.


So that's a cheaper way to possibly fix poor heating without having to go to a mechanic, as they will most likely do what I had already done four times which was a full coolant system flush which didn't work.

I've done much the same with all of my cars, just to keep the heater core passages as clean as possible. Flow rate is not that high in the core, so they tend to clog if you don't keep keep the coolant system clean. I've used a 50-50 mix of Purple Power and water to soak the core (a trick I got from Fred Lesinski... a sort-of Fleet enema for your car's bunged-up arse). I don't leave it in the core more than 30 minutes, and then flush it with clear water for about 10 minutes.

One caution in hooking up a garden hose to your cooling system: be very careful not to apply the full pressure of a domestic water supply to your cooling system. Domestic water supplies can run at 60 to 90 psi, and if you've clamped off the expansion tank and bypass the pressure relief, you may be fixing a lot of leaks later.

One other note: you don't need to remove the flow restrictor if the core is clean. It's there to restrict flow to balance the system for a target flow rate.

That is one reason why I just cracked the valve open for the garden hose. But I was reverse flushing it, so both hoses from the heater core where disconnected, so any restriction would have been in the heater core, and the original restrictor was also partially blocked that's why I removed it and didn't put a new one in right away, but I do plan on eventually putting a new one in place.

another method I was told about that might be more iffy was to use an air compressor blow gun and "gently" blow compressed air though the core.

I've followed up flushing with compressed air and then flush again. It does blow out some additional crap. Use the same cautions regarding high pressure (keep it under 20 psi) and it does no harm.