I replaced the heater core on my 1986 convertible, and am still having issues getting heat. After getting the heater core hooked up I noticed that the vacuum operated coolant valve was not actuating, so I opened and closed it a couple of times and it began leaking profusely. I ended up using a cutoff disc to just delete it (away from home, no time to wait for parts) and noted when it came off that the valve appeared to be stuck open anyway.

I then checked the supply line coming from the intake (pressure side) and got no coolant flow on startup, then a little as the engine warmed up. I chalked it up to the thermostat at the time and assumed that the thermostat must control heater core flow. I think that was the wrong assumption, and looking back on it a spurt of hot water caught me off guard and I hurriedly stuffed the line back on without really watching it.

I have since noted that there is still minimal heat even after the car is up to temperature and running, whether idling or driving. I had installed a filter on the pressure side, but tonight I took it off and noted that coolant flow is minimal both with and without the filter installed. Here are pictures of the fluid flow back to the water pump after the heater core:





I don't have pictures of it, but this is similar to the flow I noted earlier coming from the intake. There simply is not much water flowing to the heater core. With the engine up to 230 degrees, I felt a hot supply line and a cold return line from the heater core, indicating very low flow.

I spoke with an old auto shop manager friend who worked during this period, and he mentioned that almost every GM small block back then spun a clockwise water pump, and that it is highly likely that this car with 105,000 miles on it has had its water pump replaced. Since this engine runs the water pump counterclockwise, it is therefore also highly likely that someone put a clockwise water pump in it. That would explain why the coolant flow observed is so anemic.

Is there anything else this could be besides a clockwise water pump being installed on this counterclockwise block? And am I correct in my assertion that the thermostat should NOT be controlling coolant flow to the heater core?

 

The thermostat does help control water to the heater core. I would check the thermostat or even replace it if your that far in. At 230 water temp you shouldn't be able to touch the hot hose into the heater core if it has the proper flow. I would check both hoses for flow, there was a time where GM used a restrictive nipple fitting coming off the intake manifold. I would check that fitting to see if it's open, or replace it or drill it larger. (I have done that before) Checking the water pump, there should be a number on the front of the casting (I believe) behind the pulley. If its the wrong pump it won't circulate right. I had a crate engine once with the old style pump on it and the surpentine pulley wont fit on old pumps, the input shaft was larger on the old style pump, but you can make it fit if the file out the center hole on the surtentine pulley to make it fit, and yes the pump would run backwards, I'm not sure if it would cool right, I haven't tried it. I would check that water pump and pulley to see if the pulley has been altered to make that happen.

 

The casting number on the water pump is 88926178 and it's locsted between the pump and the engine. It's takes a mirror to read it.

 

Thanks again Thomas, that's quite helpful. The thermostat is brand new, I replaced it with a new 195 degree thermostat back when I was working on the cooling fan. I got some input today that there might be a "large air bubble" in the system. I can't deduce how this would be the issue, but I have noticed that despite checking the radiator level often, the "Low Coolant" light keeps coming on after driving for a minute or two. I have never left the radiator cap off until the engine comes up to temperature to bleed air out. Does that ring true to you at all?

 

I don't clamp the cap tight and pressurize it until I think most of the air is out and is up to temp, If the heater core is difficult you can bleed that separately, but you shouldn't need to. So leave the cap off until it warms up. you might fast idle it a bit until the heater hose feels warm. Some systems can be a bear to bleed. Every new thermostat that I install I always drill a small hole in the plate around the thermostat for air to bleed out, unless it's factory notched. Or maybe a short small wire in the thermostat opening the thickness of a bread sack tie to bleed air off, then it falls out when the thermostat opens. I have seen an engines overheat because of trapped air behind the thermostat with radiator full of water.

 

Since I never bled the system, I'll do that tonight and see if anything changes. Maybe that's the issue and I just skipped a step. Hopefully that saves me the trouble of replacing a water pump.

 

I had vastly underestimated how badly the system needed to be bled. I achieved this by jacking the car up by the passengers side skid bar/plate and starting/stopping the engine repeatedly while I refilled the radiator. It took over a gallon of coolant until it finally quit taking any more. I hadn't thought about it, but the radiator in this car is relatively low compared to the engine. I'm not sure it would be possible to fill all the way without inclining the car or filling from the T-stat tube. During this process I noticed that right on startup the full radiator would get drawn down to well below the coolant sensor, hence the pesky "low coolant" alarm. The radiator would slowly fill back up as it ran so I would fill it as fast I could and then put the cap back on before the coolant spilled out, then repeat the process.

After properly filling the system, my heater functions much better. Initially I was confused, because it tends to blow cold at idle and then hot as the engine is revved to higher rpms. I initially thought maybe this was because the water pump could be weak or there was still air left in the system, but I found an old post from 2004 on DigitalCorvettes.com from a user with the same issue, and he noted the same behavior. Even after a new water pump, his '88 C4 only blew warm at rpms above idle. On a side note, I wonder if he replaced his water pump for nothing.
https://www.digitalcorvettes.com/thr...-heater.26840/
https://www.digitalcorvettes.com/thr...?nested_view=1


I did notice a squeak began coming (presumably) from the water pump after it got filled all the way, I assume because the pump is now under full load. Could be the belt squealing on the pulley, or perhaps an old water pump that is rubbing. Needing to replace the pump at this point would be quite ironic.

I feel awfully silly for bothering the forum with this, but nonetheless I suppose it is now part of the Public Record.

 

I have had many times a burping problem has arose and it wasen't always on Corvettes, There is always a learning process with everything. If you think there is a problem with the water pump or pulley, you could pop the belt off and spin the pulley and idlers for any issues they could be making a noise. It's not normal for a serpentine belt to squeek because of the way its configured with the belt. That's not hard to check. Sounds like you are making headway..

 

I should have also said when you spin the water pump pulley without the belt make sure it spins smooth and there is no wobble in the pump shaft. And also check for water leakage off the bottom of the water pump housing from the weapage hole.