I've been dealing with heating problems for a few years now and I still cannot figure out what is going on - but I've made progress. This is on a '79 with "high output" no AC and a L6. I flushed the lines a few weeks ago when I changed out the T-stat. I pulled the "unit" out last weekend. And replaced the existing heater core with one about twice the size that fit in perfectly (the older one was about half the size - I thought I found the problem). I also cut and glued in new foam where the old insulation was falling apart. The cables are in good shape, the flaps are working as they should, the fan is operating, coolant level is good - still I have no heat. My shop manual shows the "in line" to come from the T-stat housing and "out line" to the water pump. Here's the odd part - I thought the "in-line" off the T-stat housing was supposed to be hot and the out line was a coolant return back into the water pump. The "out line" barely heats first when warming the system up, and the line off the tstat doesn't feel like it's heating all. I'm confident that my gasket job on the tstat did not block the hole - my TVS is working fine & and the problem existed before. When the engine is good and warm I can stick a few fingers in through the resistor cutout and feel that the core is not getting warm - or at least not warm enough to radiate heat. I tried running the motor at higher RPM thinking that the RPM's were insufficient to pump a decent volume of coolant through the core - no luck. I'm beginning to think that my water pump may be the culprit. Does the water pump create any kind of draw on the return line? I'm not having any engine cooling problems. Any of you guys ever heard of the pump being the problem? I remember a few years ago my water pump went out - I immediatley felt cold air being pumped out of the heater when the coolant level was lowering. I installed a new pump then I can't recall the heat ever being that good since. The lack of heat is driving me crazy - my defroster barely worked last year - so far this year it is not working at all. Sorry for the long post, I appreciate any constructive advice. Thanks.

 

How many miles are on the pump. I would say that could be the culprit.

I am surprised that your motor doesn't shut off though. Generally they have a safety setup so they shut off at 235 degrees Fahrenheit.
Have you replaced the hoses? Your radiator might also have significant corrosion buildup, that causes reduced flow, and a hose will collapse.

 

Did you get the right pump? The serpentine belt pump may turn backwards... The guys in the I-6 forum should know more.

 

I am not that familiar with the 6cyl but it sounds like your hose hookup is the problem. If your water pump was bad, it would cause the truck to overheat. Bypass the heater with a hose. It should be hot to the touch if it is hooked up right.

 

Yes it is the correct pump. The later FI, L6 has a long bent pressed in tube for the heater core and the connection to the fan uses a reverse threaded hole vs the four on a BC.
The pump has about 30K on it. The radiator was replaced a year and a half ago - I put in a nice three row Modine that has the extra cooling capacity vs the smaller stockers Ford used in the 300's and 302's. The hoses were done at the same time. I just replaced the upper rad hose with the new tstat.
I did have the bypass setup as you mentioned when I drove down to the part's store to get the new core last week - but I didn't check it for heat.
So, from my intuition the water pump should be directly pushing water through the core from the hose connection comming out of the water pump casting. This goes against my thought that the water exit through the top of the head was returned back to the water pump (when tstat is closed) through the heater core system - meaning the connection pressed into the water pump casting is for return, not feed purpose - which follows the illustration in my shop manual & makes most sense with the placement of the TVS location. Is there a check valve or something in the water pump?

 

I checked the spare pump I have for my pending 300 as well as the old one that came off the block and the tube that is pressed in goes without restriction before the impeller ( it dumps to the same zone as the feed line off the radiator). So I'm almost sure that the connections to and from the core are correct. Here's the strange part - the hose between the core and pump heats up A LOT warmer than the upper hose from the tstat. Also, the upper radiator hose warms once the tstat opens, yet the heater core line does not seem to warm at all - I'd expect it to warm first - especially if the tstat is closed. So, I'm starting to think there's a few possible scenarios:
1) Blockage in the tstat housing preventing coolant from returning.
2) Some type of air lock happening - I didn't attempt to fill the core or hoses.
3) Manufacturing defect in the pump impeller itself?
Any other ideas?

 

The blockage and airlock seem the most likely to me. I'd take the hoses off the core, leave one low and lift the higher and fill er up. When you have just fluid coming out the tube, lift them both, still full, and cap them. Then you know that end is good. Take your T-Stat housing back off and make sure something didn't clog the openings, same with the pump. Especially behind the plate in the pump, seen quite a few things get lodged in there. And then with the the T-Stat still off, start filling the block and watch for flow out of the bottom hose (disconnected). Just fill each section and cap it off, then you'll know which is full and flowing. Try to elminate each section independently of the others. And yeah, if you don't find any blockages or airlock situations, then you have a defective part.

 

First Did you bleed all the air out of the system? Second i have seen a water pump that was bad and the vechicle never came up to temp. It was on a 70's chrysler though. We put a new radiator and upper and lower hoses in it, including a new thermostat and the thing would never warm up. Put in new water pump and instant heat it was wierd thats for sure. But from what your saying it sounds like possible a flow problem. Do you have a temp gun by chance to take temp reading, across the radiator and along the hoses, maybe your radiator is cooling to much, also what temp of thermostat do you have in it.

David

 

I think I have a 180*, but it might be a 195* - can't remember - it is a Stant though. Sorry for the lame description- temp rises and stays right at "O" on the NORMAL scale. Today I left the cap off and let it warm up till it damn near overflowed. Water pumps are cheap so, I'll try within the next few weekends to replace the pump, "prime" the core and hoses and check the tstat housing. Keep the suggestions coming. Thanks again

 

Did any air come out. The coolant will rise when the thermostat opens. This is caused from air in the system trying to escape. You can get a no spill funnel from your local parts store to help with this as it will raise the high point above the radiator. Plus it allows you to hold more coolant when the system burbs, when you do it with just the cap off it can get messy.

David

 

Hmmm, no blurp. I'll keep the funnel in mind when I pick up a new pump.

 

did you check the hose fittings that come off the intake manifold...I had a 78 with a 460 in it and it had no heat....there was a barbed hose fitting (90 degree angle) and it was plugged solid with slime from the cooling system...Mine had about 170000 miles at the time...just put a new radiator and heater core in it...Took a water hose and blew the obstruction out of the fitting...had plenty of flow after that ....Hope that is your problem..it is an easy fix.....

 

When you refill it, fill the radiator to just above the cores. Start it up, let it idle on fast idle, until the you see the water start to move in the radiator and the top hose gets hot. Let it idle down. make sure that the water level stays above the cores. Check the heater hoses, they should both be hot.
Turn it off, fill the radiator up to a bit below the cap and install the cap.
Enjoy your heat.
Jimmy

 

The impellers "swirl" different directions for CW vs CCW turning pumps, The idiots at the factory could have put on the wrong one.

 

i had a heater control valve stuck shut on my 77, maybe give that a try