The "easiest" way I have come up with flushing the heater core on the 91-93 (possibly more) 4.9L Fleetwood's and Sedan/Coupe DeVilles is:

1. Do not drain cooling system. :tisk:

2. When the engine is cool, find the thermostat housing on the rear of the engine. Find the 1 inch hose leaving the engine near the thermostat housing and follow it to a tee about 10 inches away. Disconnect the OTHER two hoses on the tee marking which goes where.

3. Verify that the "T" is a Cadillac part that has the correct restriction in the leg of the "T" that goes toward the front of the engine. This restriction is critical to heater performance. Make positively sure that the restriction is there and that it does NOT :tisk: go toward the heater core feed hose at the firewall.

4. Get a 2-3 foot section of heater hose and connect the 2 outlets of the "T" with the hose, essentially creating a plug of the coolant from the thermostat housing. Use Clamps on them, as the pressure may push them off and get coolant or water inside throttle body.

5. The hose that goes toward the firewall/passenger side of the engine compartment goes directly to the heater core. The hose that goes down is the bypass some of the flow to reduce heater core erosion, this connects to the inlet side of the water pump and the heater core. The restriction in the tee must be in this hose. If too much water bypasses the heater core, poor heater performance will result. Blocking it off will result in rapid deterioration of the heater core and leaks inside the car. :eek:

6. Flush water from a garden hose in each of these 2 lines alternating till the restriction is clear. It should flow water at a very high rate, nearly what you are putting in it from the hose. I use a spray head on mine, mainly to give me an "off" switch on the water.

7. Blow as much of the plain water out of the heater core circuit as is possible using lung power.

8. Remove the loop of hose used to block the tee and reconnect all disconnected hoses to the tee and verify clamps are properly installed.

9. Fill with 50% distilled water and 50% your favorite brand of antifreeze. Read up on using Dexcool by searching the forums. There have been numerous posts on it. If your car came with it or not, good reading.

10. Test system for heat, I used a candy thermometer at the floor and was rewarded with 40 degree air (before flushing) to 160 degree air coming out with 35 degrees ambient temp. It worked well, and no digging to get to the core itself to get to the heater hoses, which I tried and quickly abandoned.

Tom

My 91 deville quit heating shortly after the warranty expired. The dealer wanted a bundle to replace the heater core, saying it was probably stopped up with crystallized antifreeze. I figured he was blowing smoke, and tried flushing it on my own with little success. Repair manuals were not available for this car and I didnt know about the flow restriction in the line. Good thing I live in a warm climate. :coolgleam

The "easiest" way I have come up with flushing the heater core on the 91-93 (possibly more) 4.9L Fleetwood's and Sedan/Coupe DeVilles is:

1. Do not drain cooling system. :tisk:

2. When the engine is cool, find the thermostat housing on the rear of the engine. Find the 1 inch hose leaving the engine near the thermostat housing and follow it to a tee about 10 inches away. Disconnect the OTHER two hoses on the tee marking which goes where.

3. Verify that the "T" is a Cadillac part that has the correct restriction in the leg of the "T" that goes toward the front of the engine. This restriction is critical to heater performance. Make positively sure that the restriction is there and that it does NOT :tisk: go toward the heater core feed hose at the firewall.

4. Get a 2-3 foot section of heater hose and connect the 2 outlets of the "T" with the hose, essentially creating a plug of the coolant from the thermostat housing. Use Clamps on them, as the pressure may push them off and get coolant or water inside throttle body.

5. The hose that goes toward the firewall/passenger side of the engine compartment goes directly to the heater core. The hose that goes down is the bypass some of the flow to reduce heater core erosion, this connects to the inlet side of the water pump and the heater core. The restriction in the tee must be in this hose. If too much water bypasses the heater core, poor heater performance will result. Blocking it off will result in rapid deterioration of the heater core and leaks inside the car. :eek:

6. Flush water from a garden hose in each of these 2 lines alternating till the restriction is clear. It should flow water at a very high rate, nearly what you are putting in it from the hose. I use a spray head on mine, mainly to give me an "off" switch on the water.

7. Blow as much of the plain water out of the heater core circuit as is possible using lung power.

8. Remove the loop of hose used to block the tee and reconnect all disconnected hoses to the tee and verify clamps are properly installed.

9. Fill with 50% distilled water and 50% your favorite brand of antifreeze. Read up on using Dexcool by searching the forums. There have been numerous posts on it. If your car came with it or not, good reading.

10. Test system for heat, I used a candy thermometer at the floor and was rewarded with 40 degree air (before flushing) to 160 degree air coming out with 35 degrees ambient temp. It worked well, and no digging to get to the core itself to get to the heater hoses, which I tried and quickly abandoned.

Tom
I am a bit confused, if I block off the TEE, where is the water going to flow from ? Is there another hose(s) I should be opening ?
This Tee is a Cadillac part known as Heater Control Valve $ 40.00 CAN

Bottom line, look at the heater hoses and figure a way to get coolant to reverse flush through the heater core. I did it on my son's Deville in about 15 minutes. After doing it the first time, it only took me about 5 minutes on subsequent flushes. Yes, I had to do it like three times over the year or so that he had it but it worked like a charm each time.

The hose that goes to the TEE to the core (down towards the back of the engine) will remain open (this is the core bypass line).

Katshot, did you do a 90 or older car? The 91-93's are a bear to do, as you cannot get to the core at all without removing fuel lines, which the wonderful Cadillac engineers decided to put in close proximity to the heater core, blocking sane access. Not to mention when I needed to flush mine it was 20 degrees out. Flushing was quite a chore.

The Tee is critical to be a factory one, as the tee has a restriction in it that is proper to meter the coolant through the bypass. No restriction and it mostly bypasses the core and leaves you with almost no heat.

I am a bit confused, if I block off the TEE, where is the water going to flow from ? Is there another hose(s) I should be opening ?
This Tee is a Cadillac part known as Heater Control Valve $ 40.00 CAN

NODIH,
please go to this link... and help me with this....
http://www.cadillacforums.com/forums/cadillac-seville-cadillac-eldorado-forum/66156-93-4-9-eldorado-heating.html

Thanks!

The hose that goes to the TEE to the core (down towards the back of the engine) will remain open (this is the core bypass line).

Katshot, did you do a 90 or older car? The 91-93's are a bear to do, as you cannot get to the core at all without removing fuel lines, which the wonderful Cadillac engineers decided to put in close proximity to the heater core, blocking sane access. Not to mention when I needed to flush mine it was 20 degrees out. Flushing was quite a chore.

The Tee is critical to be a factory one, as the tee has a restriction in it that is proper to meter the coolant through the bypass. No restriction and it mostly bypasses the core and leaves you with almost no heat.

My son's car was a '90.

I can see this is now 10 months old. Hopefully this will help someone.
The "T" has an arrow pointing the way for the water flow. the restrictor is on the right side of the "T" looking from the front of the car.
I am sitting here looking at a new part I got a couple of days ago from the Cadillac dealer.
It cost $27. and change and it had to be ordered.
The GM part number is 1648454.
These parts are plastic and get brittle with age and WILL break so be gentle with it.

I can see this is now 10 months old. Hopefully this will help someone.
The "T" has an arrow pointing the way for the water flow. the restrictor is on the right side of the "T" looking from the front of the car.
I am sitting here looking at a new part I got a couple of days ago from the Cadillac dealer.
It cost $27. and change and it had to be ordered.
The GM part number is 1648454.
These parts are plastic and get brittle with age and WILL break so be gentle with it.
Looking at this again I find I made a mistake. The restrictor should be on the LEFT side of the "T" looking from the front of the car.
Oreilly auto parts does carry the exact part for about $10.plus tax.
It is listed as heater control valve.

Again digging up this old thread, But this helped me alot and I want to add one thing to it. I just recently drained and refilled the cooling system with the exception of the heater core. I just got the car and didn't realize the core was blocked up. This is on a 93 Deville

I tried the above procedure in mine, but the water wouldn't flow through, water just filled the hose and came back out the top. I had to resort to more extreme measures to clear my blockage. I got a air compressor with a blower attachment, wedged it into the bypass hose going down and blew air through the core. I capped off the outlets on the T so I wouldn't lose my new antifreeze. A little green antifreeze came out the T so I knew that was fresh. However When blowing into the bypass hose alot of brown water came spurting out the hose which runs into the heater core. I then filled the hose back with water and blew again. I did this multiple times then reversed the flow by blowing into the other hose. I think I cleared part of the blockage but think their is still some junk in it. I hope the fresh antifreeze that can get into it now helps clean it up some.

I tried the above mentioned "lung:" power and about passed out lol. Had to resort to the air compressor, just kept it at a lower power so it didn't blow apart the insides of the core. Alot of sediment and black flakes came out of the core. It was really nasty. At first the water came out in spurts but over time it came out at a more constant rate. I haven't.

I haven't gotten a chance to test it yet, but hope enough of it is cleared that I can get some heat.

Anyone know of a cleaner I can now use to break down the minerals built up inside it without having to remove it? I had heard of some using CLR but just wanted any opinions.

I am glad it sorta helped. I just did my core on my 94 Fleetwood LT1 today. Seems every year I have to blast it out. I do the CLR thing in mine (It is easy to do on the LT1), but being I don't have my 91 Deville anymore I can't be sure if it would work.

I soaked my core with CLR for 20 min or so (problem with Deville/FWD Fleetwood with 4.5/4.9 is CLR likely will enter cooling system and it will foam, so it won't hurt, it will need to be 100% flushed then), and then blasted out good, and then filled up with LimeAway. I let that sit for an hour or so, and then blasted out. I think the CLR got it all, because it seemed fine and didn't seem to get anything else out.

Great ideas - I've done this many times and have found a couple things to make it a little easier/quicker.
1 - Loosen radiator cap just to release the pressure, then be sure to put it back on, tight, otherwise when you try to flush, the water will just come out the radiator opening.
2 - Rather than a piece of heater hose to connect the two open ends of the tee, I use a Visegrip with the wide, flat jaws to clamp the hose from the thermostat, just before the tee.
3 - I prefer to flush in the hose to the front of the engine first, hoping that will maybe flush back out some of the crud, rather than packing it in more solid to start. I don't know if it makes a difference, but anything might help, rather than replacing the heater core.

IIRC, the closed rad cap is cruical to forcing the water THROUGH the core and not allowing it to exit somewhere else. The open cap will allow the water to get out possibly bypassing the core. I don't have the car anymore to verify. In most cases if you are doing this, you are probably dealing with a common plugged up core....

This is a CORE flush, not a system flush. In cold weather, you might damage a hose by clamping it, so make sure it is warm first. Don't wanna risk damaging the hoses. Some grips have some pretty sharp teeth.... But it would make things easier.

I was going to just replace the core, till I figured out I couldn't get it out in 30F weather or colder without major surgery....

I just did it on a '95 DeVille (4.9L NOT Northstar) - worked great.
This car had a kinda unique problem in that it blew hot air out the passenger side of the center floor heater duct, and ice-cold air out the driver side of the same center floor heater duct. I figured it could NOT be a core blockage problem, but had nothing to lose with a 15-minute flush.
Surprise! It worked - hot air both sides.

I found my 91 was like that, the pass side was warmer than the driver's side, the core was split top/bottom for airflow, bottom half, driver's side and top, pass... Yup, same problem.... I think that is lousy myself!