Heater Core Replacement (75-91)
Article courtesy of Eric Hummel (AKA
Well you got the tell tail signs of one or all of
the following. Sweet smell when you flip on the heat, wet passenger side
floor boards, always low on coolant and fog problems on the windshield.
The good news itís not very expensive and not very hard to replace.
Most of the Chain parts stores sell the heater core for around $30
bucks. Youíre looking at an afternoon of work taking your time.
First off before you go running to the parts store
you need to check a few things out starting with your heater hoses.
Often people make the mistake of putting ĺ hoses on both lines to the
heater. This is wrong. The lower hose (at the connections on the fire
wall) is a 5/8 and you will not get a good seal with a ĺ. Itís
possible that you have a leak right there and the coolant is leaking
into the truck and you donít need a new core so check that first.
find everything is up to snuff there then you need to take some time and
look over the rest of the cooling system. Iím the type that likes to
do things once and not have to deal with it again for a while and at
this point I would be looking for other items that may need attention in
the cooling system. Your going to have to drain off some of the coolant
to do the job so you might want to inspect all the hoses and take the
time to replace any that are questionable while you have it drained.
When is the last time you changed the coolant? Most coolant has a 2-5
year life expectancy. Howís the radiator cap? Is the expansion tank
working? How many radiator clamps look like they have seen better
dayís? The water pump all right? There is a hole in the bottom of the
shaft housing that if the seal is bad coolant will be seeping out of.
Might want to loosen the belts and see if there is play in the pump that
would indicate itís going to fail soon.
few minutes and make that list of stuff you need and head off to the
parts store and pick it up. Donít forget to grab a gallon (or 2 if
your going to drain the block) of coolant and hit the Grocery store and
get a couple gallons of distilled water (not mineral). You really donít want to use tap water. Tap water
has a bunch of chemicals and minerals that will become acidic and eat at
the cooling system so spend the $1 and get the distilled that will be
mineral and chemical free (this is also what you want to use in your
battery for the same reasons) and it will help extend the life of the
hoses and radiator.
job can be done with common hand tools that you already have. Youíre
going to need a Ĺ inch and a 5/8, and 7/16s deep well socket and
ratchet. Some needle nose pliers. Screw driver, 9/32ís, 5/16ís nut
drivers or sockets. If you got air tools, fire up the compressor.
heater core is located in the truck behind the glove box. Itís housing
is attached to the firewall by several captive bolts that go through the
inside box, the firewall
and through the blower/AC housing and holding both on.
GM uses a putty type seal where the boxes contact the firewall
and as long as it is not disturbed it will not require replacement. I
was quite surprised to find that it was still pliable in my 1975 truck.
If you have been fortunate and not had a lake of coolant in your floor
you want to pull the door sill and fold the carpet back out of the way
incase you have any coolant spill in the removal process. Take some
absorbent rags and make a dam to catch any and to protect the carpet. If
you were not so fortunate and did get a bunch of coolant in the carpet
you may want to pull the front carpet out hang it over the fence and
rinse it out and let it dry for a few days. If you got nice paint get
your fender cover out and throw it over the passenger side fender.
started: drain off some coolant if youíre not going to need to do any
other repairs you need to
drain about half a gallon. There is a drain petcock located on the
driver side of the radiator near the power steering box on most trucks.
You can take a piece of hose and slip it over the end of the petcock and
drop it in a bucket to make the job less messy. On many of the trucks
that have the return line to the radiator you can take the radiator cap
off and see the coolant level and you want it to be at least 4 inches
below the return line fitting. Pop the hoses free from the retainer clip
on the fender and move them around to get as much coolant drained out of
the core as possible. If you have an air compressor you can disconnect
the hose from the radiator and blow some air into it and force the
coolant back into the block. This will help keep the mess to a minimum
when you are working inside. Finish up by removing the hoses from the
heater core at the firewall.
of the irritating parts of the job. All but one bolt is easily
accessible from either the engine compartment or the inside of the
vehicle. One nut is located below the blower motor and the inner fender
is in the way. Some have cut holes in the inner fender but unless your
truck is a rust bucket you shouldnít have any problems just unbolting
it and getting it out of the way. Take the jack out if itís hold down
is on the fender and remove anything else that may be secured to the top
of the fender. On the 73-79ís itís just the jack. Might be a couple
other things on the newer trucks. Looking in the fender opening there is
a bunch of Ĺinch (socket size) bolts that run around the inside lip of
the fender opening. Pull them all. There are 3 more to the core support
and one to the bottom of the battery tray assembly. There are two
5/8ís (socket size) bolts that attach the inner fender to the
firewall. It should just about fall out now and you can leave it sitting
on the tire.
||loosen Speed Nut
the inner fender is out of the way itís time to get in the truck and
disconnect a few things. Start by removing the glove box door. You will
find several bolts along the bottom edge of the dash and usually they
are 9/32ís wrench size. Over the transmission hump is a diffuser that
has one 9/32ís bolt holding it to the distribution box that needs to
be removed. From the driver side you may want pull the duct to the
driver side dash vent. Donít have to but I found it easier. If you got
a rats nest of wires wrapped around it you may want to deal with it. Pay
attention to it when you reinstall the box. It has one Philips head
screw that holds it directly below the steering column through the fill
plate. Itís a little bit of a wrestling match to get that duct out.
The trick is to pop it loose from the vent and push it towards the
passenger side and pull it down past the parking brake. Itís pretty
flexible so bending it some wonít hurt it just donít over do it.
While on that side there is a vacuum pot that controls the distribution
flap for floor/dash that has one or two vacuum lines, depending on the
year, you need to disconnect. This pot is located on the side of the box
and near the gas pedal.
Back to the passenger side. On the trucks with FI
or Electric spark control you will see the ECM box mounted to the
underside of the dash visible through the glove box hole. I have not
done a core on this new of truck but you may have to pull the ECM/s to
get enough room to pull the inside box loose. Donít forget to
disconnect the battery before messing with these boxes if you do find
they are in the way and need to be removed.
from the passenger side you will need to remove a 9/32 bolt that holds
the defroster ducts to the top of the distribution box. It is on the
front of the box between the ashtray and glove box opening.
You will also see a cable going to the top of the box that
controls the blend door (other end is to the bottom/temp slide on the
controls) This is retained with a clamp with a 9/32ís bolt. Try to
mark the position the cable so that you can get it back in the same
place. If this cable is not in the correct place it will cause issues
like low or no heat or not being able to turn the heat fully off so take
a minute to mark it. Remove the bolt and there is a clip that retains
the end to the blend door lever that varies from year to year. If itís
not obvious how yours is retained leave it alone and once you start
dropping the box it will give you better access to the cable to
disconnect it. There is also a second vacuum pot located in the same
area that you will need to disconnect and if you have trouble reaching
it you can again wait till you start pulling the box since it has some
slack in it.
youíre ready to pull the box. From under the hood you want to go
around the box and blower and remove the speed nuts (stamped steel nut)
that you will find. I want to say they are a 7/16 deep well socket size.
As I recall there are 3 along the bottom that with the inner fender out
you can reach through the fender opening and 2 possibly 3 along the top.
Be very careful not to lean on that box or break it loose from the
firewall and loose the seal. It should stay put by just the sealant.
From the inside there is one bolt that is located between the box and
the passenger side kick panel. The insulation may be blocking your view
but itís there and I believe itís a 1/2inch socket size and you will
need about a 6 inch extension. The box should now be free other than the
sealant. Give it a tug and it should pop off. At this point you can
rotate it back and lower it down and get at the blend door connection
and Defroster pot to disconnect them if you were having problems. You
will also find out how well you got it drained because any coolant left
in the core will now be on the floor. It will take some twisting around
to get the box clear of the dash. Donít yank , see if itís hung on
anything. Other than getting it at the right angle it should come up
from under the dash pretty easy.
|Pull the Box
||Fixing a Seal
that you got the box on the ground you can do an archaeological dig
through all the junk that found itís way into the vents! Itís fun
and educational! I found 2 keyís that are GM but donít fit the
truck, some paper scraps, a faded out button, a big old hornets nest,
(my truck spent quite a few years with no top on it) a few beer caps,
nail file, a couple safety pins, some screws, several business cards,
and some change.
out you will see the heater core is held in place with a metal ring that
also retains the blend door. As I recall there are six 5/16ís socket
size bolts that need to be pulled to remove the core from the box. Once
this is out you probably want to hose the box out and clean it up. Check
that the flaps are operating correctly and give it a good once over.
Check the foam seals for the doors. Mine were ok but if yours are really
ratty and falling apart you can probably find some weather striping at
the local home store that would serve to make a replacement.
is the reverse as they say in the shop manuals. A couple things to pay
attention to is that the cables and vacuum lines are in good shape. You
probably want to vacuum the crud out of the A/C box that will be
accessible in the truck. When your putting the box in watch carefully
that the under dash insulation and cables and vacuum lines do not get
between the box and firewall. Donít forget to hook up the control
lines and cables. If any get left off you will have operational problems
and you also will have a vacuum leak causing you a run problem with the
engine. Be very careful that the studs that go through the firewall
donít catch the Blower/AC box and knock it loose when you are
installing it. Also pay careful attention to the water connections to
the heater core. The core is made of brass and the pipes are easily
bent. They could be slightly bent right out of the box and need to be
ďtweakedĒ to match the position of the old core.
If you do need to bend them put something round like a socket
wrench handle in the tube and use it to tweak it so that you donít
flatten out the pipe. When you make the hose connections remember that
the bottom pipe (the 5/8ís) is the inlet and it goes to the intake
manifold. This is important so that it will bleed any air out of the
core. Also after running
your truck for a few dayís you want to top of the coolant and give a
quick check that you donít have any leaks.
your time and itís not a bad job. Yes it has a lot of steps but it can
only go together one way. Make
sure you got plenty of time to do the job. It took me about 4 hours
including me being anal about cleaning all the ducts out. I also have
air tools and that speeds up the process of pulling the inner fender a
great deal. If any of the speed clips break that hold the inner fender
in I have found them in the Help section at the local parts store. Some
home stores also sell them.