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Is changing the heater core on a C4 really that bad? (1/18)
 10/12/10 9:41am
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, - Canada

1992 Corvette Black on Black ZF6 6-speed LT1 EM headers/Corsa exhaust

Joined: 3/22/2009
Posts: 34

YES IT IS!!!  

For month's now, I've known that my heater core's day's were numbered.  What started off as just an occasional smell of anti-freeze every now and again, over time, gradually turned into fogged up windshields every morning  and eventually, over the past month or two, ended up turning into "steam" pouring out of my dash vent every time the car came to a stop at an intersections/stop sign.

I'd been putting off replacing the heater core for the longest time due in no small part, to all the horror stories I'd read on this forum in regards to removing them. 

Last  week, while vacuuming my car, I discovered my passenger-side floor mat was completely soaked with anti-freeze. Needless to say,  I was NOT a happy camper and knew I couldn't put it off any longer.

Looking back, I guess it was probably a good thing I did the work now, since summer is quickly coming to an end and before you know it, the weather's gonna get colder. I hate working on my car in the cold and I NEED to have my heater working in the colder month's.  Better that I did the work now while it's still warm...

I didn't bother to look through a repair manual as in my experience, they usually only add to the confusion and make matters worse.  I don't know who writes those things, but more often then not, they usually make things appear to be much harder than they really are and generally only succeed in confusing the hell out of you. LOL

I started off loosening the 2 screws and two bolts holding the surge tank in place.  Then I carefully moved it over so I had access to the 2 heater hoses underneath leading to the heater core through the firewall.  

The hoses were caked on pretty bad so I had to slice them down the side about 3/4" with a box-cutter knife in order to pull them off.

Because there was still some antifreeze in the lines, I made sure I slid a tray underneath the car to catch any if it spilled.  With the two lines pulled out, I simply wedged them off to the side pointing upwards so no more coolant would spill out of them.

(Though the picture doesn't show it because I took it later on, the heater core ends stick out about 1" from the firewall)

Now that all the outside work was done, I made my way inside of the car.

I removed the 2 screws holding the hush panel into place, then gently disconnected the wire that runs to the small light that's attached to panel.  I squeezed my head into the foot well and peered into the dark void behind my dash.  This was definitely NOT going to be a walk in the park....

Aside from being darker than ape sh*t in there, it was cramped, hot and there was about a million wires all over the place making access to the heater core and housing screws all the more difficult.

***I suppose if I could give anyone looking to tackle this job just one piece of advice, it would be this***


I didn't because I'm a moron and like making things difficult for myself.  By the time this little project was over, I had twisted and contorted myself into positions I never knew the human body was capable of.  

(Just to give  you an idea how cramped and uncomfortable it is in there with the seat still in place, go out and open your passenger side door and look inside.  Not very much space in that foot well is there?)

Now imagine having to stick your head way up in that foot well so that the top of your head is right up against the fire wall.  All those wires back there are going to be hanging just low enough to be resting on your FACE. Now picture your lower back/@ss  arched up and leaning off the very edge of the seat and your legs/feet sticking out the roof-line.  This is NOT a natural position for the human body to be in.  LOL

Do yourself a favor and learn from my mistake.  Take out the darn seat!!  It will make a world of difference.  

Once I'd finally positioned myself in there, 3 of the bottom screws were immediately visible and relatively accessible.  I used a 1/4" wrench and socket with a flexible extension to get at the screws.  I believe they were 9/32".   You also have to unhook a bunch of clips GM decided to use to hold a bunch of wiring to the plastic shell that covers the heater core.  They all have to come out before you can pull off the cover.

Next, I took out the glove box lid.  I was really hoping I'd have access to the top screws this way, as I didn't want to remove the entire dash like most others have had to do.

There are 6 screws holding the hinge/door in place.  Remove them and carefully pull away the door and set it aside.

Next you will find two philips screws holding the latch in place.  They also happen to hold the entire tray in place as well.

Remove them and pull out the tray.

Right away I saw that at least one of the top screws was visible and accessible using a wobble extension.  Good stuff.  4 screws down, one to go.

The 5th and final perimeter screw is way in the back on the right side just below the windshield.  I was totally blind on this one, but after finding it with my fingertips, I used a 1/4" wrench with an adjustable extension and amazingly enough, found the head of the screw the first time in.  So far, things were going pretty darn good...

Before the cover can be pulled out, there are 3 small plastic hose's that direct air to the various vents  behind the dash that need to be disconnected. They just pull right out and can be slid back into place later.

Unfortunately, as I found out later, not only are there the 5 perimeter bolts to remove, but there's a hard plastic hose screwed into the middle of the plastic cover with two screws holding it in place.  

You can see in the picture below the hole in the cover.  That's where the hose goes into.  Just above and below it, are two screws holding it in place.

The bottom one, I got from underneath.  The top one was a MAJOR pain in the butt and probably added about 1.5 hours to the job.

This one single screw was a real head ache, but eventually, I was able to get the little b@stard out!  :lol:
With that out of the way, I now had the joy of trying to pull out the cover through the foot well. A job easier said than done. All the wiring back there is in the way so I had to take my time and move the wiring around as I pulled out the cover .

With that done, I finally had access to the heater core.  You'll notice right away there a thin metal "strap" along the bottom of the core with a screw on one end of it.  This little arm/strap basically just holds the heater core in place.  Remove the screw and swing out the arm and pull out the core.

There will still be some anti-freeze inside, so if your floor isn't already soaked like mine was, you may want to put a plastic bag on the floor so your carpet wont get wet.

Looking at the old core, it was obvious it was leaking from the top corner...

I could also see that a few of the rows of "fins" were  bent/crushed for whatever reason.  It was probably put in that way 18 years ago from the factory so I guess I can't really complain since its lasted this long...

Before I installed the new one, I made sure the new core matched the old...

Then went about putting everything back together.

I noticed afterwords while trying to install the new core, that there was another little bracket at the top of  the inner housing which I'm assuming was there to hold the heater core into place just like the little arm/strap at the bottom.

As hard as I tried, I was not able to slide the new core back into place with this "hold-down" bracket in place.  I decided just to remove it.  With that out of the way.  The core went right in.  I tried to re-install the upper bracket but couldn't. I couldn't see it and couldn't even reach it if I did.

I ended up leaving it off since there's really no need for it.  The core is held in place by the bottom strap and the inlet/outlet pipes extending out of the firewall towards the engine also keep the core in place.  Then there's the plastic front cover which also helps keep the core in place as well.  Bottom line is, that upper bracket is totally over kill and NOT necessary.  Good riddance. 

Once the core was put back in, I started up the car and let it idle to reach normal operating temperature.  Turned on the heater and inspected the outside lines as well as the core itself, for any leaks.  

All was well.  Clap

Put the plastic cover back on (another major pain in the butt with all those wires in the way) re-attached the 3 plastic vent-hoses on the inside and once again started the car.  I cycled through each of the vent settings to make sure all the vents were working the way they were supposed to.

Again, all was well and each "mode" worked as it should.   I finally re-attached the glove box liner and installed the door again.


The top right screw just below the windshield (the one totally hidden away, unfortunately, I was not able to put back but there doesn't appear to be any sealing issues with the box as I did not feel any leaks from that area even with the heater on full blast.  The 4 remaining screws hold the two haves of the box together tight enough and there's a "ridge" between the two haves that basically locks them into place when the perimeter screws are tightened.  No air leaks, no problem.  Haven't given it a second thought since.

The whole job probably took me about 5-6 hours but I know I could have done it in less time had I removed the seat and been allowed to work a little more comfortably but hey, live and learn I guess.

My heater is now working again, no more anti-freeze smell, no more fogged up windshield, I can go back to enjoying the car again.

In a nut shell, yes, it's a time consuming project.  Patience is definitely a MUST and thin arms really go along way in helping you reach some of the screws but this project IS doable in a garage with some basic hand-tools in an afternoon.  Any reports of HAVING to remove the entire dash are not entirely true.  I didn't, and everything turned out alright!

Just remember, take your time, be patient and don't forget to remove the seat.
LT1*C42010-10-12 06:42:46
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Is changing the heater core on a C4 really that bad? (2/18)
 10/12/10 7:13pm
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lanoka harbor, NJ - USA

1990 L-98 white/ red/black leather interior targa top auto 1975 L48 blue/black leather interior t tops auto

Joined: 3/22/2009
Posts: 1363

thanks for the write up. i found an easier way to do this job.  I'LL LET YOU DO IT FOR MERolling On The Floor Laughing. hope you had a case of beer to help things along. there are somethings in a shop manual that are useful. know what you mean when you say they confuse you. glad you got it fixed.


the dynamic duo

c3vr life member #97
Is changing the heater core on a C4 really that bad? (3/18)
 10/13/10 8:43am
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Bowling Green, OH - USA

1984 Light Bronze/Dark Bronze Z51 Coupe 2002 Magnetic red convertible

Joined: 10/3/2009
Posts: 30

Nice write-up.
Is changing the heater core on a C4 really that bad? (4/18)
 11/20/10 1:26pm
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Olney, MD - USA

93 Polo Green Convertible with Bilstein Shocks, Autopower RollBar, Simpson 5 point harness, Grand Sport wheels, bald tires and dirty air filter!

Joined: 10/20/2010
Posts: 44

Are you familiar with the notorious and infamous "Blend Door?"  I'm wondering if what you did would give better access to that blend door motor so that it could  be replaced.  My heater core is OK for now (after all, it's only 18 years old) but if I had to go through t he steps you went through, I'd replace my broken motor and replace the heater core at the same time.  
If they can do arthroscopic surgery, why can't they do arthroscopic blend-door replacement, or heater core replacement???
Inquiring minds want to know!!

Rarely indicted, never convicted
Is changing the heater core on a C4 really that bad? (5/18)
 11/24/10 11:54am
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woodland park, NJ - USA

1993 black/black zf6 coupe

Joined: 12/5/2009
Posts: 9

Nice write-up.  I'm going to save this one just in case.
It's been said that when the C4s were in production the heater core came down the assembly line first, then the rest of the car was assembled around it.

Is changing the heater core on a C4 really that bad? (6/18)
 11/27/10 4:43pm
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San Tan Valley, AZ - USA

87 Red Coupe, not as pretty, had 213G miles, great running engine; got 20-21 around town, 27-28 on hwy; I got to drive a Vette, every day for 6 yrs. It was my first; so now I know what the fuss is all about. (Sold it 2015 for $3000, still ran great.)

Joined: 11/29/2009
Posts: 90

Great article and pics.  You are a hero to heaters!   However, I am not that  brave, nor mechanical, so if anyone knows of a repair man/shop in the Phoenix/Mesa/Tempe/East Valley area where I could get work done on my heater, I'd be interested in having someone fix mine.  
(The temperature slide on my dash unit won't move from the Cold side to the Warm side, it's just stuck.  I could use the AC in the summer, but can't use the heater now that it's cold in the AM, what Arizonans call "the winter." 45-50 degrees isn't much to whine about, but it would be nice....  BTW, I have an 87 coupe.)

Re: Is changing the heater core on a C4 really that bad? (7/18)
 10/1/13 5:33pm
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San Pablo, CA - USA

1993 coupe

Joined: 10/1/2013
Posts: 1

This is a great write up. However, I have a questions, before pulling the heater core, don't you have to drain the coolant system first?

|UPDATED|10/1/2013 2:33:11 PM (AZT)|/UPDATED|
Re: Is changing the heater core on a C4 really that bad? (8/18)
 10/8/13 4:30pm
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, - Canada

1994 convertible, Polo Green 2 1G1YY32P9R5114269 ---------- 2006 Monterey Red Vert 1G1YY36U665125919----

Joined: 8/16/2013
Posts: 191

I would guess that that might be adviseable; if not necessary. But you already knew that didn't you. Smile

I've heard of this job and it's no fun at all. I sure hope mine stays intact for as long as I have it.


Re: Is changing the heater core on a C4 really that bad? (9/18)
 12/15/13 10:58pm
Big J's 90 vette
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Joined: 12/15/2013
Posts: 1

what year was the vette u did the core on
Re: Is changing the heater core on a C4 really that bad? (10/18)
 12/18/13 9:55pm
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San Pedro, CA - USA

1992 Polo Green Coupe 1987 Bright Red Coupe

Joined: 6/20/2013
Posts: 7

nice write up...

and it's a bitch on a good day... It took me 8 hrs to get it apart, I got the core in, then figured, "eh, I'll put it back together tomorrow (Sunday)." Yea... I got the flu and it took me another month before I got the time to put the rest of the dash back together.