Changing The Heater Core on a GMC Syclone

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I grew all too familiar with the heater core on the Syclone when just about everyone in the Florida SyTy group had me help change their heater core. Enough to have earned the nickname: Heater Core Bitch.

My first experience started after a fun little trip to work. I found a spot of coolant on my passenger side floor and realized immediately that it was the tell-tale sign my heater core was about to puke its contents all over the inside of my truck.

It is a such a common problem with our trucks that theories have arisen to explain the cause. Excessive engine torquing (maybe a by-product of boost launches), corrosive coolant, or just stagnent corrosion may be the cause. Whatever the cause, the result is an oozing warm moist gym locker in your truck.

So, as soon as you notice the stench or the dripping fluid grab some towels and put them down to prevent the coolant from soaking into the carpet, it stinks! To remove the smell if you didn't catch it in time, peel back the carpet and soak up all the coolant that you can get out of the carpet and padding. Douse with your choice of fabric de-stinkers like FaBreeze.

Disconnect the Battery


They always tell you to do this for a reason. In this case, we're going to remove the ECU, so it's best to prevent any issues by disconnecting the battery terminals before you start.

Drain the Radiator


Put a pan under the front driver's side of the truck. Get a large one, the coolant splashes all over the place cause it drains out onto the frame rail instead of strait down.

Image 89 from Changing the Heater Core on a GMC Syclone

Unscrew the plastic wingnut/petcock on the bottom of the driver's side of the radiator. It's real easy to get to if you already have the fender wells out, but you can reach it from the top if you undo the nut on the airbox, and move it out of the way.


When the system is fully drained, tighten up the petcock, and replace the airbox.

Remove the Heater Hoses


Remove the coolant overflow tank on the passenger side of the engine bay (3 - 10mm bolts). This is not a necessary step, but it allows better access to the hoses your going to fight with in a minute.


Use a large pair of pliers move the clamp bands that hold the heater hoses to the heater core a few inches down the hose.


Using that pair of pliers, clamp down the ends of the heater core's copper pipes that are inside the hose. Bend the hell out of them, just squeeze, till they are obviously no longer round. (be careful with that formed hose, its not cheap to replace). This will make yanking the hose off about 100x easier.


Pull the hoses off the heater core


Remove the Heater core's tube support ( 1 - 7mm hex screw). Its the little piece of metal in the shape of a "W" that attempts to prevent heater core breakage.

Remove Interior Trim


Remove light bulb from passenger side footwell, or disengage by removing it's fuse. This light gets hot, and will burn your arm quite nicely (my second syclone related injury).


Remove passenger side footwell trim panel (7 - 9/32 hex screws). Its that dark plastic piece just under the dash.


Remove passenger side kick panel (4 - phillips head screws).


Remove ECM (1 - 9/32 hex screw). Remove the screw, and carefully pull it out from under the dash.


Unplug and Remove the ECM. You'll need to unplug it from the harnesses, and place it somewhere safe.

Image 146 from Changing the Heater Core on a GMC Syclone

Remove Ash tray


Remove Radio Trim panel (3 - phillips head screws) The cigarette lighter simply comes off by spreading the two plastic arms away from the shaft on the back side of the trim plate.


Remove Radio. (2 Hex bolts) Remove the bolt on the far right and left brackets, then pull the stereo out, unhook the wire harnesses and antenna wire.


Remove access panel in the rear of the glovebox (2 - phillips head screws)


Remove bolt on passenger side of truck that holds onto the dash (1 - 10mm Hex bolt). Pictured in step #4


Carefully pull out on the bottom of the dash, moving it out, and down to hook it on the tab that it was screwed into. This is to aid in removing the Heater core cover that needs a little room to move.

Remove the Heater Core Cover


Removing the screws (6 - 7mm hex screws) Four are easy to get to from under the dash (2 on bottom, 2 on right side).


The top screws are accesible through the radio opening, and one in the glove box opening. These are the real pain of the group. Have fun, don't loose the screws ;)


Disconnect the Heater Vent Hoses that are connected to the top of the cover.


Cut the tape. There is some sticky foam stripping in the joint between the rest of the ducting and the cover, Cut it at the bottom(under the dash), and at the top (from the radio cavity).


Begin Yanking! Pull on the heater core cover from the bottom right side. (towards rear of the truck). Then try to move the cover to the right to disengage it from the ducting on the left side.


Rotate the cover so the bottom goes toward the front of the truck, and the top, towards the rear. This is to get it around the heater core that rests inside. You'll have to pull the "T" shaped ducting (look through glove box) out of the top of the cover to get it out. Be careful, its plastic. It will come out, just be nice, and wiggle till you manage to get that SOB out.

Image 8 from Changing the Heater Core on a GMC Syclone

Remove the Heater Core


Remove the hold down straps ( 3 - 1/4" hex screws). There are 2 metal straps holding the Heater Core in place, Remove the left one (both bottom and top screws). The right one doesn't have to be removed completely (its almost impossible to do). Just unscrew the bottom of the right strap, and bend it up.


Now, remove the Heater Core. Pull, and rotate until you can manage to get it out. It's most likely full of coolant, so be careful here. You could put the tube covers off your new heater core onto the old one to prevent spillage (if you're new one has them, that is).

Image 2 from Changing the Heater Core on a GMC Syclone

Thanks to Ivan the Terrible for the instructions I used to change my heater core the first time around.

I surprisingly haven't had to change it again (I believe I changed it in 2001). With all the dread associated with having to change the heater core, I've been very trouble free. So best of luck, hopefully your new heater core will be trouble free.

  • Coolant Overvflow Tank: 3 x 10mm screws
  • Heater Core Tube Support: 1 x 7mm hex screw
  • Interior Footwell Panel: 7 x 9/32 hex screws (can use short flathead)
  • Kick Panel: 4 x phillips screws
  • ECM: 1 x 9/32mm hex screw
  • Radio Trim Panel: 3 x phillips screws
  • Radio: 2 x small hex screws
  • Glovebox Rear Panel: 2 x phillips screws
  • Dash Bolt: 1 x 10mm hex screw
  • Heater Core Cover: 6 x 7mm hex screw
  • Heater Core Straps: 3 x 1/4" hex screws

There are 5 Comments.

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I installed this heater core in my Syclone over 50k miles and 10 years ago. I'm actually quite amazed, but it's been working fine since.


I never had to replace one. Doesn't look fun.


The newer aluminum heater cores can be a bit difficult to fit, though I'm not sure if anyone sells a copper one anymore.

FYI, if you go to the parts store they try and sell you bulk hose (straight) to replace the piece between the back of the manifold and the heater core. However, almost all that I have been to stock a hose in the proper size with the 90 degree bend formed in it. Its not exact fit as the long side is too long, so their computer won't find it, but it will work if you trim the long side down a bit. You may have to be a bit forward with the parts guy (if its not on their computer it doesn't exist!) but they should have the hose you need.

If your heater core goes out I strongly recommend you change the antifreeze and check your grounds really well as old antifreeze and the water acting as a ground path can both cause the core to go bad (try changing 3 of these in 3 years).


Use a spot of electrical tape to hold the core strap screws into a 7mm nut driver, so they don't fall out while reinstalling the straps.

Using a 7mm socket, swivel, and extensions, it is possible to access the upper inboard cover screw from under the radio without removing the radio or bezel.

Do NOT overlook removing the lower 10mm screw and pulling the IP back and downward. This extra inch makes a huge difference.

Snoman002 is incorrect about the hot-side hose on a Syclone/Typhoon. It is not a generic hose, and does not come off the rear of the intake like 'normal' 4.3's. It is a specific SyTy P/N hose that has been NLA for about 10 years and comes off the front of th eintake.

One of those hose removal hook thingy's really helps with loosening the hoses off the old core.

I've done a dozen of these. You learn something new every time you do one.

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