How to Replace the Heater Core on a 1990 GMC S15

The vehicle owner never catches damage to the heater core nearly 100 percent of the time before it actually happens. Unlike other engine components, the heater core on the S15 sits quietly tucked away under the dashboard until there’s a problem with it. The sweet smell of sugar inside of your truck definitely isn’t that new air freshener. The heater core has ruptured and that smell is coolant leaking all over the passenger-side floor area. Replacing the heater core on your 1990 GMC S15 can be done right at home with a few simple tools. The repair should take you 40 minutes or less even if it’s your first attempt at auto repair.

Things You'll Need

• Coolant drain pan
• Flat-head screwdriver
• Pliers
• Socket wrench set
• Funnel

Instructions

1. Raise the hood on your S15 and remove the radiator cap. Position a coolant drain pan under the truck’s radiator in-line with the coolant drain cock. Open the drain cock and allow half of the coolant to drain, and then close the drain cock. While the coolant's draining, instead of looking down into the radiator to see when you have reached the halfway point of coolant in the radiator, count to 45 and then close the drain cock.

2. Loosen the retaining clamps on the heater core hoses at the firewall with a flat-head screwdriver. Then pull the retaining clamps back on the hoses approximately six inches. If your heater core hoses have spring-tension clamps, use a pair of pliers to squeeze the clamp tabs together to move them back on the hoses.

3. Pull both of the heater core hoses off the inner and outer heater core tubes. Position the hoses so they’re pointed upward to avoid the bit of coolant still in them from spilling.

4. Open the passenger door and identify the heater core housing cover beneath the dashboard at the firewall. The cover is black and shaped like an odd rectangle.

5. Remove the five bolts from the heater core cover with a socket wrench. Do not misplace the bolts; place them in the cup holder and then remove the cover from its position.

6. Remove the bottom bolt on the left and the one on the right securing the metal retaining straps on the heater core. Slowly and carefully, bend each of the straps upward away from the heater core. Each strap also has a bolt at the top of them. You do not have to remove their top bolts, but if you feel more comfortable doing so rather than bending the straps, do so.

7. Remove the heater core by pulling it toward you two inches and then lower it from under the dashboard. The heater core still has coolant inside of it. Be careful so you don’t spill any more coolant in the truck. Install the new heater core by completely reversing the removal process exactly. Use a funnel to refill your radiator with the coolant in the drain pan.

Tips & Warnings

• If you have recently had the cooling system serviced it's OK to reuse the drain pan’s coolant. General Motors recommends that you service the entire cooling system on the 1990 GMC S15 every 24 months or 30,000 miles, whichever comes first. Now that you know how to replace the heater core, you can also check its condition. The heater core cover can be removed without draining the coolant for future inspection. Don’t remove the cover if you have just driven the truck because the heater core will be hot.

References

• "Chilton's Repair Manual: Chevrolet S-10 GMC, S-15 Pick-Ups, 1982-91"; Richard J. Rivele; 1991
• "Haynes Chevrolet & GMC S-10 & S-15 Pick-ups 1982 thru 1992 Repair Manual"; Robert Maddox, John Harold Haynes; 1992

Resources

• "Haynes Automotive Heating and Air Conditioning Systems Manual"; John Haynes; 2000
• "Engine Cooling Systems HP1425: Cooling System Theory"; Ray T. Bohacz; 2007