Expert: Roland Finston - 1/8/2012

Question
QUESTION: Hello Roland,  I have a 91 5th ave 3.3 w/ 200k miles.  I am having a heating issue where the heat cools down when the rpms are at idle.  I did notice the upper hose going to the heater core gets cold when the bottom one stays warm, so I guess that I have a bad heater core.

I have remove the kick panals under the dash, and can see the heater core through the duct work, but do not see any screws from underneath to remove it.  The shop manual does not say enough about the removal of the heater core, so was wondering if you could offer any help?

Thanks for your assistance and your time.

ANSWER: Hi John,
If this is a conventional unit (not automatic temperature control) then for sure you unfortunately have to remove the entire unit from under the dash to change the heater core and that is about 25-30 steps! I am not certain about the ATC type unit but if the non-atc part of that unit looks like the conventional unit as shown in your manual then that too would very likely require removal.
My suggestion would be to back-flush the core by putting a domestic water hose into the outlet side and let it flow out the inlet side (after removing the regular hoses at the firewall). I believe there are radiator flush kits that have a garden hose adapter with which to attach it to the outlet nipple firmly so you can get the full pressure going into the core backwards.
If you have to remove the HVAC unit I can xerox copy and mail that to you. Another possibility would be that a radiator shop can do a higher pressure back flush of the core. If the core isn't leaking I would not be inclined to change it out given the size of the task.
Roland

---------- FOLLOW-UP ----------

QUESTION: Thank you for that.  I will give that a try. It is an ATC unit unfortunatly.  I change the fluid every two years 20k miles so not sure why it would clog.

THanks AGain.

Answer
Hi John,
By the way, I didn't say specifically that the ATC's core can't be removed with the unit in place but it is probably the case. It is definitely the case for a conventional unit.
Another possibility is that you have air trapped in the heater core. I would be inclined to drain the system completely again, re-using the drained fluid of course, and then refill it slowly. If that doesn't do it, then a reverse flush is the next thing ro try.
Now here is a detail you may not have followed heretofor about draining/refilling:
Start by running the engine briefly and put the ATC at hottest setting. Shut off the engine. Begin by draining from the radiator bottom cock valve, but leave the cap on the radiator. Then when flow stops remove the radiator cap to allow air to enter the system, AND also remove the temperature sender on the front of the cylinder head, below the thermostat housing, so as to also allow block to drain (It has two wires, tan/black, black/light blue). When refilling close the radiator drain cock, and fill until level reaches the temp sender opening, then install the temp sender and then fill the rest of the way. Then fill the reservoir to MAX. This should minimize trapping air.
Roland