2000 ford taurus...In the last 2 years have had heater core, water pump, thermostats (2) resovoir, hoses and system flushed. Now again after 70 miles or so of driving the car overheats and anitfreeze boils out of resovoir, and it happens after only 20 miles if AC is on. Also the heater only blows luke warm air, never gets really warm. Any ideas on what to do next????

If the heater core bypass refers to the water flow to the heater core, I would wonder why you did not flush the heater core as part of the system. You can drain water from engine, disconnect heater hoses and add a short hose to the heater and force a garden hose to flush the core.

I have not looked up the particulars for which heater system is on your car. Some just use duct-work to close off air flow through the heater core and the heater core always has hot water in it. Others will use a valve to shut off the water flow through the heater core and they can be either electric or vacuum operated. Combination systems are the third system which controls water flow and duct air flow at the same time.

Some of the Premium Climate control systems with Electronic displays have a computer diagnostic system which self-diagnoses problems but you need the sequence codes and explanation of where the bad part is located.

Check the heater hoses that go into the firewall to the heater core, they both should be hot,if only one is hot and the other isn't,then the core is plugged. Remove the hoses at the firewall, buy of hose the same diameter,buy enough length so that you can flush away from the engine compartment, install it on one of the heater core pipes,using a garden hose with a jet sprayer slowly blast water into the hose, until you see clean water coming out from the other pipe. Remove the hose and install it on the other pipe of the heater core and repeat the process,you may have to do it several times to completely unplug it. This is a typical problem with a 3.0, 2 valve taurus,coolant looks brown and rusty. The entire cooling system should be flushed, if it isn't the heater core will plug again.

Well, the usual suspect is a clogged heater core.
What I do is pretty simple,and I recommend it highly.
With engine cool, remove the heater hose from thermostat area.The heater hose at the front of the engine is returning to the water pump.There should be a connection on that hose a foot or so from the water pump where you can separate the hose. Now........
you have isolated the heater hoses from the engine by doing the above.Wrap an old T-shirt or WHITE rag around the end of the hose you took off near the thermostat housing and tie it on there so you can see how much dirt you'' flush out.
Take a water hose with the common pistol grip(not the fancy one with several spray patterns) and hold it securely into the hose you separated from the water pump area.When you spray water into the hose it will BACKFLUSH the dirt into the teeshirt/rag and thus clean the heater core.
Now......what I do then is pour radiator flush thru a funnel into the same hose you flushed thru near the water pump.Let that sit in there for one hour.Now making sure you have the hose end directed down toward the ground, do the water hose flush again for a minute or so.You now have flushed your heater core and when you reattach the heater hoses,you might find you have restored a lot of the heating ability.
Of course,if your antifreeze is dirty,this is the time to do a flush of the whole system. don-0hio

have you checked the coolent level?

try flushing cooling system could be a blocked heater core.if flushing doesnt work remove and replace heatercore.

thermostat might be in the wrong way, check the hoses, check heater core, or air could still be in the system which could be a disaster

Check the coolant level.Feel the heater hoses going to the heater core to see if they are warm,or hot.If one of them is warm,or hot,and the other is not,remove the heater hoses,and flush the heater core out with a water hose.Bleed the coolant system,and you might want to install a new thermostat while you are at it.

Is the blower motor working? If not that is what it is. If it is working then check heater coil out, Could be bad heater coil.

These Tauruses had a problem with the heater cores plugging up, blowing out the head gaskets and no heat issues. Ford issued a TSB#98B26 (Tech Service Bulletin) to modify water pump and install a heater hose bypass tube. I would start by completely flushing the system with a good flush. Then reverse flushing the heater core. Refill the system and add a good additive. Also you can install the heater bypass tube as described in the TSB. The last one I did ran around $90 and 2 hours labor. Hope this helps you out.......

This happens ALL THE TIME!! Heater core is likely plugged with rust, yes. but a garden hose fix will NOT last. and the reserve probably IS cracked on bottom, as most are.They are relatively cheap from Ford, though. Have a REPUTABLE radiator shop do a Genuine Power Flush to the entire cooling system (not an automated fluid exchange many shops claim to be a "flush") and THEN have them unplug the heater in a similar fasion. Now is the time to replace the surge tank (old one is packed with rusty sediment that is usually partially plugging the cracks in it, and not very cost effective to clean anyway). On older, very rusty 2 valve 3.0 systems only other issues are the rear of timing cover gasket often leaks a little, and the water pump impellers can become eroded away (from rust) and water flow suffers. So... watch for residue or dripping from behind water pump. If minimal, a mild leak sealant added to a properly cleaned system with refill and tightening the bolts near leak will do the trick--otherwise timing cover gasket and pump replacement is needed.