I assume temp gauge on dash is reading warmed up the same as usual, if not thermostat on engine is stuck open so engine can't warm up, and neither can heater.
Next time engine is warmed up, set Heat to Hot, pop the hood and feel the two heater hoses that go into the core/firewall.
Leave engine idling.
Both should be warm but one will be warmer than the other.
The warmer one is the IN hose, the other is the OUT hose, remember or mark which is which.
If the OUT hose is much cooler then either the heater core is clogging up or the heat Valve is not opening all the way.
Not sure on a 2008 but you probably have a Heat Valve(2 connections) or By-pass Valve(4 connections) on the heater hoses.
Usually vacuum controlled, take note of the position of the valve arm now, remove vacuum hose and valve should close, see if you can move the valve manually to open more than it was before.
If you can then hold it there and feel the OUT hose again, it should be getting warmer, if so you have a vacuum leak in the heat control or a bad valve.
To check for bad valve put a vacuum hose on it and suck on it, valve should open all the way and stay there until you release the vacuum, engine can be off for this test.
If valve checks out as OK then you have a vacuum leak, which can be hard to track down on these systems because if can be behind the dash on the selector.
If vacuum valve seems to be working, opens full, you will need to Back Flush the heater core.
Loads of videos on Back Flushing heater cores.
Basically you need to drain some coolant out, not alot.
Remove the IN and OUT hoses at the engine end, not the firewall, remember which went where, mark it or take a picture.
Point the IN hose down at the ground or towards a bucket if you want to see what comes out.
Hold running garden hose to the end of the OUT hose, start off with slow flow and increase as you feel is needed.
If flow is restricted you will know
Flush until flow is good
If you think you need to clean out the core more, i.e. flow is not great.
Blow out the water in the core, you can just blow in either of the hoses, doesn't have to be dry, just most of the water out.
Mix some warm water and CLR, pour it into the core, let it sit for 20min. then flush again.
Repeat as you see fit.
Last edited by RonD; 12-02-2014 at 01:07 PM.