Ok well I've been busy with work, but I managed to cut the heater hose coming off the water pump with the restrictor. The restrictor is about 3/8" diameter in a 5/8-3/4" hose so it is significant. The inside of the hose appeared to be a little rusty but was clear otherwise. I went ahead and spliced in a hose barb to get rid of the restrictor. At idle the front and rear were warm/hot but not as hot as it should be, and althogh the heatercore feed lines were too hot to grab for longer than a few seconds, the return lines were a little hotter than before but no where close to the feed lines.
Then I went for a drive which seems to be the real test. The rear heater maintained the heat, not hot enough to shut off, but signicantly warmer. The front almost instantly went cold when driving in the 39* weather so the front heatercore is not flowing enough fluid through it to overcome the cooler outside air.
Just to test pressure across the heatercores I also pinched off the front and rear heatercore feed lines one at a time to see if the increased pressure would help push more fluid through each heater core. It made no difference with the front.
Based on this I have ruled out the restrictor and waterpump as the culprit. The reason I rule out the waterpump, is simply because the heatercore in the rear has a longer run coming off the same T for both the feed and return lines. From a pumping perspective it should take more effort/time to pump fluid to the rear heatercore than the front as it is a longer run. Which tells me that there is a flow problem in the front heater core and the rear is clearer but not completely clear as it should blow hot to shut off after awhile driving.
I am going to replace the rear heatercore as it is easy to access and if I can at least get it to blow really hot, it will heat the cabin enough to keep everybody warm and I can get by without having to take off the dash to get to the front heatercore until the summer.
Ok got a good update. There have been a few posts after mine with claims that the test might not work, but so far they have exactly as outlined.
This morning while driving to work I shut the front heat off because it was blowing cold air and I was just using the warm air from the rear to keep me warm. I went to pass some cars and when I did I heard water flowing threw the hoses in the firewall and out of curiosity I turned the wheel on the front air and warm air continued to blow, I heard the water moving sound for a a few more minutes and thought maybe the heatercore cleared out some because I took the restrictor out of the line.
During the day I thought about this video I saw on YouTube of some dude back flushing his heatercore with some CLR and thought to myself what the hell I might as well try it before taking out the heatercores.
I removed and cut the return, remember I already cut the send to the heatercore to take out the restrictor. I then blew out all of the coolant with an air hose out of the heatercores by forcing air threw the return and blowing out the send into a jug. I then poured the CLR down the return and the send. I let it stand for 10 mins then reverse flushed with water from a hose. A lot of crud filled the jug I was using to capture from the send. I then repeated the entire process, and reconnected the lines. I started the vehicle and turned on the front and rear heat.
I'm happy to report that the front heat got hot, hot enough that with the temp set to max 90* that I couldn't leave my hand over the vent without it eventually getting so hot that I he to move it away. The next test was to take it for a ride.
With the outside temps in the 40's the heater blew so hot that I eventually started to sweat, and I couldn't hold my hand over the vents without them getting too hot, I was able to turn the temp down to 75* and the heat was good inside.
So the flushing has bought me some time and I don't have to replace my heatercore right now. I know that it most likely is not 100% clear but I have really hot air blowing now whereas I didn't not last night.
If you have the conditions I described in my previous post, most likely your heatercores are starting to lose efficiency due to rust.
I highly recommend using the CLR back flush before removing and replacing the heatercores. You may get lucky and get a little more life out of them before having to replace.
BTW I put the restrictor back in place too.