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Help: Poor heat in 99 Suburban

F

fastjohnny

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Location
SW Michigan
My 'burb has always had poor front heat output. This morning, I spent a little time to further try to get an understanding of this. The rear heat output is fairly good. The front heat will do fair to OK if on recirc, but if on fresh air, it is barely warm, more of a cool-but not cold output. This is through floor heat, defrost, or middle vents. The heater hoses seem hot. The engine temp is not much past 160, prob 175 or so, driving 60-70 mph, with ambient outdoor temps mid 30's. I have never checked the thermostats to see what ones are in there. Also, when I've been running with a plow, and had engine temps significantly higher, it did not provide any better heat in through the front heater.

It's been a long time since I've been into one of these heaters, is there a blend door opening too much???

Other ideas??

Thanks in advance.
 
I'm responding primarily to "subscribe" to this thread to see what you figure out. I'm having a similar problem, although I think, from the way you are describing it, my front heat is a tad bit warmer. My rear heat sucks, though.

The engine temps you are seeing are definitely low, though. I see right around 185* with 195 stats... similar ambient temps. But, like you mentioned, that doesn't explain why your heat didn't work better with the plow.

Just another thought... when was the last time you flushed the front and rear systems?
 
Johnny, just an FYI... On the way in to work today, I listened as I pressed the "recirc" button. I could noticeably hear the sound of the air coming from the vents (I had it on high) change. It took about 5 seconds to completely change, but I could easily hear it. I'm assuming this is from the blend door (noted above) moving. Assuming I'm correct (maybe someone can verify), maybe you could check that, before you tear into the dash. Although, I guess there is no way to know, for sure, that the door is closing completely?
 
There are three doors in the HVAC system. The recirc button moves the fresh air/recirculate door. You can watch it move, its on the bottom of the heater box in the right side footwell. You kind of have to fold yourself under there, but if you look at the air intake, you will see a flap, that is what the recirculate button controls. If that works, be glad as the controller is all but impossible to replace.

There are also two other doors, the vent and blend doors. The vent door controls where the air goes (defrost, floor, dash vents) and the blend door controls the heater core to evaporator core flow. Its been a while, but I think the blend door is near the drivers footwell.

Two thinks you might want to check. The factory press on water outlet has a restrictor, make sure if you have changed it the replacement has a restrictor, also the tees to the front/rear hoses have restrictors as well. From what I understand, if the restrictors are missing the heater will not work very well. The other thing that can go bad is the heat/AC control head. The 95's were very prone to failure, the later ones are said to be more reliable. However when they fail, one or more of the blend doors may not work or will do odd things. Unfortunately, about the only way to test them is to replace with a new one, and they are close to $200.
 
Bobbie, while trying to figure out my rear heat problem (which I still am, but won't go into detail as I don't want to hijack Johnny's thread), I came across some interesting material in regards to the restrictors. I'll pass this along, as it may help Johnny and I would also be interested to hear your thoughts.

I found that I do not have any restrictors in the T's and that some guys have them and others don't. Some guys say that w/o them their heat still works just fine.

I had stopped by a Chevy dealer to try and find part numbers for the restricted T's... Couldn't find one and the parts schematic for my year didn't show it, either.

Is this something that could have changed over the years?
 
Thanks for the replies, I just finished a 600 mile trip with the family. The rear heat actually did pretty good. I think Bobby may be on to something with the evap/heater core blend door, as I recall in summer, I felt front AC did not get as cold as rear. And I think I really need to get some different thermostats, as I feel the engine is not running warm enough, consistently 170-175. What ones should I put in? Do you guys think 195 thermostats would be too hot? AC Delco brand is what I recall being the best, though I've heard some say Robert Shaw. And while I'm doing that, I figure I'll put in a restrictor in the water pump bypass, per info from heath, again IIRC.

Dennis, go ahead and post away, your thread was very informative.
 
Johnny, I picked the 195* stats based on Kennedy's recommendation on his website. The ones he sells (and what I have) are the Robert Shaw. I tow about a 10,000 boat in the hot summer and am just fine with temps (I do have an 18:1, so I'm sure that helps, though). When my engine was stock (21:1), I also used the 195* stats and towed a boat weighing a little over 6,000lbs. I had no problems with heat then, either (that I can remember, anyways - it was a while ago).

What's the "restrictor in the water pump bypass"?
 
Johnny - I just got mine working. I did a flow test into a clear container and found I had horrible flow. I took the QD fitting off and there was a bunch of strands of dried RTV collected in the restrictor. Took it out and now BOTH my front and rear heat work nicely.
 
That's awesome, I know you worked at that for a long time.

I've been too busy on other projects like dual alternators:rolleyes5::mad2: but I really need to get this fixed as well. Maybe I'll start at the QD as well.
 
Thanks, Johnny. But truth be told, I'm not sure I actually "worked" on it for a long time... I think I did more complaining and thinking than anything else! :rolleyes5:

Good luck with your alternators. Just out of curiosity, why not just one big (stronger) alternator?

Anyways, here's the last page of my other thread - I posted a picture of the amount of RTV:
http://www.thetruckstop.us/forum/showthread.php?18064-Suburban-Rear-Heat-Not-Working/page6

Here's what I did to measure the "flow", fyi:

photo.jpg
 
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