Suburban Rear Heat Not Working

[Haggar]

Dennis,

Sorry, but the description is going to get a bit complicated. I am not surprised that the local dealers don't have a clue, it is an engineering issue, not much that a mechanic would notice or be able to fix.

It sounds like you have thoroughly removed any consideration of an absolute plug in the lines, or even a considerable restriction.

So you have to figure out now whether too little water is making it to the rear heat, or is it moving too fast. (the too fast is much less of a possibility, this is not usually a consideration in automotive applications...)

IIRC you have 1 hose coming off the thermostat header. This is the "supply"line. The other line is connected to the water pump. This is the return line. (I could be backwards on the supply and return.... Sorry, my memory is failing me on that point) The supply line has the hot water with the pressure of the water pump "pushing" the fluid. This "supply" line feeds two heat exchangers "IN PARALLEL" (see discussion of parallel vs series in electrical definitions http://en.wikipedia.org/wiki/Series_and_parallel_circuits) (notice that when a system has two parallel paths,if the resistance of one path is too great, all the current (hot water) will take the other path)

So when the water is coming out of the supply line it has the choice (great-- free will applied to a non-entity) to go to your front heat exchanger or your rear. Luckily it doesn't make this choice like humans would (totally random), it is influenced by the forces of nature. The fluid, like humans, will absolutely try to follow the path of least resistance. SO, if the path of least resistance is to flow through the front heat exchanger, it will. NOTHING is forcing water to the rear heat exchanger, all the water supplied is going to the front since this is the path of least resistance. (this is why everyone was concerned that there was a restriction in your rear system, for some reason our systems front exchanger has enough internal resistance to force a portion of water to the rear exchangers with no other work or thought about it.)
Your system for some reason does not do this. What you can do is use one of the "hose block off pliers", to restrict the flow of fluid to your FRONT heat exchanger to see if you then move a portion of the fluid to your rear heat exchanger. (you have falsely made the front exchanger have a higher "resistance" therefore the path of least resistance is the rear) If that works WOHOOO! just make a fitting to reduce the flow to the front. (edit, one possibility that your system is "different" ..... your water pump is failing causing a low flow condition. there may not be enough flow to fill both heat exchangers.... it is quite possible)

Another possibility is to put the heat exchangers in "Series" (take line from thermostat housing into front exchanger, exit from front exchanger to inlet of rear exchanger, rear exchanger outlet back to water pump) This will "ignore" the difference in resistances.... But the rear heat exchanger will only ever get as hot as the outlet of the front exchanger, but you will have flow. This is not recommended but it would not be too bad, the water coming out of the front exchanger is probably only 10 to 15 degrees cooler than the water going in, so you would still have heat, just not "great" heat.

I hope this makes sense. sorry for the long post.

Haggar

 

[Brooklyn Tow]

Just a stab in the dark here...(WAG)...could it be that the rear line(s) have to be bled?

Sorry if it sounds stupid, but if your brakes need bleeding, with the master/booster "pushing" the fluid thru, would bleeding in this scenario come into play?

 

[btfarm]

Good point Louis and I think Haggar is on the right track with the "path of least resistance" theory. I'd try bleeding it and then see about putting a restriction in the front core supply (you could do the return as well) to see how that does. There's a reason the supply line to a heater core is always bigger (3/4" ID) than return (5/8" ID). Slower flow to provide more heat exchange. For a test restrictor a piece of 3/8" pipe stuffed in the return line of the front core would probably work.

 

[DennisG01]

All good points.

Louis - I did bleed the lines (actually a few times), so I think from that standpoint I'm good.

Mike - that's great idea about the small pipe! It was mentioned up above about installing some type of restriction to the front core, but the only thing that had really come to mind was an inline shut-off valve. The problem (I think) with a valve is that even with it wide open, it's likely to have more of a restriction than your pipe idea. Being as the front heat heat isn't exactly "setting anything on fire", I certainly don't want to decrease the heat output.

Haggar - No, absolutely not complicated at all - thank you! You're correct about the supply line - it comes off the t-stat housing. The return simply dumps into the top (starboard side) of the radiator. Speaking of "path of least resistance"... I wonder if it may simply be due to the weight of the coolant in the rear circuit?

I should get a chance to feel the lines tomorrow and I'll try pinching off the front core's supply line. Hooking it up in series is an interesting idea - I can try to measure (with a IR gun) the inlet and outlet lines at the front core.

Oh, by the way... "great-- free will applied to a non-entity"... Nice one! I actually did "LOL" on that one!

 

[DennisG01]

Here's what I found... I ran the truck in the driveway at 1050RPM's for about 20 minutes (approx 185/190 showed on the dash temp gauge and I could see that the t-stats were opening and closing by the slight movement in the needle). I then took IR readings behind the tailpipe right before where the rear supply/return goes through the floor and into the core. I got 115/95, respectively.

Then I pinched off the front supply (although, it was probably only about 75% pinched off with the particular clamp I had). I now measured 157/135. Using my hand as the "sensor" for the air exiting the rear (middle seat, roof, actually) vent, I could tell there was a difference in how warm it was coming out between the two scenarios. However, even in the "pinched off" scenario, the air coming out the rear vent was still cooler than the air that comes out the front vent when at idle.

Even with the front supply pinched off (again, guessing that it was about 75% closed), I still had good heat coming out of the front vents. So, it seems that the recommendations to add a restriction are dead on. It also seems I don't have to worry about a drop in front vent temp by adding the restriction. Does that sound correct?

Mike, were you recommending also putting a restriction in the rear return line? So that the fluid stays in the rear core a little longer?

I wonder if there still might be, as some recommended, a restriction (partially failing rubber hose) in the rear supply? I just keep wondering why it worked before and know it doesn't, when nothing has been changed... except for the quick connect fitting at the t-stat housing, which I don't think would cause this problem.

A winter front (radiator grille cover) can only help, right? At least with how quickly everything warms up?

 

[NVW]

The quick connect fitting is a restriction in itself, the fitting you replaced it with may flow more.

But like you said I can't see it being a problem.

 

[Brooklyn Tow]

Another WAG....did you check for any TSBs??.....And do you run a winter front?....Oh, you said the truck was up to operating temp.....Maybe like Haggar said "water pump"....Sorry for the WAGing.

 

[NoDak]

I'm in general agreement with the theory of "no restrictions" should work or at least not cause your "lack of heat" problem. I suspect the restrictions in the QD and the rear heat Tee's is to limit flow and thus peak temps. You certainly don't need 185 deg F to make a heater work comfortably. You might also have better control of a comfortable temperature, working with a lower temperature core. The restrictions may have also been used to limit the fluctuations in vent temperatures in situations where you are going up and down hills or stop-and-go traffic. We all know that going up and down "long" hills at sub-zero temps will take you from t-shirt weather to survival gear, lol.

So, if the '99 diesel Sub was intended to have a 0.260" diameter orifice in the QD and approximately 0.203" diameter orifice in each of the rear heater tees and we can assume the factory get-up worked for both heaters... DennisG01 must have a significant restriction to coolant flow or some other problem. Just me thinking out loud?

We also have a '99 5.7L Suburban, the rear heat is slow to work. We usually leave the rear fan off until the front gets comfortable. The rest of the gang really hates to have "wind chill" in the back rows. But, once things are hot, they are hot and all is good.

 

[DennisG01]

Louis - Last time I checked for TSB's, I don't recall anything related - the only thing was about the rear brakes. It's funny you mention the winter cover... I actually just came up with a way to save myself $50 on a winter front about 20 minutes ago... and, no, I'm not using cardboard! :lol: I already have a plastic, snap-on grille cover - it has little tiny, 1/8" holes all over it. I use it when I know I'll be in a very "buggy" place. I'm simply going to screw in some male snaps and sew up a cover with some heavy-weight, outdoor vinyl. I'll attach female snaps to the cover and voila!

Out of curiosity... how do I remove that quick connect fitting? I played around with it a few minutes ago, but it was taking me longer than I thought it should for a "quick disconnect" so I stopped before I started forcing things. My t-stat housing was already out of the truck when I replaced it a few years ago (while the engine was out) so I never had to mess with the "quick" part.

 

[DennisG01]

UPDATE UPDATE UPDATE

I finally got things taken care of and the heat now WORKS! What prompted me to get going on this was that the front heat was now getting flaky. I checked the various servos and they were fine. It turned out that when my new water pump was installed, they got a little "aggressive" with the sealant. All of that RTV ended up collecting in that QD fitting off the t-stat housing. I pulled it all out and the front heat was blowing nice and hot... then I checked the rear and.... Hooray! It all works really well!!!!

Hopefully this will help some others that are experiencing a similar problem.