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Low temp thermostat causing lukewarm heater output?

6K views 42 replies 14 participants last post by  Jimpala95 
#1 ·
My Caprice, with a warmed over LT1, has always blown lukewarm air from the heater. My mechanic blamed the 160F thermostat, but my Mustang Cobra has a 160 stat too and has no problem blowing HOT air from the vents. But of course they're two totally different cars, so not apples to apples.

I've flushed the heater core a couple times in the 8 or so years I've owned the Caprice, and it seems like it maybe blows a little warmer air after I do so, but if there is a difference, it's not significant. The car has an F-body cluster in it and the temp gauge needle typically stays halfway between the first notch (160) and second notch (not denoted), although it sometimes goes above the second notch in the summer.

My gut feeling is that the heater core is partially clogged, but I figured I'd see what the consensus is before I commit to a heater core replacement project.
 
#2 ·
It wouldn't be difficult to hose flush the core to eliminate that from the equation. My SS (with a 160) does not blow super-hot, but I've found cleaning the fan to do more for keeping things warm.
 
#5 ·
As mentioned, I have previously flushed the core and if it's made an improvement, it is barely noticeable.

What fan are you referring to cleaning? The blower fan seems to move a sufficient amount of air--it's just lukewarm.
 
#4 · (Edited)
The heater core is a coolant filter. so if you use drinking water and low coolant % the crude will grow and the heater core starts filtering .

distilled water and 60-65% antifreeze . about 4.25 Gallons total
block drain a must do.
I typically use tap water and 50% antifreeze. (Sometimes if I am thinking ahead, I'll use filtered water from my fridge.) 60-65% seems high--why do you recommend so much?

What do you mean by "heater core starts filtering"?
 
#6 ·
The blower fan seems to move a sufficient amount of air--it's just lukewarm.

Understood, same here - but a bit better than lukewarm. At the slightest drop in hi-speed or any hint of hum or vibration I drop the blower and clean the mouse litter and debris. Goes with garage queen territory.
 
#8 ·
My car has a 160 and has no problems with the heat. In fact, even in the below freezing temps we've been having here it gets uncomfortable if I keep the heat turned up more than 1/3 in to the red on the dial.
 
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#9 ·
FSM calls for 50/50 mix of "water" and Dexcool. (I switched my Impala SS's to green coolant). Even my 2001 Corvette called for "drinkable water" in the FSM.
 
#11 ·
That drinking water use in the GM manual is ancient technology.

I changed all this back in the late 1980's .. in today's world clean water is not allowed in vehicles made today .. reason is corrosion . its all distilled; also why pay 12-14 bucks for 50% mix of coolant solution when the distilled water is 2 bucks a gallon ?

if you insist on using your cali drinking water to save 2 bucks then you will help some child in China making radiators/heater cores for your ride...

another GM screw up is Dexron III... I got rid of that back in 1999 ... used synthetic mobil transmission fluid GM approved.

took GM 7 years to figure it out [2006]why ALL these transmissions 4L60 trucks/sedans all failed under warranty LOL....
 
#10 ·
Anyone knows to use distilled water in cooling systems. It's common sense even for those of you who may have been asleep in high school chemistry class. Steam irons and car batteries also get distilled water. I know, your father "used tap water for years and never had a problem," but this is the truth regardless. The amount of minerals in tap water varies greatly from place to place so some tap water may cause less mineral build up and also water softeners alter the chemical composition of the tap water.
 
#12 ·
Mine has never had good heat here in Chicago winters. I have the same output today as always and this past weekend had installed a new ac Delco water pump, 180 degree thermostat, and coolant flush. Fan doesn't kick on early and confirmed with a non contact thermometer. Heater core replaced 10k miles ago. It's been the same way for the 16 years I've owned it. At one point I blocked 90% of the radiator with cardboard and still made no difference on the highway when outside temp was below 35. The gauge doesn't zero out anymore, it starts out at almost half and only goes up a tad past half but before the red. I've gone threw a few temp sensors over the years too with no results.
 
#13 ·
as I said many times before both heater core hoses must be hot. if not then the heater core is crapped up. if both hoses are hot then its the temp controller.

my 96 the temp is 145 deg f .. as some have stated too hot after running at max..

I had the same issue in my 2000 silverado low heat sure enough that heater core had crap in it . both vehicles 1 yr old GM used as they say clean drinking water ..THANKS GM .. no problems now .. need the high heat when you drive in Blizzards LOL....
 
#14 ·
I'll check both heater core hoses when I get home today. I have a non-contact thermometer gun around somewhere.

Point made--I'll go buy distilled water next time I do a coolant change.

My car has a 160F thermostat not because of overheating issues, but because it's got a bunch of boltons and is around 425hp at the crank. More power generates more heat.
 
#16 ·
I'm a big fan of a 50/50 mix using distilled water. The reason I don't go any higher is that antifreeze doesn't allow your car to cool as well as pure water. The more antifreeze you add the worse your radiator will be able to get rid of heat. If you drive your car in the summer that may be important. The only reason I could see to running more antifreeze is if you live in an arctic area. a 50/50 blend will not freeze until somewhere around -40 F. I've also had luck back flushing my heater core and then running a coolant system cleaner for a few hundred miles. If the lower passages of your heater core are blocked then no cleaner will help because if they're blocked then the water/cleaner mix won't flow through them. Backflushing with a water hose seems to work for me.
 
#17 ·
Just to chime in on the heat output with the 160' thermostat.

I've had my car in central Pennsylvania, and in Texas, and had decent heat in both winter climates. Obviously not as good as a 180 or 198, but it was decent, enough to make the ride comfortable in both areas.
 
#18 ·
I checked the heater core hoses. The bottom hose was too hot to cold for more than a second or so; the top one was warm but I could hold it as long as I wanted. (I couldn't find my non-contact thermometer so I used the less-scientific method.) This was after I'd driven the car for about 20 minutes without using the heater--ambient was around 40F.

Does this sound like the heater core is probably partially restricted?
 
#19 ·
both hoses hot the core is flowing properly . one hose hot the other not then the core is not flowing.

on the OEM set up the hot coolant flows thru that core even if its 100 deg F out and the ac is on max... so driving around for 20 min and one hose not hot means you must do like I said before .. clean drinking water is not clean .
 
#20 ·
Problem with drinking water is what is put in it to make it "drinking" water. Don't even flush the engine with tap water. The only water that should touch it is distilled water.

Tap water from ground water is especially bad because it has iron, sometimes limestone and other minerals that will collect on the inside of your piping.

Tap water that is from another source generally has things such as chlorine which is corrosive and can destroy the protective chemicals in the coolant. It can also have things such as Fluoride for making teeth stronger or phosphates which sole purpose is to coat piping to prevent metals from leaching out of it. Guess what this does to your tubing in your coolant system...

Filtered water generally just takes the chlorine and some other chemicals out and may filter out some bacteria but anything dissolved in the water will stay in the water and pass right through the filter. Most minerals are dissolved in the water so you still MAY get the Fluoride and Phosphates.

A good example of Phosphates is Flints water system. We see what happened to their water because of corrosive water without phosphates added. If they would have been added they would have coated the pipes disallowing the lead from leaching out.

So if this doesn't scare you away from ever considering tap water from touching your car, be prepared to be replacing radiators, heater cores and engine blocks... May take 10 years for it to show up but it WILL show up eventually... I have had vehicles that have been affected by using tap water instead of distilled.

Note that I work in the field of water treatment so I have had water components beaten into my head for many years...

Rodney
 
#21 ·
50/50 coolant freezes at or very slightly under -22°F / -30°C. Those who need an even lower freezing point already know that over 70% antifreeze is a counterproductive waste.

Many of us could probably get away with increasing our H20 percentage to as high as 65% H20 / 35% antifreeze, which freezes about 5°F / -15°C.
However, less antifreeze means insufficient corrosion protection, especially with our aging cooling systems.
So far as coolant additives are concerned, RMI-25 has maintained an outstanding reputation for several decades.

Coolant filtration will likely become more important to those of us who wish to keep these cars running strong.
The iron block coupled with the aluminium in the rest of the cooling system guarantees galvanic erosion will always be an issue (RMI-25 helps fight this too).

The collective experience of the ISSF [especially prior to August '09] proves the heatercore has a weakness for collecting contamination.
Replacing the heatercore without BACKflushing the entire cooling system will eventually lead to replacing the heatercore again, it's just a question of when.
(May have something to do with the fact that while the engine and radiator flow top to bottom, the heatercore flows bottom to top?
Reversing this without figuring out how to maintain the the variable flow restrictor's protection is beyond me for now.)
Regardless, the dirtier the cooling system, the worse the heatercore performs, and the more often it needs replacing.
 
#22 ·
And now for something I haven't read yet.

A clean heatercore in good shape still can't do its job if the air meant to go thru the heatercore somehow avoids it.

My admittedly meager experiences with
a) deleting my AC relay and using the 'MAX' setting to recirculate the cabin air through the heatercore again
and
b) studying the differences between RPO C60 manual HVAC, RPO C67 electronic HVAC, and RPO C68 automatic HVAC
lead me to think of 2 possibilities, there could still be more.

1. Our heatercores are supposed to include a rubber / foam ring. If the ring is missing, corroded, damaged, flattened, etc, air cheats its way around the heatercore. Obviously, NOT the heatercore's fault.
2. If your car gets good heat at relative air speeds of 0MpH, but either a stiff wind or forward motion quickly reduces the heating effect, then the problem may be somewhere inside the HVAC.
Seriously consider taking apart the dashboard and instrument cluster, the temp blend door would be my 1st guess, but there may be more to it.
 
#23 ·
[SNIP


My admittedly meager experiences with
a) deleting my AC relay and using the 'MAX' setting to recirculate the cabin air through the heatercore again...
Are you referring to the HVAC selector, or the fan/motor speed selector?

If it's the HVAC selector, I have never thought about doing that, because I'd like to recirculate the air with different output locations than what the MAX setting does.

I like using the MAX setting for recirculating the air in hot summer days, so the system can better keep up.
 
#25 · (Edited)
I flushed my car's cooling system and specifically the heater core. I got a lot of sediment from the heater core, and heat came back nice and strong.

I then filled the coolant, and when I checked it the next day, I found that the reservoir tank looked brown and gunky inside. I flushed the coolant again, and what came out of everywhere else looked pretty much fine. Upon closer inspection, there are a couple of chambers in the reservoir tank that have sediment built up in them.

I filled the tank part way, held it up, and shook it, at least half a dozen times. I also blasted water into the fill hole on the top and the drain hole on the bottom at least that many times, too. I kept seeing a little bit of sediment in the reservoir near the fill hole. Eventually it was mostly gone, but holding up the reservoir to the light, I could still see some sediment in those two chambers. It looks like it's on the top of the reservoir, not on the bottom or the sides. Presumably when the coolant level gets high enough (like when I fill the reservoir to the brim right after flushing it), that encourages some of the sediment to break free.

How do I get that stuff out?!
 
#26 ·
I've read about folks using ice chips, as well as marbles to act as an aggregate to help break up the residual sludge/contamination in the tank.

You could always use a mild degreaser and soak the tank for a little while if you have the time.

The last choice would be to take it to a machine shop and have them 'Cold Tank' the reservoir. The cold tank used the same degreasing agent (Citri-Kleen) as the old school hot tank, except it wasn't heated. We would clean valve springs, pushrods, rocker arms, valve covers, timing chain covers, oil pans...etc in the cold tank. I would think this would be a reasonable cost of ~ $20, but who knows as I stopped doing this work 25 years ago after destroying my back in the process.
 
#27 ·
It's winter, and my heater output is back to "barely adequate". I'm sure that more of the crap from the reservoir has made its way into the heater core. I even had the reservoir off back in October, and spent a bunch of time attempting to clean it. I would consistently get small amounts of particulates out, but never enough that it appeared to make a noticeable change in the deposits deep within the reservoir.

New reservoirs are under $100 on Ebay and Amazon (and other places), and that's including the cap and the sensor. I could probably get $40-$50 for my used one on Ebay (since it's in great physical shape) from someone who's got the wherewithal and perseverance to get it cleaned out.

Of course, with it being below freezing, that complicates working on the cooling system--don't want to do it in the garage and make a mess; don't want to do it outside when it's so damn cold. But regardless, I think it's time to just get a new reservoir.
 
#28 ·
As a note, I thought I'd mention that I have a 160' thermostat installed and when Temps we're in the 20s before Christmas, I had 125° Vent Temps.

Have you measured the temperature coming out of your vents for $hits & giggles?
 
#30 ·
As a note, I thought I'd mention that I have a 160' thermostat installed and when Temps we're in the 20s before Christmas, I had 125° Vent Temps.
Have you measured the temperature coming out of your vents for $hits & giggles?
I've used the eternally-effective "hold my hand in front of the vent" method. Back when I last flushed the heater core, the air coming out of the vents was hot--not sure if it was 125*, but it was almost-uncomfortably warm.

My hand says that the current output is closer to lukewarm--the interior warms up eventually but it takes a while, and back seat passengers (if I had any) would probably still be chilly.
 
#29 ·
you need to drain the block remove the KS then hose out the block until clear. then on the plastic pressure tank you use an acid cleaner a cleaner used for swimming pools and vinyl .

allow a few hours for the acid to eat at it .block all the holes.

if you use no distilled water then I understand why this is cold temp on heater out.

my 96 the air temp at vents is 145F up here in the cold north and I do have the 180F stat.

the next 6 days temp will be here 15-0 F so I am not stupid to install a 160 F stat.. perhaps the impala princess will not be used during these extreme temps ...

also you buy 100% antifreeze for about $12 .. then you go to food store and get 2 gallon of distilled water for $2 not $12 ...

I use 65 % dexcool in the impala but on my older vehicles green coolant same deal 65%..distilled 35%... no issues
 
#31 ·
Fairly certain from reading back over this thread that your heater core likely still has a lot of built up debris that isn't ever coming out. Heater core probably needs replaced,and better maintained in the future. Mine's a 160 stat,and has plenty of heat...
 
#33 ·
his problem returns because of the water he uses ...heater core as he stated works good after a cleaning then it stops putting out heat after a period of time.

also when you flush out the core and you have flow dah there is no restriction ..LOL..

he should put a few coolant filters in the system and replace them every 1000 miles ....rather than use the distilled water and not do a complete block drain ..
 
#32 ·
Every time I've flushed the heater core, the water looks crappy for probably a good 10-15 seconds. I hit the output side first, then the input side, then the output, and finally the input. When I'm done, the water that comes out looks clear. There is an immediate noticeable improvement in the heat output. I'm pretty confident that the heater core itself isn't bad or permanently plugged up, but I'll post up after I get the new reservoir and flush the system.
 
This post has been deleted
#34 · (Edited)
RMI-25 appears to be intended as a corrosion-removal additive.
FAQ | RMI-25

It states that the corrosion gets converted to soft materials that get caught in the coolant reservoir. Maybe somebody had previously used RMI-25 (or something comparable) and the deposits in my reservoir are the results of that cleaning procedure.

There is no corrosion currently in my Caprice's cooling system. The block was recently rebuilt, the radiator appears clean and, well, I have no reason to suspect that the heater core is in different condition than the radiator.

RMI-25 says to only use mineral-free water if running straight water in the cooling system, but it doesn't specify if mineral-free water is required if running a water / coolant mix. When I did the coolant in my Mustang a couple months back, I bought a bunch of distilled water and that's what I used, but I don't recall if I used distilled water in the Caprice.

Are you suggesting that RMI-25 would remove the deposits from my reservoir?
 
#35 ·
IDK how a solid corrosion material that chemically gets "softened" and winds up in the reservoir "evaporates".

I would think you would have to conventionally flush the cooling system once you run this stuff. Kinda like the radiator/cooling system cleaner (Prestone) you pour in and drive for a period then flush normally. The crap just doesn't "dissolve" within the system...unless I am missing something about this RMI stuff...

maybe it would work using hot water and pouring into a removed reservoir and repeating to clean the reservoir once removed.

Maybe something like CLR would work also just for tank...

19 years ago the dealership flushed my Dexcool and replaced the reservoir under warranty to the green coolant. The Dexcool turned into brown sludge due to its chemical reaction to the anti leak pellets GM put in 96 cars
 
#38 ·
19 years ago the dealership flushed my Dexcool and replaced the reservoir under warranty to the green coolant. The Dexcool turned into brown sludge due to its chemical reaction to the anti leak pellets GM put in 96 cars
you 19 year ago coolant dealership experience with the green change over from dexcool formula #1 that will cause what occurred to your coolant system..

!
No Jcat...the dealership did completely flush the Dexcool. Removed, and replaced, the knock sensors and the reservoir tank. The brown sludge was due to Dexcool...not switching to green. The service mgr. went into detail of the tedium they did to "completely" flush system by back flushing block, radiator and heater core separately. My car was one of many they did this service procedure on under warranty.

Caltrans is the Calif State agency that does road repair. My neighbor is a mechanic in their fleet yard and said they had to change their fleet of Dexcool vehicles to green. Brown sludge clogging cooling system and plastic cooling parts getting brittle, and GM reimbursed them for the procedure.

50k mi later when I pulled the motor for a H/C install the block was pristine as was the rest of the cooling system. I have since installed a new motor and the cooling system remains clean with my original heater core.

FWIW I have a 160 stat and EWP...heater works fine....although rarely used here in SoCal

cooling system is not immune to clog using conventional green or Dex3 if either of those have not been maintained but Dex will last longer. YMMV
 
#36 ·
you 19 year ago coolant dealership experience with the green change over from dexcool formula #1 that will cause what occurred to your coolant system..

the block has to be completely drained and flushed out as well as the pressure tank . then put in back in those days, legal the prestone acid flush .. run it for about an hour then drain everything again flush it all out then install the neutralizer repeat the process of drain flush then install the green coolant . a very timely process which I am sure the dealership did not do . so you had the system get mucked up and we all know mixing these is a bad day, takes even more work to remove that mix damage..

we are now in the dexcool formula #3 spec.. #1 did eat at the plastic GM engine parts .. GM got sued ,, intake manifolds got eaten as well as other parts ..that is why #3 formula was created.

when people buy used cars/trucks and the previous owner changed the coolant from OEM that adds to the pain and if a person just brings to a service center for maint. they will install the wrong coolant !
 
#37 · (Edited)
What am I suggesting?

Add [an] 8oz [bottle] of RMI25 to your cooling system. Full stop.
It's not nearly as effective as a triple-backflush and/or chemical cleansing, but adding RMI25 to the cooling system 2-4 weeks before any kind of backflushing helps get a lot more contaminants out.

Frankly, shouldn't've bought the 2.56 wagon in my sig in 2004.
Among other things not pertinent to this thread, when I 1st flushed it, it looked like bad coffee, rotten cream clumps, and rusty cornflakes.
Back-and-forth-backflushing the cooling system [til the water ran clear] only lasted 6 months. There was much less, but always some considerable contaminant buildup in 6 months.
That, plus the fact that heater performance always improved afterward, was enough to make me feel like another few rounds of back-and-forth-backflushing in 6 months seemed like a good idea.

In my case, I think I was making the mistake of not backflushing the engine, radiator, and heatercore SEPARATELY.
Despite removing lots of contamination, obviously, some/enough of it was just migrating to some hiding spot in the system (lower block? heatercore) to continue to be a problem.

However, in my case, backflushing each component separately was just not logistically feasible, nor was installing a coolant filtration system (the ones using rolls of toilet paper are ingenious).

Adding 8oz of RMI25, however, is almost as easy as farting.
6 months later, the next backflush was almost perfect.
Another 8oz of RMI25, another 6 months, a perfectly clean backflush.

The OTHER contributing factors to my wagon's not-quite-impressive heating performance were:
a) lacking a foam insulating ring around the heatercore
b) hot/cold blend door not seating perfectly
c) missing that little plastic variable restrictor thing
d) all of the above in unison

Neither my 160° tstat, nor my HDMeziere H2Opump were nearly as responsible as 'a', 'b', 'c', & 'd', my heatercore was simply not permitted to do its job.

Nonetheless, RMI25 is one of the few additives that I can't recommend enough.
Almost universally recommended over at bobistheoilguy.
 
#39 ·
I have in my 1996 IMP dexcool . purchased it NOV 1995. still dexcool. @20K miles on it when the water pump starting weeping and still under warranty I did the work and purchased the parts NOT a GM dealership .. so what happened was the coolant did not create any black goop in the system reason is I used distilled water/drained flushed the block .. reason I use distilled water is because my 1983 GM sedan had green coolant and when I used drinking water in it I noticed radiator corrosion and scale build up .. I having family members with a repair shop told me use distilled water no drinking / tap water that was in 1988.. since that time never had any cooling failures or any coolant looking like muck .

the other problem that will also create this is having a repair shop add the wrong coolant .. that happens every day .

there are today many types of coolant dodge/GM/toyota/VW etc... you mix these not a good day ...you have to use the exact proper coolant Dodge is really tricky as some model years use different coolant ...

just so you know there is a TSB GM stating do not use these coolant sealant pellets as it will cause radiator/heater core restrictions . last time I did my WP and the opti when I flushed the heater core core was clean but I saw small pieces of those 22 yr old pellets .. not much but surprised still there .. coolant change 4 yrs most times complete 4.25 gallons ..
 
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