My Car , my Suzuki SideKick ® is an Ice Berg or like a block of ice. 
The Cab heater, has weak heating. MPG at horrible?

The ENGINE  runs cold, the cab heater don't work or is weak?     Cut to the Chase and get the right thermostat !
Why?
Read on;
I will call the Radiator the RAD, and the thermostat the STAT.  (yes, lazy)       For those with a leaking CAB HEATER CORE SEE THIS.
brrrrrrr-caption

I cover all the possibilities on my overheat page, but this page focuses directly on the frigid  problem.
You lack cab heat from your cars interior cab core heater box.
Safety: Never remove the Radiator cap on  a hot engine.

First off, lets tick through all the possibles of cab heater failures.  (CHECK ALL fuses first , if fan fails)
If the motor can't hold 180F or more , fix that FIRST !  Use an IR gun , to the STAT housing. (gun will  read  about 10f lower)

  1. Running a G16 thermosat in the J18/20 engine, or J18/20 STAT in the G16,  tell you parts seller he /she is on drugs. (if you use the wrong themostat it will surge, hot /cold)
  2. The motor is not running 180 °F  or above ( 180,192,195° are the three sizes of thermostat)  Here is mine reading 170° ,normal.
  3. The heater core in the cab is blocked (clogged up) inside, to lack of system flushes every 2 years. (1.8L this KILLS THE STAT  operations)
  4. The heater fan is dead or the fuse to it, or the fan switch is bad or last the fan motor resistor block in the fan housing is blown open.
  5. if the cab fan good, skip that ,but note the fan must work on high even with a bad resistor block.  See 95 Schematic for wiring on FAN.
  6. The Thermostat died after 5 years and now sticks wide open, or randomly as all OLD stats do.... GET A STANT ® brand STAT.
  7. The last person who installed the thermostat did not read the TSB and used the wrong gasket. All Suzuki manifolds changed in 92.
  8. The clutch fan is seized and and over cools the radiator at all times,  it must free wheel, that fan ,when it is cold outside.
  9. Thermostat is floating up and down in the newer 92+ manifold and most of the coolant is just sneaking past it. See TSB link above.
  10. The heater box DAMPER is stuck closed.  (no coolant water valves on Kick core water feed lines, like other cars , only a damper.).
  11. The cable that operates this Damper is bad, broken or has fallen off at either end or the sheath is not clamped down. Rendering  damper useless.
  12. The Bottom of the block cab heater hose output hose fitting is blocked, because the water jackets are full of mud. (rust, scale,sludge,etc)
  13. The Cab heater core return line is blocked at the suction port.
  14. It's just too cold out side and the radiator gets too cold,  below 0 °. F, (-18F) [about], one can just block 1/2 the radiator (front) with cardboard.
  15. If using Recirculaion mode of heater , the car will get hotter faster, but with A/C missing , windows will steam up fast.  Check Recir, damper valve .
  16. If dash air flow is restricted (cab fan  flow seems weak), see flex hose #46 below and look, and vacuum out this debris and check for flow from the fan into #46 port.
  17. A classic slow thermostat is shown here, see Aqua  blue line,  5miles driving. and  slow as slow can be.... and sucks extra fuel ever inch of road driven.
  18. The heater core leaked and the PO (prev. owner) bypassed it.
  19. In colder Climate run A new STANT 195F thermatat.
  20. Themostat is upside down, or bleed hole not up side? (high side) or wrong thermostat,  per #1 , the bypass tubes are clogged?
The above seems complex, but it is not.

It takes only 3 things, to be warm driving.

  1. Engine gets up to full temperature. 180F (82C) minimum ! MUST HOLD FULL TIME once reached, NOT EVER VARY.
  2. You have perfect flow through the cab heater core. (no blockages)
  3. You have a running heater (HVAC) Fan, at least on the HIGH setting and air flow is brisk. Hint: try all modes, to see if the  recirc  mode, runs better than vent mode?
The coolant forms a loop and the heater forms another , if either loop is blocked by scale, mud , or rust or other junk the flow will be restricted.
Low flow causes poor performance.
Poor heater op's is caused by , lack of main loop water temperature (180f min) or lack of flow to heater core or blocked core, or lack of air flow through core.
Troubleshooting:

Here is the thermal gram of my 1996, 16v sidekick using the IR gun.

Note the 10 deg. thermal drop on the heater hoses, at 60F ambient.  On a really cold day this drop will be more.

If the fan in the cab runs, then good, proceed on to the next tests , but if it don't , you will have  to repair that now.   The Schematics are all here.
The first test is the check the temperature of the thermostat housing with an IR gun.  (it should be 20 deg less than the STAT rating, here is 195 F stat.)
If you can't find , barrow , rent or buy a Harbor Freight IR gun for $10 then you must use  the old shade tree tests.
Drive car at least 10 miles and get it fully hot. (the hotter the better)
Now, place your hand on upper radiator hose ( Outlet hose from engine) and fell it.  if you can hold your hand on the hose then the thermostat is working.
Warning stay away from the fan.
If the hose is nice and roasting hot, (way hotter than shower/bath water 105F to 120F) and feels too hot, then  the thermostat is working.
If the thermostat is working and the motor is hot and the coolant is hot ,then  you have core failure.

By core failure I mean the core is not flowing coolant as it should.
If I had that problem , I'd immediately do my Fall semi annual coolant service which includes a full flush of the system and the engine block and I would take of the 2 hoses on the heater core circuit and back flush them forcefully.  Make sure they  are free flowing.
Force clean water from the tap to  each heater core hose 1 at a time ( not full pressure ,just a steady gentle flow) 
If the core tubes are blocked and you can't get them clean this way , there is no other  way to fix this issue other then removing the Core using this link page, for HVAC.
The only trick I know to help clean a core is to find real radiator cleaner, that is now banned by the freakn EPA.  but can be found commercially.
The new EPA flush is the useless orange oil found as solutions to all things , it seems.

It is a caustic soda.( mix some up and Fill the core with it (plugged up at the hoses) and let it sit for 1 hour.  Then flush it again.
safest is backing soda.  (  these are what others have done, not me, nor am I recommending it, this information is just history)
Then sodium hydroxide (also called LYE or soda ash)  in the form of DRANO, that has the aluminum chunks in it , that is sacrificial. (eats scale and not pipes).  
This can wreck the pump seals , if they are weak.  Todays cleaners are orange oil based and are near useless.
I'd do this to a  RAD , sitting on the deck, but not the cooling system, total.  The water pump don't like this trick !

With engine dead cold, can you spin the fan?, and not feel the  fan slip?  if its jammped up, it will be a fridged motor. Replace fan clutch. (if leaking its bad too)

Electrical:
If the fan don't spin, check the fuse to it and check that resistor block for open resistors. (use  an ohm meter , with power off (key in pocket)) (the resistors, good, will be shorted)
Check that the Fan switch works (it is just a simple mechanical switch) ,  12vdc power enters and exits on the speed pin you have selected.  Nothing magical here at all.
If you have 12vdc up  to the fan and it don't spin like crazy , it is bad.


Cables:
Just like a bicycle  or motorcycle, the cable sheath must be clamped to the body so it can move or all bets are off.
The cable center wire must move with the lever at the lever.
The cable center wire a the "business" end must also move with the lever.
don't the lever move trick , you can hear the damper pop closed with a loud THUMP.
if not? , the damper is INOP!

Thermostat:
As stated in my overheat page that there is a TSB (trouble service bulletin) covering this issue.  Read the TSB all Suz. PN's are there too ! look.
In 1992 (or near) the Intake manifolds were changed on all 1.6L (1.8L all have this) engines.
The Thermostat well  1.6L  was increased from 1mm to 4mm (deep well) in the mid 90's , and use a simular seal like the 1.8L
If you fail to install the thermostat with the below O-ring surround seal the Thermostat  will float, if it floats you will freeze. (seal is about 5mm think)
The 1.6L  thermostat bleeder hole must be to the front of the car and all air burped from the top of the engine. (happens automatically with the bleed set right)
1.8L the bleed needs to be UP.  1.8L is called a SPORT Suzuki.
1.8L the  must not have a clogged or reduced cab heater core, or tubing or the return path to the STAT will fail to hit the STAT thermal sensor, and the STAT WILL FAIL.!

See 1.8L coolant path details here.     For 1.6L paths  look here.

Air behind or below any STAT will cause the gage and the STAT to FAIL.
Rules: 1mm well = paper gasket only,  4mm well = oring ONLY, never ever both.
Rim well is the thermostat  housing (lower)base rim. outer top edge, do not run paper on a 4mm rim or it will run  frigid. (and not hold 195F)





I hope I didn't miss a cause and that this page helps in some small way. 

Happy Kicking !
coreview

92-95 typical HVAC box.  (Cami or Canada made). N. American autos.
rev:3  12-12-09  (3-2015 added J18 horrors and link)