Coolant leaking from 1994 Sonoma. Is not near the water pump or radiator, but more towards the back of the block. What might it be?

Check the radiator with an inferred temperature gun. Check each tube to make sure that all tubes have the same temperature. If they don't, then you have a radiator that is partly clogged.

Well, if the coolant is leakimg from the HVAC drain, then you need a heater core, no matter what. You really have to jack it up and look, but I can give you a couple of pointers......

1. The water pump is leaking. That happens a lot. The coolant is leaking near the front of the car, near the center, but ever so slightly towards the drivers side. You need a special tool to replace it and I think gm has an updated cover and gasket.

2. The water pump housing is leaking. The gaskets that seal it to the block are made of plastic and deteriorate often. The coolant will be leaking at the center of the car, maybe up by the water pump, or back between the engine and transmission.

3. The heater hoses. I have only seen this 2 time, but the metal pipes corrode along the fire wall.

4. You could need a radiator. I have seen the side tanks leak a few times.....


Just A couple of places to look, but you have to get under and look....

Vehicles: Any with the above symptoms

A cracked cylinder block will cause either:
(a) motor oil contamination of engine coolant
(b) coolant contamination of motor oil
(c) white exhaust smoke, due to coolant seeping into one or more cylinders.
(d) more than one of the above
(e) all of the above

Oil infiltrating into coolant is easy to see - drain some coolant out through the pepcock at the bottom of the radiator, and place it into a styrofoam coffee cup. Oil droplets floating on top of the green (or orange) coolant are easily seen. Or visualize oil by looking into the top of the radiator through the radiator cap.
Coolant infiltrating into and mixing with motor oil will permanently damage the engine (seizing it up through loss of lubrication), and must be prevented.
A leak from above the front suspension is, as physicians like to put it, "nonspecific", with the most likely cause a water pump seal or hose/hose connection.
A leak from near the fire wall will usually be a heater core hose, or hose connection.
A coolant leak on the same side of the engine as the water pump is a leaking water pump or water pump seal until proven otherwise.

To rule out everythng else, here's the 1999 Honda CR-V Troubleshooting Guide for Coolant Loss/Coolant Leaking:

Priority Action Part Type Cause
1 Inspect Water Pump - Worn, Cracked or Leaking Water Pump, or Water Pump gasket.
2 Inspect Head Gasket - Leaking Head Gasket.
3 Inspect Radiator - Leaking Radiator Hose(s).
4 Inspect Radiator Cap - Worn or Damaged Radiator Cap.
5 Inspect Radiator Hose - Ruptured, Cracked or Leaking Radiator Hose.
6 Inspect Freeze Plug - Leaking Freeze Plug(s).
7 Inspect Intake Manifold Gasket - Leaking, Worn, or Damaged Intake Manifold Gasket.
8 Inspect Water Outlet - Cracked, Leaking or Damaged Water Outlet.
9 Inspect Heater Control Valve - Leaking or Faulty Heater Control Valve.
10 Inspect Radiator Drain Pepcock - Loose, Damaged, or Faulty Radiator Drain Pepcock, or Pepcock O-ring.
11 Inspect Engine (DOMESTIC ONLY) - Cracked Cylinder Block Leaking Coolant into at least one Cylinder, causing white exhaust smoke.

In a properly working system, the coolant expands as the coolant temp rises. The excess coolant flows into the overflow container by the hose that runs from the bottom of the overflow to the top of the radiator, under the radiator cap. When the coolant begins to cool down, it contracts,drawing fluid back into the radiator from the overflow. The water pump has a "weep" hole located near the bottom of the pump housing that will leak coolant if the pump is going bad. In some engines, the 3.0, the water pump is located under the camshaft timing belt cover, and is a BEAR to deal with. However, the coolant should be pulled into the radiator when the engine cools, so check that the hose from the top of the radiator is attached to the bottom of the overflow and that it is not blocked or pinched. Have the system pressure tested to verify that it has no leaks. Hope this helps. Dano

If the leak is not to bad, you can add 3 table spoon of common kitchen table black pepper. The pepper will plug up the hole and when it's time to clean up, it's just a simple flush and the pepper will not plug up any water passageways. Try not to use any radiator stop leak, they end up restricting the water flow and your going to have to use radiator flush to get it all cleaned up which can also damage the heater core creating more work for you. Good luck and hope it's a simple fix once you have some time. Keep me posted.

is it out of a flange or out of the block?head gasket? if its a leaky flange they usually have o rings that tend to rott out after 100k plus.and they make this stuff called stop leak it works real well,and if its real bad and your in a pinch the egg always works

Replace the hose from the radiator to the reservoir.and make sure the reservoir is not leaking.

Yes, there is a "bypass outlet" coolant hose that connects from the water pump inlet pipe (metal pipe that runs from back of water pump along engine block to lower radiator hose) to the intake manifold.

I am not sure about the "black box" - not enough detail.

try to run the water pump without hoses, and add water in the 'in' side, if is powers out the other side, then it's fine. how is your water/coolant mix?

i cant tell you exactly what it is cause im not ther but you have to go under the car and detect where the leak is coming from....try radiator stop leak if it is a smaller leak it will stop it for a couple weeks. if it is a large leak as i think ur saying go under the car get a rag and clean off the fluid. keep on looking towards the top(on water pump, on engine block by intake manifold)and see where its coming from...i had the same problem it was my water pump...if its the manifold and its small get hight temp RTV sealer an try that....could be loose hoses or crack in radiator...you gotta look. good luck