Mystery Coolant Leak: '96 Mazda B2300 (2WD)

Howdy folks! I’ve been tearing my hair out on this one. Sure could use some help. My dash temp sensor says my engine is about to go nuclear. Took it to a mechanic, they did a coolant pressure test (including testing the radiator cap pressure) and said everything checked out OK. They sent me to a radiator specialty place. The radiator people said my radiator was fine, so they replaced the ground temp sensor sending unit thingy, but my dask sensor still says engine’s gonna go nuclear. They took the actual temperature of the engine itself and said it checked out fine. I went to the dealership. They tested the actual temp of the engine and said, ‘Yes, your engine is kind of hot after all’ so they replaced the thermostat and its gasket. Dash temp sensor STILL says engine’s going to go nuclear. They tested the dash/instrument cluster sensor and said it’s good; it’s not lying. Dealership sent me BACK to the radiator people, I had the radiator flushed. My dash temp sensor still says, yeahe you guessed it. OK, all this stuff took place about 6 weeks ago. Yesterday I checked my coolant reservoir. NOTHING. Bone dry! So now I’m like, ‘Well, OK, there’s a coolant leak…uh …SOMEWHERE.’ Thing is, I can’t for the life of me tell where. There’s nothing on the ground, no visible evidence of a leak anywhere. I checked the rug on the passenger side for signs of coolant fluid to see if it’s the heater core. Nothing. Could it be internal? What do I need to have done at this point? I feel like I’ve already been to the places most likely to be able to help me. Please help! Thanks, librarylady

You, or someone, needs to do a test to determine if there is carbon monoxide leaking into the combustion chambers of your engine. The coolant may be getting pushed into one or more of the cylinders due to a slightly blown headgasket or a crack in the cylinder head itself. The test I use uses a tool (available from Autozone loaner program) that draws coolant vapors from the radiator or catch bottle through a blue liquid. (You have to buy the liquid, enough for eight tests. Do you plan to continue driving this car till you’re 90?) If the liquid turns yellow as the vapor bubbles through it, you have a leak. It’s probably not to the point where you will see vapor and smell maple syrup coming out the exhaust pipe.

Coolant does not just disappear. It either leaks out, which is visible, or it burns in the combustion chamber, due to a cylinder head gasket leak.

Have someone test for a blown cylinder head gasket.

There is also the possiblity that the coolant is leaking into the cranck case. If this occurs you won’t see any drips. Is there white smoke coming out of the exhaust?

There is also the possiblity that the coolant is leaking into the cranck case.

I agree, but that is pretty easy to check, if your oil looks like a “milkshake” it is a sign that coolant is leaking into the crankcase.

no, my oil looks fine, not milky or diluted


Someone should pressure test the cooling system if it has not already been done.

no, my oil looks fine, not milky or diluted


That’s a good sign, but I agree that you need to have the system evaluated to determine where the coolant has gone. Let us know what they find.