Jim Grant's Tech Tips
'93 Mazda 323, Head Gasket?
'93 Mazda 323 with only 75k miles suddenly overheated (temp went to max)
at about 85MPH freeway speed. I stopped 2 minutes later at the nearest
gas station, hot air (not vapor) was coming out of the radiator, and I
had to add 1/2 gallon of coolant. After 1 week (about 100 miles) I had
to add another 1/2 gallon of coolant, and the remaining coolant in the
expansion bottle was clean. After another week , I noticed white exhaust
smoke when I started the engine or when I accelerated from idle. At the
same time a noise like valve clicking was heard for about 2-3 seconds
when I accelerated. When I checked the coolant, there was oil in it, but
no coolant in the oil. After another 100 miles the engine started to stall,
so I checked the compression. It was equal on all cylinders, but the spark
plug on cylinder #2 was shorted by carbonized oily deposits, so I replaced
the plugs and all looked fine. After about 70 miles, spark plug #2 was
dirty again. The engine was losing oil slightly (about 1/2 pint per 200
miles, half of that appearing in the coolant) and the oil was just lightly
blackish, so I replaced the valve stem seals and cleaned up the plugs.
Everything looked fine for the next 60 miles, then the smoke came back
and the plug was dirty again. The engine has never overheated again since
the first incident. So having burning oil and losing coolant, I suspected
the head gasket and took the head off. The gasket looks fine except it
came off easily next to cylinder #2, but no burn marks neither on the
gasket ring around the cylinder, nor on the head or the block. So it is
not obvious that the oil and coolant were entering the cylinder past the
head gasket, and the oil was mixing with coolant because of the gasket.
The head doesn't have visible cracks, neither in the compression chamber
area, nor elsewhere. Now I have to check piston #2 rings for burning oil,
that makes sense, but where was oil coming from into the coolant? Is there
a crack in the head, in the high pressure oil line? Have I overlooked
With an overheated engine and engine oil entering your cooling system,
I would be questioning/inspecting the cylinder head more closely. Why?
The cylinder head is the weakest link and it has coolant and oil passages
that are very close to each other. Due to the location of these passages
it is not a problem that can be seen. To diagnose this condition will
require pressure testing the cylinder head. Be sure to tell the machine
shop that is performing the pressure test about the engine oil entering
the cooling system. That way they will focus on oil passages. One other
item. Does your vehicle have an engine oil cooler? Engine oil coolers
use engine coolant to transfer the heat. If the cooler is leaking internally,
you can have engine oil in the coolant too.