Here are the quick facts:

1994 Mazda B4000 4.0L, 2WD, 5-speed, 109k miles on it. Well maintained.

Runs fine, has not lost any power at all since overheating issues started.
No visible oil in coolant.
No visible coolant in engine oil.
Does not loose coolant.

Radiator is 3 years old. Still looks like new. Clean inside, except some rusty looking gunk built on the radiator cap.
Radiator hoses are 2-3 yrs old.
No coolant leaks anywhere.

Originally, Fan clutch, fan blade, thermostat and antifreeze were replaced in 2011, only 10k miles ago. (yes, this truck is not driven much)
I used NON-OEM parts.

Fast forward to few weeks ago, .. we noticed that when we drove the truck in heavy traffic for longer periods of time... the temp gauge will start to creep up. I had to pull over once because it was going pretty high. The rest of the short local drives, there were no issues.
Then, on a recent trip to local recycle yard, it started to overheat again, going uphill.... going downhill and...even going 55mph on the freeway... temp was on the high side.

This is when i decided that it is time to replace some cooling parts.
I bought and installed a brand new water pump from Napa, new thermostat from Napa, and new fan clutch from Oreilly Auto parts (Murray brand).

Drained the radiator, replaced pump, thermostat, fan clutch, and one of the hoses going to heater core. Put brand new 50/50 coolant mix. I burped it all and took it for a drive.

Well... truck overheats again. :( City driving... or highway driving... it gets above middle level on the temp gauge. If i let it go close to "H", i start to hear/see hissing and some steam from the radiator cap. The gauge is not lying when it shows close to "H". It does overheat for real. I burped it more... 3 or 4 times. Still no luck. If i drive it for more than 10-12mins or so, it starts to go above mid level on the temp gauge, slowly.

So... i decided to completely remove the Napa thermostat, since i read about crappy new non-oem thermostats, getting stck closed. Same result. It still gets hot.

****
Something VERY interesting i noticed is that the rubber hose that goes from the new water pump to the heater core, almost DOES NOT have any coolant coming out of it when the truck is running. ?!?!?! I removed it from the heater core with the engine running and it was barely dripping antifreeze. Why is this happening?
I am pretty sure the water pump is moving because i can see antifreeze circulating inside the radiator when the engine is running and i remove the radiator cap.

I noticed that the new fan clutch i installed is spinning a bit too easy...regardless if the truck is cold, warm or overheated. It does NOT seem to lock at all - ever. So, today i went to parts store and requested a replacement for it. Got the same brand (Murray), another new fan clutch. This one is stiffer, but it still does NOT lock completely when the truck is hot and the truck still overheats slowly, like before.



Basically, to summarize, i've replaced the fan clutch 3 times now. I have a new water pump installed, newer radiator, i am running with no thermostat at all, and my truck is still slowly overheating... and for some reason my heater core hose does not get any pressure/coolant !?! Yet, I do get heat in the cabin and the AC seems to work fine.

Btw, using a garden hose, i ran some water through the heater core while it was connected on the other end to the engine and water did come out of the water pump side heater hose. No clog detected. Yet, when the engine is running, nothing comes from the heater core hose attached to the water pump.


What am i missing here?! I've been dealing with this old truck for 10 days now... and i am pretty much running out of ideas. Any help is well appreciated.

Thanks!
Dan

 

Did you replace the radiator cap

 

Heater core is the cooling system bypass on the '94 4.0l

Just to test, remove heater core from the system, so either remove both hoses from heater core and hook them together or, easier, remove one heater hose at the core then remove other hose at the engine end and loop the first hose there, so "clogged up" heater core is not being used.
Coolant should shoot out of one heater hose and it is sucked in on the other so depends on which hose you tested, as to what you would see.

My '94 did the same thing, replaced most stuff you did, then read about the heater core.................$28 later with new core I was good.

Thermostat needs to be in, always, and use 192 or 195degF thermostat.

 

No. Rad cap is not new.

I bought a new aftermarket cap, from Oreilly auto parts....but was fitting very loose, not a good fit, so i put the old one back.


I am thinking to go to the local Ford dealer and get an OEM cap and OEM thermostat.

 

Sounds like an idea.

Could the transmission be causing these symptoms possibly?

 

Not a manual trans.
If automatic then also probably not, trans fluid should run about same temp as rad coolant, a little warmer when towing.
If trans fluid was running hotter then you would be complaining about shifting.