radiator question

[gerretw]

I have a 68 chevy c 20 longbed. It was sitting for a long time before I bought it. It has a305 out of a 73 buick in it. After much putzing around to get the temp gauge to work, I finally had one of the posters here mill down a sender to screw into the block. It started working and showed the temperature was out the top - I popped the radiator cap - red red coolant spilled out. I changed the t stat, and it seemed better but erratic. I had the block and radiator flushed, filled with green coolant and the needle stayed on the bottom left. So far so good.

Then, recently I was topping up the oil and I took a look in the radiator, and it was a gallon low. And red, not green. I added a gallon, and I kept driving it, then 3 days ago the gauge shot up to the right -- not in the red, a couple ticks below it. This morning I drove maybe 10 -15 blocks and it shot up again. I was thinking the radiator was plugged up again. That my driving it around for a few hundred miles loosened up a bunch of scale.

Does this make sense?

G

[NGN]

Is your truck a automatic and if so is it possible the trans fluid is leaking through the radiator tank.

[gerretw]

automatic

[jgrote]

Originally Posted by gerretw

automatic

Your radiator is bad, and your transmission is about to be. Google SMOD, and you'll get all kinds of results for modern cars that are notorious for this. (Xterra, Ridgeline, just to name a couple.)

[guyryan100]

Agreed, red=transmission fluid leaking into the coolant

[Accelo]

Fluid goes into the coolant system when driving as the transmission pressure is higher than water pressure.
When the motor is shut down it, antifreeze, moves into the transmission because of the residual pressure in the coolant.
Antifreeze/water attacks the glue that holds the friction material to the clutches.
If it's actually the transmission coils in the radiator the transmission is toast.
Motors are typically OK, but even the smallest amount of water or antifreeze toasts the transmission in short order.

[gerretw]

When I googled smod it was pink, not red. That doesn't seem likely. I think the previous owner used water, not coolant, hence the rust. I'm going to take it to the radiator shop and get it repaired. My hope is that the block isn't damaged by running it with water.

But what is more worrisome, it's losing coolant. Blown head gasket? Freeze plugs? I was planning on rebuilding the motor, but now I wonder about dropping in a used motor. The rebuild shop has a 4 week delivery time, so I will have to deal with that when I have some off street parking.

Ideally I'd like to rebuild it.

G

[72SB]

OP

check your trans fluid...if the coolant cylinder/coils in the radiator for the automatic trans leak than trans fluid gets in coolant and coolant in trans. Both bad.

My $.02 is old radiators are not worth saving. Buy a new one. Flush the motor a few times separate, including heater core. Pull T stat to do this and pull block drain plugs. Get new T stat

Your trans is already likely toast or at a minimum it and the torque converter need to be completely flushed, including transmission lines. Trans shops and dealerships have a pump/flush system for this...although IDK if permanent damage has been done to trans already

Your motor itself should be fine

[dmjlambert]

In the late 70s I had a 70 Olds Cutlass my parents handed down to me. For years would turn the green antifreeze red within a week of flushing and filling the cooling system. I would flush it for hours with thermostat removed and running with a hose in the radiator neck. Nothing I could do and no number of coolant changes would prevent it from turning red pretty much immediately. It did not lose coolant, no leak, no overheating, but it bugged me. I drove it that way for years and other than the rust in the coolant it ran fine. A year or so after I sold it to a friend his dad changed out all the freeze plugs because he thought that would solve that problem with the antifreeze. It did. The coolant was nice and green for years after that. I always thought that was mysterious.

[Ironangel]

Your initial "gallon low" may have been the result of trapped air in the block and expansion overflow from (overfilling) filling the radiator up to the fill neck. An unsealed coolant system such as in these trucks will force coolant out as it heats up and expands, our trucks are notorious for that, and when you check the coolant level again it appears low. Dont be fooled by this, simply run it at the level it stops pushing coolant out. Your plugs will tell you if there is a bad head gasket, take a look at them. If and when you decide to rebuild that motor (I wouldn't rebuild a 305) take those heads to a machine shop for a pressure test and magnafluxed for cracks, those heads are notorious for cracking, especially if the motor was allowed to overheat at one time or another. Change the fluid in the transmission! May as well drop the pan and replace the filter and inspect what has gathered and laying in the pan. When you refill the transmission, before you attach the coolant lines to the radiator, you can attach 24" 3/8 hose to both lines on the radiator fittings. Then drop the return line hose (upper fitting) in a new full gallon or 2-1/2 gallon container, drop the outgoing or pressure line into and empty 5 gallon bucket. Have wifey or a friend start the truck and watch the fluid coming out into the bucket, the moment it becomes clean fluid stop the motor. Thats how you can change the trans fluid and get all of it including what was in the converter. I buy my fluid in 2-1/2 gallon cans because I rebuild my own transmissions, but you dont want to suck the new fluid container dry and you'd be surprised how fast that transmission pump can suck up a gallon of fluid! So have your helper ready to kill the engine. The red your describing is puzzling because in all my years (64 of them) I have never seen green turn red. It's either rust dissolved in the coolant, or transmission fluid. I did find this interesting similar dilemma...Good luck! https://forums.justcommodores.com.au...ed-red.197504/

[gerretw]

I will get the transmission serviced to make sure all is well. I went to the radiator shop - he said my cap is shot which explains the coolant loss, and asked where I bought the t stat. I said auto zone - he said he has frequent customers complaining of t stat failures - he suggested o'riley or NAPA. I drove the NAPA swapped it out and the temp is where it should be. Thanks for your input.

[PbFut]

It is possible the Tstat was acting up from ATF in the coolant. Please have the Tranny checked ASAP. Generally the Tranny Line pressure over comes the coolant pressure and forces ATF into the cooling system. When you shut down, the coolant is still hot and under pressure and the tranny line pressure drops to zero. Coolant is then forced into the Tranny. This goes on until the tranny eats itself up as the coolant increases in the ATF and you loose friction protection. A very costly repair. I had a 06 Trailblazer SS do it to me twice.

[gerretw]

I will but the red color is rust - not oil based. When you look at it close up you see little dots of rust.