Slow coolant overflow

[olddesertrat]

Hello Everyone. Have a 1997 K1500 Z71. 200,000 miles on truck, bought it a few months ago with a new 5.7l vortec crate engine in it.

Have checked threads here and elsewhere. Can not find my problem, so thought would give you guys a try.

Problem: Coolant is slowly filling up overflow tank as I drive. One core radiator...so as overflow tank gets near full, the truck starts getting hot. Does not appear to loose any coolant. Burp system from air bubbles. With radiator cap off and system warmed up, no bubbles after burping at idle. rev engine, coolant level drops..no air coming out open radiator cap. Use 50/50 coolant/water mix.
Just needed to drive 550 mile round trip from Las Vegas, NV to Phoenix, AZ and back. Right around every 80-90 miles the temp gauge goes to around 210. Soon as possible pull over, stop truck. Check overflow tank, near full. Bleed air out Stant radiator cap(have replaced twice, did not fix problem). Engine not steaming with cap off. Take empty coolant bottle, put under battery. Disconnect overflow line from radiator and put into empty bottle. Wrap paper towel around overflow line on overflow tank and pressurize tank by mouth. Coolant in overflow tank flows into coolant bottle until overflow tank at correct level. Pour coolant in bottle back into radiator. Turn on truck, burp system. Close everything up...drive another 80-90 miles...repeat. Repeated this 7 times in 550 mile trip.

Again, I stop truck before it overheats. And does not appear to loose any coolant. Just slowly empties radiator by filling up overflow tank at normal operating temp. Have replaced Stant stainless steel 15 psi radiator cap twice..no change. Water pump was not replaced when engine was replace(how stupid is that?). Have a new water pump and will replace it. Also thinking of getting 2 core radiator. Am hoping these will fix problem....however would like to hear if anyone else has seen this problem and can tell me the problem is.
Also, what 2 core radiator would you recommend?

Thank in advance for any help

 

[magimerlin]

Well first off 180ish to 210ish on the gauge is factory temp on almost any small block... second gauge could be off a bit... does not sound like it's overheating...

I'm still leaning towards radiator cap area.... yes I read you replaced it twice... check the area it mounts to. the gasket area that covers the hole in the lower part of the radiator cap area... if that isn't sitting correctly the water will bypass that and will drain into the recovery tank..

sent from what use to be a great country...

 

[LSUfan71]

Your gauge should show the high temp at 260. Water boils in atmospheric pressure at 212º F, and your radiator will be at atm plus another 15 psi, so 210 is not a problem. A new water pump can't hurt.

 

[olddesertrat]

Well first off 180ish to 210ish on the gauge is factory temp on almost any small block... second gauge could be off a bit... does not sound like it's overheating...

I'm still leaning towards radiator cap area.... yes I read you replaced it twice... check the area it mounts to. the gasket area that covers the hole in the lower part of the radiator cap area... if that isn't sitting correctly the water will bypass that and will drain into the recovery tank..

sent from what use to be a great country...

Thanks for reply Magimerlin:

That was common sense I was looking for. Forgot to mention that when doing city driving, when truck is stopped, inside cab you can clearly hear loud gurgling in overflow tank for a few minutes. Also, coolant in overflow tank does not drain back into radiator when engine cools down, like it should.
All these problems point to radiator cap not having good seat. So looked at radiator filler where cap gasket sits.
Result: Right at neck, area has sharp ridge. Also different color areas. As if painted different colors, then colors started chipping off. Thought that is strange, as radiator filler is black plastic.
took some fine sandpaper and sanded around neck ridge. Turned out to different colored layers of tar build-up. Sanded down to black plastic. Ridge around neck still there, now just wider for more area to gasket to sit on.
Test drove 10 miles on highway, stopped. No gurgling. Got out, opened hood. Right next to overflow tank could hear very low bubbling in overflow tank. That would likely be normal overflow of expanded coolant into overflow tank after driving.
So looks like sanding off tar gave better seating for cap gasket and likely fixed problem.
However......Former owner did many very stupid things to truck. Those different color layers of tar buildup worry me. Good chance former owner put radiator leak stop into radiator and those layers of tar buildup are also inside radiator cooling fins. With 200,000 on radiator, will just go ahead and replace it with a 2 core.

Side note: Realize the temp #s on instrument cluster temp gauge is not accurate. Scale start at 100 degrees, three small marks, one dark mark, three small marks and dark mark at 210.
Middle mark should then be 210 -100 =110 110/2 = 55. 100 + 55 = 155 for middle dark mark on gauge. for small marks, 210 - 155 = 55. 55/4 = 13.75. So each small mark = 13.75 degrees. My gauge usually shows one small mark above middle dark mark. That would be 155 + 13.75 = 168.75 degrees. That is clearly wrong. Trunk has standard 195 thermostat, so should operator at normal temp of 195 degrees. 195 - 169 = 26 degrees. So gauge is 26 degrees low. That means 210 on gauge is really 236 degrees in engine.
Too hot, but still below coolant boiling point. That is why no steam when I stopped to drain overflow tank and refill radiator.

Already have new water pump. Does not need replaced at moment, however have no idea how many miles on it. So will replace it to be safe.

Now need to shop for two core radiator.

Thanks again thanks for replys maginmerlin and LSUfan71.

 

[michael hurd]

The gauge may not be accurate. A manual method of measurement can show if the gauge is indeed in error.

 

[magimerlin]

Issue with your temp logic.... thermostat is in the intake... the sensor for the temp gauge is in the drivers side head between cylinders 1 and 3... get a laser style point and shoot temp gun.... then compare to the gauge and the spot where the sensor is..... the tstat is designed to open at a certain temp to allow the coolant to cycle and give it time to cool off in the radiator... that's it... it's like an on off switch... hot opens it, when the cooler water gets to it it shuts, then starts over... that allows time for the hot coolant that just went to the radiator to cool down... then the cycle starts over again....

Dam I need my Internet for my computer.... typing a lot of info sucks on a phone...lol..

sent from what use to be a great country...