1970 LS5 overheating

[Fred R.]

Please help
She is heating up to 240+ after 30-40 minutes of normal driving. Stays between 190 and 210 then for no apparent reason shoots up and starts spewing fluid and steam out the overflow. Has done the same thing the last 3 times out.
Thanks

[Mark F.]

Re: 1970 LS5 overheating

stuck, or intermittently malfunctioning thermostat ?

[Fred R.]

Re: 1970 LS5 overheating

I tried it in the pot of water on the stove. Seemed to work but not taking a chance on such an easy part to replace.

[Mike T.]

Re: 1970 LS5 overheating

Fred - Any mechanical work done recently? Were the temps checked with an IR gun or the dash gauge only?

[Fred R.]

[Mike T.]

Re: 1970 LS5 overheating

Doesn't sound like anything has changed and since you are losing coolant, the dash gauge is probably fine.
I remember using my old Chiltons and Motors Manuals years ago for diagnosing car problems and they typically listed
all the possible contributing items that could cause the problem with the most likely ones at the top of the list and just like the others have
mentioned, I'd be looking at the thermostat. I know you said you already did the pot of water test and it turned out okay but you also said you
were going to replace it anyway...I agree, why not. When I got my 66 NOM L72 a couple years ago, it ran fine but had a couple of annoying problems that I attributed to over heating. The temp gauge (and IR gun) both showed the temps were reasonable (into the 190's and steady after a 20-minute run) but I found the carbs secondary side was leaking out fuel after shutoff. That turned out to be a defective heat riser. It moved freely when cold or hot but...when hot, the spring never did relax, allowing the damper to fully open. The exhaust gases were being pushed back up and across the intake manifold, heating up the carb. The second thing I came across was after a short run of maybe 20 or so minutes and back in the garage, the radiator overflow hose dripped out some coolant. Not a lot but as much as a cup or so. The radiator appeared to be in nearly new shape as did the radiator cap. The cap was a repro and for the hell of it, I tested it and found it only held '1' pound before opening up. Replaced that cap with a new GM cap I got some year ago and all has been fine since. You're losing quite a bit of fluid so not sure those two items I just mentioned will be the cause but they are relatively easy to verify. Good luck.

[Fred R.]

Re: 1970 LS5 overheating

Thank you Sir!

[Patrick H.]

Re: 1970 LS5 overheating

New radiator cap?
Recently filled the radiator?

I have seen brand new poor quality "restoration" radiator caps.
And, it is common for us to overfill the cars, resulting in them overflowing once parked. They do eventually find their own level.

[Mike T.]

Re: 1970 LS5 overheating

Fred - I reread your first post and am surprised it takes so long before you see the coolant spewing out. If the thermostat were defective, I'd expect the overheating to show itself a lot sooner, like half that amount of time, not to say the thermostat is no longer a 'suspect' but just a few other places to check and good chance you already have checked but here's what comes to mind.
Fan Belts - Water Pump - Fan Clutch - Vacuum Pod on Distributor (faulty and maybe creating less advance to the dist) - Exhaust System Blockage.
Hope it turns out to be something minor in the end.

[Justin S.]

Re: 1970 LS5 overheating

Plugged radiator? What's the temp delta between upper and lower hose?

[Jack M.]

Re: 1970 LS5 overheating

Besides the other items mentioned... don't forget about the possibility of a collapsing hose.

[Ronald W.]

Re: 1970 LS5 overheating

Could be a blown head-gasket. Can be checked by your mechanic. If you are a DIY'er there are hundreds of Internet articles on how to diagnose HG failures. I've used the "RELD Head Gasket Tester Kit Combustion Leak Detector" with success.

[Bill B.]

Re: 1970 LS5 overheating

Fred, is your thermostatic fan clutch operating properly? The fan is supposed to run closer to the engine rpm when the sensed air thru the radiator is hot. I found mine was just spinning slowly no matter what the radiator temp was and no matter what engine rpm. I ended up replacing my clutch with a ZIP Products reproduction Schwitzer unit for small block. My temp gauge now never read so low that I could ever remember. Overheating now a thing of the past for me!

[Mark E.]

Re: 1970 LS5 overheating

Fred, is your thermostatic fan clutch operating properly? The fan is supposed to run closer to the engine rpm when the sensed air thru the radiator is hot. I found mine was just spinning slowly no matter what the radiator temp was and no matter what engine rpm. I ended up replacing my clutch with a ZIP Products reproduction Schwitzer unit for small block. My temp gauge now never read so low that I could ever remember. Overheating now a thing of the past for me!

If you pay attention, these cars tell you how they're going. You can see when the thermostat opens and closes by observing the temperature gauge, and you can hear the fan clutch engage/disengage.

[Justin S.]

Re: 1970 LS5 overheating

Fred,
Any updates?