73-87 Chevy _ GMC Trucks > Engine/Drivetrain

Weird cooling issue

(1/3) > >>

Dave78:
I have an 81 k20 with a 350 and heavy duty cooling rpo. It has a weird issue I've been unable to troubleshoot and fix.

When it starts from a cold engine, it will heat up to 220-240 degrees within 5 minutes or so if I don't let it idle up to temperature. Then it will sit there for a while (10 miles or so) until it suddenly jumps down to 180 or so. If I let it warm up from idle it will never get over 180 and will stay that way no matter if it's 40 degrees or 100 outside. The colder the ambient temperature, the hotter it will spike to. I've verified the external temps with a laser temp gun. The heads temps are accurate with the coolant gauge.

I'm running one of those high flow thermostats with 2 1/16" holes drilled in it and 180 degrees. 50/50 antifreeze, fan shroud, slightly rich, pretty much stock engine. My heater hose runs from the intake manifold to the heater core and then to the top of radiator. Water pump isn't weeping. No bubbling in coolant. No puking or steam and I have a new rad cap and overflow tank. Not losing any coolant either. I ran a 195 standard style thermostat in it for a while but it was in the late winter/early spring so I let it idle up to temp before I drove it and so the issue was never really a problem. In the summer though when I wouldn't let it heat up before driving I'd notice the coolant gauge would hit 250 or so before it would drop to 190-200. Can't quite figure out what the issue is with it. The temp sending unit is in the driver side head between cylinders 1 and 3. I'm guessing it's not really that big of an issue but it still bugs me nonetheless.

The two pictures are of the temp once it has heated up and dropped and the other as it spikes.

JohnnyPopper:
I think you need to push the red button

Then the black really fast















I think your T-stat is sticking for some reason. Did it do that with the 190?

bd:
Any chance the T-stat is upside down?

Dave78:

--- Quote from: JohnnyPopper on September 27, 2021, 06:30:31 PM ---I think you need to push the red button

Then the black really fast

I think your T-stat is sticking for some reason. Did it do that with the 190?

--- End quote ---

Lol the big red button is from the previous owner it goes to these obnoxious farm n fleet special led lights. That red button will blind you at night. I never use it. The black button run the washer sprayers, which are like pressure washers lol, it doesn't mist the windshield, it blasts them clean.

I was thinking it was sticking but I've tried 4 different thermostats because that's what one would think after all the troubleshooting. Outside of getting a crappy failsafe unknowingly, all of them have passed the boiling water test. Seems like it's slow to open and heat up but with the stewart high flow 30 dollar deal, it still does the same but less dramatic. It's also not upside down. Thought it might be grabbing on the intake manifold or something but once it seems to pop open it's good until it sits overnight to cool and there's nowhere for it to bind in the intake.

I'd like to go back to the 195 because of better heat and runs the engine hotter but if I have to jump in the truck and take off, I'm sure hitting 250-260 isn't going to keep the head gaskets together very long. I've read elsewhere that installing the temp sender in the intake or water spout neck is the easy solution and forget about it and the head temp spot was just for an idiot light that came on at 260+ but I dunno. Is a temporary jump up that high normal during initial warm up before the thermostat can get warmed up to open?

All the other 350sbcs I've had took 3 years to warm up in the winter and barely could stay above 180. I'd never even think about cooling issues with these engines. I've had one struggling along with 35000 gvw attached to it in 90 degree weather and didn't have any cooling issues. It's beating me.

bd:

--- Quote from: Dave78 on September 27, 2021, 10:22:45 PM ---I've read elsewhere that installing the temp sender in the intake or water spout neck is the easy solution and forget about it and the head temp spot was just for an idiot light that came on at 260+ but I dunno.

--- End quote ---

This is bad information.  The cylinder head between cylinders 1 and 3 is the factory location for all temperature light/gauge switches/senders.  The cylinder head location ensures sensor immersion in liquid coolant that is circulating as long as the engine is running.  The issue with a sensor passively located in the water outlet is that the actual engine temperature doesn't register until the t-stat opens allowing the coolant to flow past the sensor.  In other words, relocating the sensor, at best, merely hides symptoms.

What is the cold, static coolant level in the radiator after the engine has cycled through a couple of heating cycles?  Is there any air under the cap?  Is the water pump belt in good condition and suitably tensioned?

Install a radiator pressure tester in place of the radiator cap on a totally cold engine before starting in the morning.  Start the engine and monitor how much time elapses to build 20 PSI pressure at ~1,100 engine RPM.

Navigation

[0] Message Index

[#] Next page

Go to full version