Author Topic: Weird cooling issue  (Read 466 times)

Offline Dave78

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Weird cooling issue
« on: September 26, 2021, 08:42:22 PM »
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.

Offline JohnnyPopper

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Re: Weird cooling issue
« Reply #1 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?
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1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
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Offline bd

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Re: Weird cooling issue
« Reply #2 on: September 27, 2021, 06:47:30 PM »
Any chance the T-stat is upside down?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Dave78

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Re: Weird cooling issue
« Reply #3 on: September 27, 2021, 10:22:45 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?

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.

Offline bd

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Re: Weird cooling issue
« Reply #4 on: September 28, 2021, 01:16:48 AM »
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.

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.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Dave78

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Re: Weird cooling issue
« Reply #5 on: September 28, 2021, 02:02:57 AM »
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.

I'll try the rad cap thing when I get a chance. The radiator is always full to top when cold. It doesn't take long before it slowly overflows to the surge tank. You can run maybe 15-30 seconds before the coolant starts spilling out of the rad with the cap off. Might not even be that long. I'm also not losing a noticeable amount of coolant.
« Last Edit: September 28, 2021, 02:04:48 AM by Dave78 »

Offline Shifty

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Re: Weird cooling issue
« Reply #6 on: September 28, 2021, 09:31:02 AM »
I think you need to push the red button

Lol the big red button is from the previous owner it goes to these obnoxious farm n fleet special led lights.
Dang!  I was hoping for a warp-speed button...

 ;)
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

Offline bd

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Re: Weird cooling issue
« Reply #7 on: September 28, 2021, 09:44:00 AM »
I think you need to push the red button

Lol the big red button is from the previous owner it goes to these obnoxious farm n fleet special led lights.
Dang!  I was hoping for a warp-speed button...

 ;)


Or an ejection seat....


Back to the dry stuff.

You can run maybe 15-30 seconds before the coolant starts spilling out of the rad with the cap off. Might not even be that long.

This might be early indications of a cylinder head gasket just beginning to seep combustion gases into the water jacket; more pronounced when cold before components expand during warmup.  Is there unusual white smoke puffing from the exhaust shortly after starting the engine, or does the exhaust produce a sweet odor? 

Definitely perform the pressure buildup test described earlier.  Add a cylinder compression test to your diagnostic steps.  In follow-up, your diagnosis might benefit from cylinder leak-down tests on all cylinders of the cold engine with the water outlet and t-stat removed.  Watch for air bubbles escaping from the coolant crossover below the t-stat seat.  If bubbles appear, you will identify which cylinder is the culprit before teardown.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Dave78

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Re: Weird cooling issue
« Reply #8 on: September 28, 2021, 11:09:05 AM »
No errant white smoke just a puff of blue smoke on start up from leaky valve seals. It makes it fire off faster lol.

I'll see if I can rent a compression tester to try it out. I don't have my full tools and stuff since I moved so I'm almost tempted to just pull the heads and throw gaskets on it and see what that does. Then again, I've been driving it like this for 10k miles so I might let it ride and wait till it goes boom and throw in a junkyard engine.

I've also got a suspicion the weiand aftermarket intake might be an issue. It has 2 threaded holes for the heater hose or a temp sensor or whatever and the heater hose in running off the one offset from the thermostat housing and farther away. I wonder if the water is bypassing to the heater core quicker on the passenger head and less hot water is passing over the thermostat causing it to open slower.That passenger head also runs a good 10 degrees cooler during the initial warm up it seems.

Still doesn't make sense why the block and heads and everything can be up to temp and I can go 10 miles without the needle moving off 220 and then suddenly it jerks down to 180.

This isn't the exact intake I have but close enough.
[img] https://static.summitracing.com/global/images/prod/large/WND-8016_MF.jpg[ /img]

I should also add it is completely unpredictable. Sometimes I can barely let it warm up any and it never goes over 190. Other times I've let it warm up 4-5 minutes idling with the choke and and it still hits 220-230. It's also more prone to overheating in colder weather. When it was 105 outside I could jump in and start driving instantly and never break 210. Sometimes it jumps from 220 down to 190 back up to 200. Jitters around and eventually jerks down to 180. I'd think the gauge was shotty but I'm getting accurate temps from the laser gun that match the gauge. Minus the gauge doing fluttery 10-20 degree jitters.

Another oddity, turning the heater core fan on results in an immediate jump of 5 degrees hotter. I have no idea how running the heater makes the coolant run hotter. This happens even after the thermostat is fully open and has stabilized at 180.

« Last Edit: September 28, 2021, 11:17:14 AM by Dave78 »

Offline Shifty

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Re: Weird cooling issue
« Reply #9 on: September 28, 2021, 12:16:29 PM »
Maybe consult a priest?  ((I don't have anything constructive to add, but I do want to see the suggestions)
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

Offline bd

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Re: Weird cooling issue
« Reply #10 on: September 28, 2021, 01:24:22 PM »
Another oddity, turning the heater core fan on results in an immediate jump of 5 degrees hotter. I have no idea how running the heater makes the coolant run hotter. This happens even after the thermostat is fully open and has stabilized at 180.

Use a voltmeter to validate the engine, cab, and blower motor grounds to B-.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)