Weird running hot issue

[IronJen]

Ever since I put the rebuilt engine in my Suburban a few years ago, it has been rock-solid on temperature. It normally runs right about 180 all day long, but may get all the way up to 190 going over Spring Mountain pass.

Well, I was in Vegas yesterday and it was running like it's usual self, that is to say, like a top. Just out of habit, I always glance at the temp coming up the mountain and no surprise, it purred right along at 190. Got to the top and figured it was smooth sailing the rest of the way home.

So I'm just be-bopping along, doing my thing, and I glance down and all of a sudden the temp is up to 215. I know that's not technically too high, but it's quite a bit higher than this thing normally runs. My first instinct was to pull over and put some water in it (I always carry some water), but as I was slowing down, the temp dropped back down to about 200. I decided to proceed with caution and monitor. At this point I was about 25 miles from home so I slowed down to about 65 and just watched the temp. This is where things got weird. The rest of the way, the temp continued to fluctuate between 200 and 215 every minute or two.

When I got home, I popped the cap and added about 1/2 gallon of distilled water. I expected that to be the end of it, but I ran a couple errands this morning and it did it again. Oddly, the temp was once-again, rock solid about 180 all the way over to Walmart, but on the drive home hit 215 a few times. With the addition of the water, it would go down to 190, but would never get back down to 180 and stay there like it normally does.

I gave the hoses a quick once-over and didn't find anything. Maybe thermostat?

 

[Mikeee54]

Thermostat or temp sensor would be my first suspicion.

 

[Harley]

Id almost put money on the temp sensor being faulty. Verify with a IR temp gun.

Also, never fully trust the dash gauges. They are only there to give a rough idea of whats going on. If read direct from the ECM they can be off.

I run a ScanGauge III and ignore the dash gauges.

https://www.scangauge.com/products/productsscangauge-iii/

 

[MJW89519]

My HHR did exactly the same a couple years ago, almost identical change in temperatures. Turned out to be the plastic blades in the water pump had come apart. New water pump solved it.

 

[NYECOGunsmith]

I've had air trapped in the cooling system give those symptoms , air bubble gets in the vicinity of the temp sensor and it would jump up in temp. Bleeding the air out solved the issue.

 

[Fogie]

overheating, if it really is, on the freeway is almost always thermostat

 

[IronJen]

So here's the latest.

I spent some time this afternoon checking it out again. Water level was low... again. Not sure if I didn't completely fill it yesterday, or I just lost more driving it this morning.

What I did find was the return hose from the heater core was separating internally. I found no signs of leaking under the vehicle and I checked the floorboards and found no water inside the truck.

Since it's summer and I haven't sorted out all the problems in the dash created by the PO, I just shortcut the heater circuit and jumped directly from the water pump back into the manifold. Since this is an old school SBC, I'm not so worried about an impellor failure. I'm sure it's possible, but I've only heard of such a failure with the stamped steel impellors.

I also checked both the upper and lower radiator hoses and saw no evidence of leaks from them. They are both relatively new as I replaced them when I installed the rebuilt engine.

As long as I had things apart, I did remove the T-stat and I drilled four 5/64 holes around the outside. I know it's frowned upon, but in the event of a T-stat failure, I'd rather have *some* water flowing than none at all.

I'm going to throw my scan tool in the truck for tomorrow since it reads live data, I will be able to monitor and compare the temp the ECM says versus the dash gauge.

I do need to drive in to Vegas again tomorrow so I have two gallons of water with me in the event I lose water. But if any of you wouldn't mind being on stand-by just in case things go wrong in town, let me know.

Thanks guys.

 

[Harley]

Fill and pressure test the cooling system. The water is going somewhere.

 

[tbirdsc]

Fill and pressure test the cooling system. The water is going somewhere.

I hate to be a Debbie Downer, but the beginnings of a blown head gasket comes to mind.

 

[IronJen]

I hate to be a Debbie Downer, but the beginnings of a blown head gasket comes to mind.

Better not be. These later SBC always crack heads when they get hot. If a head gasket is blown, you can bet good money the head is cracked too.

 

[Fminchey]

Pressure test the whole system and the cap

 

[tbirdsc]

It's been a while since I had to deal with it but a quick and dirty check is to fire it up with cap off and check to see if you can see any bubbles coming up. I can't remember if you need to be up to running temp with the t-stat open for full flow or you will see it right away.

 

[tbirdsc]

Better not be. These later SBC always crack heads when they get hot. If a head gasket is blown, you can bet good money the head is cracked too.

I am not a Chevy guy, but if it is the beginnings of a head gasket starting to go and if you temp gauge is reasonably accurate, I would think at this stage you did not get things hot enough to crack a head.
The better part of valor maybe to cancel your Vegas run (if you can) until you know for sure because if it blows big time out in the middle of nowhere, you could do a lot more damage as you mentioned.

 

[Gullwing]

Yeah if you are adding that much coolant to it, then it is going somewhere. Check the oil, check the tail pipe...
Pressure tester as brought up before would also help.

 

[IronJen]

Oil is clear and it still runs like a top. I find it hard to believe there's a head gasket leak. Not saying it's not possible, just that the ONLY symptom I've found so far is the wonky temp gauge. Had I not looked at it, I would not even notice any change. Truck is still running like a top.

As for water going somewhere, yes, that's true. But that also assumes the level was topped off to start. I cannot say that for a fact. The problem, as it is, with it running consistently at 180* is that it just never gets hot enough to push into the overflow. For all I know, it may have been a 1/2 gallon low all winter.

 

[IronJen]

I am not a Chevy guy, but if it is the beginnings of a head gasket starting to go and if you temp gauge is reasonably accurate, I would think at this stage you did not get things hot enough to crack a head.
The better part of valor maybe to cancel your Vegas run (if you can) until you know for sure because if it blows big time out in the middle of nowhere, you could do a lot more damage as you mentioned.

I can't really cancel my Vegas run, but if I need to, I do have alternate transportation. I'm going to strike out extra early in the morning so that if I encounter problems, I have time to return for said alternate transportation.

And so far as a Chevy guy/girl goes, I've been around them a very long time. The one thing these "smog" motors are known for are thin head castings. They are in danger of cracking sitting in the driveway on a hot summer day. (OK, that may be a little hyperbole, but it doesn't take nearly as much as it should to crack one of these heads.)

 

[Maxcarp]

If the water pump is leaking out of the weep hole,it can be hard to detect if the leak is just starting due to the seal getting worn out.Due to its location its hard to tell it its leaking unless the coolant is just pouring out.This is where I would start.

 

[Harley]

Keep in mind, sometimes leaks wont show unless the system is pressurized.

 

[NYECOGunsmith]

Dye and an ultraviolet light make it easy to find leaks like that.
For the weep hole leak where it is hard to see, I stuff a short piece of cotton string just barely into the hole, if the string gets wet, there you are.

 

[IronJen]

So I did my trip into and back from Vegas today. I can conclude one of two things. Either the problem is transient and I didn't duplicate the conditions, or it is resolved.

The only two things I did, aside from topping off the water level, was to replace/bypass that heater hose and drill those holes in the T-stat. I can't for the life of me, figure out why bypassing the heater core would solve the issue, unless I have a leak in the core I haven't located. So I have to assume the T-stat was the problem. Of course, I didn't solve the problem, I know that. The T-stat is still failing and I will need to replace it, but the highest temp I saw all day was 183* on the way back, coming up Spring Mountain.

In fact, drilling the additional holes created a small problem of its own. It was still pretty cool crossing the pass this morning, on the way down the temp dropped below 140* and the truck went back into open loop, even with me doing 80 in the foothills.