So I've been putting some miles on this engine after it sat for years and I've got a problem that I would like some help with. It's got a new water pump and thermostat, for now the heater hoses are capped off. I'm running a manual fan switch. The water temp gauge is in the drivers should side head.

So everything seems normal except for some reason if I drive down the highway my water temp will drop to about 150 degrees. It acts like the thermostats is stuck open. Yesterday I pulled the t-stat and put it in a pot of water and watched it. It started opening at 180 degrees like it should, then closed when it cooled down.

So why is this happening?

Any ideas? I'm going to hook up the heater core hoses but I'm not sure how it's going to help the cooling issue.

Have you checked the temperature sender for accuracy? Just a thought as I haven't found anyone with a similar problem. (Even those with 160F thermostats don't see that much cooling except during the winter.)

Yeah back when I had a 160 thermostat it would still stay around 170-180. This 180 degree one should stay around 190. I don't get it it. I took temp readings after a drive using my temp gun. The water pump housing, and all coolant hoses were in the 130's and the cylinder head itself was about 180. That's about the temp the water temp gauge was showing.

I'm just running water, no antifreeze. Maybe I'll just pull the sender and see if I can check its accuracy. I have to add some heater core hoses anyway, so that'd be a good time to check it.

Any other ideas though how this could be happening?

I pulled the water temp sender. I heated a cup of water to 190 degrees and dropped the sensor in and watched the gauge. It climbed quickly to exactly 190 degrees. I measured the temp of the water using a very accurate instant read meat thermometer.

So, something must be hooked up wrong. Incorrect heater hose routing or something? Would that cause this?

If I'm at a light and the temp gets to 200, within minutes of cruising down the road, it'll drop to 150 or so. Something is wrong, just not sure what.

So I hooked up the heater core, I now have heat. When topping off the system, I started the engine with the radiator cap off. Doesnt the small stream of water shooting out at the top of the filler neck come from the T stat being open? If so, mine does it when cold. Just took it for a drive and the problem is still there. Somehow water is always bypassing the T stat. Weird problem to have.

The small tube in the cap housing goes to the recirc tank, the next small tube down goes to the steam vents at the back of the heads - generally if they are flowing a solid stream of coolant then the air is purged from the system. But.. your symptom of low coolant temperature reading may be consistent with air in the system - the sensor can only measure water temp - hot air or steam isn't seen by the sensor.

When I fill an LT1 I jack the RF corner up so the filler neck is the highest part of the system, fill the radiator, fill the recirc tank to the cold line, then run the motor at idle with the cap off till the expansion from heat starts to push coolant out of the filler, then put the cap on and then crack the 2 bleed screws till a solid stream of water comes out - first put a rag under them to keep coolant off the opti. Warm it up good, let it cool overnight then recheck the recirc tank level - the system will pull some coolant out of the tank as it cools.

CA prop 65 warning - some things I say or do may cause cancer or other bodily harm, reader beware.

I'll check the routing again but I think you misread. I didn't have low coolant. I needed to hook up my heater core lines, so obviously I lost some water when doing that.

The system was full and I had no air. But for some reason it acts like I have a thermostat that's stuck open and I have no idea why.

I checked the sensor for accuracy and also put the T stat in water on the stove and watched it open at 180 degrees.

Let's say I totally eff'd up the coolant line routing. Is it even possible to create a situation where somehow water is bypassing the T stat?

No misread - I edited it to clarify as low temperature reading.

When you had the heater removed, did you cap/plug the water pump ports? That would be good and proper. If you tied the 2 ports together with a hose, that would not be proper, and might be the cause of your symptoms.

Ahh ok. Yeah the two ports were capped, not tied together.

So I installed this engine like 2 years ago and bled the system back then. I've been starting it and putting around the neighborhood every so often. The temps would steadily climb as you would expect. Driving around the neighborhood wasn't enough to keep it cool for long (no fan at the time). Now that the car is almost done, I've been driving it almost daily for the last 200 miles. Running errands and stuff like that. This is the first time I noticed this problem (obviously because I'm able to drive it faster than 20 MPH). It responds exactly the same every time. Running down the road will cool it off to 150 ish. As soon as I come to a stop, it will heat up. Once it hits 200, I flip the fan on and it cools down rather quickly, as it should. This afternoon after a drive, I pulled into the driveway, let the temp get to 190, then flipped the fan on and watched the gauge closely. Temp started to drop and never hesitated or anything. It eventually got to about 170 degree. With a 180 degree t stat, it should drop to 180-185 and not get any lower.



Anyway, just trying to give as much info as possible. Heater core is hooked up and the problem still persists. So idk, I'm stumped.

Sounds like your thermostat is stuck open or opening too soon, try a new one.

So you should only see that stream when the T stat is open? I don't see how it's possible. I've pulled that thing out twice and it's closed and like I said, I watched it stay closed in a pot of water until 180 when it started to open.

How are you measuring coolant temp - ie what gauge and where is the sensor located?

Drivers side of the head. Autometer elite gauges. The sensor is reading accurate. I tested the sensor with a cup of hot water.

Is the new thermostat an LT1 specific model? It is different than a std SBC thermostat, effectively has 2 valves.

It's supposed to be. It looks like an LT1 t stat. Maybe tomorrow I'll just pick up another from autozone to rule it out.

Fixed!!!





I don't get it. The one on the right is the culprit.

The one on the left is $10 from autozone. Works great. Anyone know what's different about the one on the right?

Main difference I see is the spring on the bottom. Maybe the smaller spring had less rate allowing it to compress/open sooner than it should?

Where did the one on the right come from?

Autozone maybe? It might be a fail safe. Maybe they don't work in these cars?

Looks like you have LT1 specific Thermostats:


Stock 180*: