I have a 1951 chevy pickup that I just purchased with a 1974 350 engine. It runs great but has a odd problem. When started from cold it overheats to 220 /240 degrees prior to the tstat opening. When the tstat opens the temperature drops to 170 and repeats the cycle again a time or two then keeps fluctuating between 175 and 195. I thought the original tstat was bad so I replaced it with a 180, no change. I tested the old 180 tstat in a pan of heated water and it started to open at 180 and full open at 220 and it closed as it should. I have run the truck several times with the heater on to make sure the air would get out, still no change. I can see the water moving in the radiator when the tstat opens and stops moving when it closes so I think the WP is working. I would appreciate anyones ideas or experiences with this problem.
Where is your temperature gauge sending unit? If it is in the head you will get all kinds of odd readings. Does it show warm almost immediately? If so, that's your problem. If it's in the intake near the thermo housing like it should be then I have no idea.
Just to clarify my first post, the engine isn't actually overheating, right? (No m***ive cloud of steam, no chugging or bucking or spitting, etc.) I am ***uming the problem is with the gauge reading temps straight from your head, which is where the sending unit for idiot lights go, rather than from your intake manifold which is where temperature gauge sending units go. When I first got my '53 on the road (1978 350, about the same motor you are running) I had the gauge sending unit in the head location and it read hot and with a lot of fluctuation just like you said. The solution is simple and getting another sending unit to install in the manifold or moving the current one there but the threads are a different size. If, like me, you have an aftermarket manifold on the engine a mid 1970s Caddillac sending unit is the correct size threads and the correct ohms for your gauge.
Dave thanks for your response, Yes the sending unit is in the head on the drivers side. And no there is no cloud of steam or chugging or spitting. There is a sending unit location (not in use) next to the tstat housing on the manifold. I will try installing a new sensor, the tube from the sending unit to the gauge is not long enough to reach the correct location. I will let you know how it goes.
pretty much the same problem w/ one of my cars w/ a 350. Ive never been able to figure it out myself. jeff
Some of those gauges have a coil of tube up under the dash. If you unwind the coil it will reach the T'stat housing. About half the ones I have had were like that.
I had one that did that. I drilled a tiny hole in the 'stat and that seemed to fix it. My theory is the coolant on the temp sender side gets hot before the hose side and the slight leak (the drilled hole) helps it to warm up evenly. Hope this helps. Blue
If I am understanding, your guage is connected to the head. It should be connected or reading on the intake next to the thermo housing... If the issue continues, consider getting a 2 stage thermostat.... I think TCI and Rober Shaw makes-um. They open partial at a given temp, then open full at the rated temp... The 1/8 hole drilled in the thermostat is helpful on Fords because of the housing position (Sideways) it wouldn't hurt, but the 2 stage thermo's come with a relief.
Get a infared thermometer and check to see how hot the engine really is. It sounds like maybe you have just a bad gauge.
It's not the gauge, it's the location of the sending unit! Temp gauges aren't meant to read from the head location on a SBC V-8. That's for idiot lights only. You can run a gauge there, but it will show hot quick (because that's where the heat originates) and it will fluctuate a lot as you drive because that area of the head heats up and cools down faster than the engine as a whole. Drilling the thermostat might help, but only because the water would be circulating a little bit as soon as you fire it up. Pappaw60: if it doesn't bother you and you know nothing is wrong with the engine there is no reason you can't drive it that way. Just don't freak out every time the number spikes up - it's only a problem if it doesn't go right back down after a little while. Just remember the water has to make its way all through the motor, water pump, radiator, and back again before it gets to the head where that sending unit is. But the manifold outlet will give you a truer, steadier reading.
I checked the temperature with a infrared thermometer at the head location where the sensor is currently located and at the unused port on the IM next to the thermostat. There was a 50 degree difference between the two at the time the thermostat opened (180) at the IM. I am going to relocate the sensor to the IM and put the 1/8" hole in the thermostat. Keeping my fingers crossed
I kinda like to see the gauge fluctuate a bit, let's me know things are working as they should. As long as I know why it's doing it I'm OK.
relocating the guage to the intake next to the thermiostat will give you a more accurate reading, and I am sure will solve your problem...
I've always drilled a small hole in thermostats. It seems to me that a little flow will allow the warm coolant to get to the thermostat sooner on startup. With no flow, the heat has to transfer through the fluid. The coolant in the head is getting to 250 before the stat opens. Once the stat opens and coolant can flow, the coolant temp in the head will be close to manifold temperature until the stat closes again. I don't like the temperature in the head changing 70 degrees in such a short time cycle.
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