Before my engine rebuild, I would start the car after it had been sitting for a night cold I would get this strange behavior. The water temperature would slowly rise to just below the red (260 degrees) in perhaps 2 minutes, and then it would sink in about 15 seconds to the normal 190 degrees. The whole time it was showing that it was running hot, it was not pinging or doing anything abnormal. I changed the thermostat but it didn't seem to help. This was all a while ago.

Now I have had my engine completely rebuilt at a very good performance shop. My radiator it turns out was weeping everywhere, so that was replaced with a good quality stock replacement. All of the hoses were also replaced. I was having essentially the same problem, it would spike occasionally when the car was started but this time often after it was somewhat warm. I would drive somewhere, and the car would have about an hour or more to cool. Then I would get back in and start it up, and the temperature would be high for a while, regardless of speed or anything. Some times the temperature would go up and down a few times and then stay at the optimal range. Other times the temp would stay warmer than usual, even on the freeway and would not return to normal until maybe the next day or after the engine had cooled down completely and warmed up again.

Now, I went to my radiator shop, and spoke to the owner at length. He gave me a quality 160 degree thermostat which I used. For the first few days it kept the temperature at just above 160 perfectly. But after a week it began to do the same temperature spikes upon startup except only up to about 220 degrees. Now after several weeks, it has spiked all the way up to 260 again.

This problem has me completely stumped, there are only a few things that haven't been replaced. The aluminum waterpump, which was put in about 25 k miles ago, and the fan clutch, both of which seem to be working perfectly. The fan turns by hand when it warm with some resistance. The heater is working, so that shows that the water pump is circulating. The temperature sender I believe is the right one, it came from pacific corvette and was not cheap. Under normal circumstances the temperatures stay at just the right temperature, except during the summer it used to get a little hot just idling in the heat with the ac on. I know my cooling system has the capacity to cool the engine just fine, because it usually does. I am having a heck of a time figuring out what is going on with these temperature spikes. I have never had any steam or any indication that it is actually running really hot. Anyone have any idea what is going on? Thanks in advance for any advice.

 

Could be air in the system causing steam pockets.

 

Does the spike occur after you have driven around a bit, turn it off, then start it back up? If so then that is normal. When the coolant isn't flowing then the temperature climbs...that's why electric fans continue to run even after shutting the engine down. As long as the coolant drops to normal after about 5 minutes then everything is OK. This is another reason why an overheating engine with coolant loss should not be turned off...it should remain running while additional coolant is added. The only time the engine should be turned off in an overheated condition is if the thermostat is not opening.

 

You aluminum water pump doesn't have a bypass hole on the drivers side does it?

Without the bypass hole NO water circulates untill the thermostat opens, then everything works properly.

To fix it take the thermostat out and drill three 1/4" holes in it to let the water through even when the stat is closed.

Check out Stewart water pumps web site..Particulary this page
http://www.stewartcomponents.com/techtip3.htm

 

I vote for 427V8. I also have those holes drilled in my high flow thermostats. They make so the pump is always moving just a little coolant. I was understanding what you were saying at first.

If your senders is located between the exhaust ports it's going to read artificially high. But as V8 stated until your stat opens your going to read the max temp and it opens when you see the temp go down.

 

There is a small bypass hose going from the top of the water pump to the intake manifold. The thermostat does not have any holes drilled in it, perhaps I will try that next.

 

I will bet you that you have your by-pass hose removed. Put the by-pass hose back on and your problem will be solved.

 

You should take your Vette to a shop that has a Infra Red Temperature Gun, and verify the temperature of your cooling system. I suspect a problem with your temperature gage or sending unit.

 

Thanks for the ideas guys. Perhaps I will be drilling the thermostat this weekend.

 

Before you do anything get a master guage to check exactly what the temperature really is......john :chevy

 

Agree with 427V8 and 73-454. The bypass hose is absolutely necessary with the thermostat is closed, to allow coolant to flow. Make sure that it is not only connected that nothing is restricing the inlets to the manifold and the pump.

Take your car to a shop that has the temp gun and find exactly what your system is running at. Make sure that the upper rad hose to the lower hose has at least a 30 degree temp drop. Even though you replaced the radiator there may be a problem. Hope like hell that the installer made sure none of the styrofoam that is used for packing protection got into the upper or lower hose pipes. Trust me it can and will happen.

Stewart waterpumps drills 3 3/16" holes in a Robert Shaw to allow any steam pockets to pass when the thermostat is shut, also allows extra flow.

 

Just for a test...take the thermostat out and see if the spike happens without a tstat. If it doesn't then you know that it's due to the thermostat. Personally I always pull the tstat out and leave it out (ducking beer cans). Buying a 160degree tstat like the racers do is a waist too, cause the engine normally runs at about that temp without a tstat. Oh, you mean the tstat manufacturers who want to sell tstats are telling you that running without a tstat will cause the coolant to flow too fast to cool down the engine and possibly harm the engine? The physics and experiemental data indicates that higher flows will cool the engine faster...and guess what...there's the water pump manufacturers selling high flow pumps that say the same thing!

 

Thanks for all the responses. Now to find an infared thermometer...

I do have a bypass hose that runs from the top of the water pump to the front of the intake manifold.

 

Well if you do have a bypass there is no need to be a drillin the thermostat...

I guess I'd make sure there WAS circulation around the engine when it was cold....

 

The bypass goes from the top of the water pump to the intake, it does bypass the thermostat at all. Apparently the bypass hose I have gets rid of bubbles and cavitation in the water pump itself.