Hello,

I have the following behavior. After cold start it takes about 5 minutes until the coolant temperature slowly rises to 240-250° F (red area) and then drops to about 210° F within 5 seconds. This happens 2-3 times. While driving, the temperature remains relatively constant at 190-220° F.

A new thermostat (Gates 33008S -180°F) is installed, I already drilled 2 small holes into the thermostat due to air in the system.

I already checked the thermostat with hot water -> ok, opens at 185°F.
Check the coolant level -> ok.

Engine is a 383 Blueprint stroker with aluminium heads in a 1972 corvette.

Thanks in advance!
André

 

use an infrared thermometer or a second gauge to verify the stock temp gauge. If thats correct, then check for loose wires or center electrode of the sender. If thats correct you could possibly have air trapped in your system. Someone else will have more ideas

 

What is the timing set at?
Initial at idle with vac advance disconnected?
Total at 3000 rpm?

Craig

 

Hello,

Initial timing at idle (750 rpm) = 14
Total timing at 3500 rpm = 33

André

 

That seems to be ok.

The factory gauge in my car was off 8-10 degrees. I checked it with a IR gun as Rescue Rogers posted.

 

This is VERY typical of a air lock (bubble) in your system somewhere. I bet your gauge is actually ok and within a few degrees from being correct. Your T stat is working off of hot air bubbles vs hot water, hence the swing in temps and then comes back down to a somewhat normal temp. The 220* is not normal for a sbc, 195 -200* is very normal with a 195* and everything in the system is good.. I bet if you get on it hard to red line a few times you will also blow out the water pump mount gaskets to block and that wouldn't surprise me. You are building too much pressure in the block. Anything over 18 psi is too high in a sbc, the 15-16 psi cap will not blow either. If this issue continues, I would drain the system again and use the (burping spill free funnel below) with the from end raised and fill it back up and watch the bubbles come out trough the funnel fluid. Don't let the funnel go all the way down while running motor or you are just wasting your time. Let the motor idle for as long as it takes to get all the air out.

I would also watch the temp gauge, but it should be ok. I've had to run mine 40 minutes and longer to get the air out sometimes. Squeezing the hoses while the motor is idling will help get the air out. ANY air is not good in the system. Make sure you turn ON your heater (full hot) as well to circulate fluid through the entire system. If the issue still continues after doing this... You may need to go to a vacuum evac fill where you put the system under vacuum to suck out all the air and then back fill it with anti-freeze. It should be fine after that.

I've also had to go to the extreme on mine once and took an old T-stat and removed the center so only the outer housing was left, just a large hole to give some restriction with a high flow pump vs no T-stat and burp the hell out of the system and then just run it with my hole T-stat for a while while driving around. Everything was good after that and the gauge would remain rock solid at 150* no matter what. I also have an aluminum radiator. Then just install your normal stat and press on. Hope this helps. Since Ron Davis radiators is here in the Phoenix valley, I called and talked with Ron Davis for about an hour about this subject and he was VERY helpful and a great guy to talk to with 40 years of experience on the subject.

Shop around, they are cheaper elsewhere.

 

Hello,

yesterday I changed again the T-stat with a jiggle valve (MOTORAD 2000160) to remove air pockets. I did a vacuum evac back-fill with anti-freeze to suck out all the air.

Now it takes about 5 minutes until the coolant temperature slowly rises to 220° F and then is drops suddenly to about 170° F within 3 seconds. While driving, the temperature changes between 170 and 220° F.

Is this ok?

Thanks in advance!
André

 

try with no t-stat. briefly. see if you still get temp spike and drop.

 

I vote electronics. Because a new T stat did nothing and the fact that the temps dropped so drastically, it has to be electrical. The cooling system could never cool and drop 50* in three seconds. Not going to happen.
So, its either the sending unit, Lord knows how old that is, or the issue is behind the dash with the gauge / circuit board.

 

Hello,

the temp sensor is one year old (ACDelco G1852) and it was tested before installation with a multimeter.

Without a t-stat the temp remains relatively constant at 170F.

 

If the system runs constantly at 170 without the t stat then I would say you see the t stat sticking until 220 then it pops open and it cools to 170. buy a new t stat

 

The t-stat is brand new and I checked it before with boiling water, it opens at exactly 185F

 

Are you sure your gauge is correct? Our stock gauges are not known for being correct and can require a resistor to correct them... Check it with a IR temp gun.

 

If you have an AIRGAP manifold or similar, and the sender is in the head, it will need to get hotter to open the T-stat when you first start because cold air keeps it cool .After everything warms up it will function normally.

 

Well, considering what happened after you did some checking and it's still doing odd things. I would go with the electrical side as well now. I would definitely check the temp sensor connector and maybe even replace the pin, it may not be making good contact with the sensor. Easy job and should only take a few minutes. I would also check the the temps with an IR gun to at least give you an idea if it's really getting hot or not in relation to your gauge. As long as it's NOT actually overheating, it should be rather easy to find the issue. Good luck and post back up results.

 

What aluminum heads are you using? Correct head gaskets used?

The Vortecs require a bypass for the heads to get early water flow and air pockets out.

 

 

amoellen - you are running yourself aound in circles.

I suggest one of these inexpensive remote temp detectors before starting any work. Just to verify that it is real.

https://www.harborfreight.com/121-in...ter-63985.html

 

Did you put any sealant or teflon tape on the temp sending unit (threads into the cylinder head)? The sender body & threads are the electrical ground point for the temp gauge system. If that connection (via threads) to the cylinder head is 'iffy', the resistance measured by the gauge could just be floating around, instead of giving a stable reading..

 

^^^THIS^^^
That is a VERY good point and should have thought of that, but I just assumed maybe too much. It wouldn't be the first time this has caused issues by using teflon tape and messing up the sensor ground.