I have a 76 carbed 308. The water temp reading is causing concern. The engine will run just fine at about 195 and then all at once the temp reading goes to about 220. Stays there for a minute then all at once goes back down to 195. I know that both radiator fans are blowing. I have not been able to corelate the fans turning on with the immediate reduction in temp reading. I thought it looked like an electrical problem from the current surge when the fans turn on, but there is no electrical connection between the blower fans and the temp gauge or the temp gauge sender..except through ground. Does this sound like: 1) Bad or failing temp gauge 2) Bad or failing temp gauge sender unit 3) Bad or failing thermostat I am beginning to think it is the thermostat. It's almost like the thermostat is stuck and then when it gets hot enough(around 220) it unsticks and pops open letting the water through. The only thing that would argue against this is that the water temp reading drops very quickly. Like 2 seconds. Not sure if cooler water would reach the temp gauge sender that fast. Anyone seen or experienced this problem? Am hoping to tap into any experience here on the chat forum so I don't have to start blindly replacing parts. Thanks in advance for any thoughts on this. Mark PS. Car has new water pump, but thermostat is not new
Or 4) cooling system not completely full -- have you confirmed no air present at the two bleed locations?
You might also have a problem with inconsistent operation of the cooling fans...that is they run...but not always at max speed, due to age and wear.
IMO, sounds like you have some trapped air in the cooling system, frequent problem for many, so you aren't alone. The other is a possible sticking thermostat. The final culprit, could be a faulty expansion tank cap. Get a pressure tester, pump up the air pressure to above 15psi, then watch and see if the pressure holds for several minutes. If it holds, great. If not, you have a pressure leak in your cooling system somewhere. The 308 series used two different types of thermostats: one was mostly aluminum and the other was half brass and half aluminum. You want the latter design IMO. Hope this helps.
This exact thing actually happened to me last year. I threw the accessory belt and the car overheated on me. One of the little hoses under the intake manifold sprung a leak as a result. I was able to pry on a belt in my driveway so I could drive it over to the shop and have it all fixed right. It was a week before I could get it over there and by then I just forgot to check the coolant level. On the way over there it was doing EXACTLY the same thing. It would swing way right, hang there a little bit then drop fast to "normal". We didn't change the thermostat and the fans are fine. It was just low on fluid and had air all in it. Once we replaced the hose and topped off the system it was perfect again. That was 12 months, 3000+ miles and three track events ago. Still no problems. I bet if you fill it and bleed it the problem will disappear.
OK. When the water pump was replaced, there was some issue with the level of the coolant. We overfilled and then it burped for a few days. I may have to bleed the system. Actually, the symptom looks very much like a bubble in there. I will see if I can play with that situation for a while and will report back. Thank you very much for your help. Mark
maybe a crazy suggestion but it worked for me. One of the veteran members here told me to give a swat to the right side of the dash covering your instrument panel. I had topped off, bled and burped my system. Gave it about three little whacks and presto. No more problem. I drove it to Auburn and back to Birmingham on saturday and not a peep. Ran great.
Wow. I am willing to try anything. Is the heater core under the passenger side dash board? Is there a possibility that there is a bubble in the heater core? Not sure I want to hit it too hard....all of the fuses and relays are under there. Can you be more specific about where to hit it? Seriously. Like I said, I am willing to try anything. Mark
I am assuming that when you are filling and bleeding you system you have the heaters wide open. You should.
air in the system. with the car cold , remove the cap and check the fluid level. open the bleeder screws on the thermostat housing and the radiator if you have both. bleed the air out. close the system and drive the car. have the heaters on. ( the early cars dont have the thermo housing with the bleed screw. my 78 had it, while my 77 does not. the extra one on the thermostat housing is VERY helpful to have) with the car warmed up you can : 1) wait for the car to cool some (but not too much) and remove the cap from the tank SLOWLY as it will still have pressure on it. and them bleed the system warm. with the cap off, and the radiator bled, close the bleed screw and start the car and let it run WITH THE RESERVE CAP OFF. when its warm and the thermostat opens, carefully open the bleeder screw on the radiator again and bleed off any trapped air. 2) ( dangerous) i use gloves and as soon as i shut off the car, (hot) i slowly open and remove the cap, then i start the car and beed it while running. hot coolant causes chemical burns (dont ask) . the needle should not jump to large temp extremes, this indicates sending unit issues, fuse block problems or air in the system.
