Saw this mentioned somewhere but can't find it now. My 1970 350/315 Skylark Custom convertible runs right around 200 degrees on the highway at 55-60 MPH. If I try to move up to 65-70 MPH the coolant temp starts moving up. It's gotten to 220 and I slowed down, temp came down. No problems at idle or in stop and go driving. The only things I have done are change the four blade fan to the seven blade clutch fan, with new clutch, that came on the AC cars (mine is not AC) and a 180 degree thermostat. Also cooling system has been flushed and new coolant used. As far as I know car has the original radiator in it. Anyone have any ideas how to cool it down?
Was this at road speed creep condition not taking place before the 7 blade fan got installed? If so here’s why. once the car is above 30 mph you should not need a fan! That is if you have enough rad volume, and water flow, then as long as enough air can pass thru the rad all should be well. I think the issue you have now is that even if the fan clutch is working as it should in regards to temperature, you 7 blade fan is blocking off too much needed air flow.
The 66 Cat still does this despite a new alumn radiator. Difference is mine will creap more in traffic and stop n go.
As several have said, when temp climbs on the highway as speed increases, the radiator is blocked. Time to replace it.
Also - consider whether your temperature is really rising. Where is your thermocouple located? Could there be an air pocket near the thermocouple which is preventing an accurate reading (and changes with RPM)? Could your thermocouple be being affected by airflow in a way that you're actually getting an inaccurate reading at lower speeds / airflows? Is it near the exhaust manifold? If you are confident in your temperature measurements, then it comes down to flow obstructions (water + air), air pockets inside the engine, coolant mix (you don't want localized steam pockets), and lastly whether you need a larger radiator (typically not the case). -Bob C.
The temperature creep was happening before I installed the new fan and clutch. That's why I installed them. This fan/clutch combo is the factory setup if the car had AC. The fan is behind the radiator, not in front. Don't see how that blocks air flow through the radiator.
Temperature creep with increasing highway speed is classic insufficient radiator. If it is in fact the original radiator, there is no way to clean it effectively. It needs a new core, or to be replaced.
A friend of mine had temperature creep on his 1960 falcon with a 302 swap. We discovered that the previous owner put a aluminum shroud that mounts to the radiator that essentially blocked off about 30% of the air flow. So we tore that off and got a manual traditional fan and a correct shroud. He also adjusted the timing to spec. It seems to be running significantly cooler now
once you get a new or a very clean rad the temps will stay at 180 I never had trouble again once I put in a new rad
The 68 Riv / 430 had the same issue. On a 80-100 mile highway drive it would get to the point of boil over when I came to a stop. Was super frustrating and this went on for a long time despite a new 4 row copper-brass rad, new HD clutch fan with the correct blade and shroud. What finally cured it was the Dewitt Aluminum radiator. Sometimes I wondered how people drove these things back in the day and didnt have the issues.
I ordered a DeWitt 2 row on January 28 from OPGI. Was told it would ship from the manufacturer in 3-4 months. I have contacted OPGI and DeWitt. Haven't gotten a response yet. Don't know if I should start looking elsewhere.
I would check out the Cold Case radiator. I purchased one last year, after reading reviews posted on this site, and have been very please with the results. I think they are far less expensive than the DeWitt radiators and they seem to be readily available. One member just did a review of it: https://www.v8buick.com/index.php?t...gma42a-aluminum-radiator.378722/#post-3314973 https://www.summitracing.com/parts/rad-gma42a https://www.amazon.com/68-72-GM-Bod...utomotive&sprefix=gma42a,automotive,74&sr=1-1
