I have a over heating problem. I have replaced the water pump, thermostat, upper radiator hose and the coolant temperature sensor. They were all in need of replacement, but my car still over heats when going up hill only. I checked the fins on my radiator and there are three medium potatoe size areas where the fins are badly bent. Could this be causing my over heating problem?

My other question is if the fins are bent can this cause the transmission fluid to get to hot. My reason for this question is because when the car gets to normal operating temperature, but I have been driving for some time my transmission during first and second gear shifts hard. When the car is cool it does not?

5 Answers 5


Firstly, you don't mention what car you have, which can help with diagnosis as we may be able to point you towards known issues with certain models.

Bent or missing fins will cause the radiator to work less efficiently than normal, but that sounds like a fairly small area, so shouldn't be causing you a major problem. It is probably worth flushing the radiator out if you haven't done so already.

The engine radiator is usually totally unconnected to the transmission fluid, so I doubt the two problems are related. Do you have a transmission cooler? If so, what condition is that in? Have you checked the condition and level of the transmission fluid?

  • Your point about the engine radiator being totally unconnected to the transmission fluid isn't totally true. Most cars use a radiator that also houses a transmission cooler even if they opt for an additional transmission cooler. That said, I think even a drastically overheating transmission shouldn't have much impact on coolant temperature. Vice-versa may be true though.
    – atraudes
    Mar 20, 2017 at 21:08
  • Arg, I just saw that this is a response to a really old answer. Sorry for the resurrection!
    – atraudes
    Mar 20, 2017 at 21:09

Small areas with bent fins on radiator are not a big problem.

The other causes for overheating could be coolant fluid needs replacement (its cooling properties degrade over years). Also I would inspect inner surface of lower radiator hoses - see if they have traces of mud or other flow obstructions. My cockpit radiator suffers from that - its clogged with anti-leak mixture I used and needs replacement, until that it is really working at 20% of its limit.


Bent cooling fins can most definitely increase engine running temps considerably; and you do have a significant amount of blockage. Only a few years ago my 23 year old pickup needed a new fan and fan clutch. I had grown accustomed to the temp meter running mid scale to slightly above mid scale, rather than below mid scale, and I had just chalked up the hotter running to engine aging. With the change of fan and clutch the temp characteristics did not change. Shortly after I replaced those items, the radiator developed a leak. So I replaced it and, with a fin straightener I bought along with the radiator, I straighened all fins, including in the AC exchanger through which air had to travel to arrive and be sucked though the radiator heat exchanger. I was amazed to find that the vehicle's normal operating temp returned to the same, characteristically low (below gauge mid range) operating temperature that it showed when brand new. Snce then there has been not the least occurence of hot running, even with radiator substantially underfilled.

So do make it a practice to periodically inspect and striaghten fins. Also, take water hose and nozzle and flush/back flush insects and debris from those fins, too.


to check fins for stoppage use a water hose while engine is running and normal temp--spray water onto radiator and watch to see which tubes do not evaporate hose water--tubes that are not stopped up will dry up quicker because hot water from engine is running though them while the stopped up ones will not evaporate--normally these are the lower part of radiator--


If you have an automatic transmission then it is likely running fluid to an integrated cooler within the radiator. Most passenger vehicles cool the automatic transmission fluid in this manner. If ATF gets too hot then it will degrade and can cause shifting problems and premature wear on your vehicle's transmission. If this is a concern then an external automatic transmission fluid cooler can easily be installed on most vehicles with simple hand tools for under $50. Other causes of fluid related shift problems are too low fluid, too much fluid, and old fluid (will be a dark brown color on transmission dip stick - should be bright red). Too high fluid will cause cavitation creating air bubbles which is something to avoid in any hydraulic system. Always keep the fluid level in between the high-low marks on the dip stick.

On the issue of engine temperature. The engine is under a much greater load when pushing a vehicle against gravity (up hill). This causes greater pressure within the cylinders during operation. Increasing pressures lead to greater heat. If your cooling system is no longer able to dissipate the heat efficiently then engine temperatures will rise above normal. If the radiator fins are significantly bent then air flow across the radiator will not be sufficient.

Often overlooked is the radiator cap. The cap will have a rating on it and a spring loaded plunger. When the system heats up pressure within the cooling system will increase. Fluids under higher pressure have a higher heat capacity (higher boiling point) so if your cap is worn out the fluid cannot retain enough heat before boiling. Boiling is also bad. So cheap insurance is to replace the radiator cap when issues arise to see if that helps or whenever the radiator is replaced. FYI, always use a cap with a pressure rating listed by the manufacturer or suggested by an auto parts retailer if that cannot be found. A higher pressure cap is not better and may cause damage to your system.

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