QuestionI have changed the radiator, water pump, hoses, thermostat, sending unit, and back flushed the engine and it still overheats after driving fast on the freeway. What about more additives to the radiator?
AnswerRadiator additives won't solve the problem, and one of the best, from Redline, will help by just a few degrees, and mainly on ultra high performance engines.
The best coolant for a street car is a 50/50 mix of regular anti-freeze, the green stuff.
If you have a cooling problem, then you have to find the problem, not band-aid it.
Under band-aids I include additional electric fans. the car was designed to run without it, and you should be able to make it do the same thing.
Here are some ideas.
If an engine overheats while running down the highway on a flat level road, that is a big indicator that you may have a leak in a head gasket. It is possible to have combustion gasses get into the coolant without the coolant getting into the combustion chamber (one way). This is why it overheats at highway speeds, but not at idle where most coolant problems happen. Those gasses just superheat the coolant.
It is possible to check for this, first, look for air bubbles in the radiator (be careful taking off the cap of a hot engine!),but since it happens mainly at speed, this is hard to do until it gets worse.
You can get the coolant tested for combustion gasses.
here is a site that mentions a test kit you can get from a NAPA store:
http://www.troubleshooters.com/tlitthypothesis.htm
Here is another site that talks about a test:
http://www.testtoolsinc.com/instructions.htm
Now, the problem may NOT be a bad head gasket, and the tests above won't show which head it is, so you may have to replace both, or wait till it gets so bad that you get white smoke in the exhaust and that sweet smell from the tailpipe. In that case pull plugs and you will be able to tell which one is leaking. In fact, pull your plugs and look now, and smell your exhaust anyway, just in case.
NOTE: this leak CAN happen without using a lot of extra coolant, so not having to add coolant isn't necessarily needed.
Also, you might want to check for coolan flow, a rebuilt water pump can still have bad parts sometimes. A new thermostat can stick partially or totally closed.
A flushed engine can still have junk at the bottom of the block restricting flow (pull a freeze plug on each side to check for junk in the bottom of the block.)
you should also check that the bottom hose isn't collapsing at speed. THis happens if the hose doesn't have a wire support inside, or if the support is rusted and gone. If you can easily squeeze the hose it should be replaced.
What rear gear are you running? If a very low gear (bigger number like 3:90 or 4:11 etc, then are you turning a lot of rpm's at highway speeds? 4000+ is too high, and you should either slow down or get different gears, or an overdrive tranny.
How is your fan blade? Is the factory shroud in place? These can be problems at low speeds though, so are not suspect at highway speeds where airflow through the radiator should enough.
An auxiliary electric fan may bring the temps down, but they should not run at highway speeds, so that is another band-aid.
Finally, how good is your radiator? If it is new, did you get the right one with enough cores? A 390 creates a lot of heat, you need a good radiator, maybe you got one for a 6 or 289? A quality four-core radiator that is proven to flow correctly might solve the problem. But again, if the radiator is seriously wrong, it should show up at idle too.
I hope I have given you some ideas.
Don
