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HI yea low pressure can cause over heating ... higher pressure holds down the temp .. just an FYI the coolant level in the radiator when cold should be about 1" below the cap to allow for expansion any more and its to low and could cause overheating.. If you run a catch can it can hold any over flow and it well suck it back in as the motor cools and you won't have any loss..

I'd add a new cap just for peace of mind... 16 lbs.


good luck take care be safe
tim
 

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1990 Ford Mustang LX 351M powered!! Project Cherry Bomb!!
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3lbs can really make a big difference... It will really increase the temp at which the coolant will boil. Higher temperature the coolant will boil mean less apt to boil over and more coolant stays in the system to keep it cooler..
 

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Nah, run the 13 and see what happens
well i just bought a new cap b4 i read this. its a 13# do i need to take it back for a 16? does 3 # make that much difference? the one that was on it was 16# but it was really old.
3# will impact the boiling point by 9°. In your case... probably not a big deal considering you are running a 160° t-stat.

Zero pressure can impact coolant flow... typically at idle. I've seen plenty of cars over the years that lose heat in the winter at stoplights due to a bad cap. These cars would also run pretty hot until you got them moving.
 

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Won't make much difference.
Cam break in is going to create some heat. Have you got the truck out on the road yet?
Pics of truck?
 

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I'll repeat what some others sprinkled-in for focus. Do a search for how to properly do each one:
  • Verify your displayed temperatures are accurate. We find more problems are bad readings than issues.
  • Test your thermostat for proper operation, cracking open at rated temperature and fully open within ±20°F higher.
  • A 13# cap is correct for a 160°F thermostat rating, allowing boil-off before engine damage.
  • Rebuild, modified or old? Check your actual TDC and verify your timing marks with a piston stop.
  • Get a vacuum advance on that thing, as highway cruise is retarded (adding heat) without it. Until then:
  • Rev the engine in neutral to find where your advance is all in. Set your all-in timing to 36-38° when above that rpm for now. Report where idle timing settles to at that point.
  • Use an IR gun to measure temps everywhere, and for clues of hot-spots or differential.
  • Set your warm idle mixture for the richest that will give the max manifold vacuum reading.
  • Read your spark plugs after 10 minutes at consistent high-cruise. Key-off from cruise and coast to the side to read a couple of them. Be safe.
  • Lean is not hot if your timing is set for it. Without vacuum advance, it's not getting enough, so your plugs should be light to dark tan.
  • Verify fan and shroud fit, and test the fan clutch if you have one.
  • Note under what conditions the temperatures rise. This will clue you to what is weak in the system at that point.
  • Report any findings or changes for more specific suggestions.
  • Good luck!
 

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my 545 blower car had the same issue now mine is 545/871/aluminum heads/ mez elec water pump/ msd ignition/38 degrees of advance dist is locked/7 qt pan milodon/hv-hp oil pump hot or cold i run 65 psi and i spin the blower 20% over i switched to a dual pass radiator i dropped 30 degrees and i use a 22lb cap trans is on its own cooler i now run 205 at 95 degrees out
 
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