Radiator too large, tstat too high?

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Sooper Pooper
Sep 5, 2010
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The 26th State
Drove truck into town to get the leak test kit this morning.
White smoke startup @ 34*F, clean exhaust sometime before engine temp hit 180*F. Smells normal, no sweetness before and after warmup.

Seeing your exhaust is normal at 34F.

Radiator cap had leaked at cap seal by the time I got home per usual, wasn't actively leaking when I checked but the neck and everything around it is wet with coolant.
I Let the truck cool for a few hours and then attempted radiator leak test once block and radiator cold to touch, test failed. Discovered or created radiator leak on driver side at plastic/aluminum seam @ ~15PSI.

Well there you go, you've got a bad radiator.

I don't earnestly believe the leak at the radiator seam existed previously,

Why? You have a leak..... you found it, why would you think this is somehow not the leak you're looking for???

it starts to leak fluid noticably in a constant pouring stream before 10PSI now and the radiator cap appears to be good for up to 16PSI as rated, yet had still leaked before I got home at some point.

Either I'm not understanding what you're trying to say, or you're not thinking this through. If the radiator has a leak, no cap of any rating will make it not leak.

Maybe the tool is out of calibration and I inadvertently pushed it to ~20PSI? The radiator cap tests lead me to believe otherwise..

Calibration doesn't matter much for a leak. You're not trying to measure the pressure it takes to make the leak, as much as you're trying to find a leak.

So for now, the leak test is a dud bust

No, you successfully found the leak.

I'll note that the plastic clip on the reservoir return line above the cap seal is a tad loose, I can pull, push and twist it on the stem until it hits the swell. I'll swap this for a screw-type clamp.

Leave it alone, it's fine. Pressure it gets is minimal. I replaced mine with a steel constant tension band clamp, only because I bought a whole bag, for something else, and had extra. I still have the original in my parts stash, and it has around 300K miles on it.


I'm Awesome
Sep 16, 2020
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Try using English. You broke the seal, then replace it. Oh you meant it relieved pressure, as it should. Don't use slang, it doesn't mean the same to everyone.
Your halfway right and wrong.
You proved you have a broken radiator through testing. Unless you you broke it, it's a defect. Your pressure test wouldn't damage it as it has a higher burst pressure as safety factor. As you drove it around airflow blew coolant all over. Why would you ignore what the testor proved in testing the system. Cap worked as designed. So did the radiator test.
What lines do have plumbed into the trans cooler? You have auto or manual ?

Erik the Awful

Supporting Member
Supporting Member
Aug 9, 2019
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Choctaw, OK
Fire it up and watch down the radiator neck. Is it blowing bubbles?
Smells normal, no sweetness before and after warmup.
No sweet smell does not mean no leak. My '99 Suburban had been overheated to hell and back. It ran perfectly smooth and didn't have any sweet smell in the exhaust, but it sure pumped the radiator full of combustion gas. It had a cracked block.

This is a simple test. Take the cap loose and fire it up. Five minutes later check it.


OBS Enthusiast
Jan 25, 2023
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If yours is not under warranty, 1-800-Radiator is significantly cheaper than autozone/napa/oreillys...

1 (800) 723-4286


I'm Awesome
Nov 11, 2020
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a few years back i did a similar upgrade on my squarebody, went to a 34" rad. to my surprise the rad I got was copper. Well 4 rads later i still had a leaker. ended up taking to rad shop and they replaced the rad cap neck, soldered and "baked" the rad to the tune of $500.... I know the gmt400s came standard with plastic tank/aluminum core but if you have a copper rad, just get it recore'd. not worth ******* with this off shore JUNK