My 85 F150 with a 351W 4vHo started leaking antifreeze out of the seam on the passenger side of the radiator. At that time my heater core also went out, so I replaced that. When I refilled the radiator, I only filled the radiator. And I also took vice grips along the seam to see if i could tighten it down. The leak then stopped out of the seam, but I didn't know what fixed it. So a few days ago I put the remaining fluid in the overflow tank so it could fill the radiator. After It pulled some fluid in, i noticed that It started leaking slightly out the seam of the radiator again. It wasnt pouring out, so I left it sit for a day. The next day i took the radiator cap off, and fluid still shot out at me. I tried to manually open the valve on the cap by sucking on the end of the overflow tube and nothing would come through with the cap on.. With the cap off its clear.. I put on a 13 lb cap from my mustang, and other caps from various cars and trucks, and they all let fluid go through. But the thing is they all leak out of the cap. I don't know what the deal is with this thing, but I dont think my old cap is working right.

 

Are you sure you don't have a small leak around the radiator neck that allows coolant to leak out? Have you done a pressure check and cap test using a tester? This is the only way I know of to determine if and where you have leaks and also to effectively check the cap. I think your seam leak is probably due to mineral buildup at the seam which forces it open. Common problem with soldered tanks and I am sure with the rubber gasket sealed tanks as well. How many miles on this radiator and can you see if you have mineral buildup at the radiator tubes?

John

 

The radiator has never leaked around the neck previously. And it doesn't leak when I use my original cap. I don't have any testers so I havn't pressure tested it. I think the truck probably has 106000 miles on it, but I don't know if anything has been done to the radiator prior to me getting it. When I first got the truck, the fluid was pretty bad. Real dark, and kinda rusty. I flushed the thing with water quite a few times, and the new fluid has stayed pretty green. I changed the hoses, and they were rust colored inside.

 

My bet is that you have a radiator that is in pretty bad condition, given your description of the coolant and hoses having rust. Best bet is to pressure test the system to determine where the leaks are. If it turns out the radiator is the problem you might have to have it recored or replaced based on cost. You might be able to get a pressure tester on the lender program at Autozone. Otherwise, you might have to take it to a radiator shop to have it tested. I am not clear as to whether you replaced the cap with an exact match or with ones from other vehicles thinking they would match. Get a replacement cap that meets the spec of your truck since while a cap can look the same it may not be the same spec. as to sealing surface, relief, etc. Let us know what you find out.

John

 

I think the problem is in my cap. After working with it, I was able to get it to release the fluid when it heats up. It doesn't leak out of the radiator anymore. When I switched caps, they were a 13 lb cap from a 71 mustang, a 15 lb cap from a 71 Dodge truck, and a 14-18 lb cap from a 93 Chrysler new yorker.
I will see if anybody i know has a tester to make sure.

 

jpjahn,
Consider this. Purchased new rad. for my truck. After start-up and heat, leaked at the pressure cap. Turned out that the locking wedge tabs at the filler neck that the pressure cap engages, one of those had snapped, but it was not visible with cap removed because the tab relaxed into it's normal position, but put on and twist the cap, and it would pull that tab and the cap would tilt in the filler neck, causing the gasket to un-seat. Check that filler neck real close, especially if it is made of plastic.

 

I had a problem very similar to yours. It did not turn out to be the cap. Go to a junkyard and buy yourself a radiator, make sure to have it pressure tested. Then, buy a radiator back-flushing kit. Swap the radiator and flush out the system.