Radiator cap or filler neck problem?

[94SL]

I replaced my radiator with a new, lifetime warranty "Spectra Premium" radiator from NAPA. Actually, the only one they carry for the 4.3 V6. Managed to solve a leak with the trans cooler fittings, then had to solve a coolant leak on the passenger side. I snugged the bottom rad hose, the overflow line, the heater line(?), still leaking. Looking a little closer, looked like it was my brand new AutoZone rad cap. So I went back to NAPA, and bought a new cap. They had two to pick from, so I got the locking cap, NAPA brand. At first it held good, on just little short trips around town. Yesterday I took a 20 minute cruise, and when I pulled over and checked under the hood, I found a little bit off coolant had escaped from under the cap. Not as bad a leak as before, but definitely a leak.


So, any ideas? Are radiator caps getting to be a crapshoot in terms of quality? Is there a better radiator cap I can order from some where? Is it an issue with the filler neck of my brand new radiator? Just looking at it, nothing obviously wrong with the filler neck I can see. A buddy suggested making a rubber gasket to slip inside the cap overtop the existing rad cap gasket, to hopefully improve the seal. Could try it, but might cause other problems, maybe?

I guess I could live with it if I have too, but would really like to not have to worry about it getting worse.

 

[454cid]

I bought a new cap several years ago, from Orielly that didn't seal. When I installed it, I noticed right away that the spring didn't have to compress much at all. It felt much different than the old cap. It didn't hold pressure. I don't recall if it actually leaked or not... probably did, or maybe it ran hot? I replaced it with a Stant, and it wasn't quite like the previous cap, but did last many years. I've since replaced it again with an OEM cap I got on ebay, and it fits really well.

Let me see if I can find the link... the guy had a bunch of them, and probably still has some.

 

[454cid]

Looks like he's out, but here is the post where I talk about it.

Radiator cap problems

https://www.ebay.com/itm/GM-OEM-NEW...83.l44720.c10&nordt=true&rt=nc&orig_cvip=true

It looks like there is more than one version of the RC27. I have the black plastic covered cap, which is what our original caps looked like. I'm pretty sure my original cap (gone years and years ago) was a 16lb cap, not a 15lb cap like my current one.

 

[arrg]

I had the same problems with the leaking generic caps. I put the this AC Delco RC27 on, and the leaks stopped.
https://www.amazon.com/gp/product/B000C9SP4W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[randomguy]

I put one of those Spectra radiators in my truck and reused the old crusty cap. No coolant leaks but the transmission cooler lines were a pain in the ass to get to seal correctly. The radiator is okay, wasn’t my first choice but the radiator took a dump on the opening day of deer season.

 

[df2x4]

I've had issues with cheap parts store caps. I run OEM ACDelco caps on both of my trucks, like what @454cid posted.

EDIT - Both of my trucks are '97s, though. They use part number 15982188. Just looked up a '94 GMC SL C1500 with the 4.3 on GM Parts Direct and the GM part number for that factory radiator cap should be 10409635, which looks to be the same as the RC27 that @arrg posted.

 

[df2x4]

Here's a link to the 15982188 I use on my '97s from Amazon. I'm assuming this is standard issue for the '96+ Vortec trucks, it's what both of my trucks' factory caps always looked like.

https://www.amazon.com/gp/product/B000C9CEOO

But like I said, I think you want the 10409635 that arrg posted.

 

[PlayingWithTBI]

Just a thought, are you sure the hose to your overflow tank isn't plugged?

 

[bowtie-72]

Whenever I need a cap I just snag a nice OEM one of the same type at the local Pull-a-Part. They practically give them away.

 

[98 Nitro]

Here's a link to the 15982188 I use on my '97s from Amazon. I'm assuming this is standard issue for the '96+ Vortec trucks, it's what both of my trucks' factory caps always looked like.

https://www.amazon.com/gp/product/B000C9CEOO

But like I said, I think you want the 10409635 that arrg posted.

That's the one I bought last year to replace a generic one, works good.