Solved-cylinder head vapor vent line was clogged at the filler tank preventing steam bleed off and forcing steam back towards the water pump displacing coolant and causing the system to build up pressure and to act as if the rad cap was weak

Original post:
While sitting in traffic, the car ran hot one morning. Not super hot, around 220 when it normally runs about 200. I stopped at the autoparts (O'Reily) store and added some 50/50 mix and while at it replaced the radiator cap. Everything was fine for the rest of the day but on the way home it ran hot again.

I stopped and alot of water was missing. I figured there was an air pocket so opened the bleed screew and I added some more coolant mix and back on the road. Now it seems that anytime the car gets to 190-200 degrees I start losing water through the radiator cap to the overflow tank. I went back and exchanged the cap and same problem. The water is not boiling (no burping and gurgling sounds) and stops flowing shortly after I turn off the car. Anyone have problems with the OReily caps or might there be a reason I might be exceeding pressure causing the cap to blow off.

Car Info: 96 LT1 (stock everything) with stock remote fill near the firewall passenger side.
Replacement Cap: Murray Ultra Part# 7816

 

stuck t-stat

 

Replaced the TStat yesterday. The car is staying cool until it runs out of water from the overflow.

 

Is the cap, regardless of part number, the right pressure rating? I know it's somewhat high, like 18 psi. I have a pressure tester so I can test if caps are right. The cap must be rated at a pressure higher that the pressure at which anti freeze will boil at the engine's highest operating temp + some, since the coolant can get hotter at points inside the head and you don't want steam pockets in there. So the cap's pressure rating prevents boiling but it doesn't sound like you have that problem.

If the engine starts up cold and there's no air in the cooling system as the coolant heats up it expands, causing the pressure cap to relieve and coolant to go into the overflow bottle. How much depends on the difference between the min & max engine temp. This is normal. When the engine cools off it sucks the coolant back in. However, the outer (Larger) seal between the cap & radiator must be good or the radiator will suck air back in, not coolant.

So the fact that more coolant is going into the bottle might just be because the engine is getting 20 deg hotter than it used to.

Does it run cooler at highway speed, when ram air, not the fan cools the engine? - maybe it's a fan problem.

When the engine cools off does it suck all the coolant back into the engine so that the bottle level goes down and when you take the cap off the radiator is full? - that would be good. If not maybe the outer cap seal to the radiator is bad or maybe - and I hate to say it - there's a head gasket leak adding exhaust gas to the cooling system and pushing the coolant into the bottle, in which case the engine also would not suck it back in.

 

220 is normal when stuck in traffic. You are probably filling your overflow past the fill line causing it to overflow when the radiator is hot and coolant runs from the radiator to the overflow. When your car is cool, remove coolant from the overflow until it is at the full line. Drive away and don't panic when the coolant temperature hits 220. 235 is another issue.

 

Cap is a 16lb cap. Water seems to be getting lost. I keep having to add water. I tried this when got home.....opened the bleeder on the water neck and started the engine. I kept adding water until water came out the bleeder and I .closed it. I left the cap on and let the water circulate, I could see it rise and fall and occassionally let bubbles out. I kept adding water until it was at a stable level. The whole time, temp stayed at 200-201.I opened the bleeder valve again...just air and the water level dropped. I added water again until water came out. I did this numerous times with the same result.

 

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