95 XJ - Coolant boiling -again-!

Hey, I'm not sure what is doing this......first let me start off by telling you what is new:

Thermostat
Thermostat Housing
Water Pump
Upper and lower radiator hoses
Radiator
Radiator Cap

Yet again, my coolant is boiling. I was driving at normal speed's (35-45 mph) with maybe one instance of 4,000 rpm.....the rest was 1000-1500 rpm. I noticed the temp increasing, and getting really close to the start of the red zone. I pull over, turn it off, and all of the coolant boils out of the radiator, into the coolant resivoir. Gushes out of the overflow vent too.

What is causing this? Oil is normal level and not "milky".

I can't think of anything else to do!

Was filled with 50/50 prestone and some Water Wetter.....so it's not like the mixture was wrong.

Thanks!

Which new radiator cap did you put on? A cap that is too light will pop the radiator cap too soon and piss it out to the overflow. Or maybe just a bad radiator cap

13 psi "normal" cap. Same thing was happening with the 13 psi "pressure release valve" cap. I'm not sure why the pressure was building anyway, though.......

Normal = 16psi, right?

Well, it's what came up on the AutoZone computer for my year. Can anyone confirm 13 or 16 PSI?

Radiator condition?

Rev

Your next step is to pressure test your system, if you can't build pressure it will boil over every time.
13# is correct

Radiator is new as of July.

A few things to check. If you notice the hose coming off of the thermostat housing that goes to the heater and the other that returns to the pump. You need constant circulation through that hose, either through the heater or the bypass at the heater valve. It keeps a flow of coolant going around the thermostat, to keep a more constant temp. in the motor. Instead of pulses of over hot, then over cool coolant. In worst case scenerios a batch of cold coolant keeps the thermostat closed and there is a batch of boiling coolant behind the cold coolant (no real reason they should mix much at the thermostat). You need a gasket with the cutout, for the heater hose at the thermostat housing (OEM type), a universal gasket doesn't have this and works poorly.
Having this circulation around the thermostat restricted or plugged, makes the temp. gauge take some pretty wild swings. As will installing the thermostat backwords. A plugged or faulty heater valve is also a possiblility.
Minor coolant leaks can cause a lot of grief. They may drip and evaporate and hardly be noticed. But especially in the closed system, when the motor cools back down, it actually sucks coolant back into the radiator from the recovery bottle, that was pushed into the *recovery tank* by expansion. A very small leak, sucks air much more easily than it leaks fluid, so when the system cools down and goes into negative epressure it sucks air into the coolant system, that often gathers near the rear top of the block and will eventually cause that portion of the engine to seriously overheat which will boil your coolant, which can surge.
Another thing to check closely is the tubing to the recovery bottle from under the radiator cap. The tubing gets hard and cracks and may have a small leak that sucks air. The tubing may be tied down to tight (or kinked) and restricted. The coolant may have enough pressure to go one direction and not the other, like a valve. The tubing could be to soft and also act like a valve. I usually replace this tubing with thick walled tubing.
A head gasket leak, form a cylinder into the coolant system can also cause this, the exhaust gases super heat the coolant and at the same time gather at the high points in the motor, overheating parts of the motor, which can then boil the coolant and cause a surge.

I'm looking at my stash of 4 factory OEM rad caps still in their bags, 1-52079880AA dated 12/03/02, all 4 say 16lbs, quick look outside on both the XJ and both wranglers, same part number, same pressure. 13 is too light.

RichP said:

I'm looking at my stash of 4 factory OEM rad caps still in their bags, 1-52079880AA dated 12/03/02, all 4 say 16lbs, quick look outside on both the XJ and both wranglers, same part number, same pressure. 13 is too light.

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I thought 13 sounded wrong, thanks for double-checking Rich.

Can I buy a 16 PSI radiator cap from you? I threw my origional away (stupid!) when I changed the radiator back in July. Why do AutoZone, Napa, and Advance Auto list 13psi as the correct radiator cap?

Also the gasket I have for the thermostat....It's just a cutout for the large cicle, and another cutout for something else (I'm guessing the temp sensor?)

I'm not sure how you could seperate the heater coolant and the regular coolant as there is nothing for a gasket to seal up against.

Get it from the dealer, price is maybe $2 more for the tstat and factory RTV'd ready to go gasket. Same with the rad cap, I think it was like $5 at the dealer. Certain things I only use OEM, based on the PIA factor of having to replace it.

I don't know what you are saying is bad about my thermostat and the gasket?

The Gasket looks like this:

A problem I noticed with my XJ is that the electric fan wasn't turning on. Run it in idle and see if it ever kicks on. It should also kick on if the A/C is on. If not, I'd check the relay. My relay was fine and I was never able to find out why it wasn't kicking on so I just wired a toggle switch to the relay. Now I can turn it on whenever I want. Sounds like you've checked everything else, so I hope this helps.

Electric fan is working, with A/C or if it gets hot. Just replaced the viscous fan clutch just now as a precaution. Also replaced the radiator cap with a 16 psi pressure-relief valve Stant cap. That's just about everything now.

PS-
There's no difference in automatic operation of the pressure-relief-valve caps and no-lever caps, right? I just thought incase you needed to take the radiator cap off in a hurry, that the pressure-relief-valve would be alot safer in terms of quickly releasing the pressure, rather than getting a hot gush of coolant in your face!