Radiator cap questions - lack of pressure - JeepForum.com
 
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post #1 of 6 Old 01-18-2017, 11:53 AM Thread Starter
8v-of-fury
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1998 ZJ 
 
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Radiator cap questions - lack of pressure

So on a recent thermostat change and subsequent coolant burp.. I noticed that after taking it for a drive and getting up to temp, there was still a bit of the heater core gurgle. Pull in to the drive, and squeeze the top rad hose, its soft. Weird. I take this as a safe time to open the little rad cap on the "flush T" I installed in the heater core hose. Although the coolant was hot, there was no system pressure. I rigged up my funnel and hose and added the coolant mix.

I also ran the engine with the funnel hanging and full. After a bit I noticed the level going down, so I added more. However, this was filling the coolant reservoir tank by the PCM as I noticed after I filled the funnel for a second time.

Isn't the cap supposed to keep the tank cut off from the system until pressure is needed to be bled there? And then also when the pressure subsides and it wants to suck the coolant back in to the system from the tank?

I feel as though with no pressure after a drive I either still have a HUGE air bubble somehow trapped somewhere or a dead radiator cap not holding back ANY pressure. As possibly evident by it filling the reservoir bottle via the heater core hose filling.

Thoughts? I am going to get a new cap and see what it does, as I am sure the one that is on there is near original.. if not at least 10+ years old.


'98 ZJ 4.0/242, Locked d30/d35, 31x10.5x15, 2" BB w/ Bilstein 5100's
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post #2 of 6 Old 01-18-2017, 12:10 PM
StPaul59
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Not sure about the 4.0s, but getting air out of the V8s after draining the rad is a beech. You practically have to fill through the thermostat housing or water pump tube to have an easy time getting the air out and flowing. I had mine up on ramps and using the funnel and still was a pain to get the coolant going..
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post #3 of 6 Old 01-18-2017, 12:21 PM
vlackattack
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When I changed the radiator on my 4.0 I just parked it with the front uphill and the lid on the reservoir off and ran it until it got up to operating temp. and haven't had any problems with it, that was like 2 years ago. I would try bleeding the air again, but a new radiator cap isn't a bad idea if it's that old.
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post #4 of 6 Old 01-18-2017, 02:03 PM
kg6mov
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It sounds like your radiator cap isn't functioning properly, which will make it hard to burp the system properly.


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post #5 of 6 Old 01-18-2017, 08:51 PM Thread Starter
8v-of-fury
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Coolant shouldn't be going to the reservoir tank with a "cold" system should it? Indicating a bad rad cap, perhaps.

I didn't drain the rad, or even the heater core really. I drained what was required by taking the thermo housing off, and whatever came out came out. So the head drained down a touch I suppose. ANYWAY I filled the system back up VIA the heater core hose going to the heater core first before the rest of the system. The thermo housing nipple was blocked off with a rubber plug.

New cap in the morning regardless, so we'll see if that helps gets the system pressurized.

'98 ZJ 4.0/242, Locked d30/d35, 31x10.5x15, 2" BB w/ Bilstein 5100's
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post #6 of 6 Old 01-19-2017, 12:23 AM
Uniblurb
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Agree it seems like your rad cap is malfunctioning or not sealing if it's letting coolant added to the heater hose flow into the reservoir tank.

But maybe your coolant is still cool enough where the cap may not be designed to seal until it heats up more? On engine/cooling system cool down there's a vacuum created by the engine cooling down and the coolant is then drawn back into the cooling system from the reservoir tank. Know you stated the same thing and the cap seal has to be opening when cool.

If you look at the below photo the nipple in the rad neck is above the seal. Sorry if the photo is of a XJ instead of a ZJ but best one I could search out. That cap seal has to open/release pressure once the hot coolant psi on the cap is reached along with opening on cool down to pull coolant from the reservoir.

Good idea on buying a new cap and they're cheap. Hope you get your heat to work better and good luck!

Here's the photo (courtesy of chaddwick2005) of the rad neck with nipple/hose to reservoir above where the cap seals.
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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