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'73 CJ cooling system pressure

The cooling system in my '73 CJ5 with a 258 does not hold pressure. A new cap allows the system to vent through the overflow when it's hot. There are no external leaks and there is no coolant in the oil. The cap makes faint venting sounds when the upper radiator hose is squeezed.

I've tried two new caps and I believe the problem is the filler neck height. The radiator codes show it's original (Blackstone, left factory in June '72). The NAPA parts book shows the same cap is used for CJ '72 - '86 but Quadratec shows the std one is only '74 - '86. Can someone measure the height of the inner seal to the outer neck on a vehicle in this timeframe and send me the measurement? Mine is 3/4" and shows no signs of modifications.

Thanks.

Lee

Reply to
LeeB103926
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3/4" it is, on my 86.

Maybe your cores are plugged so the top tank has too much pressure on it?

I just flushed mine and it will flow the garden hose on full in the top hose hole out the bottom with no backing up. You might want to remove the hoses and try yours.

Do you have a clutch fan on it? If so maybe it has worn out. When the engine is hot and shut down a good clutch will stop the fan instantaneously almost.

I have also seen and had a couple years back, head gaskets blow so that exhaust goes into the rad pressurizing the crap out of it. That I could see because of smoke and tiny bubbles, many many tiny bubbles once the t-stat opened. Some t-stats have a bypass hole that makes bubbles when the engine is just warming up, but once open these head gasket bubbles turned to foam. I got photos of my 258 doing that....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

LeeB103926 wrote:

Reply to
Mike Romain
3/4" on my 81 CJ7
Reply to
Greg

The cap may be new, but is it the right pressure rating. I recently had the same problem with an "82 258. Bought a new cap and trusted the counter person. The cap was rated lower than manufactures spec. Kragen wouldn't exchange it when I brought it to there attention. Lesson learned, don't trust a counter girl that is younger than the vehicle.

Reply to
K Horner

Update: the cap was the problem. The spec cap is apparently incorrect on several brands -- the center vacuum vent doesn't have a spring or it has a diaphram that doesn't correctly close here at 6000ft so there is no opportunity to hold pressure in the first place. I put the cap from my F-150 on it and it holds pressure, so I went and bought another one of those (13# vs 15# so I'll lose a few deg F in the boiling point). The F-150 cap has a light spring on the center vacuum vent which holds it closed so pressure can build on the outer spring.

Thanks for the ideas and measurements.

Lee

Reply to
LeeB103926

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