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PostPosted: Sun Nov 24, 2019 2:05 am 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 13305
Location: CT
We are all familiar wth weakened LOWER radiator hoses which can collapse while driving, restrict coolant flow and cause overheating at high speeds. That condition can be a real bear to diagnose because it only occurs while your engine RPM is fast enough for the water pump to pull cool water from the bottom of your radiator so fast, that a weakened lower hose collapses under the"vacuum" (difference in air pressure). It's literally a Venturi effect inside your lower hose.

At low speeds or at an idle the hose remains normal so your engine stays cool, but at high RPM the lower hose collapses and your engine overheats. While you look under the hood at idle the lower hose is normal, so trying to diagnose that overheating problem can be frustrating. Many lower hoses now have a coiled spring inside them to combat such collapse.

But far less frequent is a collapsed UPPER radiator hose. Those are apparently so rare that I just noticed the very first one in my experience last week. The upper hose in most cars is under pressure most of the time. It carries very hot coolant from the hot engine block, thru the thermostat, into the top of the radiator to be cooled, so the internal pressure will always threaten to burst an upper hose rather than collapse it. When I noticed a collapsed UPPER hose on a parked and cold ZX I couldn't picture how that could happen. When I checked the coolant level I found it appeared normal, but the radiator cap opened with a WHOOSH. I assumed it was pressure, but it turned out to be vacuum.

Research showed me either a bad radiator cap or a plugged return hose from the radiator overflow reservoir (usually both) will cause the upper hose to collapse as the engine cools off. Here's how that happens:

As engine coolant heats up it expands and bleeds out into the overflow reservoir. When the engine cools the coolant contracts, causing a slight 'vacuum' inside the sealed cooling system. That vacuum should pull the overflow reservoir's coolant back into the radiator, restoring the radiator coolant level back to normal.

It's normal for a little hot coolant to overflow out of the radiator. In the days before overflow reservoirs that expanded coolant simply dripped down the overflow tube alongside the radiator and onto the ground under the car. The responsible driver simply kept adding coolant to his radiator to top the level off before driving each day.

But if the overflow reservoir's return hose is clogged, the coolant makes a one-way trip; the reservoir traps the coolant and won't release it to return to the radiator. Each time the engine cools completely, it causes a little more deficit in engine coolant volume, eventually resulting in a small 'vacuum bubble' btwn the radiator cap and the cold/closed thermostat. That, of course, is exactly where the upper radiator hose is. The resulting 'vacuum' inside the upper hose causes it to visibly collapse inward while the car is parked and cold.

A properly-functioning radiator cap is designed to release pressure or 'vacuum' in either direction. By opening the (faulty) cap I relieved the vacuum under it, causing the WHOOSH and letting the collapsed upper hose fill with air. Altho the coolant level appeared correct it wasn't; the reservoir had trapped the coffee-mug worth of coolant which the cold radiator wanted back. The clogged return hose wouldn't let the reservoir drain back into the radiator, and the faulty radiator cap wouldn't relieve the 'vacuum' trapped btwn the cap and the thermostat, causing the upper hose to visibly collapse inward.

So a collapsed upper radiator hose on a cold engine indicates one (or probably both) of these problems. Replacement of the radiator cap and cleaning of the overflow reservoir's return hose is the solution.

*[I suspect an incomplete filling of the radiator and block, resulting in a cooling system airbubble, might cause the same upper hose collapse when the engine cooled. But that would result in a constant overheat at all speeds and the coolant level under the radiator cap would be obviously lower].

Who else has experience with this?

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1970 240Z


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