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Coolant leak, not straight forward

Hoping for some pointer here. I have a 92 toyota pickup, 2.4L 4cyc engine, 192K miles and I have been loosing coolant and she runs rough (high idle)in stop and go traffic. Here is the history. Coolant leak noticed about a month ago, loosing about a gallon a month. No obvious external leak, no evidence of coolant in the oil. Took it to the shop and they tested both the cylinder compression and the coolant system. Both passed. They added a “sealant” to the coolant. No change except gunk in my coolant now. Replaced radiator cap. Radiator itself replaced 3 months ago. Never overheated. No white smoke. I am thinking either head gasket or some other gasket maybe? Is there any way to test a bad for a bad head gasket other than compression tests? Any pointers?

How are the radiator and heater hoses? If they are still original, your on borrowed time. Is the coolant reservoir hose in good shape, or does it have any stress/age cracks? Also, check under the rug/floor mats. If you smell coolant in the cab, and the floor is wet, it may be a leaking heater core.

Also, I’d check the vacuum lines for cracks/leaks to solve the high idle. I’m also suspecting an intake manifold gasket leak as a possible cause of both. On this engine, the coolant runs through the intake manifold on the front side, and the thermostat is actually housed on the intake manifold.