Cold water is leaking on to hot side causing luke warm water

While it doesn't seem likely, if you've changed, or had the thermostat changed.... I'd say you got a faulty thermostat. I am curious as to what your temperature gauge says the temperature of the engine and coolant is.. If your gauge is showing hot, I can think of two more possibilities... the water pump isn't pumping right... and, if the water pump isn't working right, the problem can be a cracked head or block, which can cause an airlock in the coolant system. Check the coolant or radiator cap.

Yes it can, leaks , low level coolant, stuck thermostat can cause this issue

If one of the hoses to the heater core warm and the other is cold then the heater core is restricted still. Try disconnecting the hoses at the core and flushing to core with a hose go from one side to the other you can likely clear it out and save it. Also check the radiator cap and radiator it sounds like the system is not holding proper pressure. If this is a 4.3 they have a vary large cooling system and the radiators are knowen to have problems not holding pressure they are leaking and you can't even see it. It leaks in a fine mist and is despursed under the vehicle when driveing . For now i think you can save the heater core but it sounds like you are going to have to do a total mantintance on your colling system when the weather allows

Also check that by-pass valve(s) is/are, in proper position at back of tank.

Yes, change the cartridge, it sounds like the cold is bleeding into the hot.

you could be having a problem elsewhere in house system! do you have a garden hose hooked up with a "Y" hose? or something in system were hot and cold connect...then a valve after? what happens is the hot water can back siphon through cold lines..its rare, but this happens, and is dangerous! water from hot water tank is full of bacteria, and if you drink it , well thats not good!

Modern shower valves have an "anti scald device" in them to prevent scalding from to hot water.

They come preset at 120 degrees.

Sometimes they can start to malfunction and all you get is warm, not hot water, they can also be intermittent.

This will only effect the shower valve though, not the other faucets.

Hi blondi_1452,

I'm Harvey the Master Plumber.

Your question is: Why would the cold line water be luke warm?

In 30 years I've seen this maybe 10 times. It is relatively rare. But each time it was a cross connection between the hot and cold water lines. It could be this fixture is close to the hot water heater or the cross cnnection is in a faucet or it is in the way the water lines are piped. Anyway I would have to closely inspect the system while running the water in the fixtures in different combinations. Once the cross is located the easiest fix at this point is installing a check valve at the right point. You might consider utilizing thr Repair Directory to find a repairman. Make sure he has solved this type of problem before.

Feel free to contact me again!
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Thank you,
Harvey your Master Plumber