My heater core is bypassed, car overheats, leaks when started and shakes, lexus is200

lost of possibilities, but without looking at it I would suspect the thermostat is stuck closed. if this is true it would only overheat when the vehicle is hot. Thermostat would require replacement.

There are two reasons that could be the cause. The most common is that the blend door in the heater control box is non-operational and is not routing the air from the blower through the heater core. The other possibility is that the heater core is clogged and not getting coolant flow. Actually, a third (more remote) possibility is that if the car is used, a previous owner may have bypassed the heater core if it was leaking. You would have to check the heater hoses under the hood to make sure they are being routed through the firewall and not connect to each other, bypassing the heater core.

Check your heater core behind dash. You probably will see a leak int the floor board. You can bypass it under the hood for now by connecting the two radior hoses together that go into the passenger side firewall.

Was the heater core leaking? Have you checked your thermostat? By passing a leaking core should stop a running hot problem if that was your only leak. Does your cooling fan come on? Has the car been run extremely hot?

Water pump. The impeller can come loose from the shaft and not circulate enough coolant. These can throw you off, because when you remove the pump the impeller will be tight against the shaft. It is when they warm up they begin to slip. If the heater core is not leaking don't bypass it.

Check the coolant level for over heating issue.If the coolant is low then car will overheat.Also if coolant is getting leaked then also due to that car overheats.Also if there is blockage of coolant in heater core tube then it can be one more reason.Disconnect the inlet and outlet hoses on the firewall that go to the heater core, and flush the heater core out thoroughly with a garden hose.The heater not working,for this check the heater blower motor ,If the motor is faulty then heater will not start.Other cause of heater not getting started is faulty thermostat and faulty coolant temperature sensor.This are the possibilities that are to be checked.Thanks.

ENGINE NOT GOING TO START IF GET TOO HOT FIRST THING I WOULD REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP THEN ADD COOLANT UNTIL COOLANT LEVEL CORRECT THATS WHEN THERMOSTAT OPEN UP AND TOP HOSE HOT.CHECK FOR LEAKING RADIATOR HOSES CHECK TOP RADIATOR HOSE CHECK BOTTOM RADIATOR HOSE.CHECK FOR LEAKS AROUND THE RADIATOR LOOK DOWN BELOW COOLANTS FANS CHECK FOR LEAKS AT RADIATOR CORES OR LEAKS AT THE RADIATOR PLASTIC SIDE CONTAINERS. CHECK FOR LEAKS AT WATER PUMP WEEP HOLE AND CHECK FOR LEAKS AT THE HEATER HOSES AROUND ENGINE BLOCK TO THE HEATER CORE HOSES. THERE IS A REASON CAR OVERHEAT.IF CAR OVERHEAT WHILE DRIVING YOU HAVE FAULTY THERMOSTAT OR LEAKING OUT COOLANT OR BLOWN HEAD GASKET.IF ENGINE OVER HEAT WHILE SITTING DURING A LONG IDLING PERIOD.YOU COULD HAVE FAULTY COOLANT TEMPERATURE SENSOR OR FAULTY COOLING FAN FUSE OR RELAY.HOT WIRE COOLANT TO SEE IF IT RUNS IF NO REPLACE FAN MOTOR. IF YES FAN MOTOR COULD HAVE FAULTY WIRE OR PCM FAILURE. CHECK ENGINE CRANKCASE IF OIL LOOKS LIKE MILK SHAKE ENGINE HEAD GASKET LEAKING.

You should be able to look down in between the motor and the firewall and see if the heater hoses are attached to the heater core. Most of the time when they are bypassed it is because the heater core has started to leak. Instead of replacing the heater core (not a fun or easy job) they just bypass it so they won't leak coolant. So, if it is bypassed your probably looking at replacing the heater core in order to get heat.

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Too much blockage in heater hoses or heater core.
Bypass valve may be stuck on "bypass" .

1. Remove bypass "Tees", but don't discard.
Replace with straight-through hose fittings.
The core may start leaking under the water pressure since there is no bypass hose.

Once the heater starts working, put Tees back in place. You need bypass protection or the engine will overheat.

You can bypass the core or fix it that's up to you but I would change or remove the thermostat first. If it still overheats then start looking at the water pump and radiator. Hope this helps. Good luck.