Q: Expansion tank cap keeps cracking and the car overheats in stop and go traffic.

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The car is old and from the east coast so she's got a rusty undercarriage. Has had several problems but still runs nicely. Awhile ago it overheated for the first time. So bad that fluid sprayed everywhere. I took it in and a hose had come loose they reattached the hose. Immediately after leaving the shop she overheated again so I put water and coolant in the radiator. That worked for awhile but just kept overheating so I took it to another place. They did a bunch of work and replaced the radiator cap. Still overheated! I noticed the expansion tank cap was cracked so I bought the part myself and replaced it. Boom. No more overheating!!!! For about 6 months. Today, she overheated while I was sitting in bumper to bumper traffic. First thing I check? That freaking new cap was cracked in the same way as the old one. What is causing this? Some have told me there's coolant in the engine, or the thermometer is messed up. The fans don't seem to work (but they do sometimes). Help!! :)

My car has 180000 miles.

It is impossible to tell without doing some further testing. Your issue may be caused by a compilation of several different components. A run of 6 months is pretty good to not have any issues. Slow traffic can cause the engine to heat up quickly. High heat leads to high pressure which can cause the cap to fail. When you say that "a bunch of work" was done, what exactly was fixed? What wasn’t?

The radiator cap is meant to relieve high pressure situations, and it may have failed to vent. The fan is critical for the decrease in temperature when sitting in traffic. When the vehicle is moving, the air coming through the grill works to help cool the coolant in the radiator. The fan is in charge of this task when there is no air flow.

I recommend having a certified mechanic, such as one from YourMechanic, come to your location to determine why the car is overheating.

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