Hello all, what is the purpose of that cap? it's right on the top front of the engine. Mine was cracked and leaking coolant and the other day I got a restricted performance message and the car was running like crap. Would a small leak in that area cause the car to go on restricted performance? I tried sealing the cracks on the cap with some high temp silicone. The car is running fine now but the engine light is still on and there's still slight coolant leak coming from that cap. Thanks for any suggestions.

 

That cap is where you would normally refill the cooling system after a drain and is a common leak location. It is hard to loosen or tighten a 10+ year old plastic cap without cracking it. While it can be replaced for about $20 everyone replaces the entire plastic housing, which will eventually leak anyway, with an aluminum one for less than $70. If you do that you might consider painting it black to match the engine. A bright aluminum one looks a little out of place in the middle of an otherwise all black engine.

A leak at this point can cause faults with the knock sensors located under the intake manifold.