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What's wrong with the new radiator cap?

I have a 2000 chevy malibu. Recently, I replaced the coolant reservoir cap. First thing I noticed was that no heat came out of the vents. Then the engine overheated. I put the old cap back on. Things returned to normal. What could be wrong with the new cap?

I have a sneaking suspicion that the problem was a failure to properly fill and void the system of air rather than a failed cap. I’ll even guess that when you refilled the system you did not warm it up with the heater on (which opens the fluid valve) and thus left the heater core full of air.

A cap, assuming it’s the correct cap, can only fail in one of two ways: it can fail to release pressure into the reservoir beyond it’s rating, or it can hail to hold pressure up to its rating. Yes, a cap can be tested.

Where did you get the cap? Does it look much different than the original?
Some aftermarket parts are junk, or just don’t fit properly.

OR, the new cap is rated for the wrong PSI. A cap with lower than specified PSI will open to release pressure too soon, killing to cooling capacity of the system. I suggest you return that cap as defective and get another.

Thanks for the replies. From what you guys are saying, I have a theory that the defective cap was causing coolant to overflow, and engine overheating was due to low coolant level. Here is the full story.
On Friday, I took the car to Shop A for a brake repair. I asked the owner to also check the cooling system. According to the receipt, he topped the coolant and replaced the coolant reservoir cap. He mentioned to me that the car ran hot and told me to keep an eye on it. The car did not run hot before, so I didn’t pay too much attention.
I drove the car home Friday night. On Saturday morning, there was a pool of coolant under my car. I thought they must overfilled the coolant tank, and again ignored it.
I made a few short trips over the weekend. It rained. I didn’t noticed if there was more coolant leaks. On Sunday evening, while driving with my wife, she pointed out that there was no heat coming out of the vent. I noticed the temperature gauge was near the red line. Then there was heat coming through the vent and temperature needle dropped somewhat.
Monday morning, half way to work, my car over heated. I stopped at a parking lot for about 30 minutes. Since Shop A wasn’t open, I took it to Shop B after the engine had cooled and left the car with Shop B.
Shop B did a diagnostics and came up with a long list of items to fix. Nothing on the list would cause engine overheat, in my opinion. I paid for the diagnostics and left.
I called the owner of Shop A. He asked me to bring the car back. It’s only a couple blocks from Shop B to Shop A. At Shop A, we put the old cap back on and ran the engine for a while. the temperature was normal.
I left Shop A and drove to a Jiffy Lube. They flushed the cooling system and put a new cap on. The engine is running fine since.

Given that your gauge hit the red, I will keep a close eye on oil/coolant levels and also under the oil cap for foaming. There might have been some damage.