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93 Geo Prizm is vapor locking

Yes, it’s true. But why? Nothing comes up on codes.Okay on highway. But in stop-n-go traffic, I watch the temp gauge rise to normal op temp and them it gasps and then dies. The car can’t fart. But why is it vapor locking in the first place? When it dies, I remove the gas cap (which flies off) and lift the hood to cool it down and I’m good until I too long at a light again. If I can go a few good blocks without having to stop again, it cools down quick enough to buy me more blocks. Who’s got the magic answer?

I don’t think you have vapor lock. Vapor lock occurred on vehicles where the fuel was pulled from the tank. Too-hot fuel lines would boil the fuel in to vapor, and the pump would lose its prime (the line would get locked with vapor). Your car has an electronic fuel pump that pushes fuel (at a pretty high pressure) up to the fuel injection system.

I think you either have a bad fuel pump, bad fuel pump relay, a combination of those two, or simply a plugged fuel tank vent or malfunctioning emissions canister purge valve.

I also don’t think you’re suffering vapor lock. I suspect that a component of your engine, probably the coil, is overheating and you’re losing spark.

As to the gas cap flying off, you need to have your evaporative emissions system checked. Your tank should be breathing through the charcoal canister, and the purge system should be allowing fumes to be ingested into the engine. These things combined should be not enabling excess pressure to build in the gas tank.

try driving a few blocks with the gas cap off?

You don’t have vapor lock, you have a problem with the vapor recovery system. When you fix that the rest will be fixed.

Are you in the habit of topping off the tank when you fill it?

Thanks everyone. I’m going to run a pressure check and vacuum check tomorrow to test the fuel pump. I’ll have them see if air passes through the charcoal canister to see if it is plugged.
Fuel tank vent? - I’ll have to look into that.
Coil overheating - what coil?
I have driven with the gas cap off and it helps a bit. I also just replaced the fuel pressure regulator, and that has helped a lot, but it still dies. I can drive a lot farther, only now it dies regaredless of what the temp guage says, so I have no warning to pull over!
I don’t fill the tank beyond half anymore because that just made it worse. Like there was more gas to make more vapors or less room in the tank to store the excess vapors. But, no, I don’t over-fill it.
Thanks again for your feedback!

Your car has separate problems. The “coil” is the ignition. Ignition coils are known to, sometimes, become erratic (or, fail) when they get hot from operation, or engine temperature conditions. When the ignition coil becomes cooler, it “heals”, only to malfunction if it gets hot, again.
The ignition coil can be tested. When cool, it may test fine. To simulate the heated condition where it fails, you can run the car around the block, park it (idling) with the hood closed until the engine temperature reaches that temperature where it starts to have that problem. To hasten it to that condition, play heated air from a hair dryer over the ignition coil until the engine starts to have problems. Then you can do these electrical measurements on the ignition coil:
To avoid testing the ignition coil, a replacement costs about $45.

Have you checked for spark and fuel pressure after it dies? If you haven’t then there isn’t any point in guessing what’s wrong until you do so.

I checked it when cold and it’s okay. I’ll do the hot test next time it dies. I am getting ingnition codes now. THANKS!

plugs, wires, dist. cap are all fine. Fuel pressure is all fine, too. Thanks for your suggestions. They are much appreciated!

WHAT trouble codes are being displayed by your scan tool? P03XX?
Several sensors can become faulty when the engine (and sensors) become heated, such as the VSS (Vehicle Speed Sensor, CkPS (Crankshaft Position Sensor), the CPS (Camshaft Position Sensor).
Some people will remove a sensor, and heat it. While heating it, they are watching its resistance with a volt/ohm meter. Any sudden changes to the resistance is an indication that that is the behavior which has been causing the stalling.

HELLOKIT - YOU ARE A MECHANIC GOD!!! IT WAS THE IGNITION COIL!! When it stalled, I hurried and pulled of the dist cap and checked it again and sure enough the ohms were too high and erratic. Replaced and runs fine - even has more power on the hills. THANK YOU THANK YOU THANK YOU!!! I highly recommend that everyone in car-talk land listen to hellokit because he’s GREAT! Or are you a she? If you’re in CA, I’m bringing my car to you to fix next time.

BUT, now for the other half of the fun. You’re right that there is more than one thing going on with my car. The same issue that started all this many months ago is still there. And that is that it chugs when it switches from 1st to 2nd gear. It’s an automatic. It chugs, as soon as it changes, then I just keep my foot on the gas where it was to begin with and it recovers and takes right off after 1-2 seconds. It mostly happens in morning or when cold/foggy. If I take off faster from a stop light, it pulls through that much easier and either chugs less or not at all. A mechanic has recently looked at my gears and says they’re fine, but I take that with a grain of salt. What’s your magical answer?
(By the way, this is the problem that the dealership couldn’t figure out and told me to keep driving it till it got worse and then it’d be easier to diagnose!!)