It’s terribly frustrating when your car’s engine will start and run wonderfully when it’s cold, but keeps stalling out when it warms up. You come up to a stop sign or a stoplight, and it dies. Sometimes it even happens if you let off the gas while the car is moving. Whether it happens while you’re driving or as you come up on a stoplight or stop sign, it can be dangerous. You need to figure out what’s causing the issue and fix it as quickly as you can. Here’s some places to start looking.
Hot Outside and In the Engine Compartment? Think Vapor Lock
When I hear someone telling me their engine stalls coming up on stop signs or lights, but only after it gets to normal operating temperature range, I immediately start thinking vapor lock. This is a condition where the fuel pump isn’t delivering enough pressure and the fuel boils in the fuel lines. Usually this only happens when the outside temperature is pretty warm, heating up the fuel in the lines up to the engine compartment. Once it reaches the engine compartment, the higher temperature there causes the fuel to boil. When this happens, the fuel is vaporized. Whether you have an injected or carbureted engine, it’s not going to run on only fuel vapors.
If you have an injected engine, the way to check this is to rent or borrow a fuel pressure tester and follow the included directions to test the fuel pressure at the fuel pressure regulator. Simply disconnecting the fuel line and looking for liquid fuel to spurt out isn’t enough. You have to test the pressure and it needs to be at least 40 PSI, but preferably 50 PSI. Perform this test when the stalling happens.
With a carbureted engine, remove the air cleaner and look down the throat of the carburetor as you crank the engine and move the throttle. You want to see drops of fuel or a spray of fuel as you move the throttle linkage. This test also needs to be done while the stalling is taking place.
Overheated Fuel Pump Relays on Injected Engines
Fuel injected engines use electric fuel pumps to generate the amount of pressure that fuel injectors require. When the stalling happens, check to see if the fuel pump is receiving power. If not, check the fuel pump relay. If the fuel pump relay isn’t closing and delivering power, check the automatic shutdown relay. The easiest way to test these is to look at the numbers on the relays, and swap them with other relays with the same numbers.
Overheated Ignition Coil(s) Cutting Out
This usually doesn’t happen with newer cars because coils don’t usually fail very easily, but it’s something that’s actually the easiest to test. Pull a plug wire off the plug, stick a Phillips screwdriver into the plug boot, hold it near a good ground, and have someone crank the engine as you look and listen for a good spark. If it’s anything less than a strong spark that you can both see and hear quite well and you have a coil problem.
If all of the above tests give you the results that they should, then you need to look at the ignition switch overheating. Feel the ignition key or the ignition key lock itself. If it’s noticeably hot, there’s extra resistance developing as the circuit inside heats up. There are a number of reasons why your engine could be stalling at idle when warmed up but at least now you’re closer to finding the culprit.