Why does a Car Start Hard When It Is Hot?

Maybe it has happened to you, it has to me in my 91 Mercury Capri XR2 turbo. You drive around town or on the highway for some time, pull over briefly or stop at a store, hop in the car and the car turns over but starting is difficult and hard, something one associates with cold starts in the morning. With my car, it starts right up in the morning, but, when the engine is hot or 180+ degrees F, it is a more dicey issue. Sometimes, it is not an issue, other times, it is a major concern. What is going on?

First, it has nothing to do with the Turbo, if you have one. It has to do with fuel pressure and sensors that regulate it and fuel vapor in the lines when the engine is hot. In some cars, it is a sporadic issue and weather does impact it. In other cars, some have to wait 20-35 minutes for the engine to cool, to reset various sensors.

While the nuances of cars are different, one can inspect and test some universal items commonly associated with this issue. Names may be different, but the function is the same, as are the engine codes on the car computer that help diagnose.

On fuel injected cars, consider the following and evaluate if a part needs to be checked or replaced.

• Fuel pressure regulator - a on 91-94 Mercury Capri, it is located on the end of the fuel rail on the left, The fuel rail holds the fuel injectors.
• Fuel pump test connector- on a Capri, it is near the right front strut in the engine compartment.
• Fuel Pump switch- in a Capri, it is in the Volume Air Flow meter under the air cleaner lid
• Fuel pump relay-in a Capri, it is center, near the PCM and instrument panel
• Fuel pump - in the gas tank
• Fuel injectors can become faulty and will not operate if the pressure is not correct. If the engine is not getting gas, it will not start.
• Main Relay for the Fuel pump- if it goes bad, you will not have pressure in your fuel system
• Spark Plugs- if they are too cold\hot, it can contribute to the problem.
• Ignitor- in a Capri and many others, it is part of the distributor. If it goes bad, you will have starting problems
• Leaking fuel injectors- if you smell gas or see fuel seeping from the injector site, that needs to repaired because you are also losing pressure. The area around injectors should be totally dry.
• Some cars have a Thermo-time switch with is a sensor that opens and closes that go bad.
• Check your Intake Manifold Vacuum psi
• If your fuel pressure drops after stopping the engine rapidly, even by 5 psi, there is a leak somewhere.

Since you know the car starts fine when it is cold and runs fine, idles fine, accelerates fine, then you know the fuel pump and most everything is working. The change is when the engine is hot and you park and then try to start. Therefore, fuel is not getting to the injectors in sufficient amounts, suspect the fuel pressure regulator, look at the injectors, hoses. 

In my Capri, when it happens, starting is rough and the idle is not stable for a little bit. I step on the gas until it stables. Once stable, the car runs perfectly fine. Just not enough pressure.

Some easy fixes to try first are:

Turn key on, not starting the engine, for 5 seconds. Allow the fuel pump to built pressure, then try to start the engine. If it starts, that is your issue. If not, suspect insufficient fuel pressure getting to the fuel injectors.

Use spark plugs that the car specs recommend. Gap is important. Of course, the coil, plug wires, distributor cap, rotor should be OK also.