Hot Start Problems With Your Car: What to Do
If you have ever been stuck in traffic for a period of time or having driving hard and your engine temp is hot when you stop and park and then try to start, yet it fails or stumbles barely starting, you have a hot start problem. Your car may start fine when cold or when warm. Maybe even if hot, but if consistently has problems starting when hot, consider replacing your Fuel Pressure Regulator (they are in every car with fuel injection). Many cars also have a Fuel Pressure Regulator Control Solenoid. In a 91-94 Mercury Capri, there are both and they exist in many Japanese cars.
They both should be looked at for failure. The Fuel Pressure Regulator (FPR) is mounted to the return line end of the fuel rail with the fuel injectors. It is controlled by manifold vacuum and maintains a specified air pressure of 34-40 psi in a Mercury Capri. All cars are different. When the intake vacuum is low, the fuel demand is high and the spring inside the regulator causes the valve inside to start to close which increases fuel pressure in the fuel rail. If the vacuum is high, there is low demand for fuel, and the valve opens so the pressure in the fuel rail is low. The pressure within the fuel lines increases as engine vacuum drops. There is a diaphragm in the center of it with fuel pressure on the lower half and engine vacuum on the upper half. If the diaphragm ruptures, raw fuel will leak into the intake manifold and cause a rich condition affecting fuel mileage and hot engine starts. If ruptured, it will act much like an applied choke on a carburated engine (aiding cold starts and hindering hot starts).
The fuel pressure control valve (PRCV) creates the vacuum needed for the FPR to work, thus, if it is failing, so will the FPR. This PRCV is used in hot starting. During a hot start, the PRCV cuts off the vacuum from the manifold going to the FPR. This permits an increase in fuel pressure and allows more fuel to the injectors. Without this, you car would turn over and over before slowly starting (the car should start fairly quickly hot or cold). Once the engine is going, the vacuum is allowed to the FPR and the fuel pressure is lowered to spec. This valve was meant to help restarting a hot engine. It raises the fuel pressure for an amount of time, when restarting, to enrich the fuel mixture. The Hot Start condition is caused by the intake air temp sensor (IAT) reading artificially high when the engine has been shut off and restarted a short time after due to the high underhood temps and no airflow around the sensor. This causes the ECM to deliver less fuel causing the engine to run too lean until enough air passes around the IAT to bring it down to normal temperatures. So, the ECU uses this solenoid to allow the FP regulator to see atmospheric pressure instead of vacuum pressure increasing fuel pressure to overcome the momentary lean conditions on a hot restart.
The easy way to see if your FPR needs to replace is to remove the vacuum line from the FPR (usually on top) so to stop the vacuum generated by the PRCV. Start the engine. Observe to see if any gas is dripping out of the spout. If there is, the FPR needs replacing. You could also hold the vacuum hose up close to the nipple and watch the fuel be pulled through the FPR. Same conclusion.
If you have problems starting your hot engine (hot temps outside also cause it), find the FPR, usually close to the fuel rail and injectors, remove the vacuum hose. Start the car. It should help because the vacuum is no longer there and more gas will be available (basically, your car cannot start because the fuel is too lean due to heat, so the car requires more fuel to start).