This article presumes that the reader has access to the above documentation. Our intent is to point you at the right areas of the manual for further investigation. See also a related article about Cold Start Issues and No Start Conditions (to be written).
Theory/Diagnostics:
It is not unusual to see cases where a DeLorean engine will fail to start, or be difficult to start, when warm or hot. There are several common causes. With some methodical troubleshooting techniques you can avoid replacing the wrong parts.
The most important factor in maintaining the ability of the engine to start when warm or hot is the retention of fuel pressure in the fuel system. The Bosch K-Jet is a mechanical Fuel Injection system, and the injectors are opened by fuel pressure of approximately 48 PSI. (Contrast this to a more modern Electronic Fuel Injection system where the injectors are actually tiny electrical valves that can open and close regardless of fuel pressure).
When the engine is started cold, there is a single electronic injector that opens to allow an extra "shot" of fuel into the engine during starter operation. This speeds up the presence of fuel into the cylinders. This "shot" is not present with an engine/coolant temperature above 95F (35C). Refer to Cold Start Issues for more information on this circuit.
When the engine is turned off, the fuel system is designed to maintain fuel pressure at about 40 PSI for as much as 8 hours (specification is only 10 minutes per the manual, but as a practical matter it should hold for at least 30 minutes). If pressure is not held, the heat from the engine ("heat soak") will cause the fuel in the injection lines over the engine to boil, i.e. filling the injector hoses with fuel vapor rather than liquid fuel. When this happens, the vapor will not allow sufficient pressure to open the injectors, and will compress behind the injector nozzles instead. Eventually with enough cranking of the engine the vapor may purge through the injectors and the engine will start, but at higher temperatures the car battery may run down before the engine is able to start.
Refer to the service manual page D:02:02 for instructions on connecting a fuel pressure gauge to troubleshoot the system.
If you don't have a pressure gauge available, you can get a coarse approximation of fuel pressure by removing the air cleaner assembly and pressing lightly on the flap at the center of the intake manifold assembly. Do this the first time without running the engine to get a feel for the pressures involved. When the engine has been off for several hours, the center flap should offer no resistance to a gentle push. With the engine running or just shut off, you'll feel a pressure back (this will stall the engine if you try it while running). You should still feel a noticeable pressure for up to 30 minutes.
Common Failure Modes:
There are 5 potential fuel system failures that can cause the above problem to occur. It is important to do some troubleshooting, as just "replacing all the suspected parts" can be unnecessarily expensive. These failures are explained in order of how common they appear.
1 - Fuel Accumulator P/N K100519: The fuel accumulator is a pressure reservoir located between the fuel pump and the engine. Internally it is a rubber diaphragm over a spring.
Due to the use of the rubber diaphragm, the fuel accumulator is subject to deterioration over time. The life of an accumulator will depend on the formulation of the gasoline used and the presence of stale fuel for long periods of time. The life of an accumulator can be from 10 to 20 years under normal circumstances, but as short as a few years if the car is parked for a long period of time with oxygenated (ethanol) fuels.
The accumulator will typically fail gradually. If you notice the car beginning to be hard to start when hot, initially after 30 minutes but with the period of time getting shorter over time, this is the likely suspect. It is also the likely suspect if the original unit is still in the car! An original fuel accumulator will typically have hard black hoses attached, but it's common for the black coating to have deteriorated and be falling off, exposing the cloth covering underneath. This is usually an indication of an original accumulator but it's possible that a prior owner replaced the accumulator without replacing the hoses.
The fuel accumulator is physically located inside the center section of the frame, between the gearshift lever and the transmission. It is accessed through an oval hole underneath the car. The replacement part includes an instruction sheet on how to best replace the unit.
CAUTION: The following tests will expose you to liquid fuel. FIRE HAZARD/FLAMMABLE! Do these at your own risk in a well ventilated area. The most conclusive test of the accumulator is to disconnect the hose from the end of the accumulator that faces the rear of the car. Plug the hose. Connect a second section of 1/4" hose to the nipple on the accumulator and direct it into a suitable container. Start the engine. After an initial spurt of fuel, no fuel should flow from the accumulator. If it does, the internal diaphragm has failed.
2 - Fuel Pump Check Valve (not normally a replacement part): On original DeLorean (Bosch-style) fuel pumps, there is a check valve at the top of the pump that holds pressure on the system after the pump is turned off. The earlier style "long neck" pump has an internal check valve that is somewhat failure prone. The later style "short" pump has an external, replaceable check valve between the hose and the pump, although the valve itself is not generally available as a service part. This later valve has a very low failure rate.
Failure of the Check Valve is characterized by very sudden pressure loss after turning off the engine (fuel pump). If you notice the pressure dropping to zero within 15-30 seconds, this could be the issue.
CAUTION: This test carries a risk of cracking or otherwise damaging the fuel feed hose, so if the original hard black (or orange replacement) hoses are in place on the pump we do not recommend doing this. A more conclusive test of the fuel pump check valve involves clamping off the hose from the fuel pump as close to the pump as possible. To perform this test, after noting how fast you lose system pressure, simultaneously clamp off the feed line at the pump AND turn off the engine. Note whether or not the system now holds pressure for a longer period of time.
3 - Fuel Distributor P/N 102807 Internal Pressure Regulator - The fuel distributor contains the primary pressure regulator for the fuel system. There is a tiny o-ring inside (about 1/4 inch diameter) that can occasionally fail and cause loss of fuel system rest pressure. The O-ring can be replaced in the field. It is not listed on the parts section of the service manual, if you suspect this issue contact your DeLorean Motor Company Dealer (click "see our locations") for details.
The O-ring is a low failure item, and failure is usually caused by attempting to service the regulator. If you have not touched the fuel distributor, this is likely NOT the issue.
Failure of the primary pressure O-ring is characterized by very sudden pressure loss after turning off the engine (fuel pump). If you notice the pressure dropping to zero within 15-30 seconds, this could be the issue.
4 - External Fuel Leaks anywhere in the fuel system. If this is the issue, you will typically see fuel leaking on the ground and/or smell gasoline. This typically happens after service where the technician leaves a fitting (fuel filter, pump, accumulator) loose.
5 - A lean idle setting (CO setting) condition can also cause balky starting, especially when hot. This may come on after fuel distributor replacement or other fuel system service that has changed the operation of the system enough that a CO setting is necessary. Refer to the article on Setting the Engine Idle Mixture for information, as well as DeLorean Service Manual section D:04:11
Revised DAS 6/15/2016 [extensive re-write, links added]