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 Hot 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 cold. There are several common causes. With some methodical troubleshooting techniques you can avoid replacing the wrong parts.
Obviously an engine needs both fuel and timed spark to start, as well as a functional starter. This article assumes that all are present and that the car will eventually start, or starts normal when hot. For a car that will not start at all refer to No Start Conditions (to be written).
Cold Start Circuit:
For an engine to start cold, an enriched fuel mixture is necessary. At coolant temperatures below 35 degrees Celsius, [95 degrees F], the Cold Start Circuit (Technical Information Manual page 56, Service Manual page D:01:10) furnishes extra fuel to start the cold engine.
This function is activated by the ThermoTime Switch [TTS] P/N 102125 . The Thermo Time switch activates the Cold Start Valve (Injector) P/N 102372 when needed. This valve is an electronic fuel injector which sprays fuel directly into a port in the intake manifold that routes it to all 6 cylinders, pulled in by engine vacuum while cranking.
This system only adds fuel while the starter is cranking the engine, up to 30 seconds. There is an internal heater that turns off the valve after a period of time no matter what the engine temperature is, to avoid flooding the engine. Once the coolant temperature, OR the TTS internal heater rises above 95 degrees, the TTS ground opens and disables the cold start injector.
Failure of this function is often caused by the failure of the ThermoTime Switch [TTS] P/N 102125. CAUTION: Avoid trying to power the circuit externally as improperly applying battery voltage to the TTS directly can destroy a working switch. Failure of the Cold Start Valve is very rare.
It is possible to troubleshoot this circuit using a test light and ohm meter.
Cold Start Valve Diagnostics:
The Cold Start Valve is located at the top of the engine toward the left side. The top of the valve is blue plastic. The first quick check to see if the cold start valve is functional is to disconnect the blue connector from the top of the valve. Inspect the pins on the valve looking for corrosion, and inspect the pins on the connector looking for corrosion and verifying that one of the pins has not backed out of the connector shell.
Next, disconnect the grey connector from the warm-up regulator and connect it to the Cold Start Valve. Attempt to start the car. If the car starts, IMMEDIATELY disconnect the grey connector from the Cold Start Valve or you will flood the engine. This verifies that the valve is good and you can move on. If the car does NOT start immediately, don't continue cranking or you will flood the engine. You either have a defective valve or some other problem. Remove the valve and repeat the test, observing whether or not fuel is spraying from the valve.
Thermo Time Switch Diagnostics:
Testing the TTS is a bit counterintuitive as it appears to have two connection pins but actually uses engine ground as a third connection. To test with an ohm meter, set the meter to a low (Rx1) range. Unplug the TTS and note that there are two pins on the connector. One of these pins is the connection to the heater, the other is the connection that grounds the Cold Start Valve when cold. The heater should measure about 50 ohms to ground, and the switch connection should measure a dead short (0 ohms) to ground when cold and open (infinite) to ground when hot. NEVER apply a test voltage to the TTS.
The usual failure of the TTS is failure of the internal switch contact. If you get an "open" reading on the switch connection, and the cold start valve test started the car, you can be pretty sure that the switch needs to be replaced. If the heater in the TTS fails, the car may have a tendency to flood at middle temperatures but the car will start normally most of the time.
CAUTION: The connector on the TTS is keyed so that the wiring plug will only go on it one way. If the outer shell of the connector is damage, the plug can be inserted the wrong way. This will immediately destroy the TTS.
Ignition Start Boost Circuit:
There is a circuit related to the starter that bypasses one of the ignition ballast resistors to compensate for battery voltage "sag" during engine cranking. If that circuit fails, the ignition spark will be weak during cranking and the engine may be hard to start. This can be a problem during cold or hot start conditions. The problem is more prevalent if the battery in the car is old, or the battery/engine ground connections are in poor condition.
This circuit starts from a blue/yellow wire at the starter. About 5 inches from the starter there is a connection that very often corrodes.
The other end of this connection is at the ballast resistors mounted at the top left of the engine compartment. The voltage from the blue wire from the starter activates the relay that is positioned next to the ballast resistors. When cranking the engine the lower ballast resistor is bypassed by the relay, increasing the voltage available to the ignition coil during cranking.
The common failures in this circuit are the connection near the starter mentioned above, and the relay mentioned above.
A quick check to see if this is the problem is to use a clip lead jumper (i.e. a wire with an alligator clip at either end) to connect the + side of the ignition coil to the battery jump post. If the car starts with this jumper in place, you need to repair the rest of the circuit. DO NOT leave the jumper in place for long, it will eventually damage the coil and run down your battery, as this leaves the coil powered all the time.
Written by Warren Wallingford, DeLorean Motor Company (Texas)
Rewritten/expanded DAS 6/22/2106