12.3 - Code 3, Coolant temperature sensor. ( ,  )

Code 3 says Coolant temperature sensor.

On the 1990 the codes are different. However, on my car there are two coolant temperature sensors located almost next to each other on the thermostat housing (top front of engine).

One drives dashboard temperature gauge via dash board computer. Other is used by ECU. The latter is the problem. You can find out which is which by disconnecting each and seeing if temperature gauge goes wacky (warm engine required).

Dashboard gauge sensor has just a single wire and other has connector. If the connections look OK (e.g. remove and clean). You can measure resistance with meter. Do when engine is cold and hot. At 25C (hot day-cold engine) it should be about 2500 Ohms. Hot engine (80C) about 300 Ohms

12.39 - Fuel Fail 39, EGR temp sensor circuit ( Brett Gadzinsky,  )

The ECM looks for a low egr temp signal when the egr valve is closed, and a high temp signal when the egr valve is open. The ECM also looks for an over temp signal when the egr system is enabled.

The egr temp sensor is located behind the throttle in the bottom of the intake manifold, close to the exhaust gas entry point.

The egr temp sensor is a negative temp coefficient thermistor :
Resistance decreases with temp increase.
Resistance, 122f 560,000 to 710,000 ohms.
Resistance, 212F 76,000 to 94,000 ohms.

Conditions for flagging FF39 :

- Engine coolant temp above 180F,
- Throttle position within expected range for engine speed,
- Engine speed 1700 to 3000 rpm,s,
- Load site range 2 to 7,
- Egr enabled, indicated temp greater than 525F for 64 milliseconds,
- EGR enabled, indicated temp under 212F for 60 seconds,
- EGR disabled, indicated temp greater than 122F for 5 minutes.

Possible faults:

- Blockage in EGR transfer pipe between egr valve and intake manifold,
- Blockage in intake manifold EGR adaptor port (90% of problems),
- Disconnected, leaking, blocked, pinched vacuum hose between,
- EGR valve and the egr vacuum solenoid,
- Defective EGR solenoid valve (stuck),
- Defective EGR valve,
- Defective egr temp sensor,
- Open or short in egr temp sensor wiring.

Most modern cars use exhaust gas into the intake tract to reduce combustion temps and NOx because exhaust gas is inert. It has no fuel or oxygen.

The EGR system ONLY operates when the engine is up to temperature and the engine is under light or moderate loads.