I suggest you take it to a reputable shop with equipment that can perform real time diagnostics. Your problems may be due to a faulty PCM or PCM ground wire, a faulty MAP sensor or a defective coolant temp sensor even though it may be new! Rebuilt exchanges are available through most parts stores so you can save about 50% over dealer prices. Usually a tech will install a new resister in place of the defective coolant temp sensor feedback resistor in the circuit board, and it may perform well for a week or so or longer. Most come with a 30 day warranty but beware of parts sources that do not accept the return of electrical parts. If you have a shop do the job, they should warrant their parts and workmanship, so that may be the best solution.
It may be a temp sensor mounted at the rear of the head of your 305 engine and it would provide the signal to the the dash temp gauge, (if equipped). If you don't have a temp gauge, then it is the switch mentioned by Jack in his reply.
The coolant temp sensor mounted to the water pump, supplies a supposedly accurate temperature to the PCM. The signal from the coolant temperature sensor, aka: "thermistor", varies it's resistance with the change of temperature measured in the coolant stream and signals the PCM. The PCM supplies a signal of aprox. 5 volts to the coolant temp sensor and a feed back signal is returned through a resister in the PCM circuitry which in turn measures the voltage. When correct, it is interpreted as high voltage when engine is cold and low voltage when engine is hot*.
*source: Chilton Total Car Care manual for GM models: Camaro /Firebird model years 1993-1998. #28284
Though the source may not be specific for your engine, the info is valid to an extent for your application.
I have experienced similar problems with Chrysler Corp. E-Class models built from 1982-1990, and it seems to be an industry-wide phenomenon, regardless of make. They would have built a better mouse trap had they moved the PCM out of the engine compartment, away from the heat and vibration, but the bean counters know copper wire is expensive, so we, the consumer must suffer the consequences.
If you do need a replacement, the number on the side of the PCM is your reference for parts availability. The supplier may need to send your PCM out for repair (about 2 to 3 weeks, in some cases), so try to find a good used unit if you can. It is about half the price of "repaired" unit and it will allow you to keep your car on the road if you do need a replacement. Almost all go bad, sooner or later, so call it cheap insurance if you intend to keep the car for a long time.
I hope this helps.
Monday, July 16th, 2007 AT 10:29 PM