My 1996 neon starts ok when it's cold. After reaching its normal operating temperature by driving it awhile, I'll stop the engine. Waiting approximately 10 minutes, I have to crank the engine several times before it starts up again. The computer has no codes registered and the car works fine when driving around. The fuel pump is new, crankshaft sensor is new, the map sensor is new, had the fuel injectors professionnally cleaned and the battery is fully charged. If the car sits outside overnight, it always starts immediately the next morning. The compression is good since I replaced the piston rings 10 months ago. I suspect a coolant temperature sensor. Can my suspicions be correct.
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Possible coolant temperature sensor. It relays info to PCM which in turn regulates fuel needed for different engine temperatures. If sensor is faulty, it may be telling PCM that engine is already warm, when it's actually cold and needs a richer fuel mixture.
If that problem goes away once the engine has reached operating temperature, my guess would be a non functioning cold start injector.
Since the warm up phase is controlled by the ECU, you will not be able to adjust the mixture of fuel and air to create a richer mixture while the engine warms up.
Have your mechanic scan for fuel trim - long term and short term - and test the cold start injector as well. This can be easily done by any decent scanner; when activated, you should be able to actually heat the injector work, if it does not, it probably needs to be replaced.
If the sensor detects a problem, it sends a trouble code to the ECM and your "check engine" light will come on. IMO, your symptom sounds like "cold engine" issue. Myself, our 98 Toyota Rav4 will idle rough and engine stalls when put in drive, but the problem goes away when temperature shows the engine had warmed up to operating temperature. FYI the temperature sensor monitors the radiator coolant temperature. Normal engine temperature is when coolant reaches 120 degree fahrenheit. At around 130 to 140 degrees fahrenheit, the coolant thermostat opens to let radiator coolant flow from "bypass" mode to "enter radiator" mode. If it takes longer than 10 minutes for your engine to warm up, its possible the coolant thermostat is stuck in open "enter radiator" mode. When thermostat is close, coolant radiator bypass the radiator to speed up engine warm up, but if its stuck in open, it will take longer to warm up the engine. Another factor is ambient temperature. If its cold below 50 fahrenheit, it will take longer to warm up the engine.
It could be a plugged heater core or heater control problem. I assume the blower is working, but the air is cold. The first thing to check is the coolant level, make sure there is coolant in the resivoir. Next see if the temerature gauge reaches the normal operating temperature range. If the gauge reads cold most of the time, it would tell you that the thermsotat is faulty and not allowing the engine to reach full operating temperature. If your car doesn't have a temp gauge, the blue cold engine light might stay on for an extended period of time. The thermostat is supposed to not allow coolant flow in the radiator until the engine reaches operating temperature. If you drive the car for a couple of minutes, from cold start, the radiator or upper hose should be cool until the engine reaches operating temp.
You will not see any vacuum from the egr solenoid until the engine has reached normal operating temperature and certain other driving conditions have been reached (like throttle position and road speed).
If the engine runs rough when warmed up, check the CTS (coolant Temp sensor) when hot and cold and if no fault shows there, start checking input sensors for engine and cam position, engine timing, RPM etc.
If the EGR valve was leaking, it is only adding air to the exhaust stream and usually, manually opening the valve and blowing out the seat with compressed air should fix the issue.
This can also be an issue with clogged injectors. When the engine is cold, fuel enrichment supplies more then normal fuel amounts to encourage even running. When the engine warms up (normal operating temperature), fuel enrichment mode is turned off. If your injectors are clogged (even if only slightly) fuel starvation will cause rough running. Fuel enrichment overcomes the starvation of clogged injectors. Try adding a propriety injector cleaner to your fuel tank and watch for an improvement. If you do see an improvement, have your injectors cleaned and tested professionally. If you locate any other issues with engine sensors, replace them and ensure the new components are adjusted correctly if required..
check engine coolant temperature when it gets warmer, or at operating temperature, and verify that the temperature gauge is showing warmed up to operating temperature. If the coolant temperature sensor tells the computer the engine is cold it will over fuel and miss and the engine will die. When cooled down it will run fine until it warms up. Also could be coil if it starts missing after warmed up.