1998 DeVille Hard to cold start,runs rough

Probably a lean fuel issue . Engine coolant temperature sensor reading warmer than it actually is causing a too lean mixture , restricted injectors, restricted fuel filter , weak pump or bad regulator on return systems causing too low of fuel pressure , Mass Air Flow contamination .

Lean air fuel mixture probably , major vacuum leak , low fuel pressure , clogged fuel filter , dirty or bad MAF sensor - mass air flow sensor . Y ou should hook up a fuel pressure gauge an check pressure an flow .Spray starting fluid or carb. cleaner into the engine an see if it starts then .
046 Engine Hard to Start or No Start Scan Data for No Start

THERE ARE 2 TEMP SENSORS ONE IS FOR THE LIGHT OR GAUGE AND THE OTHER IS FOR THE ECM. IF THE ONE FOR THE ECM IS FAULTY IT CAN CAUSE HARD COLD STARTS

check fuel pump and filters , make sure you have good fuel. maybe the computer is taking time to reset fuel injection when the engine is cold.

Have your temp sensor checked. It may not be setting off a code but a sign of a failing temp sensor is hard starts, rough idle, until the car heats up. It sends a code to the ECM to help adjust fuel mixture. If it sends gals readings, the computer will adjust the fuel-air ratio and cause vehicle to run too lean until warmed up. Some people have had to keep pumping the gas pedal to keep it going from a bad temp sensor.

Oxygen sensor only report the condition of the engine the rough running condition can range from a worn sparkplug wires to worn sparkplugs to valve problems to injector problems. with the cadillac deville a tune up should be the first attempt to smooth the rough idle with the inspection of the cap and rotor cap and rotor do have a tendancy to arc internally and create misfire so does sparkplug wire that can leak spark voltage to ground and create misfire. the spark plug wire especially if they are brittle and hard can create issues

a code for lean or lean condition typically indicates a vacume leak ,lean refers to too much air or not enough fuel too much air or more air than expected "leans" the fuel mixture, usually a vacume leak or a leak in the air intake system is the problem also i would have the coolant temp sensor checked out

Pretty hard to diagnose without a scan tool to check the fuel trims. rough guess is either a vacuum leak or bad coolant temp sensor. But I sure wouldn't replace either of those without seeing the results from the scan tool

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..

How old is your Battery? at the Temp. Stated above an Older Battery even though Good may not have Enough Cold Cranking Amps Left to Kick her Over.