Rough idle at engine start for 1 or 2 minutes, then runs fine all day long afterwards

cadillac
seville

#1

Yesterday after starting the car after a day of no driving the car ran a bit rough and this lasted a few minutes but eventually leveled off and ran smoothly again. I drove the car for a few hours stopping a few places here and there and still ran smooth, do I need a tune up or could moisture in the fuel line mean I need fuel additive? Could it be one of the spark plugs is going and if so do I need to replace all plugs and wires to get better firing?


#2

Unfortunately, there are at least a dozen possible explanations. It is not likely related to the items you mention.

The on-board computer is of limited help in diagnosing cold running problems because during cold operation, the system is running open-loop, and the computer is not reading and comparing a lot of its inputs yet. An electronic component that is out-of-spec when cold may not be apparent to the computer.

Based on my limited experience, common reasons for rough idle when cold include:
Vacuum leak at intake manifold that closes when metal warms up.
Bad temperature sensor (there may be several temperature sensors)
Moisture somewhere in ignition system.
Bad cam position sensor or leaking seals on a VANOS engine.


#3

How old is your Seville? (miles and years) Is all the maintenance, as listed in the owner’s manual up to date? How long has it had this problem? Off hand I would guess it is a temperature sensor and your car does not know it is cold when you start it.


#4

I would add to that list a leaking fuel injector.


#5

I had a similar problem with a 1990 Ford Aerostar. Coolant was gettng into the combustion chamber in one cylinder. When the engine was cold, this woud foul out the spark plug. Fortunately, the Aerostar was under warranty. The head gasket was replaced, but when it happened agian, the dealer found that there was a crack in the cylinder head. At this point, the entire engine was replaced because the cylinder wall had become scored. Again, this repair was done under warranty. I hope this isn’t your problem, but it is a possibility.