Ford: Rough Idle and Coolant Temp Increase, intake manifold gasket, oxygen sensors

Question
Hi Ken,
I purchased a 1996 Ford Contour for my son's first car...This vehicle has a Z-Tec 2.0, 4 Cyl. with almost 200K miles and the previous owner has provided all the repair reciepts since 2000...the vehicle was very well serviced!  I have replaced many parts, mostly all mechanical (struts, springs, bearings, brakes, etc)...the vehicle is in very good condition and the overall operation is above average...(there is no exhaust smoke at start up, or under acceleration), but the motor idles rough after warm up, and moreso, when the transmission is in gear vs. idling in park (this "roughness" does vary-sometimes the motor idles smoother than other times-when in gear).  There is also a hesitation when the throttle body is first opened (accelerating from stop), but no missfire under acceleration...The coolant temperature maintains at about 60% of the "normal" range on the gauge when driving, but edges upwards to the high end of the "normal" when vehicle is idling (if left to idle long enough, the guage needle will almost reach the red area on the guage)...The coolant temperature sensors, thermostat, and water pump are new and both oxygen sensors are new.  I did replace the intake manifold gasket, and throttle body gasket.  The check engine light is on and a code of P0420 is the only code displayed by the PCM...The exhaust is in good condition (quick visual evaluation to date), but the pressure at the tailpipe seems weak, and I suspect a minor leak may be possible at some point along the exhaust system...I have not closely inspected the exhaust system to date...I have inspected all vachumm hoses, all connections, etc. and am confident no vachumm leaks exist, and no faulty electrical connections exist...I suspect the catalytic converter is "tired", but, could this be plugged??  The intake cam was missaligned, and I corrected as needed and the timing belt was recently new, and a new serpentine belt was installed...I suspect a control or sensor is malfunctioning, but I am seeking some advise from the "pro"...I have purchased a factory shop manual, but not the powertrain and emissions manual...I appreciate any advise you would have, and thank you for reading and perhaps responding to my request...
Chris in Wisconsin...

Answer
Mr. Raverty,

For idling rough, I would check the idle air control, which is located by the throttle body. I would also check for vacuum leaks any way. Take some WD-40 and spray it at the base of the intake manifold where you changed the gasket. If the idle goes up when you spray it, you have a leaky gasket. The only other thing that I would check would be the fuel pressure regulator, which is on the fuel rain up next to the intake manifold in the rear on the driver's side. As far as the exhaust, did you check the oxygen sensor? There may be one before and after the catalytic converter (they started doing that in 1996). As fo rthe temperature going up, did you check the sensor for the electric fan? It may not be coming on when it is supposed to. It is possible that the catalytic converter needs to be replaced with that many miles on it and i would recommend changing it as well. Hope that this helps and if you need further assistance, please do not hesitate to contact me. Good luck.


Ken