Your scanner should show when the system is in closed loop and switching between rich and lean. If you find it is staying mostly lean, check for a small exhaust leak ahead of the front oxygen sensors. Between the pulses of exhaust gas, the momentum develops little pulses of vacuum that can draw in outside air through that leak. The unburned oxygen will be detected by the O2 sensor which will tell the Engine Computer to hold the injectors open longer to enrichen the mixture. No matter how much fuel it adds, there will always be that unburned oxygen from the leak. The O2 sensor doesn't respond to fuel, only to oxygen.
You can also look at the fuel trim numbers. If they are high positive, the computer is adding fuel in response to something, possibly related to your coolant temp. Sensor. If the numbers are high negative, the computer knows there is too much fuel entering the engine and it is trying to reduce the amount but apparently not having much luck. Fuel pressure too high is one possibility. Fuel leaking through the spring-loaded diaphragm in the pressure regulator is real common on GM products but I've never heard of that on a Chrysler vehicle. A leak in the vacuum hose going to the pressure regulator will cause fuel pressure to go up just as it would under acceleration when manifold vacuum is low.
The MAP sensor has the biggest say in how much fuel enters the engine. Check its hose for leaks that would make it read low vacuum. That is a sign of acceleration which results in a richer mixture. The MAP sensor on newer engines is bolted right to the intake plenum and there is no rubber hose. These sensors don't really cause too much trouble like they did in the late '80s.
Wednesday, June 30th, 2010 AT 2:39 AM