Has the O2 sensor(s) been changed? Here's a simplified version of what goes on. With the closed loop fuel injection system, the oxygen sensor in the exhaust monitors the oxygen content of the exhaust gases. The sensor then produces an electrical charge based on that content and the fuel injection computer uses that signal (as well as several others) to adjust the length of time the injector stays open. In the "old" days the O2 sensors generally needed to be replaced every 50,000 miles. Now days I think that's more like 100,000 miles, but either way they do wear out so to speak. When the sensor starts acting up, the FI computer doesn't provide the proper air-fuel mix. Both too rich or too lean will decrease fuel economy as well as power. Most newer vehicles have two O2 sensors, one in the exhaust pipe before the catylitic converter and one after. If everything is working as it should, when the sensor starts acting up, the check engine light will come on. Most often, when an out of range sensor signal is detected by the FI computer, it switches to a default mode, or "limp home mode" as it was called in the early days of fuel injection systems in the 1980s. In this mode, rather than the injectors being kept open based on sensor input signals, injector on time is determined by arbitrary defaults programmed into the computer. In the default mode, the engine will usually run normally or near normally, but emissions can increase and fuel mileage decrease. Normally the onboard diagnostic system will throw a code if the O2 sensor starts failing, but not necessarily. It did on my 2003 Caravan, but never did on the old '91 Caaravan. Just for fun, you might disconnect the battery cable for a few minutes and then reattach it. The loss of current will cause the FI computer to revert to the default mode, after which sensor inputs will be "remembered" as you drive it more and more. Getting the computer back into a condition where it has adjusted it's settings to suit the current state of engine tune and such can take anywhere from a few miles to 10 or more. By current state of engine tune I mean such things as ignition and valve timing. If the timing chain is worn and loose, that will throw valve timing off, which will affect engine vacuum, one of many external inputs that various sensors feed to the computer. A condition such as this can effect the ignition timing needed to optimize power, fuel economy and emission control. If any sensor is bad, starting over from scratch may trigger a fault code.
Something else that can cause a fuel injected engine to run too rich is a bad fuel pressure regulator or check valve in the return fuel line. If they restrict return flow, pressure will be increased at the injector, which will allow too much fuel to enter. With a closed loop FI system, a minor problem could be adjusted out by the O2 sensor telling the FI computer to cut back on injector open time. If the problem is too great, the computer can't do so. There should be a Schrader valve somewhere on the fuel rail that will allow yo uto attach a pressure gage. Old R-12 AC gages attach perfectly. Not sure what the normal operating pressure is for your particular vehicle, that's a question for someone who knows more than me. Caravans of that same era have a normal operating pressure of 49 psi and I would expect yours to be pretty much the same. Sorry for the long manifesto.
