1988 2.8 w/auto Starts well hot or cold, runs strong, at highway speed you would not know anything is wrong. Usually idles fast but will vary and when at or below 1000 RPM it bubbles and has to catch itself by raising the idle speed. At idle and slow speed it smells very rich. Have not noticed anything like a spark miss. When noticed as running rough at slow speed it is like a carbed car when you leave the choke on to long. Hit the throttle and it jumps and comes out of it. No notable hoses loose. Moving the throttle position sensor by hand at idle causes bad stumbling, but releasing returns to fairly normal rich, fast idle. No odd engine noises. 2 tanks of gas reveal only about 10MPG. Wow very rich. I will do a pressure leak down on the fuel rail. My picks for suspects are oxygen sensor, throttle position sensor if I do not find leaking injectors. Question, what have you guys found relating to this? Thanks
The MAP sensor is the sensor the engine uses the most to determine how much fuel to inject. If the MAP sensor isn't seeing full intake manifold vacuum that can cause the engine to run quite rich. There is a nipple right next to the MAP sensor that is plumbed to the MAP sensor and the fuel pressure regulator. Inspect it to make sure there are no cracks in the plastic line or rubber elbows. While you are there look to see if there is any fuel in the vacuum line that runs to the fuel pressure regulator as sometimes the diaphram in them leaks fuel and that fuel gets sucked into the intake via the vacuum lines.
I'd try to find some way to get access to a scanner that can capture data while driving. Sounds like it's running open loop, most likely due to a bad sensor. A bad ECT (engine coolant temp) sensor can cause this. I think that the ECM wants to see an engine temp over 180 (may be 170) before it goes closed loop.
Also be aware a failing stat not allowing the engine to get to temp can cause a drop of around 10 mpg. I've had two cars that the stats failed that way and my mileage dropped to the mid teens.
*** UPDATE *** I purchased a cable from reddevilriver (excellent product and service) and I used the TunerPro RT that was supplied. I had to leave the jumper on full time to start and maintain a connection. A 15 mile drive had the O2 listed as lean the entire time and the O2 value stayed the same. During the middle of the drive it went into closed loop for less than a minute. There were zero rich/lean transitions. I put in a new O2 sensor and a new 195 degree thermostat. Went for the same drive, flipping between rich / lean dozens of times a minute and many transitions. Does not smell as gassy so I filled the tank to check mileage. Before I could drive much the alternator failed (again - 4 years ago). And the storm in northern Indiana delayed FedEx and the garage is around 10 degrees. Fixed and now I am driving again. Being winter and warm up times I will consider things normal if I get the high teens for MPG. *** This is the second alternator I have changed V6-Automatic and I pull the wheel well liner and DO NOT have to disconnect any suspension. Treat it like a puzzle and twist move twist some more and it will come out. Thanks everyone.
