Rmbb1981 Posted July 24, 2015 Report Share Posted July 24, 2015 Hey guys, I have a 92 Cadillac DeVille with the 4.9 liter engine. I bought it about a year ago. It's a bit shaky on idle and surges as I slow down for red lights. It has pretty bad gas mileage in the city but it's mileage is as it should be on the highway. The on-board computer gets the right mileage on the highway, which is like 30 MPG when I drive (I drive slow) but in the city, where the computer tells me the mileage should be around maybe 15 MPG, it gets maybe 10 MPG. I rule out the possibility of the city mileage I'm getting being normal. Now what might be wrong? The idle is really not that bad, it's just a little shaky and the surging is not terrible but it's always there, if I am coasting along going like 10 MPH or so the car will gently cycle power on-off almost like someone is cutting air supply in a cyclic motion. I have checked the on-board computer for a little info. Absolute Manifold Pressure at idle is 34 (normal I guess) so Im guesing no vacuum leak. There is no long term fuel trim to look at but from the short term fuel trim that I can look at it does seem the car is running rich in the city but not on the highway. Also the 02 Cross Count which, I've heard, should at least be 10 is often much less than that in the city whereas it is up between 15-35 on highway. I have replaced fuel pump, fuel filter, spark plugs, spark plug wires and PCM. Any thoughts? Quote Link to comment Share on other sites More sharing options...
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