92 DeVille 4.9 V8 running rich bad mileage shaky idle

[Rmbb1981]

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?

[KHE]

Does the idle speed control (ISC) motor ratchet and make a grinding noise when you *smurf* the car off? If so, the surging at idle could be caused by a closed throttle switch that is not making contact. There is a ton of information in the archives on how to check this - one of the icons in the onboard diagnostic system will indicate the status of the closed throttle switch so you can check it from the driver's seat. Simultaneously pres and hold the OFF and WARMER buttons on the climate control panel and the codes will be displayed on the fuel data center. When you see .7.0, the system is awaiting further input.

Another possibility is the fuel injectors. There is a way to turn off the injectors one by one while in diagnostics to see what cylinders have no affect on the idle quality when the injector is turned off - that is how a bad injector can be identified.

Post any codes that are stored in diagnostics - there might be a code that would offer a clue to what's going on.

[Rmbb1981]

Hello!

Yes yes! When I turn of the car the black little box with the nail coming out of it makes a grinding noise and goes back and forth a few times. The throttle never goes to 0, it bottoms at 1.9 or so. Should it go to zero?

I don't have a Service manual so I don't know exactly what to do after it gets to seven. I have been able though to figure out what all the parameters between 01 and 25 are in diagnostics mode. Is there an easy fix for the ISC?

[KHE]

With the engine running and the diagnostics activated, one of the lights that is near each of the button icons on the climate control panel indicates the status of the closed throttle switch. I do not recall which one but there is a lot if information in the archives - search ISC or idle speec control. If you need to replace the ISC motor, it's not a hard job but it is best to determine if it is bad before replacing it.