92 Deville 4.9 V8 running rich bad mileage and shaky idle

[richardbergquist]

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?

[936CD69]

Check the TPS (throttle position sensor) for correct adjustment. Enter diagnostic, go to switch tests and make sure the Idle Speed Control nose switch passes, if bad can cause the surge. Although rare on 92-95s, injector leak comes to mind. Perform an injector balance test while in diagnostics to check. You can do  a search here to find the impedance range on the injectors which may or may not identify a bad injector. I am assuming you have a factory service manual to assist.

HTH,

Craig

[richardbergquist]

Hello, I don't have a service manual no. I know how to enter diagnostics yes but to test the Idle Speed Control how exactly would I go about that? What is a nose switch and how can I tell if it passes (what does it pass?)?

How do I perform an injector balance test in diagnostics mode?

[936CD69]

Richard, the diagnostic procedure is a step by step process clearly spelled out in the FSM. It would take pages here to explain it all, but it is quite intuitive when you follow the FSM procedure. You can find the manuals on ebay for around $35.

[richardbergquist]

Well, I can't really afford one right now. They're closer to $100 to get where I live with imports and customs.

[Scot Minesinger]

You really cannot afford to not have a service manual.  That $100 is going to save you major money.  It is the most important tool.  Others are likely to chime in on this.  I have a 1970 Cadillac, and although simpler, there is just no way without the shop manual.  I had a 1992 Cadillac with the 4.9 for a short time, and it did not idle super smooth at 102k miles, but like yours ran well on the highway.  Please reconsider the shop manual purchase.

[richardbergquist]

I will probably sell this car in less than 8 weeks so no I won't get the service manual. I considered getting it last year but now it's too much money and too late.

[TJ Hopland]

My first thought is a bad fuel pressure regulator.  How well does it start both cold and hot?  Hard starting is a sign of a leaking regulator as is the poor idle and mpg.  You usually don't get codes because the computer does not monitor the pressure, its just looking at the O2 sensor and trying to keep things in range. 

The regulator is a round device on the fuel rail kinda by the distributor.  Its got a short piece of vacuum line on it.  Pull the line off and look / smell for signs of fuel.  There should be none.   If it is wet with gas or smells like gas it needs to be replaced.   Often times the hose will turn to goo if its been exposed to gas.   Its not an expensive part but the fuel rail has to come off the engine to replace it and there are a few things in the way that have to come off or at least get moved out of the way so you can lift the rail.    When I did mine I replaced both the lower and upper injector O rings and the valve cover gaskets.