Very poor MPG 1992 Deville ?? HELP!!??!!
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HT4100 4.1, 4.5, 4.9 Discussion, Very poor MPG 1992 Deville ?? HELP!!??!! in Cadillac Engine Technical Discussion; I recently just purchased my 1992 Deville from a guy who was getting rid of it because he had aquired ...
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    Cynical's Avatar
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    Very poor MPG 1992 Deville ?? HELP!!??!!

    I recently just purchased my 1992 Deville from a guy who was getting rid of it because he had aquired a newer car. It has a 4.9 engine and I filled the tank full on thursday and have not even drove 150 miles or so and I only have 4 or 5 gallons left. It shows that I am only getting on average like 11.5 miles to the gallon. Anyone know what could be wrong? Perhaps a tune up would fix this problem??? Any help is appreciated. Thanks

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    First, do a tune up, INCLUDING a change of the oxygen sensor...this is a must! It's a $20 part, and is the method by which your engine computer checks how much fuel is burning within the engine.

    If that doesn't change anything, you may have a leaky injector.

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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    I remember when my #5 injector went out, I was getting like 11-12MPG. Does yours have a rough idle at all?

    What kind of mileage does the car have? I'd recommend a full tune up also. Do the plugs, wires, cap and rotor (if necessary) and the O2 sensor.

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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    I would not touch distributor parts for a while. People often get new problems as soon as they disturb didtributor. Move one step at a time.

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    chevelle is offline Cadillac Owners Enthusiast
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    Before you do anything perform a simple test. Fill the tank again and reset the electronic fuel data center. Zero out the fuel used and average MPG when the tank is filled. Drive the car until the tank is low again. Fill up and record the amount of fuel added, the mileage and the data from the fuel data center...i.e....the fuel used and the average MPG. Calculate the actual MPG from the gallons added and mileage. That should match the electronic average MPG from the fuel data center. Similarily, the actual fuel used should match the electronic fuel used from the fuel data center.

    If the actual and electronic values match then it will indicate that the injectors are not leaking and the fuel pressure regulator is OK, etc.... If they do NOT match, then you are looking for a place unmetered fuel can enter the engine.

    Recommendations for replacing O2 sensors, wholesale tuneups, etc. are just shotgunning hoping to find the problem. Rarely successful and very expensive. I would wait until you know what the problem is...or if you have a problem.

    If you just drove short trips in the city and stop and go driving and like to hit the accelerator hard it is not too hard to believe 11 or 12 MPG actual. It is a relatively heavy car with a fair amount of power. It costs fuel to accelerate hard and stop and go. That is just the way it is with any car. If you aren't used to larger or heavier cars than the MPG might surprise you. Compare that to the cost of the car and how much it will cost you for gas and for the car to see what the total costs are. People panic and sell a lower MPG car for peanuts only to make payments on a much more expensive car just because it gets a few more MPG....they never save as much on gas as the car costs to get better MPG.

    With a 92 one thing you would certainly want to do is to check or have someone check the EGR tubes under the throttel body for cleanliness. Hold the throttle blade wide open with the engine off and the air cleaner off. Look down the bores. The two tubes sticking up from the bottom of the intake manifold should be clean and NOT clogged with carbon. If they are clogged rod them out with stiff wire and/or a round brush and carb cleaner. Clogged EGR tubes can affect driveability and fuel economy and are often overlooked.

    I assume the engine starts and runs smoothly and there are no severe driveability issues. If not, then a "tuneup" is not likely to do much for you.

    If an O2 sensor is failed it will likely set a diagnostic code telling you it needs to be replaced. Replacing them just as a matter of fact is pure BS....unless you sell O2 sensors.

    One thing to check is for trouble codes. Turn the key on, engine off. Simulataneously press and hold the OFF and WARMER buttons on the climate control panel until the onboard diagnostics activates. It takes about 5 seconds. When the onboard diagnostic system activates release the buttons and write down the trouble codes that display on the FDC. Post back. That will tell if there are any operating deficiences noted with the sytem.

    Checking or measuring or estimating "fuel economy" from the fuel gauge is very misleading. Similarily, unless you reset the electronic Average MPG when you filled the car the data displayed will include all the idling and driving done before you got the car. The only way to know your MPG is to fill the tank, measure the miles and gallons and calculate it ..... and if things are fine it should correlate well with the electronic data.

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    I replaced my o2 sensor without a code being set and went from 9-10mpg up to about 13.8. The way i understand it is, If it is present and sending a signal then it will not set a dtc. But it can be present and just worn out and the computer would think its ok, just cause it to run rich.

    After I replaced my o2 i pulled a spark plug out and it was black, indicating a overly rich condition. While I didnt get around to replacing the plugs, I did replace the factory plug wires at 90k or so. I now get about 19mpg, but the route I travel now is longer and more highway driving then my 13.8 from before.

    The o2 is about 20$ and labor can be done yourself. I would clean the egr as mentioned first, but the cost of a new o2 isnt to bad

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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    I use to drive a 83 oldsmobile delta 88 and that thing ate gas but it got better mileage then my caddy is getting, hell even the 74 deville I had got better mileage then I am currently getting. My brother owns a mechanic shop so I can get work done on the vehicle its just a big pain in the ass getting them to actually look at the car (dont know if you ever have had family do you like that or not lol). I filled the car up again with about 15 gallons of gas yesterday and average I am getting now is like 8.4 miles a gallon, and it says I only have about 122 miles roughly and this is after I reset it. The car seems to run okay, doesnt idle rough or anything. I will see if my brother can take a look at it soon and Ill note back. Thanks for everyones help

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    Whats the mileage on the car? That sounds just like my problem. went from about 12mpg to 9mpg or so over the course of a month.

    replacing my o2 sensor fixed most, but not all of the problem. The o2 is a $20 part at autozone or similar, and almost anyonce can replace it. Only tool needed is a open end wrench. You may start with it in the meantime while you wait for them to look at it. If it works then cool; If not its cheaper then 15gals of fuel, so you wouldnt be out a ton, Im betting it would help somewhat though.

    I assume you checked for codes using "off and warmer" and have none?

    Mine drove fine aswell, just sucked the gas. Your in the same spot I was, when; I got mine i wasnt sure exactly how it was supposed to act, I thought the driveabality was fine so nothing was wrong. I can say though that after a few new items it drives worlds better, still some work left to do aswell so who knows how it will drive in the end

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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    Chevelle--
    I don't understand your comment about O2 sensors...first of all, on all OBD I cars (pre-1995 for GM), this sensor is supposed to be changed every 30,000 miles. After that, they get lazy and inaccurate, squirting way too much fuel into the engine. The 02 sensor one of the most critical emission and fuel control device--for an analogy, imagine yourself driving down a curvy road...you watch the road, steer the vehicle while still watching, and make corrections to the steering based on what you see the vehicle doing--this is called "feedback." This is exactly what the 02 sensor does for the fuel system. Driving around with a lazy O2 sensor is like driving around with cataracts in your eyes.
    To cynical: one other thing you should check is your 02 cross counts (parameter P.1.6 on the diagnostic code readout), and parameter P.1.8 (can't remember what it's called right now). When cruising, your 02 cross counts should be between 15-25 (mine hovers in the 20-28 region on the freeway), and parameter P.1.8 should read around 128. If parameter P.1.8 is less than 128, it indicates that the computer is sensing a rich condition and is commanding less fuel to the injectors...the root cause of which is most likely a bad injector. If P.1.8 is greater than 128, the computer is commanding a longer pulse width (more fuel) for the injectors as it has detected a lean condition.
    I'd guess your cross counts are very low (i.e. less than 8) and your P.1.8 parameter is less than 128.
    This sounds complicated, but it's a really, really easy, non-invasive test to tell you why your car is running the way it is. To check these parameters, start the car, then hold Off and Warmer together for about 3 seconds on the climate control display. All digits will illuminate, the service engine and vehicle soon lights will come on, and the fuel computer will first state any codes, followed by .7.0 when it says .7.0, hit the "Lo" button, then press "Hi" until you get to parameters P.1.6 and P.1.8. To end the test, simply push "Auto" and your car is back to normal.

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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    By the way 128 (or thereabouts) is normal for P.1.8 and more O2 cross counts is better.

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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    Thanks awadecki, I will try that out and post the readings hopefully later on today. You might know the answer to this ..? The back end of my car is pretty low and drags over speed humps, is there anyway get a code to figure out if something is leaking or not working correctly? Thanks

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    Quote Originally Posted by Cynical View Post
    Thanks awadecki, I will try that out and post the readings hopefully later on today. You might know the answer to this ..? The back end of my car is pretty low and drags over speed humps, is there anyway get a code to figure out if something is leaking or not working correctly? Thanks
    I have that similar problem aswell, The first thing I would ask is if you hear a compressor running when you get in or out of the car? Thier is a compressor under the hood to auto level the back of the car when weight changes. If you never hear this pump then it may be unhooked or messed up, therfore the back of your car will not raise up.

    It could also be worn struts, those may or may not give a light on the dash
    here is the thread I started on it
    http://www.cadillacforums.com/forums...ut-no-sss.html
    I havent gotten around to checking to see if the bulb was pulled on mine yet, maybe some of that will help you

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    chevelle is offline Cadillac Owners Enthusiast
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    O2 sensors do not need changing every 30K...whoever came up with that? An O2 sensor will run for a long long time unless it is contaminated with leaded fuel or silicone in the gasoline or something like that. You are correct in being able to check the closed loop operation and the amount of correction it is applying. I have never seen a situation where the O2 made the car run THAT rich...there is a limit on how much it can correct and there are diagnostic tests looking at the O2 activity and reaction to fueling changes that are pretty good at determining a failing O2 sensor.

    Not that they cannot go bad but they are much more reliable than people make out and they do not need to be routinely changed. Really, unless they are contaminated somehow they will basically work forever.

    Your analogy of driving with cataracts is close...LOL. Maybe driving with blinders on would be closer. The system is still "driving" but the O2 correction may be limited or offset due to contamination. Its not like the system has no idea how much fuel to deliver. It still will run fine in open loop operation. Just the O2 might bias it a bit. The fuel injection system does not have to have the O2 sensors to operate in other words.

    The fuel injection system has a pretty good basic fueling map in it and the closed loop system simply trims the fuel delivered to keep it right at 14.7:1. It's not like the O2 input can drive the engine so rich that it smokes black or something like that.

    I just believe in diagnosing the problem and fixing what is wrong rather than a wholesale change in parts as is being recommended. There could be a stuck injector causing problems or something like and what good would changing the O2 sensors and tuning up the whole engine do? Advice for how to diagnose is much more helpful than wholesale part change recommendations. What you put in for checking the O2 crosscounts and integrator value is right on. But....it sort of disagrees with the idea of just replacing the O2 sensors.... If you suspect the integrator is less than 128 (pulling out fuel due to a rich condition) ,which is what I suspect also, then the stuck injector idea is closer to reality than replacing O2 sensors. The fact that the O2 is reading it as rich indicates the O2 is fine.

    BTW....OBD2 started in 1996 for everyone, not just GM, because it became a Federal requirement then. And there is nothing different about O2 sensors and their operation between OBD1 and OBD2 cars. OBD2 simply required a standardized set of onboard diagnostic tests that were uniform in readout across the industry. The system still works exactly the same.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    Hey Chevelle--

    Sounds like we're getting a good discussion going that will help many people out. 1) If you check any of the Chilton's or Hayne's manuals for older vehicles, they'll state that you should replace the 02 sensor about every 30,000 miles...for only $20, it's a good practice in my opinion. 2) There actually is quite a difference between the older 02 sensors and the OBD-II ones: the newer sensors are electrically heated for faster warm up so that they can monitor emissions more quickly. They also have different internals and only need to be changed every 100,000 miles. The older, OE-type Delco sensors were intended to be replaced more frequently.

    To Cynical: No, no code will show for your low "rear end," but my guess is worn out shocks, which may or may not have now led to sagging springs. Codes are primarily set for engine control devices or body computer module (i.e. climate control) problems.

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    Re: Very poor MPG 1992 Deville ?? HELP!!??!!

    awadecki,

    Mine counts far less than 20-28! One time I tested a new O2 sensor on my car and checked how many counts I got. It could go about 10+, but I have never seen it going above 20. It usually stayed around 5. I thought that's quite quick (every 200 ms or so, right?).

    Also, I am not sure if a manufacturer uses different O2 sensors for pre-OBD-II cars. I agree with you about the heated O2 sensor, but I don't think it helps to extend the life of the sensor much, unless you drive only a few miles a time. My understanding is that the sensors made long time ago (pre-OBD-II era), MAYBE the technology had not come far enough so the lifetime of an O2 sensor was limited, but once the technology has advanced, the lifetime became longer that happens to be after 1996 (OBD-II time).

    Well, I am not sure on all these. But I tend to agree with chevelle.

    By the way, with that low cross counts, I get 17-18 mpg. For 100 % highway driving, I got about 26 mpg without AC at 70 mph, not too bad.

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