Falling MPG - 1990 Deville/Fleetwood
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HT4100 4.1, 4.5, 4.9 Discussion, Falling MPG - 1990 Deville/Fleetwood in Cadillac Engine Technical Discussion; My car gets decent highway mileage but I noticed that the MPG goes down on long drives. At constant 70mph ...
  1. #1
    caddynewby is offline Cadillac Owners Member
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    Falling MPG - 1990 Deville/Fleetwood

    My car gets decent highway mileage but I noticed that the MPG goes down on long drives. At constant 70mph I get readings of 25-26MPG, then after an hour it drops to low 20s. What could this indicate?

    Other issue when the car is hot in slow city trafic, I occasionally get power interruptions on acceleration, as if power was cutting for very brief sequences, making the car jerk forward on mild/strong acceleration; I need to accelerate very softly when that happens.

    Car has 37,000 miles and otherwise runs great. Any ideas appreciated.

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: Falling MPG - 1990 Deville/Fleetwood

    Does it throw any codes(Seems like the number one question after the opening question, doesn't it)?

    A few things come to mind and will require some testing to narrow down. First, the engine losing MPG when fully warmed up could be bad/failing O2 sensors, faulty CAT, and possibly faulty/failing injectors. Those things come first to mind, but diagnoses should not be limited to just those things. That's why we query about codes.

    Next, the stumbling acceleration can be a maladjusted TPS, bad MAP, injectors, or even something as simple as a fuel pressure switch.

    If you've run codes, post them, please. As far as the fact that the car ONLY has 37,000 miles means nothing when it's 21 years old. That's about 1800 miles a year which means a whole lotta' sittin' goin' on. Everything falls apart faster when it isn't used

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    drewsdeville's Avatar
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    Re: Falling MPG - 1990 Deville/Fleetwood

    Loss of mpg's and stumbling on acceleration when hot sounds most like a possible EGR issue to me. That's my first guess.

    As is always recommended, it would probably be a good idea to clean out the EGR passages under the throttle blades.

    As mentioned, with the vague description, it could be anything at this point including the above sensors, vacuum leaks, or fuel and spark delivery issues. Any other symptoms? Even minor ones?

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    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: Falling MPG - 1990 Deville/Fleetwood

    OBD-1 is a joke. I've been through a shit ton of problems, hours of diagnostics, and maybe twice has a code actually clued me in to what the problem was. I've even disconnected things just to see how it would handle it only to get laughable results.

    I have to roll my eyes everytime I turn the headlamps on at dusk and the D.I.C. screams at me: "HEADLAMPS ARE ON"!!!!!!!!!! Yeah, no shit, I just turned them on!

    End rant.

  6. #5
    caddynewby is offline Cadillac Owners Member
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    Re: Falling MPG - 1990 Deville/Fleetwood

    Thanks for all the replies!

    The only code I occasionally get is the E49 and EE49 (air diverter valve?) which I clear immediately when it comes up; seems like it comes up in warm weather and after driving at slow speeds. The car runs normally when this error code is triggered, I simply dislike seeing the yellow light on the dash!

    Only one other issue with the car, so minor I wouldn't mention it: the dash gets very hot sometimes, burns my fingers to clear the codes. Pure speculation on my part: this could be due to mildew on the electrical connections behind the dash since the car was in storage for many years.

    I should also mention that I have one of two exhaust studs missing on the rear manifold (which connects to the main pipe) just ahead of the cat. Could have some bearing on O2 readings or air-to-cat issue. I have no clue where the sensors are located on the exhaust system.

    Car is in storage now, I will check these issues when it comes out. Thanks for all the essential leads!

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: Falling MPG - 1990 Deville/Fleetwood

    The A.I.R. diverter valve may be screwing with the system also. Unless you smell exhaust gasses in the cabin of the car, the missing manifold bolts shouldn't be an issue, although I'd replace them anyway...

  8. #7
    caddynewby is offline Cadillac Owners Member
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    Re: Falling MPG - 1990 Deville/Fleetwood

    Quote Originally Posted by the recluse View Post
    The A.I.R. diverter valve may be screwing with the system also. Unless you smell exhaust gasses in the cabin of the car, the missing manifold bolts shouldn't be an issue, although I'd replace them anyway...
    I do not smell exhaust gas, whether inside or outside the car. The only issue is that the car sounds like an old truck when I step on it. The one stud in place is enough to make a tight fit, but as exhaust pressure builds the seal may be weaker, leading to the noise. With a light foot, it sounds normal, the exhaust system is doing its job as it should. I will attempt to replace the missing stud but it'll not be fun: the driver side stud broke inside the flange. Some fancy tools will be needed to drill out the remaining bit, I'm not sure whether access will be an issue. Worst case scenario would be to take off the rear manifold, but that would be a serious PIA. For sure the exhaust will come off to attempt drilling out the stud so I'll send it to my local exhaust shop to examine the cat.

    How can I disable the AIR system? Also, where is the fuel pressure switch? I should change that anyway along with the fuel filter, since I'm having the occasional fuel issues I mentioned (I'll do that before replacing the injectors).

    The funny thing is that the car runs just fine most of the time, only on long highway drives and extended slow traffic in hot weather that it displays the said problems.

    I appreciate the input guys!

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    drewsdeville's Avatar
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    Re: Falling MPG - 1990 Deville/Fleetwood

    Replace the manifold bolts. If you can hear exhaust escaping, that also means that fresh air is getting in (sort of a sandblaster effect), throwing off O2 readings, eventually causing an incorrect fuel trim.

    You also risk warping the manifold over time if those studs aren't replaced.

    The air system can't be disabled without putting out the check engine light. The diverter valve is a pretty reliable unit. Usually one of two things happen: the check valves fail, letting hot exhaust to the diverter valve, burning out the diaphragms OR the solenoids that control vacuum to the diaphragms fail.

    I recommend you get a junkyard diverter valve (a new one is hundreds of $$$) and replace the 2 check valves with new.

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    caddynewby is offline Cadillac Owners Member
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    Re: Falling MPG - 1990 Deville/Fleetwood

    Quote Originally Posted by drewsdeville View Post
    Replace the manifold bolts. If you can hear exhaust escaping, that also means that fresh air is getting in (sort of a sandblaster effect), throwing off O2 readings, eventually causing an incorrect fuel trim.

    You also risk warping the manifold over time if those studs aren't replaced.

    The air system can't be disabled without putting out the check engine light. The diverter valve is a pretty reliable unit. Usually one of two things happen: the check valves fail, letting hot exhaust to the diverter valve, burning out the diaphragms OR the solenoids that control vacuum to the diaphragms fail.

    I recommend you get a junkyard diverter valve (a new one is hundreds of $$$) and replace the 2 check valves with new.
    I will study the design of that AIR system to fully understand how it works then try to identify the problem.

    About the exhaust, I just want to make sure it is clear which stud is missing, it isn't one of these studs that go into the head (that would be a major PIA!), but rather this one. Still, it will not be fun to extract this!

    I guess there isn't much point experimenting with all the possibilities before I sort out the exhaust and AIR issues, which could be causing the problems states above?
    Attached Images

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    drewsdeville's Avatar
    drewsdeville is offline Cadillac Owners Master
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    Re: Falling MPG - 1990 Deville/Fleetwood

    Quote Originally Posted by caddynewby View Post

    I guess there isn't much point experimenting with all the possibilities before I sort out the exhaust and AIR issues, which could be causing the problems states above?
    Exactly. Take care of the known problems and see where you are at before continuing with anything else.

    I wrote up a fairly detailed explanation of the AIR system that included the causes and effects of it's failure. It should show up with a search for E49 or E049

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