Lazy MAP Sensor?
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Cadillac DeVille 1985 to 2005 including:
1985-1992 Fleetwood, 1993 Sixty Special, All FWD Forum Discussion, Lazy MAP Sensor? in Past Cadillac Vehicle Discussion; This thread is a continuation of the thread titled "Distributor Problem 4.9l" that I started a few days back. In ...
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    mc_marto's Avatar
    mc_marto is offline Cadillac Owners Fanatic
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    Lazy MAP Sensor?

    This thread is a continuation of the thread titled "Distributor Problem 4.9l" that I started a few days back. In that thread I describe a mild hesitation happening on kind of a light acceleration in closed loop. Cadillac_al suggested it might be a lazy MAP sensor. I checked the values through the on-board computer parameter p.0.2 and when the engine is shut off it reads normal atmospheric pressure of 102 kPa = 14.7 PSI - exactly 1 atm. That is normal.

    When I start the engine and it's idling the value jumps down to 38 kPa, when a gear is engaged it reads 41-42 kPa, which I also think is normal since when the engine is idling there is high vacuum, resulting in lower pressure. What I don't find normal is that when I rev up the engine or accelerate in D the value jumps to 45 or 50 kPa, but in less than a second returns to the initial pressure reading of 38 or 42 kPa (depending on whether a gear is engaged or not).

    My logic says, that, if I increase the throttle opening and keep it constant the vacuum should decrease resulting in a higher pressure number (the value of which is going to depend on the actual opening) and the number should stay there and not change unless the throttle opening changes.

    In my case it jumps up but in less than a second it changes to the initial pressure value.

    Does that indicate a MAP sensor going out of range (maybe internal pressure leak), or is there a flaw in my theory?

    I have checked the vacuum supply line for the MAP and there are no leaks present.

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  3. #2
    onecool is offline Cadillac Owners Fanatic
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    Service manual has a chart that might prove useful.

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    mc_marto's Avatar
    mc_marto is offline Cadillac Owners Fanatic
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    Re: Lazy MAP Sensor?

    I was trying to look for something like that in mine, but was not able to find it. Any reference of which section it could be listed under?

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    onecool is offline Cadillac Owners Fanatic
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    I look tonight when I am at home.

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    onecool is offline Cadillac Owners Fanatic
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    I lied I'll try to remember to look tonight.
    Doh!!

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    onecool is offline Cadillac Owners Fanatic
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    I forgot to look again but I did a quick Google search. and found this chart. its general but it should work for most cars.
    Attached Images

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    onecool is offline Cadillac Owners Fanatic
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    New cart.
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    dennis93coupe's Avatar
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    Re: Lazy MAP Sensor?

    Here's an explanation from the FSM if it helps.


  10. #9
    mc_marto's Avatar
    mc_marto is offline Cadillac Owners Fanatic
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    Re: Lazy MAP Sensor?

    Thank you for the charts onecool!

    Thank you for posting this Denis! I read the same thing by the way, but the thing I'm not certain of is whether the reading should stay spot on to where it is without a change in the throttle opening or it could deviate. I couldn't find no information regarding this in the FSM.

    I got an extra MAP sensor off a '94 Deville from the Pick'n'Pull. Well, I've been interchanging my original and the "new" one and tell you the truth the readings are pretty much the same. So, my MAP is probably fine. Maybe there is another sensor chiming in when in closed loop to cause this hiccup condition. And, I don't think it could be clogged fuel injector filters, or the injectors themselves. I have Ohm-ed 'em out and I have 3 injectors with lower resistance than 17 Ohms, but I don't think that rules them out. I have all of the power that I'm supposed to have, and can easily smoke other cars on the road . If it was injectors, I wouldn't have much power, and I would experience hesitation throughout the RPM band, correct?

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