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20mpg drop, no CEL... bad map sensor?

Discussion in 'Civic and CRX - EF' started by mrhaasguy, Jan 8, 2007.

  1. mrhaasguy

    mrhaasguy New Member

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    My question is, is 4.17V approximately 5 volts? I have tested the voltage at the map sensor, and the manual describes that both of the voltages should be about 5 volts. My results were 5.00 V pretty much on the dot, and 4.17 V. Is this "close enough" to 5V?

    As a little background info, my vehicle is a 1991 Honda civic std with an MPFI swap that I did myself (I took my time, did it right, and it started the first try with no real problems) Other than the swap, the setup is pretty much stock. That's right, stock pm6 fuel system in a d15b1, and I am aware of the fact that it runs a little bit rich. Not a huge problem if the car were to be kept in closed loop properly.

    Well, during the summer, shortly after I did the swap, everything was running beautifully; mileage was between 35-42mpg, which is what I'd grown to expect. I had a cheap air/fuel gauge, that I used STRICTLY to tell how quickly my o2 sensor heated up, and when I pushed the gas too hard, which obviously kicks it into open loop.

    As the temperatures began to drop, however, so did my economy. I went from about 41-42 mpg highway in the summer to under 25 mpg highway, as low as 18mpg normal driving.

    Watching the air/fuel gauge, the reason for this mileage drop is apparent: When it is cold outside, the car stays in open loop until a good 20 minutes after its been warmed up, and then kicks it into open loop again whenever the cars gets above a random speed, again depending upon the temperature. If it is a bit warmer, say 45 degrees, this speed will be as high as 60-70mph (3k-3.5k rpms). Colder, such as 15-20, and it will be as low as 40mph (2k rpms) Eventually, if I am on a long trip, after approximately 4-5 hrs the car will be functioning normally, keeping it in closed loop until I exceed 80mph (4k rpm)

    There are no CEL codes, never have been in quite some time (except when I unplug some wires during testing) I have tested the hell out of my oxygen sensor and tps systems, and they all perform flawlessly. I have also tried a different ecu, with the same results.

    So, could the map sensor be the issue (or the wiring to it?) Or does my car just have a severe gasoline addiction?

    Any advice would be greatly appreciated, as I have been trying to deal with this for several months now to no avail.
     
  2. welfare

    welfare Ancient Member

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    By all means, try another map sensor, you can get one from a scrap yard for next to nothing,
    but I think it's your o2 sensor, there are no real definitive tests for the 1 wire o2 sensor, but you are explaining that it takes longer for your a/f gauge to read closed loop, and outside temp is also having an effect.
    O2 sensors aren't cheap, and the older ones like yours are really only good for 50kmi beacsue its not self heated like newer ones. But I would say you have already wasted the cost of a new o2 sensor in gas a few times over.
     
  3. mrhaasguy

    mrhaasguy New Member

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    I had a problem with the o2 sensor during the summer; it kept throwing a code 1. Replaced the o2 sensor, no results. Turned out to be a loose ground wire. Tightened that up and checked to make sure the voltages the sensor was sending was the same as the voltage the computer was receiving, and haven't had a code since.
    Could the o2 sensor be bad after only about 6 months, maybe 2000 miles?
    ...well, it is a bosche.... :(
     
  4. welfare

    welfare Ancient Member

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    after 6 months an o2 sensor could go bad, but you say only 2000 miles, that is unlikely. . . Im not sure what to tell you then, sorry
     
  5. asmallsol

    asmallsol Super Moderator

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    How long has it been since you changed your fuel filter. I had a fuel filter go bad and I basicly had a linear decrease in my gas mileage for about 3 months. At one point I was getting mid 100 miles on a tank. Finally after doing everything EXCEPT the fuel filter, I changed it and my mileage bumped back up to 360+ a tank.
     
  6. racer_XX

    racer_XX New Member

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    try to check if ur o2 sensor is reading rich mixtures. rich is around 1-1.3 V. if ur map sensor was bad u'd have poor engine performance overall not just gas mileage. correct me if im wrong but open loop is when the engine is still not recieving all the signals from the sensors i.e. o2. but when it warms up and starts sending voltage to the ecu it begins closed loop and stays in closed loop except for specific things such as WOT. if im wrong correct me.
     
  7. mrhaasguy

    mrhaasguy New Member

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    I changed my fuel filter a couple of months ago, when I did the mpfi swap, so I don't think theres any problems there.

    As for my oxygen sensor, I hooked up a multimeter a couple of days ago and it generally sat at .8volts (it didn't ever go as high as 1-1.3) Once it gets "warmed up" to where the computer accepts data from the o2 sensor (30min+ of driving, well after the engine is at operating temperature, and even then it works sporadically), it'll generally fluctuated from .6v to... umm... -.1v .... why its negative I don't know...

    I don't know a whole lot about open loop and closed loop, but from what I have gathered, even after closed loop has begun, the ecu can kick it into open loop whenever it sees fit. My reason for this thought comes from an instance that has happened numerous times: Sitting at a long red light with my o2 sensor barely warmed up, in open loop but the voltage moving very slowly. Eventually the o2 sensor would cool off too much and the gauge would go up and sit on rich, like it does during open loop. Once I get moving again, after maybe half a minute it will get into closed loop and start fluctuating again like it should.

    And finally, for my theory, I changed the map sensor, no change. Checked all the wiring, everything is good. There isn't any problems there.

    I must say, at this point I am completely stumped. The car is giving the impression that there is no problems, absolutely no codes, drives fine, and everything I have checked that has absolutely anything to do with the open/closed loop business has turned out to work fine. The only thing I can do is throw a look of disgust as I watch the numbers on the pump get higher and higher....

    Thank you all for your suggestions, but I don't think I'm ever gonna figure this thing out...
     
  8. welfare

    welfare Ancient Member

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    Maybe just try the o2 sensor, what can it hurt??
     
  9. mrhaasguy

    mrhaasguy New Member

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    Wow. Major update.

    So I was driving the 10hr trip back to my school, and after about 2hrs the car died. First time thats ever happened to me, and was actually pretty interesting because its a manual and it kept cutting in and out until I finally came to a stop.

    But anyway I got it towed, and it turned out to have a bad fuel pump. By this time, it was pumping a good 10-15lbs less than it was supposed to be, resulting in alot of stuttering and "almost starting" upon cranking, but no starting

    Talking to the mechanic a little bit, he suggested that this may have been the problem with the o2 sensor. The fuel pump may have dropped pressure gradually and caused that whole system to freak out and kick into open loop (although there was no code...) But anyway heres to hoping...
     
  10. welfare

    welfare Ancient Member

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    Makes sense, not getting enough fuel would cause the system to try to dump more fuel to compensate, sometimes it would have thrown a code 43, sometimes it won't, I guess in your case it didn't.
    get that fuel pump fixed and see where you are at from there, being that your system ran lean when not gettingenough fuel it may have damaged the o2 sensor (too much heat) so keep an eye on it.
     
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