'04 Ford 3.9 MAF Sensor

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  • Modis500
    Senior Member
    • Sep 2012
    • 358

    '04 Ford 3.9 MAF Sensor

    Need some help here. Got an '04 Ford Freestar 3.9 with the awfully annoying P0171. Will make this short and sweet. Watched data numerous times and found intake leaks(IMRC shaft seals, upper plenum gaskets replaced) Customer has had tough time with this one, and just finished up the intake job tonight. Watching data, shows bank 1 @ 22% LT, and short term is -3 up to +33 when revving. So we have removed all intake leaks on this thing, but still getting the above FT numbers. So went to the MAF, and backprobed the MAF Signal and here's what I found. Kind of getting stumped on the MAF sensors, and had a post last month on a Toyota 1.8L with same code. My assumption would be on a V type engine like this V6 i'm working on, wouldn't you get a 171/174 at the same time? Or am I over thinking this? Oh, by the way, Fuel pressure is ok. Is this enough dropout on sensor to condemn? One is slight acceleration, one is quick acceleration.
    Let me know
    Attached Files
    "If you aim for nothing, you'll hit it every time!"
    Zig Ziglar
  • Crusty
    Senior Member
    • Dec 2007
    • 2450

    #2
    Good that you found and fixed the intake leaks. Remember the fuel trims should be manually reset. Codes should be cleared and then the vehicle driven with several hard accelerations followed by some long decelerations for the ECM to relearn the Baro. Also check for any exhaust leaks. Even exhaust leaks PAST the sensors can be "pulsed" into the area of the O2 sensor as each cylinder cycles.
    See how many emissions monitors you can get to "complete" and then check for either "pending codes" in OBDII, or take a look at some Mode-6 data for the O2's.
    Just some food for thought....

    Comment

    • greasybob
      Senior Member
      • May 2008
      • 1590

      #3
      Just wondering what are the fuel trims for bank 2 ? Did you snap the throttle to get the scope shots ? A good throttle snap should have a double hump and show peak at around 4.5 volts. Her is an example I found in my scope files, not the best but it gives the idea.
      Attached Files

      Comment

      • Modis500
        Senior Member
        • Sep 2012
        • 358

        #4
        Bank 2 FT numbers were around -5 to +8 Short Term and -4 to +9 Long Term, so nothing crazy, but it's just that Bank 1. And like i said, no air leaks, pinched off hoses to Brake Booster, purge valve, and EGR and nothing changes, so no problem there. Did a snap acceleration, but did not get the double hump like yours, it goes up then plateaus, and then drops off as I let go of the throttle. Also, trying to upload an image from SSC, but still learning that whole deal.
        "If you aim for nothing, you'll hit it every time!"
        Zig Ziglar

        Comment

        • Bob's Garage
          Senior Member
          • Nov 2007
          • 3257

          #5
          MAF Testing

          Originally posted by Modis500
          Bank 2 FT numbers were around -5 to +8 Short Term and -4 to +9 Long Term, so nothing crazy, but it's just that Bank 1. And like i said, no air leaks, pinched off hoses to Brake Booster, purge valve, and EGR and nothing changes, so no problem there. Did a snap acceleration, but did not get the double hump like yours, it goes up then plateaus, and then drops off as I let go of the throttle. Also, trying to upload an image from SSC, but still learning that whole deal.
          What are LTFT & STFT # at a steady cruise , say 40 to 50 MPH?

          The key word is steady. Cruise control helps. The ranges you gave are too wide to be steady. I test MAF sensors with a software V.E. calculator. On that engine I would expect about 80%. I also expect all 4 O2 sensors to go high and stay high on a WOT RUN, until you let off. If not, it shows fuel starvation if they go low at WOT.

          The fact that you are only setting a P0171 seems to be a normal occurance. The P0174 will come, I expect.

          Can you perform a V.E. run? That is, set up a custom data list with IAT, MAF g/s RPM and maybe O2S & LTFT, STFT? Cull out the rest of the PIDs. Start out on a deserted road and capture data. From an idle , go WOT till after the last gear has been achieved. Do it a couple of times. Save the movie and we can calcluate the V.E. to determine if the MAF is accurate. Post the movie here in a reply.

          You could have a fuel delivery issue ie; low pressure or restricted injectors. Not likely, but always a possibility.

          Thanks, Bob

          Comment

          • Modis500
            Senior Member
            • Sep 2012
            • 358

            #6
            Here's some more info. While in driveway, watching upstream and downstream 02's. Bank1/Sensor2 goes dead lean while revving or holding throttle at steady speed of 3000RPM in park. Appears vehicle does not have Bank2/Sensor2 as PID's show 0.000 Volts all time. So my guess now is either a dead downstream sensor which would throw this code when its trying to compensate for a very lean mixture or an exhaust leak at the flex pipe. Will try to get vehicle from customer for a short while to do the VE run you requested. Anyone have seen pattern failures on flex pipes on these minivans? Again, thanks everyone for the input.
            "If you aim for nothing, you'll hit it every time!"
            Zig Ziglar

            Comment

            • derktins
              Member
              • Oct 2012
              • 37

              #7
              Modis, I would do a hard throttle snap ... put it on the floor and keep it there until it nearly redlines ... while scoping the MAF signal wire and compare to greasybob's example. His looks like a pretty texbook example of a good sensor. The other testing method I have used for MAF sensors with a lot of success is to take a good volt meter with a fast sample rate and set it to min/max, then perform a hard throttle snap and check the recorded max voltage. If it is less than 4.0V, verify power+ground, check for air leaks and replace the sensor. The catch to this test is that you have to use a meter with a fast sample rate. Most DVOM's will miss the MAF voltage peak because they simply dont update fast enough. I generally use the VERUS scope, but have also used a Vantage Pro just set to min/max with the meter screen pulled up. This is a 5 min test and has NEVER YET let me down. It is also handy to know that this works on almost all 5v MAF sensors and you are ALWAYS looking for 4V, with the exception of Toyota, in which 3.75V and up can be considered normal. Of course, this test won't work on frequency type MAF's such as on many GM's.

              Again, this is a 5 minute test that has never let me down. Good Hunting, buddy.

              Comment

              • Modis500
                Senior Member
                • Sep 2012
                • 358

                #8
                Derktins,

                I did do this type of testing, unaware that was the actual correct testing. While doing the test, it would rev to about 5000 which is the limiter in park, and it would peak, then dip down and hold (not have the double hump effect which is correct), then drop down when I let the throttle go. Max voltage would be in the 3.35 to 3.70 range, occasionally getting near 3.9 but never towards 4.0. So I'm seeing the MAF may be bad, but why is the B1S2 going dead lean when revving up in park/no load? Have not confirmed exhaust leak, but at idle downsteam sensor is sitting around 500 to 600 mV....Still confused on that
                "If you aim for nothing, you'll hit it every time!"
                Zig Ziglar

                Comment

                • derktins
                  Member
                  • Oct 2012
                  • 37

                  #9
                  Modis, if it is getting close to 4V on a hard throttle snap then a good cleaning with MAF cleaner (not carb cleaner!) might get it back up where it needs to be. If it will not hit 4V, either it isn't doing it's job, or there is an air leak somewhere. Normally, if a good cleaning gets the sensor back up above 4V then I will recommend a new one to the customer anyways. Too many times I have cleaned one and had it just act up again in a couple months anyways. The MAF HAS to hit above 4V, no exceptions. If it's not, there is a problem... Make sure you eliminate air leaks, MAF wiring and engine efficiency first before replacing a sensor. Ford MAF's are notorious for problems and I am always suspicious of them.

                  If you are using your scope, the first hump will generally be the highest one and should definitely go over 4V. This is at the moment when the throttle opens and there is a large in-rush of air to fill the vacuum in the manifold. The first hump will almost always be higher than the second. I like to see the first hump above 4V and the second darn close to 4V. The captures you posted definitely don't meet that criteria. Assuming you have no vacuum leaks, air leaks in intake boot, worn out engine that isn't moving air, there is a MAF problem. The real key to this test in that you have to snap the throttle HARD. Don't roll the pedal to the floor, go from CT to WOT as fast as possible to get that big rush of air filling the vacuum in the manifold. If you want to know how to properly do a throttle snap, find the nearest 16 year old boy and ask him to show you!!

                  PS This test works on ALL 5v maf sensors. The only exception I know of is that on Toyotas anything above 3.75V is considered normal. Also, it doesn't work on Frequency generating MAF designs like most GM's have. I learned this test at an AVI training event a few years ago and immediately started seeing success with it in the field. I have been using it ever since and it has not let me down yet!

                  If you want extra confirmation on the MAF, look at the BARO reading on your scan tool. This reading is calculated by the PCM based on the MAF readings at WOT on virtually all ford products. At sea level it should be 160 hz. Where I am at about 500 ft above sea level, I generally see readings from 153-157. If I see a reading that is below 150, I start looking for a MAF problem, vacuum leak etc. On some cars with really bad sensors I have seen BARO PIDS as low as 130 hz.

                  Comment

                  • crackerclicker
                    Senior Member
                    • May 2008
                    • 400

                    #10
                    Originally posted by derktins
                    Also, it doesn't work on Frequency generating MAF designs like most GM's have.
                    I don't have a Snap on scope anymore to verify this, but I seem to remember that they were capable of graphing frequency. If so, that would give you a very similiar waveform to a regular MAF, but would be shown as a frequency value instead of a voltage value. Actually, I do have an old heritage Vantage, but not a frequency style MAF to test it out on.

                    Comment

                    • derktins
                      Member
                      • Oct 2012
                      • 37

                      #11
                      crackerclicker, yes my VERUS does graph frequency, I assume a MODIS will as well. The waveforms look almost identical. On most GM MAFs I look for at least an 8500 hz peak, and on 5.3's and 6.0's I like to see 9000 or more. I didn't hear that spec anywhere, it's just from personal experience, and as such I don't generally give it to anyone as a spec because I don't want to be giving out false info

                      Comment

                      • Kadibia
                        Junior Member
                        • Mar 2013
                        • 1

                        #12
                        if you have not resolve the problem yet, check the exhaust leakage critically, and change after cat oxygen sensors with known good and retest

                        Comment

                        • Modis500
                          Senior Member
                          • Sep 2012
                          • 358

                          #13
                          Finally-Scope Screen shots

                          Okay guys,
                          Finally got to get a scope shot of this MAF...so I'll post 2. Yes, they did get above 4 Volts, but look at the glitches in these pics. Am I looking at them right, as in one of them is going below 0 volts. And checked into exhaust leak, thats not really an issue. Long term on bank 1 is + 15%, and down stream bank 1 is rich at idle and off idle, which it should be, and it all checks out fine. Still confused guys= about to set this thing on fire!
                          Attached Files
                          "If you aim for nothing, you'll hit it every time!"
                          Zig Ziglar

                          Comment

                          • Crusty
                            Senior Member
                            • Dec 2007
                            • 2450

                            #14
                            Good catches with those screen shots.

                            Make sure your leads aren't just picking up a loose connection.

                            Those two spikes down in the middle of the waveform might cause a quick hickup in the motor.

                            If your connections are secure and the feeds and grounds for the MAF are good, it looks like a bad MAF to me. JMO.

                            Comment

                            • Witsend
                              Banned
                              • Nov 2012
                              • 2942

                              #15
                              Long Fuel trims of both banks should be pretty close regardless

                              If no vacuum leaks ,injectors flow test the same, IRMCs both work , MAF sensor replaced and road tested hard, yet fuel trim remains too high on bank 1 ,maybe pre cat substrate clogging and there's higher exhaust back pressure on bank 1 than Bank 2 ? Just something to consider if stumped. I pull both upstream o2s out. Start up vehicle breifly and determine if exhaust seems to jetison harder out of one hole than the other. Use hand and eye protection , sheilding vulnerable components from hot exhaust gases , If no difference I'll swap the upstream O2 sensors while they're both out.

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