Low Rpm Lean Pop Trouble

Discussion in 'Fox 5.0 Mustang Tech' started by SloNoMoRacing, Nov 20, 2013.


  1. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    See the section below on testing the O2 sensors before you swap or replace them...

    Code 41 or 91 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

    Revised 29-Sep-2013 to add back in a clogged crossover tube as cause for code 41

    Code 41 is a RH side sensor, as viewed from the driver's seat.
    Code 91 is the LH side sensor, as viewed from the driver's seat.

    Code 172 is the RH side sensor, as viewed from the driver's seat.
    Code 176 is the LH side sensor, as viewed from the driver's seat.

    The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

    The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
    "When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

    When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

    Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

    Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

    Testing the O2 sensors 87-93 5.0 Mustangs
    Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.

    Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

    Backside view of the computer wiring connector:
    [​IMG]

    87-90 5.0 Mustangs:
    Computer pin 43 Dark blue/Lt green – LH O2 sensor
    Computer pin 29 Dark Green/Pink – RH O2 sensor
    The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

    91-93 5.0 Mustangs:
    Computer pin 43 Red/Black – LH O2 sensor
    Computer pin 29 Gray/Lt blue – RH O2 sensor
    The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


    Testing the O2 sensors 94-95 5.0 Mustangs
    Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


    Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

    Testing the O2 sensor wiring harness
    Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

    87-90 5.0 Mustangs:
    Computer pin 43 Dark blue/Lt green – LH O2 sensor
    Computer pin 29 Dark Green/Pink – RH O2 sensor
    Disconnect the connector from the O2 sensor and measure the resistance:
    From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
    From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 43

    91-93 5.0 Mustangs:
    Computer pin 43 Red/Black – LH O2 sensor
    Computer pin 29 Gray/Lt blue – RH O2 sensor
    Disconnect the connector from the O2 sensor and measure the resistance:
    From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
    From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

    94-95 5.0 Mustangs:
    Computer pin 29 Red/Black – LH O2 sensor
    Computer pin 27 Gray/Lt blue – RH O2 sensor
    From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
    From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

    There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

    The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

    Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

    Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

    Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports, Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

    If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.
  2. SloNoMoRacing

    SloNoMoRacing Member

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    jrichker: Thanks for posting the o2 sensor testing procedures. I've those bookmarked for awhile now, but it was a good reminder.

    On that note, I decided to break out my analog meter and retest the o2 sensors... They are both now only switching between 0.01 V min, and 0.03 V max at idle. Last time I checked them (last month) they were switching between 0.2 and 0.6 at idle. Looks like I'm in the market for some new o2's.

    edit: My question is, why wouldn't it be throwing lean codes 41 and 91 anymore if the o2's are actually that far out of spec?!
  3. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Are you sure the batteries (if any) in your meter are good?

    Maybe the O2 sensors are dancing on the edge of the cliff, just about to fall off...
  4. SloNoMoRacing

    SloNoMoRacing Member

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    Definitely a good battery. I never store my testing equipment with batteries in it, so I had to put a new one in just for this test. Sounds like you're right about them about to fall off. I performed the Ohms test on the harness and it's right around 0.5. The engine grounds are all good, and I've actually added extra grounds strategically to avoid voltage loss with my battery being in the trunk. I'll see if I can pick up some Bosch direct fit replacements tomorrow, and report back. If I can't find direct fits, I'll just order some Motorcrafts on Monday. I'm not a fan of those universal Bosch o2's. Splicing a harness that relies on incremental voltage fluctuations to operate properly doesn't sit right with me.
  5. SloNoMoRacing

    SloNoMoRacing Member

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    Well, no dice. Still popping. I even waited for new Motorcraft sensors, but the popping is still there exactly as before.

    Maybe I can find a ProM meter on sale for cyber Monday.
  6. SloNoMoRacing

    SloNoMoRacing Member

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    So I spent last night troubleshooting the MAF, and contacted a rep at Pro-M Racing this morning to see what their thoughts were. Turns out it's not the MAF. According to even the Pro-M rep, it's an ignition problem. I ran diagnostics with them on the phone, and they confirmed the readings I was getting were accurate (acceptable anyway). I even ran the engine with the MAF unplugged, and the popping was still there with the same frequency. Their guess was that it may be attributed to spark scatter in the distributor. They advised me not to purchase one of their MAF units until I rectified the ignition issue. I've got nothing but good things to say about those guys, and I'm glad they're back in business.

    I have a known working TFI module that I will start with, and then throw in my spare dizzy if that doesn't work. I have an old MSD 6a box and coil waiting to go on, but I don't want to add that into the mix until I've nailed down this issue. I'll start testing the ignition system one component at a time to see what's going on. One thing that comes to mind is the Ford Racing plug wires that I have on there. They were new when I installed them, but I've had issues with their wires in the past. Seems like the quality control with those wires sucks for some reason (outsourced?).
  7. mikestang63

    mikestang63 Mustang Master

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    Is it a stock or MSD ignition system now? If MSD, take all that crap off and I bet the issue goes away. For the wires, bring the car into a garage or somewhere dark and with the engine running take a spray bottle and light;y mist the wire- if you see arcing then the wires are shot.

    The TFI normally shows up as a warm no start or dying out, not a popping. If you have a spare stock distribtuor, TFI, and coil, I would put that on and see what happens.
  8. SloNoMoRacing

    SloNoMoRacing Member

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    Stock ignition system, except for a Crane Fireball coil. Like I said, I'm not putting the MSD ignition box or coil on until these issues are sorted.

    I'm certain the wires aren't arcing. I have done the spray mist test just to be sure though. The issue that Ford Racing wires are known for is having too much or too little resistance, or having varying resistance amongst wires within the same set. That could possibly be happening here. I'm going to check the resistance of each of these wires tonight. My headers are also wrapped like mummies, so I'm not concerned about the boots being melted either. The wraps are ugly as hell, but I can grab a primary with my bare hand after sitting in rush hour traffic without getting scorched haha. My under hood temps are nice and cool.

    I agree that most of the time a faulty TFI module will cause issues only when it gets hot, and create a no start until it cools down. It is possible to have an intermittent issue with the TFI though. I'll swap it with my OEM spare and see what happens

    Thanks
  9. cleanLX

    cleanLX Founding Member

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    put a stock coil on it.
  10. SloNoMoRacing

    SloNoMoRacing Member

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    I do plan on putting one back on for diagnostic purposes, but just out of curiosity why? I had this coil on my previous motor and it worked brilliantly for years.

    Thanks, and happy Thanksgiving!
  11. Grabbin' Asphalt

    Grabbin' Asphalt Mustang Master

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    The more i read this the more it sounds like the bad msd distributor i had. It only surfaced when i installed a 3g alternator and it went berzerk. If you're in the atlanta area, i got a spare refurb FoMo one you can slap in to check.
  12. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    coil isnt going to cause your issues. Coil either works or it dont. Yours does, its fine Now you van ohm it out to see if its still within the manufacturers advertised specs, but unless you've got a high dollar racer where every millisecond counts, you'll never notice any difference even if its out of spec
  13. liljoe07

    liljoe07 Active Member

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    Can you take a picture of the distributor from the front of the engine and post it?

    LoL! I hate to ask questions like those. But Id rather ask and know, than to assume. Youd be surprised at how many have done that!


    That is interesting! The more vacuum the engine makes, the more the fuel pressure will drop when the vacuum line is connected. So the numbers are believable. But it seems vacuum is kinda excessive in your case. Almost make me want to question the cam timing. :chin
  14. 5.yellow

    5.yellow Member

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    what part of Florida are you in?
  15. SloNoMoRacing

    SloNoMoRacing Member

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    Woah I didn't realize I had so many responses since last week. I stopped getting email notifications.

    Here's the update: I replaced the TFI module, and as suspected nothing changed. I finally got around to taking the Crane Cams Fireball coil off tonight, and temporarily replaced it with an MSD Blaster 2 coil that I had laying around... I think it may have worked! I won't be able to start it up again until tomorrow (don't wanna wake the kid), but I'm pretty sure the issue was either with the connection to the coil, or the coil itself. Not sure if anyone here has seen the connector on the Fireball coil, but it's pretty goofy. It splices into the factory wiring, and the connector itself is a rubber block with two female bullet connectors inside of it. The coil has two male bullet connectors. Well my negative connection inside the rubber block was pretty loose. There would be no way to tell that without taking it off, and looking really closely inside the rubber block.

    I installed the MSD coil because the connections were easy to temporarily join with the existing Crane connections (after fixing the defective negative cable). Fingers crossed that this was the issue.

    madspeed: That's what I've seen too, but this may be an exception. Of course, it could have just been the loose wire. I'll know more after I drive it tomorrow.

    liljoe07: I'll take a photo of the dizzy tomorrow for you, but until then I can tell you that the plug end of the TFI module is clocked at about 4:00. I think I know what you're getting at, and it's definitely not off a tooth. Based on experience, it looks pretty close to 10 degrees initial. I stabbed this distributor while it was on the stand, before I even rotated the motor, so I'm pretty sure I got it right.

    LOL. I've seen people do some pretty dumb things when running diagnostics, but that one cracked me up.

    I installed the cam straight up (no advance) according to the specs on the cam card. I also checked it with a degree wheel and everything lined up perfectly. I'm positive on this one, because when I first installed the cam I installed it 180 degrees off (I was in a rush and lined the dots up backwards DOH). I didn't get even get the timing chain back on before my brother pointed at my mistake and laughed his ass off. It was a late night, but we decided we had better take our time and degree the cam properly after that. It's definitely in straight up with no advance.

    5.yellow: I'm in Miami.

    Thanks again guys. I'll update again tomorrow afternoon.
  16. SloNoMoRacing

    SloNoMoRacing Member

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    Spoke too soon... Still no dice. I'll try stabbing my spare Motorcraft distributor in tonight and see where that gets me.
    Grabbin' Asphalt likes this.
  17. Grabbin' Asphalt

    Grabbin' Asphalt Mustang Master

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    I'm waiting for the stabbing to begin, im standing by my choice :rlaugh:
    mine would idle great then flip out around 2500-3000 rpms, was worse when an electrical load was on given too, A/C, heat, power windows, ...was very strange.
    Last edited: Dec 3, 2013
    SloNoMoRacing likes this.
  18. SloNoMoRacing

    SloNoMoRacing Member

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    That's right, $#!+ just got real... I'm no longer installing and replacing parts, I'm stabbing and throwing them lol.
  19. SloNoMoRacing

    SloNoMoRacing Member

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    Distributor didnt do it either... Guess it's time to focus on the fuel system. I have an A9P computer that I'm going to try as well just in case. I replaced my original A9L when it fried with an 8LD, and i just want to be sure its not hiccupping.

    What sucks is that this may boil down to a simple wiring issue. I've been out of the foxbody realm for awhile now (been driving an SN95), and I'm now remembering all the wiring gremlins these cars can have.
  20. Grabbin' Asphalt

    Grabbin' Asphalt Mustang Master

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    Hmmm, well thats a bummer. Did you already say you checked the coil, wires, boots and already tried the dark engine bay looking for sparks method. Whats the fuel pressure doing when it pops??

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