Idle surge with fluctuating AFR and idling Rich.

Discussion in '2005+ Mustang GT 4.6L Tech' started by CammedS197, Mar 9, 2018.

  1. CammedS197

    CammedS197 forum member

    Hey guys,

    Been searching for about a day or so and I cannot find something similar.

    So to start off, About a month ago after stopping to get food and starting my car up after about a minute it all of a sudden started to idle at about 10:1 AFR and if I gave it some throttle it would stutter as in way too much fuel and not a spark issue. After about a minute back to normal and that little rich time was gone. Now it has done it a few more times lately and now will sometimes surge when I clutch in. Drops to 500 up to 1000rpm and settles at 750.

    The AFR will follow the idle and go up and down with it. I have also noticed that if I rev the car AFR will drop to 10 or 11 and then spike up to 17ish and back to normal.

    The only times I have ever noticed it do that pig rich idle where it wants to almost stall out is only shortly after a start up but the car is already hot. If car is cold it will not do it. But you know it just wont do it if I'm logging though.

    WOT is still strong no issues there and regular accel and driving is fine. I don't feel this in my driving but if I do a rev and rpm comes back down AFR will be around 12-13 and if I hit it again it will have that slight hesitation due to too much fuel. Last night it did it again after starting up from being driven shortly before and it only happens about a minute or two after starting and lasts for maybe just a minute.

    AFR was normal and then bam all of a sudden it went from Stoich and dropped into the 12-13 range and slowly climbed down all the way to 9.xx AFR then went right back to normal. I was only moving slow in 2nd gear due to bumpy road and kids. I wasn't able to see if being that rich affected it.

    The car is a 2006 Mustang GT. Tuned by Lito (trying to get him a log with it) and has Longtubes, detroit rocker cams, off road H, MBRP mufflers, Airaid CAI, and GT500 throttle body. All of these have been on the car for over a year. No new mods.

    This happened on 2 different tunes and I know it is not the tune. Been using Lito for 3 -4 years now. Tried on my 87 octane tune for when we get hurricanes (seems more dramatic or pronounced on 87) and then my 93 octane tune as well. Same thing for both.

    Now my plan so far: Going to clean the MAF and throttle body and swap out the fuel filter within the next few days.
    Plugs are at 40k miles, COPs are at 125k miles (I have a whole new set in my garage) Front o2 sensors are at 125k miles as well. I plan on swapping the o2 sensors at some time as well with the NTK's.

    Was also thinking the Fuel pressure sensor or TPS could possibly be a culprit.

    Going to start out with the cleaning of the simple crap and see what happens. Can a fuel pressure sensor be cleaned? The TPS on the GT500 throttle body is about 1.5 years old and about 20k miles on it.

    I'm assuming all of these possibilities are able to be seen in a log except for the COPs and plugs?

    Thanks guys,

  2. Marble

    Marble forum member

    Do not start throwing parts are it!

    Clean one thing at a time.

    I would start with the throttle body.

    The MAF, COP and fuel pressure sensor either work flawlessly or not at all. Don't mess with them. You're O2 sensors could be old, but I would wait and do those after I changed everything else, one at a time.

    Weather changes, such as extreme cold, can have effects like you describe.

    A wide band that bounces around like this is not abnormal. If the car drives poorly under WOT then I would be more concerned.
    46addict likes this.
  3. CammedS197

    CammedS197 forum member

    Hey thanks Marble.

    I'm cleaning today. But I did a lod and holy crap my Bank 1 o2 sensor was showing a minimum of -0.10 and max of over 1800.00 wtf.

    Bank 2 was min of 0.00 and max of 58.0
  4. RocketcarX

    RocketcarX 95% of my weight is fuel

    My car started doing this exact thing, I replaced the MAF with a new OEM unit (ebay take off) and got no change. I disabled my LT fuel trims and it had no effect either.

    My car does it at idle, it will max out STFT from minimum to maximum. My wide band will peg rich and them work its way over to being all the way leaned out. It oscillated only at idle and has zero affect on WOT AFR. It doesn't seem to do it in drive, only in park. My car is 4R70W swapped so the ECU has no idea when I am in park or drive other than by the load it sees at idle.
  5. 07 Boss

    07 Boss forum member

    I used to have a similar issue. Maybe I still do but it hasn't popped up. But every once in a while I would start my car and it would be pig rich and stumbling. It would clear up after a few seconds. Never did figure out what it was but I did notice it really only happened when I was restarting the car after a short stop of a couple of minutes like when picking up a 12 pack or some smokes or something. Like I said I never did figure it out. Now I have kinda changed up my start up procedure. I put the key in and turn it to on, then buckle up my 5 pt and then turn the key to start. I don't know if this lets the fuel pressure come up or whatever but it seems like it's been a long time since it has had this issue pop up and it has never happened using this start up procedure. But there is no real correlation that I can put my finger on. Like I said it only seemed to happen when I was in a hurry and making a quick stop jumping back in the car and firing it up. Wish could be more help.
  6. CammedS197

    CammedS197 forum member

    Yeah that's exactly like it is for me. Stop get a drink or pick up my son or something. But has not done it since this post lol. Cleaning the MAF and throttle body made no difference. Looks like I need to change my o2's, Lito said I could since they are acting a bit odd in parts of the log and also they seem to be acting still but still work just fine. I will turn key to "on" and then buckle up and see if it comes back anytime. Still going to swap the o2's cause pretty sure they need it lol. 12 years old and 125k miles and 80-90k cammed.

    Yeah its really weird. Trying to let the clutch out to start from 1st is difficult and car just wants to stall out but once going its fine and AFR works its way up to normal.

    Only other thing I have noticed lately is the afr bounces around much more than it ever has when I shift and or rev the car. On a shift it will drop to 11's and the jump to 17-18's and back to normal. This is all within clutch in shift and clutch out. If I just rev it drops to 11-12's and up to 17-18 then normal. That little bit is still happening but its also when I noticed the o2's acting weird too. They never used to act the way they are.

    After I swap out the o2's I will report back my findings. Other than those no other sensor is looking odd or out of wack on my logs I did.
  7. RocketcarX

    RocketcarX 95% of my weight is fuel

    I put new OEM oxygen sensors before I changed the MAF, sensor replacement hasn’t helped at all.
    CammedS197 likes this.
  8. CammedS197

    CammedS197 forum member

    Hmm weird. Have you ever reloaded the tune? Mine has been acting much better lately so idk lol. The afr is still fluctuating more than it has before but still hasn't done that pig rich idle.

    Damn thing is like a woman. You never know what they want and you're always wrong lol.
  9. RocketcarX

    RocketcarX 95% of my weight is fuel

    Yes, the tune gets reloaded every time a change is made. I disabled my long term fuel trims as well and saw no change.
  10. thump_rrr

    thump_rrr Senior Member

    Mine was the O2 sensors.
    It was showing a lean code but was running pig rich.
    For the $30 per sensor it was worth the effort after 10 years.
    Fuel economy increase alone paid for the sensors.