06 GT slight surges with throttle off

Discussion in '2005+ Mustang GT 4.6L Tech' started by Someguy, Mar 19, 2019.

  1. Someguy

    Someguy Junior Member

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    So until i have another engine built and ready to go, I'll probably hit the tracks a few times by summer time with my current engine and its setup. It's a 06 GT with 120k miles with a vortech centri blower and few other mods all previously done by last owner. It currently runs fine and pulls strong, but lately its got a problem of surging a bit after i let off the gas pedal. It's annoying enough that i hope there's a fix before i take it to the track.

    - Let's say im cruising along about 40mph and let off the gas, for the next 2-3 secs it will surge a bit as if i'm goosing the pedal on and off very lightly, and will do it about 2-3 times. I'd say it raises the rpm no more than 50rpm when it happens, but it was just enough to feel like as if you dipped into the gas pedal again.

    - There are no codes and no check engine lights. The engine doesnt make any unusual noises. It doesnt smoke or noticeably use any oil either ( I havent driven it long enough since i own it to see a noticeable amount of oil consumption).

    - I have installed a new set of spark plugs with 0.032in gap, cleaned the air filter, and cleaned the MAF sensor with MAF sensor cleaner.

    - Ive brought my car back to the tuner that previously did the tune to let him check the car out. He works on mustangs only and his shop is always packed. He looked at the tune, took the car for a long test drive with datalogging, and said he found nothing in the datalog or the tune that could contribute to what i described. He said without investigating further, at this point he's most suspicious of my BBK 62mm tb. He suggests his customers to just stick with stock tb unless they have huge hp goals to reach.

    - I've read up on people having bad experiences with BBK tb. But before i randomly just drop money onto another tb, is there any other things i could check to confirm it is my tb acting up? I wonder if it needs cleaning, but it seems like it takes a little bit of work to undo the charge cooler that bolts up to the tb to get access. it may be what i have to look at next time when i get to work on the car. I do not have the luxury of having the time or space to work on the car anytime i want. It'usually one afternoon a week so i got to plan ahead what i'm going to do and get the parts ahead of time, and hopefully i have all the right parts on hand.
     
  2. Dino Dino Bambino

    Dino Dino Bambino I have a red car

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    Your TB probably just needs cleaning but like you said, there have been enough reports of the BBK units being problematic that you're better off going back to a stock unit.
     
  3. JeremyH

    JeremyH 3V Fuel Guru S197 Team Member

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    Like said clean the TB and check for vac leaks, specifically associated with the bypass/bov. Also did you buy "pregapped" plugs or did you gap/check them yourself. Done any fuel system maintenance? Injectors/filter etc. Those are often neglected items on modified vehicles.

    I bought a bbk 62mm about 9-10 years ago when people were bashing it. Went through 2 well known tuners who couldn't get it perfect and had issues. Third tuner tuned it no problem. And have had zero issues since then and still run it today. I have cleaned it maybe twice in 10 years just to do so.

    That said it's a pretty simple mechanical device any actual issue would be motor/tps related which will throw a code or limp mode. If car was previously fine then just clean it.
     
  4. Someguy

    Someguy Junior Member

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    - At the next opportunity I will try to get to the tb and do some cleaning. And also look around for vacuum leak. Would probably try with the starting fluid method?

    - I put in new Brisk silver racing spark plugs for my hp range, and I gapped them all myself at around 0.032in. The oem plugs that the previous owner had, they were all around 0.05in gap. I thought for sure I'd see big difference after I put in new plugs. While most of the off idle stumbling, especially when engine is not fully warmed up, had been cured, the surging after lifting throttle while cruising hasn't gone away.

    - I've added fuel system cleaner in the tank and that's about it. Haven't checked injectors or changed fuel filter. I'll probably just go get a filter since I dont know when it was last changed. About the injectors, they are 60lbs. I read up on it a bit and I wonder if they may be too big. I checked receipts and last owner put these injectors on 2 years ago, maybe 4k miles on them at the most.

    - I don't recall seeing a bypass/bov. Maybe I just wasnt paying attention. Will check again next time I pop the hood


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  5. Someguy

    Someguy Junior Member

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    So I still haven't had a chance to look at the car yet. But looking at a datalog file I did when I bought this little obd2 reader that connects to a phone app, I think I may found a possible issue that had been overlooked before and worth looking into again.

    So my absolute throttle position reading had always been 13-14.5% both at idle or with car moving but gas pedal off. It goes up normally with throttle input during acceleration but never gets below 13% with pedal off. I'm no expert in doing data logging but I'd assume with no throttle input the reading should be much closer to zero? Again I have no codes or cel. Maybe it is just a motor/tps issue? If that's the case I'll just start with the tps and go from there

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  6. Dino Dino Bambino

    Dino Dino Bambino I have a red car

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    That's normal.
     
  7. Someguy

    Someguy Junior Member

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    If the reading is normal, I guess I'm back to checking the tb for cleanliness first. I also have a new tps on the way just in case.

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  8. JeremyH

    JeremyH 3V Fuel Guru S197 Team Member

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    If your system is draw thru maf still, then when you come off throttle the bypass valve opens and recircs the pressurized metered air back into the blower inlet. If the bypass is failing or leaking or has a poor vac signal it can def cause the surging off throttle issue you describe.

    There will be a valve with vac line from manifold and hoses routing from charge pipe to blower inlet tube.

    A common upgrade is switch to blow thru maf and remove the bypass valve and hose and switch to a bov that vents to atmosphere instead. So would need more info on your setup.
     
    hammeron likes this.
  9. Someguy

    Someguy Junior Member

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    I have the draw through maf and bypass valve, the setup that came with vortech kit done by previous owner. As you described I traced the vacuum line and I think I now know where the bypass valve is, but it takes some work to get to as it seems to be buried down by the charge cooler. The lines i was able to see I didn't see any defects. May need to find another day to remove the cooler, check the lines, TB and the bypass valve.

    It was hard to even find time to touch the car lately. Today I didn't have much time to dig deep, but enough time to swap a new TPS, and installed a different set of rims/tires I got off from someone a few weeks back. Went for a small test drive to see how everything works. It feels like the off throttle surging is mostly gone, at least I didn't feel any like previous where it's very obvious. Now the only thing that's noticeable is a little roughness in low rpm, like coming off a stand still or after shifting in the lower rpm range. It's in between like a slight stumble or just delay in throttle response. But once it's going it's fine and pulls strong

    Also something new that's unrelated, is the abs and traction control light came on during this test drive. I'm assuming that's because I changed from 275 width tires to 295 and they are different diameter and the computer freaks out. I'm guessing I need to go to a dealership and have some kind of recalibration done?

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  10. Someguy

    Someguy Junior Member

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    So in the last few weeks, i've got new clutch and new Torsen rear diff with 3.73 gear installed (it had 4.10 and made some noise, oil was in bade shape so might as well replace everything). Also checked for vacuum leaks and replaced the bypass valve. The surging is still happening although it's not as frequent or as apparent as before.

    With the drivetrain noises and sloppiness all gone now, what i noticed when driving now, is when coasting in gear and no throttle, whenever the slight surging happens the AFR also jumps to 11.x range momentarily and then going back to 15+. As said it happens less frequently, maybe once every 2-3 seconds and may happen again if i purposely coast for longer.

    I'm thinking to just say screw it with the BBK and go back to a stock TB, and then go back to my tuner for another tune and see what happens. Again i do not have any codes and the car drives fine otherwise. A friend of a friend that deals with lots of track/race cars had driven my car and said he thinks my AFR seems to be too rich across the board, but im not sure what he's basing his assumptions on and of course we cant compare apples to oranges.
     
  11. 1950StangJump$

    1950StangJump$ forum member

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    On your SC, where does the air filter sit? Is it exposed directly to open air?
     
  12. Someguy

    Someguy Junior Member

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    No my air filter sits just aft of the left side headlight, not directly exposed to open air.
     
  13. 1950StangJump$

    1950StangJump$ forum member

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    Okay. Sometimes having your cone exposed directly to air can cause idle or low RPM surging when at speed because the turbulent air confuses the MAF/computer.
     
  14. Someguy

    Someguy Junior Member

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    Hmm i see. Did not know that... but i dont think my filter is seeing any direct air stream that can cause this type of surging
     
  15. Someguy

    Someguy Junior Member

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    Well I'm not sure if I have found the culprit to the surging, but I discovered a new problem. I finally got around to do the fuel filter. I found that whoever did it last, not only did he installed the filter backwards, both quick disconnect fittings are also damaged probably from someone that didn't know how they work and broke them in the process of removing them. Without much effort they came right off and cant be reused to install the new filter. They were barely in place and i was just glad they didn't give me problems while I drove the car. Maybe it was a clogged filter that's giving me problems this whole time?

    Tomorrow I'm going to start searching for maybe splicing in sections of fuel lines to either end with new fittings that can connect to the filter, or whatever ways I can repair the lines to connect to a filter.

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  16. Someguy

    Someguy Junior Member

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    If anyone cared to find out what really fixed the issues a few weeks ago, here it is....

    So i fixed the fuel filter fittings and put on a new filter, no help. Then i put on an OEM throttle body, and all the surging and bucking are gone. There are no more off throttle surging when the car is coasting. Only thing i notice is the car drives like crap before its fully warmed up (i drive very easy when engine's cold). Sometimes the throttle response off the line is a little slow and sluggish as well. I'm guessing it may need a retune since now with the OEM TB the engine is getting less air than with the 62mm and is getting too much fuel? But otherwise it drives fine when fully warmed up.