Hi guys, I'm looking for a little help figuring out an acceleration issue on my car. When it's cold it accelerates unevenly. If you mash the accelerator to the floor it moves right out, but if you give it partial throttle it stumbles a bit. Warmed up completely there's a slight stumble. The car has 62k miles and it's a manual, still using original plugs. The only mods are mufflers, BBK headers, CAI, bama tune
I've checked the map sensor and cleaned it, cleaned the throttle body, cleaned the fly by wire potentiometer under the gas pedal, removed the bama tune and reinstalled it. Clean intake filter too
Your fuel trim is perfect! If you're getting that on both banks congrats on all the cleaning work. At light part throttle acceleration you should see the air/fuel ratio staying at 14 - 14.3 running on typical 10% ethanol gas. It will go up and down slightly but definitely not any different from just cruising. You can simulate light acceleration by holding steady speed going slightly uphill while reading A/F and look for any wider swings.
Interesting that for a while new spark plugs seemed to fix it. My gut feeling is something in your tune. Have you tried stock tune and intake recently?
Hi koryak,
Ok, tried taking it back to stock as hawkstang mentioned along with disconnecting the battery. Still has that weird surge.
Any other sensors you think should be tested? I can check the ignition coils with a static resistance check, but there's no way for me to test them under load where I wonder if one is failing.
Any other thoughts?
This is sort of fun in a way, the car stills runs so it's not an awful experience. It's like a little mystery or puzzle to solve and I know I'll eventually figure it out with all the guys on this forum
It's possible the spark plugs minimized it but didn't go away. It's back for sure though. I don't think it's the tune because I think I tried reverting to it and the issue was more pronounced. I'll try that tomorrow just for grins. Any other ideas?
I don't think it's the intake since its been on the car since 12k miles without issues. Plus I don't have the stock intake, didn't come with it
Hey Hawkstang, good point, it does happen in the same rpm range all the time. I could test the coil theory by borrowing Strange Mud's coils... You think he'd mind? ;-).
Seriously, I can look at the service manual and see if there's tests for those. Probably some sort of static resistance test
If it happens at the same point in the power band I think that sounds like a tune issue. You're coils are prob fine but might be worth checking. Usually if a coil is bad you'll get a code though.
Not a custom tune per se but an off the shelf Bama tune. I think I reverted it back to the stock tune in the past to test this but I'll try that again tomorrow to see what happens.
Try this... return it to stock tune, then unplug your car battery for 20 mins, then test it.
If it still does it, use your SCT tuner and skip the bama tune and use the SCT to tune it. You will answer a serious of questions when you go this route, then it will tune your car. Then test it this way. If it happens on stock, SCT and Bama, you may have a more serious issue.
ALWAYS RETURN TO STOCK TUNE BEFORE UPDATING YOUR SCT.
That sounds like a good idea, I'll try it today and see what happens. Here's a thought I had - there's a potentiometer above the gas pedal that actually operates the throttle. I tried cleaning it with tuner cleaner but couldn't access the brushes very well. It didn't idle well because the potentiometer was dirty, cleared it right up when I sprayed it with tuner cleaner. It's a nice theory but it wouldn't totally explain why it mostly happens when it's cold and only a bit while warm. I think the poor contact would be consistent wether it's cold or hot. If I get desperate I'll pull it out again and measure it with my DVM.
A question regarding the SCT tuner - I didn't know that could walk through steps so I could create my own tune? I thought the bama version was locked out to their versions only?
Its a vvt issue. The VVT system has its own set of oil pressure regulators, and when oil isnt up to temp (when the car is cold...) there isnt enough pressure to advance the intake cam (which increases torque). My old miata suffered from this heavily, i basically couldnt give it over 1/2 throttle until the car was fully warm or it would buck and hesitate.
I dont know if yours are "defective" or what grade of oil you're running, or how cold it is there, but to me it sounds like a VVT issue.
One other thing, while driving it today after being warmed up fully it's still got a funny surging issue. It's much more noticeable with the stock tune than the Bama tune. I didn't have time data log it today but I'll try to get to it tomorrow morning. Should require only a few minutes of data logging for you to see the issue; I promise I won't make you slog through a ton of code. This is really interesting, thanks in advance for your detective skills!
Hi voltwing
I'm not sure if that's the problem because this started happening within then past 5 to 10k miles, not the entire life of the car. But I'm gonna look at it today.
Koryak,
Here's a new one - with the stock tune in place it actually bogged down when it was cold this morning. An actual bog like a carbureted motor when it's choking on gas. No codes or anything. Gonna keep looking on this
Here's some general troubleshooting info: If the throttle hesitation feels sharp then ignition failure or fuel leanness is likely. Bogging or sluggishness is more like too rich fuel or possibly VVT.
I think VVT is used for EGR on this engine so cam position is important. See if there are any pending diagnostic codes for cam position error if your scanner allows this.
Again, the VVT has its own oil pressure regulators and solenoids, so if one of those started failing in the past few miles that could explain it.
It seems you're comfortable datalogging after skimming this thread. The Live link software will let you monitor desired VVT position as well as actual position. If the actuator is in fact failing then they wont match. If they do match then i'm wrong and you're back at square one anyways lol so its at least worth a shot.
ok, so I can activate the data logging function from the Bama tuner. I haven't done that before but I can figure it out. How long do I need to drive the car in order to data log it?
My last car was a vvt too and it never had hesitation on acceleration, cold or otherwise. Until a coil went bad and then the engine light flashed repeatedly and it threw a code. I do notice my mustang shifts poorly on hard accels in the first couple mins of driving though, but never after it warms up
I did get rev hangs on occasion but never since uploading the sct tune.
---------- Post added at 03:51 PM ---------- Previous post was at 03:51 PM ----------
My last car was a vvt too and it never had hesitation on acceleration, cold or otherwise. Until a coil went bad and then the engine light flashed repeatedly and it threw a code. I do notice my mustang shifts poorly on hard accels in the first couple mins of driving though, but never after it warms up
I did get rev hangs on occasion but never since uploading the sct tune.
I am assuming Bama gave you a list of items they would like to see in the datalogs? Load, AFR/lambse, rpm... if you can, when you get that datalog, try to add the VVT PIDs to that list, dont just take a log with only the VVT PIDs selected lol. Thanks.
Hi volt wing, did a quick catalog a few days ago, I'm going to try to download it tonight. Spoke to the folks at Bama, they said they'd look at it and even offered to write a special tune that might compensate for VVT issues, which was very nice. I'm not going to do that however, if I can establish that's the issue I'll have to figure out how to repair it correctly.
Hopefully I'll get that info to you soon
I finally got the data log to bama, they want me to do a complete catalog and send it to them. Something is going on with the fuel, the car keeps adding fuel and taking it away at inappropriate times. Weird. At least it still runs
Just a quick update - finally did a complete data log. The car is running a bit rich so they sent me a new tune that was a little leaner. Runs better now, less surging. There's still something weird going on, I'm gonna datalog it when it's cold and surging and see if they can find it
Here's the latest, I've been driving around on the new tune for a few weeks and all seems to be well. There's still a very slight bog sometimes when it's cold but overall it drives well. I wish I knew what caused it to run rich
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