Stumble when accelerating

What could be causing my car to stumble when taking off? At WOT everything is perfectly fine, a/f is spot on but anything at part throttle, whether taking off from a stop or roll, once the car hits boost, the a/f shoots full lean, the car stumbles and won't accelerate unless I back off and then get back on. Already replaced maf, and no vacuum leak from what i can see.
???

Ok well for starters, don’t do that anymore.

No CEL?

Not sure how long you’re seeing the lean condition, but it’ll show a little lean at tip in momentarily.

Do you have a handheld so you can datalog?




Sent from my iPhone using svtperformance.com

Do you have cams?

Bad TPS? Faulty/dirty fuel injector? Spark plugs need to be replaced or the gaps checked? Bad fuel?

Could be a bazillion different things.....a simple datalog could narrow down the list considerably if not outright pinpoint it.

Stay out of boost until you can log a controlled partial pull

Have you changed combos recently?

PPRV? I had a similar issue. Car would go lean and fall on its face sometimes when I would put the pedal to the floor. Got rid of that useless pprv. All gone. Also added the fuel rail pressure disc to prevent the sensor from popping.

No CEL or codes pull up on my X4

stock cams

changed plugs, no change

had injectors cleaned, no change

still doing it after filling up with new tank of fuel

I will try changing the tps next

Check ur coils. Make sure they are all firing. Under WOT u won't notice it. Fuel pumps will act funny under low voltage so ur FP could be failing or just dirty as well or an injector could be clogged.

c6zhombre said:

Could be a bazillion different things.....a simple datalog could narrow down the list considerably if not outright pinpoint it.

Stay out of boost until you can log a controlled partial pull

Click to expand...

Looks like the parts shotgun is coming out.

Last one like this the guy must’ve replaced every damn engine control component on the car and I still don’t recall him fixing it


“BLM is doing God’s work”
2KblackGT

svtperformance.com mobile app

Given that it doesn't seem to happen at WOT I would datalog it and see if maybe it's skewing one way or the other in the fuel trims, then has to hastily correct when you accelerate at part throttle. You don't see it at WOT because it defaults to open loop.

I would take a look at one of the 02 sensors dropping out on light throttle. The front sensors not the cat monitors.

what about fuel filter?

This sounds really similar to my issue. Car pulls fine at WOT, no lean stumble or lean tip in when you mash it WOT. Drops down to the commanded A/F and pulls.

Part throttle it will stumble and go lean(sounds like its breaking up).

I did have an issue with a couple coils going bad during the initial MAF tuning. So mine has brand new plugs, new FRPP coils (All 8 replaced).

If you find out what the culprit is let me know. I have cams in my motor, low pressure whipple bypass on my 3.4L Whipple and I am on pump 91 right now. ID 1000's, FORE fuel system, etc.

Had this issue 2 summers ago, I thought it was the maf, but it ended up being a coil. It didn't throw a code the first few times and finally it did and was a bad coil.

Hope you figure it out....

Sent from my SM-G965U using the svtperformance.com mobile app

@04torchred That can be a side effect of having low manifold vacuum caused by cams. Mine did the same when I had Crower Stage 4's. First you'll want to make sure the blower bypass valve is actually open at idle and cruising RPM. If it's working then your tuner may be able to fix it or at least improve it. I went from stage 4 down to stage 2 cams and then had it retuned by Kevin Dunn so I'm not sure what had the greatest effect but now it drives perfect!

olympic said:

@04torchred That can be a side effect of having low manifold vacuum caused by cams. Mine did the same when I had Crower Stage 4's. First you'll want to make sure the blower bypass valve is actually open at idle and cruising RPM. If it's working then your tuner may be able to fix it or at least improve it. I went from stage 4 down to stage 2 cams and then had it retuned by Kevin Dunn so I'm not sure what had the greatest effect but now it drives perfect!

Click to expand...

I'd be interested to hear more about that. I wonder if it's a combination of the high overlap on the cams causing reversion and mixture issues at low flows/velocities, and injector timing with larger cams like that. One of those things that would be interesting to experiment with- see if it likes more or less timing in that region, and pushing back the injection event as late as possible.

With the bypass wouldn't the failure mode be the valve staying shut, resulting in high IATs at idle and low load?

SecondhandSnake said:

I'd be interested to hear more about that. I wonder if it's a combination of the high overlap on the cams causing reversion and mixture issues at low flows/velocities, and injector timing with larger cams like that. One of those things that would be interesting to experiment with- see if it likes more or less timing in that region, and pushing back the injection event as late as possible.

With the bypass wouldn't the failure mode be the valve staying shut, resulting in high IATs at idle and low load?

Click to expand...

If your manifold vacuum is too low the bypass valve will remain closed causing drivability problems and high IAT's. Or sometimes you have just enough vacuum to open the valve but as soon as you touch the throttle the valve closes or flutters open/closed. I had my car tuned by 2 tuners with the stage 4 cams and it never drove well. It always wanted to stumble or buck when taking off and also stumbled when transitioning from vacuum to boost. I couldn't drive in city traffic for very long before IAT2's would reach 80C/180F due to reversion. I think a good local tuner could have resolved the driving issues but I don't have access to one so I swapped to some milder cams.

olympic said:

@04torchred That can be a side effect of having low manifold vacuum caused by cams. Mine did the same when I had Crower Stage 4's. First you'll want to make sure the blower bypass valve is actually open at idle and cruising RPM. If it's working then your tuner may be able to fix it or at least improve it. I went from stage 4 down to stage 2 cams and then had it retuned by Kevin Dunn so I'm not sure what had the greatest effect but now it drives perfect!

Click to expand...

Interesting info. My cams are smaller than the stage 2 crowers. I am 225/227 duration with 0.450" lift. I ran the ICL on the intake cams down to 109 and have the exhaust at 115-116 range.

My whipple bypass is low operating for decreased vacuum. I see about 10-11" of vacuum at idle.

I am going to try to add fuel down low in the MAF curve to see if just running a bit richer can compensate if there is some of that occurring.

I am not smart enough to know how the transient fueling works on these cars to know if adjusting those numbers would help this issue.

I have never noticed IAT2 temps that high with my car.

04torchred said:

I am going to try to add fuel down low in the MAF curve to see if just running a bit richer can compensate if there is some of that occurring.

I am not smart enough to know how the transient fueling works on these cars to know if adjusting those numbers would help this issue.

Click to expand...

The transient fueling on those strategies is a function of both the manifold volume and anticipatory fueling based on the load with failed MAF (LWFM) table. If that table isn't calibrated you can have all kinds of transient issues.