Bucking/Hesitation at low RPM
 

Hi Guys,

I'm getting hesitation / bucking at low RPM when deaccelerating. At random points under 2000rpm, the car will rock back and forward then be fine. I can also feel it in my gear stick as it goes back and forth a little as well. I have also noticed that when I let go of the accelerator and get back on it, it's no entiring smooth, it's quite rough.

I've been doing some searching underneath my car and I think i'm running an Auto Harness on a manual tranny... though i've read this shouldn't be an issue?

Mods: Fresh Rebuild, Street port, exhaust, PFC, Front Mount, Intake, Poly engine mounts, Heavy clutch...

It also happened prior to my rebuild... It could be tune related but i'm leaning towards something else?

Any help is appreciated.

Thanks,

You could be experiencing drivetrain slop. Two mounts support the engine and two hang the differential. The two are connected by a power plant frame. From what I understand, you might still experience this even with better mounts. Your differential bushings might be getting old.

Matt

It may be tune related But not in the way you are thinking .

there is an easy way to figure out if it is . When you let off the gas watch your air fuel ratio gauge , and see if the bucking starts when the wideband goes from --- to reading a number

if thats the case than it may be an easy fix , It may just be the power FC is feeding fuel on deceleration to keep the car from stalling when you let off the gas in neutral

On the commander Look for

rev/idle
look for F/C AE, F/C E/L, F/C A/C. if the point is set to 2000 on the deceleration there should be 3 of them one for 3 different load settings

you can adjust them and lower them until the car starts to stall on free deceleration from high RPM

If your poly mounts are adjustable, try readjusting the stiffness

So i've been driving my FD around a lot more recently and spent more time analyzing this issue. Its a little hesitant mainly in 3 ways..

1. Getting on the accelerator - If I de-accelerate say to 2500rpm then get back on the accelerator its not smooth at all, quick vicious

2. When cold and decelerate at low RPM (3000rpm down) its slightly jolts back and forth all the way down to almost idle speeds...

3. Letting off the accelerator - similar to the 1st issue, it jolts back and forth enough to really notice.

Does this sound mechanical related? TPS? Electrical? In all above scenarios I can feel the jolting through my shifter as well...

I am about to go through a single turbo conversion and re-tune but wanted to get some advice as if it mechanical I can try and sort it out at the same time.

EDIT: Emissions have been deleted

Thoughts would be appreciated.

What are your VTA1 & VTA2 readings? O2 feedback control on or off?

Sounds tune related. Base map on a power fc really isn't intended for much more than an exhaust and isn't the best to begin with. If you have a wideband seeing what it does when you experience the bucking/hesitation would help. As mentioned above, I would also check the tps values.

Agree TPS could be a culprit. You removed your emissions so I assume that means airpump too. Did you get re-tuned afterwards? Disabling the airpump on my car even with PFC causes the kind of hesitation you're talking about.

Managed to get the TPS readings today. Most values are out... but VTA2 closed is extremely out... i'm going to adjust this and the air idle screw this weekend

VTA1
Fully Closed - 0.85
Flly Open - 4.56

VTA2
Fully Closed - 1.73
Fully Open - 4.98

Are those readings with the engine cold? The TPS voltages are read on a fully warmed up engine. By this, I mean the wax rod on the thermo valve is fully extended so the cam for the fast idle separates. If it's not fully warm, you'll have higher closed throttle voltages like that.

Matt

It was fully warm. Values were taken after a 45 minute drive... 86c water temp

clutch in, give it a small amount of gas as you release the clutch. (almost like a mild power shift into the same gear)
sorted :nod::icon_tup:

So I spent a bit of time today adjusting the TPS and air idle screw.

New TPS readings:
VTA1
Fully Closed - 0.33
Fully Open - 4.26

VTA2
Fully Closed - 0.75
Fully Open - 4.98

I have felt a difference which is great. Up high (3k and above) it feels much better then before hesitation barely noticeable - much smoother drive. But still noticeable down low. It's as though there is a flat spot from get back on the accelerator after letting go. When I get go of the accelerator down low, the car rocks back and forth. quickly...

Its all happening when using a little/slight throttle. Even when I was adjusting the air screw, giving it a quick small rev you can hear something isn't 100% and sometimes stalled...

Next thought is a bad 02 sensor...

So I got the o2 reading when I checked my TPS readings...

o2
Closed - .11
Open - .85

Does this seem to be ok?

Hi Guys,

I decided to wait until my single turbo was done and tuned to see if there was any difference. I replaced my 02 sensor and re-tuned the car.

1. Jolting/Bucking - This is still there... occurs more when cold then warm. I can feel it quite a bit through my gear shifter as if it is electrical. If an auto harness was used on a manual transmission, would such things occur? I looked under my car and I have a auto harness and does look like a bit of a hack...

2. Letting on/off Throttle - This is also still there but is better then previously... I adjusted my TPS previously which improved it but it also caused pulsating on partial throttle throughout my rev range...

3. Another issue completely - Engine stalls when cold. I have to keep the revs up until it can idle on its own. From there the idle is bouncy, almost stalls but as the car warms up it idles better.

Any help is appreciated

did you get this sorted. i get staling issues when cold. and part throttle is very lean/hesistant.

Not yet. Have not had much time to look into it further. The hesitation I feel is TPS related so I am going to replace it and see how I go.

I recently diagnosed my TPS as bad. If you have a multimeter, probe the wire second from the top and the bottom wire for DC voltage with the key on and the engine warmed up (not running). Put the multimeter on the windshield and very very slowly actuate the throttle from full closed to full open. I did this sitting in the seat and moving the throttle with my foot for the "feel" of when the bucking occurs as well as much smoother and slower control of the throttle. If the circuit goes open or jumps at any point in time, the TPS is bad. Mine had a voltage jump around 1v and a complete dropout around 2v's. I've heard of people taking the TPS apart and cleaning the components of it, but I would much rather spend the cash and replace it with new. I have a bucking issue with mine and my new TPS will be here Monday.

Update
 

So most of the issues I mentioned previously were tune related. I recently got my FD re-tuned and found so many problems.

Cold start - Found no timing what so ever so car was trying to idle at 0 rpm

Hesitation on partial throttle - All injector related. The previous tuner didn't change any of the injector values to suit the new fuel system. This also resulted in the car stalling from time to time.

I did change my TPS and few other pieces before my tune to see if it would help but didn't.

The hesitation issue I am facing at low rpm is still occurring. I've gone over the car again and again, and only thing that seems off which could cause this, is my transmission harness. I can tell it's an auto harness to suit a manual transmission. Though I have heard many people do it without any issues / side affects.

I'll reiterate what happens.
Cruising at 2000rpm, let of the throttle quickly, car jolts back and forward. Upwards of 3000rpm you cannot really feel it. When getting back on the throttle, same things occurs though it has improved since my tune. There was quite a flat spot in my throttle.

Can the auto harness (or something electrical) cause this to happen?

My issue definitely looks like its electrical related - I have no other explanation. How difficult is it to replace the auto harness to manual?

Do you have AC? Swap your air cond. and fuel pump relays and see if that changes anything.

Also, how old is your fuel filter?

Just thinking of easy things to try.

No AC, upgraded pumps with new relays and fuel filter changed just recently changed as now running e85 - this issue has been happening for a long time though

I've been researching for the past few hours on the auto to manual swap and harnesses. All in all, it appears mine is done fine and no other people seem to have my issue.

Is there an auto component such as the AT Control unit (Which has been removed) that would be sending a signal to my transmission causes the jerking?

I have similar issues with my car... bugs me a lot! I haven't really dug into it too much yet (just getting the car back up and running after doing a bunch of work on it), but I'll go through some of the things mentioned on this thread and post my results as I can.

When I was digging through threads in the past on this, I feel like some people suggested the grounds might be bad (in particular, the ECU ground below the UIM).. I did clean up all the grounds and added extra ground connections but that didn't seem to do much.