Bucking @ low throttle / RPMs

[Artago]

Hi all, let me get right to the symptoms / scenarios:

1. Cruising at 3000 RPM in any gear with 10% throttle. I let of the throttle, RPMs die down and AEM WB shows --- AFR (aka no fuel). At about 1500 RPM the ECU injects some fuel AFR 13-15 and the care starts to buck forwards very violently.

2. Cruising through a parking lot at 2000-3000 rpm in 1st or 2nd with less than 10%+ throttle, I ease up on the throttle to like 5% and the car starts to buck again, the lower the rpms drop the more violent the bucking.

So those are my basic scenarios. They're both REALLY annoying because I basically have to keep the car above 2000 RPMS with more than 10% throttle position. Anything between 1-10% throttle below 2000 RPM the car buck EXTREMELY violently.

Any help would be appreciated. My mods are in my profile, and I would be glad to log the bucking with DSMlink if someone would tell me what to log.

Thanks all,

Tom

 

[Ninharu]

Same thing happened to me, it turned out that my TPS was on its way out. I had to get a new one and go through the adjustment steps with a multimeter.

 

[97ebaygst]

have you done a boost leak test?

 

[Artago]

Same thing happened to me, it turned out that my TPS was on its way out. I had to get a new one and go through the adjustment steps with a multimeter.

Hmm. Ok, I'll log the TPS voltage while it's bucking to see if the voltage / position is smooth and consistent. Is there any other tests I should perform on it? Like the TPS resistance test?

In the mean time, why would a faulty TPS cause bucking? Can I get a second opinion on this?

have you done a boost leak test?

Yes, but I'll test again.

Tom

 

[Ninharu]

For me the TPS was not adjusted properly. so when i started it it would be fine, but as soon as i touched the pedal while in gear it would violently jerk forward and buck. my guess is that the TPS was telling it that i hammered on the pedal instead of just lightly pressing it to accellerate. I had to use a multimeter while turning the sensor until it the resistance changed to find the point of engagement (there is a guide somewhere, i think it was vfaq). Then when i logged my TPS it read zero with out me pressing on the pedal. At least that was my problem, perhaps yours is different.

try this

http://www.plymouthlaser.com/tps.htm

 

[mikelv]

Can you pull the car okay?

What is your 50hz slider set at, try giving it a little bump.

What the hell is a 7 bolt Crank Mod? Did I miss something in the last 6 months I've been out the game? The only 7 bolt crank "mod" I know is to get rid of one.

 

[Artago]

Can you pull the car okay?

What is your 50hz slider set at, try giving it a little bump.

What the hell is a 7 bolt Crank Mod? Did I miss something in the last 6 months I've been out the game? The only 7 bolt crank "mod" I know is to get rid of one.

The car pulls ok.

All sliders are in the middle.

And the Crank mod is.... well I don't know... the guy who built the motor told me but i forgot.

Tom

 

[PieEyedPiper]

The car pulls ok.

All sliders are in the middle.

And the Crank mod is.... well I don't know... the guy who built the motor told me but i forgot.

Tom

Is this the overbore engine? That guy carves out oiling grooves in the thrust bearing I think.

Anyways, I'd be all over the TPS and of course check spark/wires etc. Also, just for my own clarification, is the engine bucking at low RPMs or is the car rocking back in forth from a fuel on/fuel off parking lot type situation (low RPMs)? You know how if you're rolling in gear(first) and you let off and then back on the car kinda bucks/rocks as the slack in the drivetrain is picked up?

Assuming it's actually bucking go ahead and try to log a situation where this is happening. And the AFR's dropping to nill/unreadable is normal because the ECU shuts off fuel when winding down in gear and then the injectors start firing again @ ~1500rpm. So that all sounds relatively normal.

So, check your plugs/wires, test your coils and TPS, log your TPS and any other relevant information. Probably wouldn't hurt to double check the timing either.

That's about all that comes to mind from the description but I'll stay tuned. Hopefully you get this all fixed up nice and easy!

EDIT: Just some other odd ball ideas that popped into my dome: fuel filter, 02 sensor and is your MAF modded by chance?

 

[Artago]

Is this the overbore engine? Yes. That guy carves out oiling grooves in the thrust bearing I think. That was my guess as well but he never actually told me that.

Anyways, I'd be all over the TPS and of course check spark/wires etc. Plugs/wires are new. Also, just for my own clarification, is the engine bucking at low RPMs or is the car rocking back in forth from a fuel on/fuel off parking lot type situation (low RPMs)? I'm guessing it's a fuel on/fuel off situation but I'm not a 100% on that. You know how if you're rolling in gear(first) and you let off and then back on the car kinda bucks/rocks as the slack in the drivetrain is picked up? Yup, that's what it feels like, that can't be good for the drivetrain.

Assuming it's actually bucking go ahead and try to log a situation where this is happening. And the AFR's dropping to nill/unreadable is normal because the ECU shuts off fuel when winding down in gear and then the injectors start firing again @ ~1500rpm. That's the point where it starts bucking. Fuel is cut, car winds down to 1500 RPMS, fuel comes back on and it starts bucking. So that all sounds relatively normal.

So, check your plugs/wires (good), test your coils (good) and TPS (see more info below), log your TPS (TPS values increase smoothly as shown by DSMlink) and any other relevant information. Probably wouldn't hurt to double check the timing either.

That's about all that comes to mind from the description but I'll stay tuned. Hopefully you get this all fixed up nice and easy!

EDIT: Just some other odd ball ideas that popped into my dome: fuel filter, 02 sensor (it's new) and is your MAF modded by chance? (nope)

Tested the TPS:

Pin 1 & 4 show 5.1k Ohms
Pin 2 & 4 show 0.5k-4.5k Ohms as the throttle is cycled from closed to open
TPS shows 0.59 volts (in DSMlink) at closed throttle.


Correct me if I'm wrong here, but shouldn't the TPS show 3.5k-6.5k as the throttle is cycled?

Also, the book (and various other sources) state the TPS should show 0.48-0.52 volts at idle. Since mine showed 0.59 I adjusted the TPS and it made a noticeable difference. The bucking is now weaker and there is less of it. So I'm definitely, on the right track.

Tom

 

[twdorris]

You can also consider using the built-in TPS dialing function in ECMLink.

TPS Adjustment

You basically warm the engine up then move the throttle from fully closed to fully open and then back again and hit that "TPS Adjust" button. It'll calculate an auto-scale for you. After that, you can check the "simulate idle switch" checkbox and not have to deal with the TPS adjustment again.

Note that even if you have V1 or V2 inside your ECU, you can still use the new V3 application (which is the version with this function built-in).

Thomas Dorris

 

[Artago]

You can also consider using the built-in TPS dialing function in ECMLink.

TPS Adjustment

You basically warm the engine up then move the throttle from fully closed to fully open and then back again and hit that "TPS Adjust" button. It'll calculate an auto-scale for you. After that, you can check the "simulate idle switch" checkbox and not have to deal with the TPS adjustment again.

Note that even if you have V1 or V2 inside your ECU, you can still use the new V3 application (which is the version with this function built-in).

Thomas Dorris

Sick. I didn't know that. I'll give it a try.

Thanks Thomas

 

[Chumpaumpalumpa]

Bringing this back from the dead. Exact same issue. Car is bucking from the on/off of the injectors and does it very violently. I got a log of the situation. Its terrible in traffic and parking lots. Its rough anytime it comes on and off the injectors. ie. let off in 3rd, WB goes ---, press throttle again and it hits hard when the injectors fire up even if you barely touch the throttle.

No boost leaks fuel pressure is good timing at 5* and all that jazz. Might be worth noting I can't get the O2 to cycle at idle. Cycles at cruise just fine.

 

[Chumpaumpalumpa]

Anyone?

 

[steve]

Log the idle switch, it looks like your flipping between decel fuel cutoff and normal along with in and out of closed loop mode.
Your airflow readings look really low to me for the engine speed.

Also check the Vehicle Speed Sensor (VSS) to make sure the ECU knows the car is moving.

 

[Chumpaumpalumpa]

Log the idle switch, it looks like your flipping between decel fuel cutoff and normal along with in and out of closed loop mode.
Your airflow readings look really low to me for the engine speed.

Also check the Vehicle Speed Sensor (VSS) to make sure the ECU knows the car is moving.

The idle switch is broken so I'm simulating it with the TPS. Does that matter? What do you suggest I adjust?