Power...Pulsing?

[bergeron007]

3400 miles on it and a few days ago I noticed that between medium and heavy acceleration it tends to, well pulse? kind of a surge slightly nothing too major but feels like I am lacking power, I would like an idea of what could cause before I head to the dealership since I literally trust non of them around me and I bought the sonic from a dealer 2 hours away

No tune, all stock
1.4 6spd man

Seems somedays its a snappy little bastard others it seems to forget it has a turbo. No codes or anything just the feeling as it drives.

 

[Opticon]

Are you using premium fuel? If not, it could be sensing detonation and pulling timing.

 

[ungawd]

Mine surges as you describe. It happens to me usually when the engine isn't up to it's usuall 200 degrees and when the outside temperature is below 20 or so degrees.

 

[sonic007]

Mine does it here and there. Ive noticed when i take off its because i get slid back and my foot comes off the pedal a little.

 

[sonic24740]

I have noticed that now also. But it started after i installed the tune. I thought it was the tune that needs to be adjusted. I still need to do a log and send it in.. i keep forgetting

I think i will to do that tomorrow.. Do Log...

 

[dvn]

Mine did it with the Budget tune. After a few revisions by Vince, it was pretty much gone. Now I have the full tune and haven't noticed any surge.

 

[bergeron007]

All good replies, I am rather tall and I dont slide back lol I laugh cause I actually slid seat forward just incase but it wasnt that, I dont know what temp the engine is cause Chevy thought of this cute little cluster but hardly reads any useful info haha it is cold here in central PA but I can not remember if its only when I leave in the morning or if its all the time after running,

I will try premium fuel next tank, which is horrible timing cause this morning fuel jumped from 3.67 to 3.85 for regular lol bastards know most people have tax returns lol

 

[sonic24740]

All good replies, I am rather tall and I dont slide back lol I laugh cause I actually slid seat forward just incase but it wasnt that, I dont know what temp the engine is cause Chevy thought of this cute little cluster but hardly reads any useful info haha it is cold here in central PA but I can not remember if its only when I leave in the morning or if its all the time after running,

I will try premium fuel next tank, which is horrible timing cause this morning fuel jumped from 3.67 to 3.85 for regular lol bastards know most people have tax returns lol

premium jumped here in md .27 the other day i was a little upset..

 

[dvn]

Chevy thought of this cute little cluster but hardly reads any useful info haha

If you have an Android phone I suggest the free Torque app and an ELM 327 off eBay for about $20. You can read just about everything with it and check and clear DTC's. I use it all the time. Even use it to check codes on other peoples vehicles.
http://www.sonicownersforum.com/forum/t4333/

 

[ungawd]

I'm tuned and have never logged yet. Maybe I'll have a chance to do that today.

 

[brandon1239]

Mine did that when my intake pipe came a little loose after the MAF. It was taking in way more air than it was registering. It'd rip for a second and then fall on its face.

 

[vento]

I had this same issue, its started occuring when it got colder out. I think it has to do with the colder/denser air causing compressor surge at high load, your moving alot more air at the same compressor speed due to its increased density. Everything Ive read states this normally happens in larger turbos, but I think if we are right on the limit of our surge range it might be causing it.

I know at first thought it might be a bad wastegate at first having trouble holding 21+lbs of boost. but there is no slop in the waste gate and the problem seems to go away if I drop a gear.

This is from the NASIOC forums:

To summarize, there were three types of surge discussed...

1) Surge due to trying to accelerate while at too low of an RPM in certain gears with larger turbos.

2) Surge due to the BOV or BPV not being able to vent enough psi during shifts or when the driver lets off the throttle without shifting.

3) Surge due to a turbo spooling too quickly and "crossing the surge line" at certain RPMs.

The surge I'm experiencing once in a while is being caused by the 3rd scenario imo. I see some surge during WOT runs in 5th gear when my turbo gets spooled to between 1.4 - 1.5 bar. It happens more when it is cold outside.

REMEDIES
1) I believe #1 above can be remedied by the driver knowing where the surge points are low in the RPM band if they have a larger turbo. I think this is probably more rare with smaller turbos, but I'm really not sure. If you know where the surge points are, you can avoid them. I know it sounds hard, but it's actually very easy. I know b/c my car behaves like this in low RPM pulls in 6th gear. Very easy to avoid.

2) An easy remedy would be to swap your BOV or BPV to another which vents more air/psi.

3) TheMadScientist indicated this could be remedied by controlling boost onset at certain RPMs so the turbo spools a little slower, thereby avoiding the surge. For me, I cannot do this right now due to the fact I have an ECU flash and AVCR to control my boost. To my knowledge, an AVCR cannot control both gear specific and RPM specific boost parameters at the same time. Somebody please let me know if I'm off on that. TheMadScientist is using a GM Solenoid, the Cobb Street Tuner and a UTEC to control boost so he can control boost at any RPM (he's not using gear specific boost control to my knowledge?).

I wanted to start this thread b/c I hogged the other thread. Experts, please chime in with thoughts or suggestions. I'm pretty open to both and am aware that some of my assumptions might be off so take it easy on me if that is the case.

Garret FAQ:

Q. What is compressor surge?
A. The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side. Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself. Common conditions that result in compressor surge on turbocharger gasoline engines are:

-A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body
-The outlet plumbing for the bypass valve is too small or restrictive
-The turbo is too big for the application