The Project Pad

Well that would be fun to figure out which one. So I have the 2.0T upgrade. Can I just use normal 2.0T coils?

Rabbitracer43 wrote:I've scene some pretty bad bucking with a bad ignition system, and that is exactly the load on an engine where it would happen

I agree. Peak torque.

Could also be overboost cut.

Why do you say that this is different from a fuel cut? What's not the same?

Boost leak causing you to go rich under load leading to misfire.

Would be surprised if it was a coil.

Spark plugs are a good thing to look at.

Rich misfires and plugs don't cause violent bucks. Fuel cuts (whether from over boost or otherwise) do. Also, rich misfires and small ignition problems are typically higher frequency events and very repeatable under the conditions in which they occur.

Of course, one person's definition of violent could be another person's definition of mild, but when the op says it will almost throw you out of your seat, it really sounds like a complete fuel cut. Also like a mid to late 2000s Gallardo shifting with dsg. What a pos.

loxxrider wrote:Rich misfires and plugs don't cause violent bucks. Fuel cuts (whether from over boost or otherwise) do. Also, rich misfires and small ignition problems are typically higher frequency events and very repeatable under the conditions in which they occur.

Of course, one person's definition of violent could be another person's definition of mild, but when the op says it will almost throw you out of your seat, it really sounds like a complete fuel cut. Also like a mid to late 2000s Gallardo shifting with dsg. What a pos.

I guess I didn't read the post carefully enough. I thought he implied it was repeatable in 4th and 5th gears. Is this a stock URS4?

So on my brothers S6 we found 2 things that were leading to violent bucking which was definitely a result of over boost fuel cut based in VCDS.

1-The N75 was bad. It was leaking all the reference pressure to the waste gate back to the suction side of the turbo.
Obviously it didn't matter if the N75 was plugged in or not.

2-034 sent us a varimap that was set to 2.5 bar when the ECU tune was designed for 3Bar.
We were hitting 32PSI of boost with a GT3071
This happened at high load in high gears most often.

VCDS, AFR and boost gauges are priceless when troubleshooting. They have helped me out so many times. Takes so much of the guess work out.

Hi All, thanks for the discussion on this...

So the bucking happens violently enough to, maybe not throw you, move you forward in your seat. Like you hit the brakes pretty hard in an emergency brake.

As for being able to repeat it, I can only repeat it during the same driving session. Example, it happened last Wed on my way to meet my boss and I could reproduce it every time I tried in that driving session. I have not been able to get the same results since then. However, I've only driven the car home and then again to the springs and back.

And to answer the question, is my car stock? No, it's not. It's mostly stock but there is a different WG spring (PO put it in, no idea what pressure it is), 2.0T coil upgrade, silicone michelin man hose, ect. But that's almost all of them from bumper to bumper.

Draky wrote:Hi All, thanks for the discussion on this...

So the bucking happens violently enough to, maybe not throw you, move you forward in your seat. Like you hit the brakes pretty hard in an emergency brake.

As for being able to repeat it, I can only repeat it during the same driving session. Example, it happened last Wed on my way to meet my boss and I could reproduce it every time I tried in that driving session. I have not been able to get the same results since then. However, I've only driven the car home and then again to the springs and back.

And to answer the question, is my car stock? No, it's not. It's mostly stock but there is a different WG spring (PO put it in, no idea what pressure it is), 2.0T coil upgrade, silicone michelin man hose, ect. But that's almost all of them from bumper to bumper.

Usually when you have an ignition issue it will get worse when it is hot out, with more boost, and with more humidity. It is more repeatable and worsens as those conditions become more severe. When this happens, usually there is moderate power loss, not total power interruption. The engine note will change tone when this happens.

Usually overboost fuel cut will happen when it's cold and your car is chipped. If the boost goes above where it normally does (because the air is more dense), the ECU will say oh crap and cut fuel immediately. Then start it up again after about half a second. This causes total power loss and the engine note doesn't change it's tone... it just gets interrupted and starts back up again.

Which does yours sound like?

Good points, this typically happens more when it's warm out. That's why I was surprised when it happened a few days ago in the early morning (before the sun was up). But last fall this happened on a chilly day early morning and then hasn't happened again till last week. But in the summer I find I have to back off a bit on the throttle because it does happen more.

So, as it happens when it's cold out (around 50 degrees) but not super cold (I've driven it around 30 and below but it didn't happen this winter and normally does not happen in the winter) but happens more often in the summer maybe it is an ignition problem? I was thinking because it happens during both that maybe it is a vacuum issue?

I also put a good ground on the car since last fall when it last happened in the early morning because I thought it was a ignition problem (bad grounds on an older car). I now have 13.8/13.9v from 13.3/13.4v to the battery so the ground is good.

I think you need a boost gauge to tell how much boost the thing is making. With a wastegate spring like that and certain chips, these cars will bounce off of boost cut under certain conditions. Going uphill can load the car in such a way that boost cut happens when it normally wouldn't.

Of course, there are other reasons the ECU will cut fuel.

Not sure if this applies since my experience is with an MC2 but do you have an adjustable wastegate? I have experienced similar fuel cut (low rpm, heavy load) when I crank down on the wastegate adjustment bolt in conjunction with using the N75 for boost control. Backing off the wastegate adjustment bolt eliminated the problem but boost builds a little bit slower. The ecu allows a certain amount of boost overshoot (air and engine temperature dependent) for a certain amount of time and if the time or overboost is exceeded then it triggers fuel cut. (Basically what Chris is saying.)

Was this resolved? More or less having the exact same problem. Check engine light was on. I took it in to have the codes read and apparently the turbo boost valve was having an issue so I had it replaced but that did not resolve the problem. If I baby the rpms up through the gears, there is no problem. It happens when I am at 3-4k on the freeway and step on the gas as if I was expecting my car to downshift like at automatic and launch forward but instead it bucks until I let off the gas. So I guess the idea is that I floor it even though I know it's not going to go anywhere and it bucks.