After installation of Garrett GT3071 turbo and new injectors I started having the problem of slight engine hesitation/jerking when cruising on highway with a constant speed at part throttle. Otherwise the car behaves normal. Acceleration is smooth, idle is OK (except it's now at 1000 rpm due to larger injectors).
When I drive at a constant speed, after a while engine starts jerking. The boost may be at 0 or even at vacuum at the time. When hesitation begins, AFR gauge starts also jumping to the rich side (or so it seems).
When I let off the throttle, it disappears, but can appear again if I start driving with a constant speed.
First we had HKS spark plugs installed, but with those the car was having really bad misfires and at cold start it sometimes worked on three cylinders.
When we put the stock plugs back, it became normal, but the highway hesitation remained. The spark plugs didn't have any unusual damage or deposits (I'm thinking of the other tread), all gaps were OK.
What could be a possible cause of the issue? Does it ring any bells?

 

I guess I was missing the mod list. So I added this signature.

 

Something needs to be ironed out in the ECU

 

sounds like injector scaling ?

 

What do your long term fuel trims look like? If you are more than a +-5% you may get some odd behaviour at times. Scaling the injectors to a smaller number will skew things richer, higher numbers are leaner. I think normally you aim for around 15% lower than the actual injector size. In my case I have an AMS intake so I tried a value closer to 20% lower (665 cc) to counter the fact it leans the car out more. Keep in mind each car is different and the Injector scaling numbers round to the nearest valid entry. If the idle fuel trims are off compared to cruise you may also have to adjust the latency at different voltages as well.

BTW I had no issues with my HKS plugs gapped at .030" out of the box on the stock turbo. The GT3076R had major hesitation and breakup beyond 22 psi on them and after gapping them to a more reasonable .024" the car now pulls rock solid to redline. After some fueling and timing tweaks based on my dyno runs this car is silly fast at even the lower boost levels.

Let me know if you have any questions about dialing in the GT30 turbos as I'm finding out all kinds of things that need to be done to the tune that aren't obvious without several days of datalogging on the street.

 

Thanks for detailed response. I hope this is only an ECU issue. My tuner locks my ECU, so I can't see the maps in EcuFlash. I will take the car back tomorrow and ask them to look at those numbers.
It's strange, but my car didn't like HKS spark plugs. Stock plugs work really great.
The car pulls very strong. I think it feels less laggy then when I first drove after turbo install. It's either me adjusting or the turbo is running in. Probably both

 

Sounds like surging and the bov opening causing the richness and then bucking. Its a domino effect.

A 3076 on a local 10 did the same thing. Your SSQV might be making the situation worse as well.

 

So, the suggestion is to remove the SSQV (which may be an option), or to tweak the tune?

 

If you still have the stock BOV, I would recommend just swapping it on real quick to see if that fixes it or makes it better.

 

Yes, I have my stock BOV. I might try swapping it.

 

So, more problems here.
I tried this and that and it dind't help. At some point the car just went into the limp mode and kept doint that whenever I went above ~3000 RPM with 123X codes.
So, I disabled those codes and discovered that there was zero boost, the car was just atmospheric.
So we looked inside and discovered that waistgate door wasn't there. It was gone. No wonder now that the car didn't boost, the WG was always open.
So much for the new GT3071R (((

 

Sounds like injector latency/scaling issues to me and maybe you're running a little on the rich side. Going back to the stock plugs (hotter plugs) would probably make it run a little better but wont iron out the problems.

Also to get rid of the cold start problem, you should lower the amount of ignition retarding for cold start.

 

Well, I'll say it louder: MY NEW TURBO BIT THE DUST!!!

 

Sorry, I didn't read your last post before posting.......

 

So the WG flapper flew down your exhaust? The rest of the turbo assembly should be fine otherwise, maybe go do an ATP transformable O2 DP and use an external gate if you don't want to remove the turbo assuming what remains of the WG flapper is sealed from the outside.

This is the second instance I've heard of that failing that way, makes me wonder if mine is going to fly off down the exhaust. I did notice when I was preloading the WGA last time that it would bind slightly if you opened it too far, I'm wondering if it's not hitting the stock gasket ever so slightly and beating itself up over time. If you can check the flapper or gasket for signs of contact that would help determine if that was the cause.