This is a weird one.

Car runs great with any negative vacuum pressure, from -20 to -1.

Car then runs great under boost, from positive 1 to the max, currently at 8psi.

Heres the problem: at ZERO (the “crossover” point) the car is stuttering and hesitating, feels almost like a miss, somekindof stuttering hesitation thing. Then, once I’m back into either negative pressure or boost it runs fine. This is most noticeable on the freeway while at normal cruising speed.

Also, this problem JUST HAPPENED after doing a head gasket job, putting on a 2 mil head gasket and installing block guard.

Any ideas on this one guys ? ? ?

 

It's probably your fuel curve on your AFC. Since you put on that 2mil headgasket, you've changed your compression and therefore your fuel needs to be changed. Does it do it at wot? or just at partial throttle?
good luck,
Jared

 

So would it need more fuel or less? I was wondering about that.

It does it under fairly heavy load, like 90mph in fourth gear, and ONLY at the ZERO mark. WOT and all other issues are fine. Any positive turbo boost, from 1psi to 7psi the engine is running great. Same with any negative vacuum readings, from -1 to -20...its just that damn ZERO mark thing!!!

 

So if wot is ok, then you probably need to lean out your partial throttle settings, especially around the rpms that you're running all the time at part throttle(2500 to 4k)
good luck,
Jared

 

leaning out ALWAYS makes me nervous, especially since i dont have an a/f gauge OR a EGT, but then again, those gauges can be pretty inacurate. Im gonna get a GReddy A/F gauge (damn thing is $300) but GReddy swears its accurate and to this point they have been right on with everything they've represented.

Whats your opinion on these gauges / which one is best and EGT pver A/F?

 

I would get both as they are both great for tuning. I suspect that at partial throttle you have the car too rich. Since you're using the V-AFC, use your low throttle map to lean things out since your wot is ok. I really would suggest though, getting an a/f and egt soon. I would never turbo a car without one. I like my autometer a/f gauge, I think that's a matter of preference, but I do recommend the Greddy egt.
good luck,
Jared

 

Damn! I think i got it! (Just like you said Jared, THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU!!!!!!!!)

I decreased the fuel at 3,000 and 4,000 rpms on the AFC (for the LVT on the NARROW THROTTLE only) and the skip miss stutter thing is gone.

I guess the problem is with the compression lowered it was too rich at the ZERO spot and choking.

DAMN! I love it when things get figured out!

 

Yeah i had this same issue....

What happens is that you get into a tooo rich situation while the ECU is still in closed loop mode and what it does is pull back hard on the fuel... so it almost feels like a pulsing fuel cut.... and like you said, only at part throttle.... because once u mash the gas, the ECU goes into Open loop and could car less about the mixture...

 

Damn! The things im learning....

thanks all