S14 SR20 bucking at constant, med-high RPM throttle

[jt1583]

I've had an S14 SR running pretty well in my S14 for about a month, stock ECU, intake, throttle, stock boost, just exhaust/FMIC. Also of importance, BOV is a Tial Evo 9 piece venting to atmosphere (working on recirc setup).

I had it running a few days before I took it to Road Atlanta, ya super rushed, and it was there I learned the car really doesn't like partial throttle at higher RPMs. This made turns 1 and 12 pretty scary.

If I try to hold the car at anywhere above 3500 with a steady foot, eventually it will start bucking pretty good, to the point I have to let off or get on it. Around a corner throttle steering or highway cruising, I'll feel this primarily through the rear end and wheels, a medium strength back and forth bucking. It's most pronounced in 4th gear and pretty much disappears in 5th.

Like I said, everything control wise is pretty much stock. I'm 100% on the cam timing, ignition base timing is right around 14 BTDC, and TPS voltage at closed is 0.49V.

Only thing I suspected was my blow off valve, a Tial Evo 9 one. I had it hooked to vacuum under the intake and thought maybe it's getting too much vacuum under this partial throttle and dumping boost to atm. I changed the vacuum source to under the throttle which doesn't see much vacuum. It seems to have helped a little, but not much.

Thanks in advance for checkin out the wall o' text, going to little Talladega soon and would like to figure it out.

[jr_ss]

Are you using the source under the upper plenum/TB or one directly located on the TB? The ones on the TB are not vac sources, the ports are actually drill right at or slightly before the throttle plate. All the vac sources run under the upper plenum and come out below the TB. You also have one on the side of the plenum, but your FPR should be hooked to that.

Do you have any vac/boost leaks? Have you checked for any?

[jt1583]

Going to check for boost leaks asap, just need to make a tester.

First orientation, I had the BOV hooked to 1 of the 2 vacuum ports on the bottom of the upper intake. This is where the bucking felt the worst and I suspected the BOV was venting under my constant throttle condition, which doesn’t make too much sense as hot pipe and upper intake pressures should be similar, helping the BOV stay closed. (BOV is on hot pipe to recirc eventually)

Most bucking/hesitation/jerking problems people have is at full throttle, and often originates from ignition or other electronics. I’m hoping this isn’t my case as the car feels great when accelerating, it’s just went trying to hold constant throttle, 3-5k RPM.

[jr_ss]

Just so you know the source you had it hooked up too is one of the only vacuum sources. The ones at the TB are not. I'd recirc your BOV as well and you may see it disappear. It doesn't sound like an electrical problem because it only happens at part throttle, but you could have a dead spot in your TPS, which can cause all types of funky things.

[Jites]

This is interesting, I have the same problem on my car. I was going to check for boost leaks but may go over every boost hose and pipe and where it is plummed into.

Most of the work done to my car was done before I got there but i swapped the DE to DET loom, ecu, coilpacks ect. as the car was once an NA SR20DE. I hadnt had this problem when it was running a custom map on the DE stuff but I do now on the DET setup

Sorry for the hijack, just though it was relavant

[Kingtal0n]

Did you try blocking off the bypass valve yet?

[jt1583]

Actually at little Talladega right now, had it on vacuum source on the way down and it's definitely bov related. Idle dips really low when coming down and I can feel the bov venting if I'm under hood just barely revving.

I hooked to the boost only source and it doesn't vent as much during shifts but it's drivable. I should've got an adjustable unit.


Now I'm wrestling with brake performance getting really bad after a few laps. Master cylinder is so damn hot, mostly from no heat shield on it or the stock manifold.