I have a freshly rebuilt turbo motor that I have got to run pretty well except for one small problem. At very very light loads the motor seems to miss. I almost can't replicate it on the street, but if I'm in 4th or 5th gear above 3k rpm and just barely on the throttle, when the car's momentum is carrying it forward for the most part, it makes the car surge or hesitate a bit.

It's pretty obvious to hear when I just let the engine run free and hold it above 2k or so rpm, I took a sound clip of the exhaust note so you can hear.

In the video the wideband gauge is reading pretty steady at around 14.0, on the street it seems to happen around that range as well, but when it happens when I'm moving the AFR's tend to bounce around a bit more, between 13 and 16. Maybe this is a little rich for so little load? My fuel pressure is set to about 42#, that's about the only adjustment I have to tune AFR off idle. I've turned it down to around 35#, that brings AFR closer to 15 when I rev it free but the misfire is the same.

Any suggestions? TPS is set correctly, timing is right on factory mark, car runs great other than that. It only has about 130 miles on it at this point, but plenty of heat cycles. Injectors are brand new, fuel pump and filter are also new. Spark plugs are fresh but maybe could use some cleaning after testing and tuning?

Thanks for any help.

Video link:

 

Is everything stock?

 

No, I have a rtek 1.8 with 800cc primaries, walbro 255 fuel pump. fuel rails are plumbed parallel, regulator in the center. turbo is bnr stage 2 and exhaust is straight through at the moment, no cat. air metering is all stock

 

Others would be best to help you but your fuel pressure numbers are at idle?

 

yes, they stay that way through the rpm range though

 

And what size are the secondary injectors? Plus is your pump a high output pump? And your car is an S4 NA? And fuel pressure should not be constant from idle to full throttle (stock is about 28 psi or so at idle and climbs close to 40 at full throttle)

 

Secondaries are the same, yes the pump flows 255lph. it's a s4 turbo.. I wasn't aware that fuel pressure climbed like that, I'm assuming it's dictated by manifold vacuum. I know pressure is supposed to rise with boost, at least with a rising rate regulator. didn't know it should be that low at idle though, maybe I'm just setting my base pressure too high.

 

Consult the FSM for the proper range to use as a base (page70 in the fuel and emissions turbo). Appears to be 28.4 psi.

https://drive.google.com/drive/folde...kMzI1?hl=en_US

 

Well, I tried turning the fuel pressure down to 28 at idle today and it didn't make much of a difference.. and then the car runs really lean when I'm driving around, it misses much more often and at different loads. The regulator does raise fuel pressure with manifold pressure, but maybe not enough to turn it down that low.

If I turn the pressure down at the regulator, do I have to give the ECU time to re-learn the fuel injection values at that pressure? Or are the maps already set and it gives the injectors the same values at a given load and RPM regardless of fuel pressure/O2 input?