I have an S5 tII and I can't seem to get rid of this bucking issue.

I have a fmic, bnr stage 2 and custom intake, boost controller, koyo radiator, removed airpump, p/s, a/c, blocked off split air pipe, relocated leading coil due to intercooler space issue, replaced all vacuum lines, and gaskets, new o rings and injector grommets, battery relocated to passenger compartment teamfc3 style, refreshed most grounds, etc.

Tested tps and it is within spec, got some strange results from leading coil resistance test (got a lot of fluctuation before it settled down at 1 ohm) refreshed ground and added an extra to no effect. Will retest leading coil soon to see if it was a fluke.

Symptoms are:

Occasionally doesn't start on the first try. Starts up great 9 times out of 10, AWS kicks in, although only maintains 3k for about 5 - 10 sec then drops to about 1700, then falls normally, sometimes has trouble with hot starts when it does have a problem, and I'll have to give her a little gas to get going. Then idle will fluctuate a little until I drive for about 5 - 10 min.

Usually idles smooth but occasionally has a slight stutter. It sweeps smoothly across the range of rmps when not under load. Vacuum pressure fluctuates between -16 and -15 at idle. There is less fluctuation with the heater on full blast and pressure mostly stays at -15.

Stutters between about 2500 - 4000 starting very minor in 2nd gear and getting progressively worse. In 5th gear it is very apparent and bucks the most violently. Seems to happen when vacuum pressure is between -11 and -6. Its worse when going uphill or on rainy days. Does not stutter at all under 1/2 to full throttle. Does not stutter under boost.

Sometimes dash lights go out and sometimes clock goes out, but I think the connectors are just loose because banging the dash in the right spot brings them back.

It doesn't seem like a fuel issue to me because you would expect symptoms to get worse under heavy load. I sprayed carb cleaner everywhere and am not detecting any leaks, although it does bog out if I spray near the acv or behind the water pump near the front oil injector. I am leaning towards electrical issues at this point. My harness is getting kind of crispy and I have already had to repair a few wires going to the acv that busted. I am not getting any codes. Anyone got any ideas for me to try? I am going to go bald from pulling my hair out if I don't figure this out soon.

 

Check the Boost Pressure sensor according to spec. Might want to bypass the Fuel Pump Relay and Resistor if equipped.

 

Yep fuel pump relay was next on my list. I did hear an excessive amount of ticking today climbing a small hill in 2nd gear. Hit the throttle and didn't get a a lot of response, but heard what sounded like injectors ticking... Wonder if it could be a clogged injector? Idle has gotten more rough and seems to be idling lower now. Maybe its not as noticeable of a problem when secondaries kick in? I replaced the fuel filter not long ago...

 

Update: Pressure sensor is within spec. Car is now idling very low and stalls sometimes. I can't drive it until this issue is solved. Also noticed that dash lights lost power completely and if I rev the rmps up they very gradually start lighting up. Could it be a ground or alternator signal interference issue? Anyone?

 

With the car running check the output voltage of the alternator and see if it is at 14 volts or a bit more. If it behaves as you mentioned then definitely check the output voltage while idling and see what you get according to how much it should normally be.

 

It turns out my issue was a sticky BAC. I took it off, cleaned it with carb cleaner, vacuumed out any dirt and debris, and slapped on a new gasket. I notice it now operates at times when it never did before, because I can hear it kick on. Soooo the BAC is another thing to check in the stuttering / hesitation / bucking symptoms line of inquiry.

 

My issue returned, but only when its raining. Must be electrical. The bac is injecting air at times it should not be (mid rpm range) and is masking the symptoms in lower gears. Still hesitates in 5th gear especially. Anyone ever had a similar issue?

 

Turns out there was a small tear in the hose going into the bac from the fitting I installed on the intercooler pipe. Repairing it and using better clamps has solved the problem once and for all. Now I just need to get rid of the compressor surge...

 

Good to see that you're finding and solving all of your idle issues on your own. Keep it up!

 

Thanks. I appreciate the encouragement!