Alright, we can't seem to find the solution to this problem.

The car is an 88TII street ported
s5 turbo
full turboback RB exhaust
walbro 255 (rewired)
marren 750cc secondaries
cai
s-afc
innovate motorsports xd-1/lc-1 wideband and gauge

Alright, here's the problem...car starts and idles fine. The car revs fine up to 3500rpm. At 3500rpm it turns to garbage and bounces back and forth from 3400-3600rpm. Almost like it's stuck there. Once it hits this point the wideband goes retarded and shows very lean. It almost seems like fuel is cutting on and off really quickly. Now this problem only occurs if you pull slowly or midpedal through the rpm range. If you go WOT through the rpm range, it goes through with no problems.

Alright, now what's been tested so far. The tps was having issues, just replaced it with one that is holding the right resistance, and sweeps through just like it should, so that's good. I've swapped afm's...no change. The timing was off a little. When the timing was reset it seemed to clear up the problem just a little bit, but it's definitely still there. We've added a large ground. The crank angle sensor is in spec, the coil packs are in spec. The fuel pressure was tested at the pump after the rewire and it was 110psi. We haven't checked the pressure after the filter or up front. This will be done tomorrow most likely.

One other bit of info. We're running straight fuel lines. There is no hose coming off the regulator on the secondary rail (the one that hooks into the intake). Could this be causing the issue?

What else can we try? Is there an afc setting that's possibly been missed? We've set it every way we know how, and still can't get rid of the problem. Give me some more ideas.

 

You need a vacuum, a real vacuum line going to the FPR which you say does not have one. That's so the fuel pressure will rise as you get into boost and other times.

Are you sure the boost sensor is a TurboII boost sensor?

Are you sure it's getting VACUUM?

You have the reverse problem of a lot of people. Usually you can go all the way to redline if you gently press the pedal and have no problems, but it'll give you problems at 3500 if you floor the pedal. Oddball problem.

 

yep, that's why I posted it...cause all the "3800 rpm hesitation" threads were the opposite of what was happening to me. Hopefully, we'll get to hook up that vacuum line tomorrow and we'll double check the boost sensor...which should be a tII sensor, I don't ever recall having changed it for any reason. Any other ideas that might be checked?

 

ok, so....the line was put back on the regulator on the secondary rail, and the bac was removed....drastically better. The car doesn't run that great when it's cold, and it on occassion will bog for a split second at lower rpms...around 2500 or so...we think the water temp sensor may be flaking out. Could that cause any of these symptoms?

 

The bog lower (1500-2500) is common to the throttle body mod, has it been done on your car???

 

yes it has...so that may be what's causing it. It's not everytime...but every now and then it'll bog a little

 

does it happen moving from partial throttle to max throttle???

 

Does this mean that you have completely removed the double throttle plates? I had a TII do something very similar several years ago and it was actually the double throttle plates getting stuck shut. On the 2nd gen they default closed. On the 3rd gens they default open.

 

Sorry to bump this kind of old thread, but my car is having hesitation under 3k rpm under a similar condition. Quick increase in throttle causes a nice hesitation followed by acceleration. No throttle body mod though :-/

TPS checked out ok. I'm not sure if the pressure sensor affects anything like timing advance or something, but I'll check that tomorrow.

edit: Is this just the "3k rpm" deal or what. I've replaced most of the grounds but not the starter ground. Is that note worthy?

 

IF the vacuum restrictor is missing from the vacuum line to the boost sensor, that can cause bucking also.

Vince

 

wait, so wtf was causing that 3800rpm hesitation?! i have the EXACT same problem. at light throttle it just bounce off that like a rev limiter. but when i mash it, it goes right through it....what fixed it?

 

not sure exactly what did it...but i can say the bac was removed...the timing was set, and the vaccuum line that was not connected to the fpr on the secondary rail was replaced. Other than that...only the afc was change I believe....now the car pulls through fine on low throttle...and pulls great when you floor it...by ~half pedal it catches and hangs between 3500 and 4000 rpm

 

Oh, another thing i noticed...on the afc...doing the throttle sweep...the car goes from 0 -100% in like a 1/4 pedal...not sure what's up with that. I think this is screwing up my lo/hi throttle point. Any advice here?