Annoying stumble at 2,000 RPM ...

[AL91GTR]

91 RB26DETT stock internals with bolt-ons, 550 cc injectors, N1 turbo's, etc... running Power FC with a tune from Eurocharged. Stock MAF's are still in place and running LS2 coil on plug.

It starts and idles nicely at 1,000 RPM (tuned for this) and timing shows at 20 Deg after it is all warmed up. Solid vacuum and it pulls pretty hard at 1.2 bar.

The problem is when pulling away from a stop it hits a little hesitation right at 2,000 rpm and then pulls smooth after that.

So far, I've checked all wiring connections, timing, obvious things and then last night I pulled the MAF's and opened them up. They both look fine but I re-soldered the connectors anyway because I was there. No change at all today. Looking at the Power FC it shows MAF voltage at approx. 1.000 mV at idle and I watched it hit around 4.350 mV when doing a hard pull yesterday up to about 7,000 RPM (hard to see when you're watching the road too).

I was going to pull the plugs out and have a look but they were only replaced last year in October sometime and probably only have around 1,000 km on them - will check them out tomorrow night.

Anybody have any other ideas here?

Thanks!

[AL91GTR]

Nobody?

I checked voltage and resistance at the TPS and found a little off spec but not much:

Voltage: Closed 0.4 V and WOT 4.04 (as read on PFC Commander). Spec is 0.5 - 4.5 so a little off
Resistance: Closes 1.0 KOhm and WOT 8.0 KOhm. Read with multimeter at the sensor. Spec is 2.0 - 10.0 so a little off

Can anybody else with similar experiences or who has replaced a TPS comment on this? A Facebook post suggested it could be a dead spot on the TPS but it's hard to determine that.

I pulled the plugs last night too and they were fine
I also checked all connections at injectors and all fine (missing 3 wire clips but everything still good)
I also checked resistance at the injector resistor pack and it's fine

[AL91GTR]

As more info in case anybody is still using this Tech forum instead of FB:

I think I have found the cause of the issues. O2 Sensors. Checking voltage while running and at operating temp and revving at a steady 2,000 RPM both read 0.08 - 0.1 V but steady voltage and not alternating (supposed to correct min 5 times within 10 seconds according to the FSM). With the key 'ON' but the car not running both show 0.04 V

I checked resistance at the sensors also and they are slightly out of spec which the FSM says mean they need to be replaced.

Watching the wideband also it is showing really lean at idle and at 2,000 RPM (like 17 +/-) so pretty sure they are pooched.

Power FC Guys: are you running your with O2 sensor feedback ON or OFF ? I need to either change these O2's out and keep the feedback on or just turn it off and re-tune. Any help from experienced users would be appreciated, thanks!

[AL91GTR]

O2 sensors? Negative. I actually had a spare set of O2 sensors sitting here so I swapped them out. No change whatsoever and my O2 feedback is off and my car has been tuned for this.

Local GTR owner Reily was good enough to let me try his TPS on my car this weekend and it also had no effect either

Boost/Vacuum leak? Thanks Al, I'll check that out next hahaha

[240zdan]

I dont have a PFC L jetro, rather a D jetro. But I have two things to contribute to this post:

I run 02 feedback off.
I have the same stumble with my PFC D jetro. Playing around with the tune never really eliminated it.
Try to enrich your Accel vs TPS values, or bring it up with your tuner. Some injectors require different values for a smooth blip up depending on spray pattern etc
I found I had to add a lot more enrichment at lower rpms than normal for a smooth blip up, but it still hesitates a bit. If its hesitating with no drastic change in throttle position then it sounds like a tuning issue as well. What is your wideband reading when this happens?

[AL91GTR]

HI, thanks a lot for the reply. Your description of 'Blip Up' is good. Pretty much sums up my issue. Seems fine when the throttle is steady.

My wideband readings are pretty much fine at idle (14 - 15) and 2.000 rpm but i'll have another look tonight and post results. I notice it a bit lean if I rev to say 3,000 and hold it there but not while driving and no stumble or hesitation at all. I'm pretty sure the issue is in the tune because it happens at exactly 2,000 every time so have to conclude one of the set points needs some adjustment. I've been crazy busy lately and haven't had time to get it back to the tuner but planning to take a day off work for this soon.

I also have O2 feedback off, Deatschwerks 550cc injectors, I just re-wired the Deatschwerks DW200 pump with 12 Ga through a relay and have confirmed full voltage with the car running (13 +V). The battery is in the trunk so the wiring is nice and short. I'm running a Tomei FPR and stock rail with 6 AN hoses.