Fixed Idle Problem, But Now Stalling At Abrupt Stops?

All the Mass Air computers have VSS as a factory installed sensor. If it is missing or non-functional, it sets code 27 or 29.

Mass Air conversions may or may not have the extra wires run depending on how well the conversion was done.

Thanks! I figured a bad VSS would set off a code, but I'm not sure I can rely on my problem setting off a code. Would a bad or weak sensor always trip a code???

I'm not sure what my next steps are here. I summarized what I've done to date and what my issue is.

Issue- runs great except at a stop- RMPs rev up and down quickly from 300 to 1200 on the verge of stalling

Repairs so far:
  • Recent O2 sensors- 3 wire OEM
  • Recent fuel filer
  • New plugs and wires
  • Timing 12 degrees
  • New IAC
  • New ERG
  • New ACT
  • New TPS- set .94
  • New Temp sensor
  • New 65mm BBK throttle body and spacer
  • No vacuum leaks
  • No exhaust leaks (at least from O2 sensors to engine)
  • Fuel pressure good
  • Engine vac good
  • No codes with engine off
  • Codes 41 and 91 running (still can't figure these out)
I know I'm throwing money at parts, but the next logical parts are MAF (I do have a used one to swap out) or VSS, otherwise I'm out of ideas.
 
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Ran car after re-adjusting idle. No real difference. I do notice when I start car and let idle in driveway, it runs fine. I can rev RPMs and sounds and works good. 15 min later I go for a ride and it operates pretty normal then will start wanting to stall coming to a stop again. Once stopped and idle evens out, runs fine again. That really seems to point to VSS to me since only when I come to a stop.

Not sure if related, but my idle is lower now than when I adjusted last night. It idled around 900 last night warm, now more like 700. No idea when this seems to fluctuate.

I think I'm going to bring my timing back to 10 degrees. Probably should have changed it to begin with until I got car running right.
 
Set timing to 10 degrees. After, car idled perfectly and smoothly at 900, then standing in front of the car, it heard what sounded like an accessory kicking in and my idle dropping slightly. I looked and car now idling around 700. Everything was off, radio, fan, AC etc. I'm not sure what would cause a load like this seeing we have one serpentine belt that runs everything. Is what I described normal? Probably attributes to my idle being lower than when I set it. Car was at op temp and probably took a minute to drop.
 
When first started, the engine idle speed is increased for a short time to insure that it won't stall when the engine is still cold.
 
Set timing to 10 degree and swapped out MAF with a different one. Drove car long enough that it should have acted up. No almost stall out:) I won't say problem solved until I drive it a little more.

Ran codes again after and on top of my standard 41 and 91, now I'm also pulling code 12. Need to research this.
 
Great, second test drive, car still tried to stall although not as often. I did put a different IAC in due to code 12 between my ride earlier (that was pretty good) and my ride now which I experienced some of the same problem. Probably should have waited to do this.

I pulled into the driveway, idle bouncing up and down, then I came to a complete stop and 10 seconds later it leveled out.
 
Reset base idle again and ran car. No stalls at all. I think my idle was set too low, although now I think it's set a little too high. Idle hangs a bit more than it did, but at least it doesn't stall anymore. I'll probably just tweak in small increments (1/4 turn) and see if I can even it out. I can't imagine i have any vacuum leaks after smoking my system twice and fixing a cracked intake, EGR and throttle shaft. Seems like I just haven't gotten idle just right. Too low, stalls and too high idle hangs.

I'll run codes tomorrow and see if Code 12 is gone.
 
Spring is here and I'm back on the hunt to fix my stalling problem from last summer!

I replaced the VSS a week ago and also the speedo gear that was fairly chewed up (23 tooth). Drove the car for 10 miles or so. Ran pretty good. I did have some sporadic issues of pulsing idle when at a stop, but not nearly as bad as before. At least the car wasn't on the verge of dying. I did make note it was about 53 degrees outside and I do feel my problem has to do more with temperature (hot outside or car hot). The car seemed to run better in the cooler weather. Could temp be related? I did put in a new temp sensor and ACT last year.

I started building a smoke machine which I should have done this weekend so I will be looking for vac leaks again.

I also want to rebase my idle again just to be sure I did it correctly last year. I do think my idle slightly too high (1/4 turn maybe).

Question: is is normal for your idle to fluctuate? Sometimes I'm at a nice 800 after letting her warm up for 10 mins, then a few miles into the drive I'm down to 700 sitting at a light, then later on I'll notice I maybe be up at 900. Normally I can rev the engine and notice it will kick back down to 700. I'm sure this points to vac, but figured I'd ask anyway.
 
Mine drifts slightly as engine load (A/C, lights, etc.) and temp changes, but there is no surge. Once the engine warms up good and the car has sat motionless for 10 seconds or so, it drops down to a steady 700-750 RPM.