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S Chassis Technical discussion related to the S Chassis such as the S12, S13, S14, and S15.

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Old 10-17-2016, 06:20 PM   #1
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96 S14 Speed Sensor (VSS) /ECU Problems

I am at wits end... I took my 1996 S14 zenki in on a Saturday for 150k oil change and also had the fuel system cleaning done at the local Walmart about 5 months ago (early June) The car is a completely stock 5 speed base model except for an injenex air intake and aftermarket wheels. It ran fine all weekend, but Monday morning on the way to work (On the highway) The car intermittently cut out like someone had flipped the ignition switch off . it would slow down, then kick in again bucking on and off. very annoying and dangerous. I figured that since I had had the fuel treatment done, perhaps a piece of dirt was stuck or something. I pulled over revved it up a few times without any issues. turned it off and restarted. It ran fine the rest of the way and for the next 4 weeks. It threw no codes, no CEL... I also switched to a newly built gas station on the off chance that i had gotten a batch of "bad gas". About 4 weeks after the first incident , it did the same thing again, almost exactly. Still no codes or CEL. Being that it was very warm and humid both mornings, I popped off the distributor cap to check for moisture. It was dry, but pretty dirty. I replaced cap and rotor. No issues for another 3-4 weeks until it did it again, in the afternoon and the weather was dry. I limped back home, fortunately. Suspecting fuel starvation issues, I replaced the fuel filter. It was pretty dirty and needed to be changed anyway. Engine ran quite a bit better afterwards, I could feel a definite HP gain and had no problem chirping the tires.... however when I took it on the highway as I was getting up to speed... cutting out again. It went away in town in lower gears and Idled beautifully. I installed a walbro 225 pump the following weekend. felt a little more powerful, but again, accelerating or at Hwy speed. Cutting out. Now more frequently, to the point that I am afraid to drive anywhere out of town.
Today I removed and cleaned the O2 sensor , the MAF and shot some MAF cleaner into the idle control valve. Car idles silky smooth. revs without issue. Took her for a test hop on the highway and no problems. I have a bluetooth dongle attached to the OBD plug. I have the Torque android app running the whole time with various engine functions displayed and everything looks normal. Just for fun I dropped a new set of plugs in her, and took her back out. This time it started bucking again. I am totally out of ideas until I noticed the (ECU?) speed display on the torque app was very erratic and showing double and even triple the indicated speed on the speedometer (which is working fine and seems to be showing the correct speed). when it hit about 120 (actually 45-60) it would cut out until the speed dropped back down. So.. I am fairly sure that the cutting out is due to the speed limiting function on the ECU. If it is the VSS, why is the dash speedo working just fine? why are no codes popping up? I also noticed my (ECU Indicated) voltage running low at 11-12.5 when it was working correctly, but was 13V+ when it was malfunctioning. I replaced the alternator (with voltage regulator) last summer 2015. Where should I be looking? Is my ECU about to go tits up? is there a bad ground wire under the dash somewhere. is the issue between the speedo and the ECU? can I disable the ECU speed sensor input without losing my dash speedo? Please help

EDIT: Checked for any loose connectors or wires, but nothing. Took it for a test spin and the speed sensor ECU on the Torque app was showing normal. no issues. It is apparently an intermittent problem.
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Old 10-18-2016, 10:14 PM   #2
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Old 10-22-2016, 07:07 AM   #3
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Update : had to work 4 straight 12 hrs shifts last week, so I haven't had a chance to look at the car until yesterday. I took another test drive and confirmed that the ecu indicated speed was very erratic and 2 to 2 1/2 times the speed indicated by speedometer and also confirmed by GPS speedometer. I am convinced that the cutting out is due to the speed governor kicking in. After digging into various forums and my Chilton manual I arrived at the conclusion that there is a short or something between the speedo and the ecu. I also saw that there is a yellow wire with a green stripe running from the speedometer to the ecu and cutting it should solve the issue. Since I had the ecu cover off already and could see the wires and not really wanting to tear into the dash panel, I decided to clip it at the ecu. I quickly located a yellow /green wire. I snipped it and a cel came on. It was the evap canister.... Looking closer and blowing off some Dust, I saw about 6 more yellow /green wires... So don't do that. I reattached the clipped wires and put the ecu cover back on. I finally just removed the cluster and there was only one yellow /green wire. I clipped it. Test drive was flawless. Ecu indicated speed stayed at 0. The mpg and some other information functions no longer work, but that is a minor issue. I had already ordered an oxygen sensor and new plug wires, so I popped those on and she is humming along like a sewing machine. Smooth as butter. I am 90% convinced that she is cured. I also noticed that my indicated voltage stabilised at a normal 13.2-4 volts. I am wondering if the clipped wire was shorting out somewhere. The wiring that I saw all looked to be in really good condition, especially for a 21 yr old vehicle. Sorry for being so long winded, but I am a nurse and my documentation /charting habits are hard to suppress. As these cars age, electronic issues are going to pop up more frequently and I hope that this post is helpful to someone else in the future. I will post again here if the issue pops up again, but I am almost certain that it is resolved.
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cutting out, ecu, speed sensor, vss

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