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Throttle Position Sensor/Switch - Question about Part #'s

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  • #16
    Originally posted by Chris W View Post
    A few years after I bought the car, I went with some car club friends to a dyno day and had my car dynoed. I was curious about just how powerful the engine was as the previous owner had chipped it and installed the exhaust cam gear. When my graph came back the dyno operator mentioned that the car was a little down on power and showed me the air fuel ratio during the run. It started out in the mid to low 12s and then at high rpm went down to the high 11s. He told me that if I could lean out my fueling I could easily gain another 5 hp or more.

    That started me down the road of chasing this problem, and it took me years of false leads and failed solutions. I started by ordering a remanufactured AFM. At first I thought it might have made a slight improvement in the car's feel, but looking back I think that might have been a placebo effect. I also heard that the CTS could be a problem, so I replace it even though the readings were reasonably close to what they should have been, and again not much improvement. I also read the common advice that when the barometric pressure sensor fails it makes the engine run lean, so I replaced it with a new one, and again there was no improvement.

    Over the years, the car was start every spring, but it seemed like every year the first time I started the car after the winter seemed to get tougher and tougher before it would finally start. I also noticed the car would buck and stumble badly when the engine was cold whenever I tried to give the car any serious throttle, and I was only getting about 225 miles on a tank of gas. Once the engine was warm everything again seemed normal. That trend progressed year after year, until one year the car wouldn't start at all. At that point, I tied the trick of disconnecting the CTS, and sure enough it would start with it unplugged. If I plugged it back in when the engine was cold, the car would stall, but if I let the engine warm up before plugging the sensor back in, I could still drive the car. Eventually, even that trick no longer worked. That then led me to suspect the injectors were either leaking or stuck, so I tried sending them some place to be cleaned, and when that didn't seem to help, I replaced them with new ones, but the car still wouldn't start.

    After all that I tried rechecking the things I tried to fix the car, and even replaced the CTS a second time. Eventually, I came across the information on this site with the correct values at various altitudes for the barometric pressure sensor. When I measured the sensor it came back at 2.1k ohms (not the 2.3k ohm I wrote above. At 2.3k ohms, the car would start, but have the bucking and stumbling problems), which would have indicated the car being over 1,000 ft lower in elevation that it actually was. Since air is denser the closer it is to sea level, I reasoned that the sensor was causing the car to run rich. That also got me thinking that disconnecting the CTS was able to make it start because it was leaning out the mixture just enough for the engine to start, at least for a few years. Maybe the Motronic has been programmed to interpret a disconnected sensor as a default temperature value such as normal operating temperature? I double checked the reading with my old sensor, which I kept as a spare since it didn't seem to be any more faulty than the new one I bought, and sure enough it also reading too low. That's when I took my original one apart and manually adjusted it to 2.5k ohms where it should be. The car's run flawlessly ever since, and now I usually get 275 miles on a full tank.

    Every other year or so I start to notice the car being a little sluggish, and that's when I know I need to readjust the sensor again. Once's that's done, I can again feel that the car has that little extra power or snappiness to its throttle response.
    What elevation are you at?



    • #17
      2,200 ft


      • hardtailer
        hardtailer commented
        Editing a comment
        Did you ever change the chip inside the ECU for a stock one during your search for what caused your engine's starting difficulties?

      • Chris W
        Chris W commented
        Editing a comment
        No, I know I have a Dinan chip in the ECU and that would have contributed to the rich condition versus a stock chip.

      • hardtailer
        hardtailer commented
        Editing a comment
        Thanks for your reply

    • #18
      Thanks Guys. Double checked the new TPS, it's adjusted correctly. New spark plugs installed. I disconnected the Barometric sensor, and no change in behavior. I'm at about 500ft elevation.

      I haven't tried disconnecting the AFM yet, will do that to see what happens.


      • hardtailer
        hardtailer commented
        Editing a comment
        Check for an intake leak. Pulling AFM won't help.

    • #19
      Disconnecting the barometric pressure sensor would just cause the Motronic to default to sea level, which would be the same fueling as 500 ft elevation, so disconnecting the sensor would represent no change to the Motronic, and also no change in your result. What resistance is the sensor registering as per the Iigomotiv procedure?


      • #20
        There's a persistent misconception that the barometric pressure sensor measures elevation. It doesn't, it measures barometric pressure, hence the name.

        Yes, in general, barometric pressure falls with elevation but there are other factors that affect barometric pressure. It's constantly changing even when you're at rest at sea level.
        Last edited by Mick; 09-08-2023, 01:35 AM.


        • #21
          Absolutely true! I checked my baro sensor at 490m elevation whilst ambient pressure was 1024mbar. Going by the list quoted by Jake I would start adjusting the wiper but I won't as 1.69kOhm is spot on for 1024 mbar.

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          • #22
            Barometric values obtained for any location reporting have been adjusted to sea level. National Weather service is interested in barometric readings for purposes of weather predictions. That way their analysis of weather is unaffected by elevation.