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My car is a 2005 Subaru WRX Wagon. It has 120 000 miles and has had all the regular servicing. Last night I did some highway driving and then hit stop/go traffic. After about 10 mins in the congestion the revs started to increase when idling (clutch disengaged and in neutral). Typically it sits at about 600rpm or so when idling, it was started out at 1500rpm. After another 5 minutes it had passed 2000 rpm, another 5 and it was close to 2750rpm. Felt as if I was holding the accelerator down constantly. I didn't have any engine management or oil lights showing on the dash, and temperature wasn't any higher than normal. This problems has been occuring whenever I get stuck for a long time in traffic and then goes away as soon as I can drive at normal speed. I suspect this may be related to the temperature rising under the hood. The car is not drive by wire.

Coolant temperature was logged at 212F when this issue was happening yesterday.

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    " I suspect this may be related to the temperature rising under the hood" Me too. – Moab Jun 25 '18 at 20:52
  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Jun 25 '18 at 21:00
  • What was the air temperature where you were yesterday @Jean-ChristopheRodrigue? – GdD Jun 26 '18 at 8:35
  • @GdD The air temperature was about 64F – Jean-Christophe Rodrigue Jun 27 '18 at 17:12
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If your car has a map sensor and developed an air leak that would explain the rising RPMs. In a car with a map sensor the ECU will interpret the additional air flow from a vacuum leak as an increased load and will add fuel causing the car to rev faster.

  • I have logged the MAP sensor during the issue and couldn't find anything really weird. The graph moved smoothly vs amount of throttle applied. At idle the reading was flat. Thanks for your suggestion. Vacuum leak could explain this indeed but would it show up on the MAP reading? – Jean-Christophe Rodrigue Jun 27 '18 at 19:04
  • It should show up on the map look at the engine load PID if you can – Robert S. Barnes Jun 27 '18 at 20:00
  • I will do that when I get the chance and will report the findings. I will also log AFR to see if the car runs too rich. – Jean-Christophe Rodrigue Jun 27 '18 at 21:23
  • If this is the issue, it won't be rich, as the ECU is adding exactly the right amount of fuel based off the air density from the map sensor. It's not like an maf system were the leak is unmetered air. – Robert S. Barnes Jun 27 '18 at 22:16
  • I will log the engine load PID the next time this happens and compare it to idle when this issue is not happening. – Jean-Christophe Rodrigue Jun 28 '18 at 17:39
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I'm going to hazard a guess that this could be an engine temperature sensor issue, as the engine gets hotter from sitting in traffic the computer is thinking its cool so keeps the mixture rich and RPMs go up.

  • It would just be very rich, but wouldn't run any faster as there isn't any additional air flow. – Robert S. Barnes Jun 27 '18 at 22:18
  • Good logic. The ECU does not have drive by wire and could not open the throttle body. – Jean-Christophe Rodrigue Jun 28 '18 at 17:37

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