I have a diesel common-rail 1.5l engined car. Lately I am facing an issue where the RPM needle is bouncing in the tachometer when the car is idling. The normal idling RPM is around 820. When idling, I can see the RPM fluctuating between 800-1000 rpm. This fluctuation is more evident when the AC is switched on. I can even hear some loud thud coming from the engine bay (due to the AC compressor) just when I turn on the AC, and along with the sound, the RPMs fluctuates even more. Few points:

  • Coolant is in the max level.
  • The issue seems to be more evident if I turn on the lights as soon as I start the engine(cold start)

Can anyone help me out on this?

  • What year/make/model of the vehicle? This just sounds like standard operation with an A/C functioning to me, but having the vehicle information may help diagnose if this is common/uncommon for your vehicle. May 19, 2015 at 10:53
  • Its a 2011 Tata Indigo- eCS sedan. Well the AC loads up the engine always but I had never seen the tacho jumping around the 900rpm mark, since I bought it 4yrs back. Also whenever I switch my AC on, the car shudders a bit with fluctuating rpm(Only during idle) May 19, 2015 at 11:10

5 Answers 5


The Problem can be with the engine not getting enough ventilation.

  • Problem: In your car the ECU is programmed to raise the rpm when it detects a stall or the RPM dropping down so if there is problem with your air filter which is not allowing much air into the engine and causing a stall but not noticeable your ECU will fight it to keep it in working range.This causes the RPM to bounce.

  • Solution : Clean up your air filter and check for any clogs.

The Air Con part is pretty standard

  • Would be no harm in checking out the throttle body assembly as well - grime and gunk could make it a bit rougher
    – Nick
    Aug 18, 2015 at 12:17

The exact cause of the issue had been found to be that of a loose alternator belt. When the engine was cold, the alternator had a slack in it and if the AC load is added on the engine, the rpm fluctuated. Solution was simply to tension the alternator belt.

Well, thanks to all for sharing the knowledge.

  • 1
    award yourself the answer :-) Feb 10, 2016 at 0:09

Or could be your mass airflow sensor because I have similar issues and that is what's wrong with my car

  • If you think it is your Mass Air Flow Sensor, get your MAS cleaned first (and correctly). Avoid using after market air filters which have oily residue in them which can get onto the sensor (especially if you have a turbocharger). Jul 13, 2016 at 8:13

Guessing. May be due to looping of 2 governors?

Your car software tries to maintain the idle rpm.

The AC temperature governor tries to maintain temperature, but some clogs in coolant path or issues with compressor leads to variable load on engine.

Make sure AC system serviced once.


One tata497 school bus engine hunting on the idle moment & low pickup.action taken: overall fuel pump,tappet valve clearance settings, complete high pressure line clean , turbocharger overall but problem not solved.

  • Welcome to Stack Exchange! Could you please edit your question to explain what action was taken regarding the Turbocharger? Thank you, in advance.
    – NitrusInc
    Apr 14, 2018 at 13:55

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