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Engine RPM bouncing up and down after cold start


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Hey guys, I've got a 2013 Toyota Avensis with the 1.8 litre petrol engine and manual transmission. After a cold start, the engine idles normally, but after driving off, then shifting to neutral or depressing the clutch, the revs start bouncing up and down. It does this for 10-15 seconds, then idles normally from then on. Note: The air conditioning and lights were turned off, so there was no additional engine load to explain RPM changing.

I've done a lot of searching on Google for what the root cause might be. People have suggested it could be a clogged up IAC valve, but I don't think Toyota Valvematic engines have an IAC valve. It could also be a faulty throttle position sensor, but then I'd expect the problem to occur all the time, not just for a few seconds after a cold start. I removed the air intake and checked the throttle body - it's very clean. I cleaned the MAF sensor as a precaution. Some BMW engines exhibited the same problem (from new) and in that case I gather it was resolved with an ECU software update.

On the occasions when it was really bad, I didn't have my phone ready to capture it on video, but yesterday I recorded two examples with somewhat minor symptoms and have uploaded them to YouTube:

 

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Check your intake pipes after air filter for cracks, also all vacuum pipes. If there is a crack or a hole, it could cause uneven idle speed. Also it does not hurt to check your spark plugs.

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2 minutes ago, Goran3456 said:

Check your intake pipes after air filter for cracks, also all vacuum pipes. If there is a crack or a hole, it could cause uneven idle speed. Also it does not hurt to check your spark plugs.

Would a vacuum leak not cause idle speed issues all the time, rather than just for a few seconds after a cold start? Actually, it idles perfectly after a cold start as well. It only does this for a few seconds after you start moving and then depress the clutch and brake to come to a stop, then it idles perfectly until the next cold start.

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So I unplugged the MAF sensor before starting the car this morning and the idle speed was rock solid - no bouncing up and down. I don't know whether that indicates the MAF sensor is faulty or whether the ECU was simply running in "open loop" mode with the sensor unplugged. A side effect of unplugging the MAF sensor was the instrument cluster lit up like a Christmas tree with five different warning lights! Plus there was an error message that said "Check parking brake system." When I got home tonight I plugged the MAF sensor back in, but that didn't clear the warnings on the dash (even after turning the ignition off). I had to disconnect the Battery to reset everything. I then went through an "idle relearn" procedure I found online, which I'm not actually sure is required on modern Toyotas, but there's no harm in trying. I'll have to wait for the next cold engine start tomorrow morning to see whether it's back to its old tricks or not.

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If MAF is offline, then the car goes by default values already programmed in. This in turn uses  a bit more fuel, as the car cant adjust the values on the fly.

 

Try cleaning the MAF with those special spray cleaners. I had similar issue last year, the sensor looked pretty clean, i did spray it good with the cleaner, and i had no issues after plugging it back in. 

Be sure you don't touch the actual sensor wire while working with it.

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I'd already cleaned the MAF sensor and it didn't help. Do you think the fact it idles perfectly without the MAF sensor plugged in points to a faulty MAF sensor?

After plugging the MAF sensor back in and disconnecting the Battery for a while, the cold idle was back to its old tricks at the next cold start.

To clarify what happens:

  1. Start the engine from cold and it idles perfectly
  2. Drive off and once the vehicle speed reaches about 15km/h you hear a clunk sound as the ABS performs its self-test
  3. Only after that, when you depress the clutch pedal does the idle bounce up and down 3-5 times
  4. It continues to idle perfectly after that

It hardly sounds like it's worth worrying about, but it's clearly not normal.

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Idling perfectly while cold, and while hot is not the same, as the car is dumping more fuel until it reaches operating temperature.

You don't mention anything about the issue when warm, so i presume it only manifests itself on cold starts.

When you depress the clutch, you apply the load to the engine, turning inside gears + Oil in transmission is cold, so, the car tries to adjust to this. If you say that once the MAF was unplugged, and you drove the car, it was all fine, i'd try replacing the sensor.  I don't really know how expensive it is, but you also might consider a 2nd hand one.

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Depressing the clutch means pushing the clutch pedal down, so the gearbox is disconnected from the engine.

It idles perfectly when both cold and hot, except for the RPM bouncing up and down 3-5 times the first time you come to a stop after a cold start. I also find it interesting that it only does it after the ABS self-check.

MAF sensors are quite expensive.

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