Rough low idle : Mass airflow and misfire error codes - Toyota Yaris Forums

LeBnomiS · 05-05-2020, 02:31 PM

Hi there. I've been having some trouble with my Yaris lately and have been trying to diagnose and fix it myself. I'm a beginner at this type of stuff, I usually only do basic maintenance (brakes, oil change, etc.), but am looking to learn more.I have a 2011 Yaris HB 5 speed manual, with approx 165 000 km (100 000 miles).

At idle, it seems to stutter a bit, and the RPMs jump around a little. It also produces a pretty pronounced clicking or ticking sound, as if something were loose in the engine. This only happens when the engine is warm, and idling around 500-600 RPM - the problem disappears when the RPMs go above 7-800.

This was accompanied by a loud whistling sound which would go away when accelerating. I found a pretty big hole in the PCV valve hose, so there was a pretty bad vacuum leak. I replaced the hose, and also took the time to clean the throttle body. I thought that would fix the rough idle, but it didn't.

The check engine light and the traction control "slip" lights are constantly on. I read the OBDII codes, and I am getting :
P0100 - Mass airflow circuit malfunction
P0102 - Mass airflow circuit low input voltage
P0300, P0302, P0303, P0304 - These are misfire codes, the 300 code indicates misfire in more than one cylinder, and 302, 303 and 304 indicate misfires in cylinders 2, 3, and 4. The low idle clicking does not sound like a misfire, but I guess that could be it.

I am thinking the problems with the mass airflow system are causing the misfires. Could that hole in the PCV hose have caused a problem with the mass airflow sensor ? I ordered a new PCV valve, just in case that got damaged.

Should I try replacing the mass airflow sensor, or maybe just cleaning it first ? I also noticed the airbox has a broken clip and didn't seem to seal very tightly, could this be cause for concern ?

Thanks ! Any help is super appreciated.

komichal · 05-05-2020, 03:52 PM

1. I am almost sure the PCV valve cannot have any impact on MAF sensor readings.
2. Cleaning up the MAF is an easy and efficient way to start. Just take it out, spray it like mad with the MAF cleaner spray (no brake cleaner!) and let it dry properly. Remember to spray also inside, not just the visible part.
3. Make sure you disconnect the battery during that (that will also reset the computer so that it can re-learn the right positions for your throttle body flap.
4. Change the plugs. If you are still driving with the original ones, 165 tkm is a massive portion even for iridium plugs that - to my knowledge - are mounted in by default.
5. Delete the error codes and observe the changes.

LeBnomiS · 05-05-2020, 05:27 PM

Komichal, you're right, plugs have never been changed to my knowledge. I will be doing that this week. Thanks for your reply !

LeBnomiS · 05-08-2020, 05:24 PM

Well, after changing the plugs, the PCV valve, and cleaning the mass air sensor, I found the rattle to be the exhaust manifold heat shield, lol ! The bolts on the bottom practically turned to dust when I tried to tighten them. I'm going to try to tie it down with a hose clamp.

We'll see if the new plugs and the clean sensor will do the trick for the misfire problem.

05-05-2020, 02:31 PM	#1
LeBnomiS Drives: 2011 Yaris 3DHB Join Date: Aug 2014 Location: Canada Posts: 8	Rough low idle : Mass airflow and misfire error codes Hi there. I've been having some trouble with my Yaris lately and have been trying to diagnose and fix it myself. I'm a beginner at this type of stuff, I usually only do basic maintenance (brakes, oil change, etc.), but am looking to learn more.I have a 2011 Yaris HB 5 speed manual, with approx 165 000 km (100 000 miles). At idle, it seems to stutter a bit, and the RPMs jump around a little. It also produces a pretty pronounced clicking or ticking sound, as if something were loose in the engine. This only happens when the engine is warm, and idling around 500-600 RPM - the problem disappears when the RPMs go above 7-800. This was accompanied by a loud whistling sound which would go away when accelerating. I found a pretty big hole in the PCV valve hose, so there was a pretty bad vacuum leak. I replaced the hose, and also took the time to clean the throttle body. I thought that would fix the rough idle, but it didn't. The check engine light and the traction control "slip" lights are constantly on. I read the OBDII codes, and I am getting : P0100 - Mass airflow circuit malfunction P0102 - Mass airflow circuit low input voltage P0300, P0302, P0303, P0304 - These are misfire codes, the 300 code indicates misfire in more than one cylinder, and 302, 303 and 304 indicate misfires in cylinders 2, 3, and 4. The low idle clicking does not sound like a misfire, but I guess that could be it. I am thinking the problems with the mass airflow system are causing the misfires. Could that hole in the PCV hose have caused a problem with the mass airflow sensor ? I ordered a new PCV valve, just in case that got damaged. Should I try replacing the mass airflow sensor, or maybe just cleaning it first ? I also noticed the airbox has a broken clip and didn't seem to seal very tightly, could this be cause for concern ? Thanks ! Any help is super appreciated.

05-05-2020, 03:52 PM	#2
komichal View User's Garage Drives: Yaris 2007 1.3 2SZFE 64kW Join Date: May 2012 Location: CZ Posts: 297	1. I am almost sure the PCV valve cannot have any impact on MAF sensor readings. 2. Cleaning up the MAF is an easy and efficient way to start. Just take it out, spray it like mad with the MAF cleaner spray (no brake cleaner!) and let it dry properly. Remember to spray also inside, not just the visible part. 3. Make sure you disconnect the battery during that (that will also reset the computer so that it can re-learn the right positions for your throttle body flap. 4. Change the plugs. If you are still driving with the original ones, 165 tkm is a massive portion even for iridium plugs that - to my knowledge - are mounted in by default. 5. Delete the error codes and observe the changes.

05-05-2020, 05:27 PM	#3
LeBnomiS Drives: 2011 Yaris 3DHB Join Date: Aug 2014 Location: Canada Posts: 8	Komichal, you're right, plugs have never been changed to my knowledge. I will be doing that this week. Thanks for your reply !

05-08-2020, 05:24 PM	#4
LeBnomiS Drives: 2011 Yaris 3DHB Join Date: Aug 2014 Location: Canada Posts: 8	Well, after changing the plugs, the PCV valve, and cleaning the mass air sensor, I found the rattle to be the exhaust manifold heat shield, lol ! The bolts on the bottom practically turned to dust when I tried to tighten them. I'm going to try to tie it down with a hose clamp. We'll see if the new plugs and the clean sensor will do the trick for the misfire problem.