Cutoff at low RPM during gentle acceleration - Yaris Verso 1.3 VVTi

[jackmilas]

Success!
New front O2 sensor improved the situation a lot!
But occasionally I get this hesitation, so it's not solved entirely yet. Perhaps it would happen 4 times out of 5 before, now it is once out of 5 and much smoother / gentler.

Could the rear O2 sensor bring more improvement / solve it entirely?

I didn't disconnect the battery after replacing front O2 sensor. I did it now and I'll report back if there's any change.

PS.: Previously I unplugged the battery a few times - there was no change then.

[Bluevitz-rs]

The rear O2 is on there to affect Long Term Fuel Trim and monitor the Cat.

[jackmilas]

I ordered MAF sensor yesterday. This is one of the oldest cars on this forum so I guess other people will benefit from my experiments :) I'm going to install it within a few days and I'll report back.

[jackmilas]

I got P0420 code today.
I will be replacing rear O2 sensor as well and I'll report back.

The car is much nicer to drive after changing front O2 sensor.

[Bluevitz-rs]

Take a reading of the O2 sensor first before throwing parts at it.


Sent from my iPod Touch

[jackmilas]

Originally Posted by Bluevitz-rs

Take a reading of the O2 sensor first before throwing parts at it.
Sent from my iPod Touch

I understand the logic of checking everything before making changes. I decided this car is old enough to change both O2 sensors and the MAF sensor, because they wear out by their nature. It costs me less than 200 EUR and I will probably recoup that in better fuel economy within a year.

Now, the front O2 sensor made a huge difference. I just finally took it outside the city and I drove fast on a motorway. Noticeably more power and from looking at fuel gauge and distance driven I get a feeling that fuel economy might somehow get closer to what the car's economy display is showing (~6.5l/100km).

... more to follow :)

[Bluevitz-rs]

I guess a 15 year old car doesn't owe you much now.

New parts shouldn't hurt if you have money to spend on them.

[jackmilas]

The car drives great, the jerk most of the time is gone but it still happens sometimes. I was thinking that it wasn't there before I changed oil and oil/air filters in November. There was synthetic 10W40 (I don't remember the make) before, I replaced it with Castrol Magnatec 10w30. Perhaps I should change it back to 10W40? Or maybe I should put original Toyota SAE 5W30 there?

[Bluevitz-rs]

I'm always sticking with 5-30. Unless your racing or live in a very hot climate you don't need a different grade.

[jackmilas]

Changing the sensors helped to decrease the severity and frequency of jerky starts under light acceleration at low revs. But I'd like them gone completely. Would any of the below cause the issue?

- dirty fuel filter
- worn out fuel pump
- blocked air vent to fuel tank

[Bluevitz-rs]

I forget. Did you confirm your TPS is around 10-12% at idle. And it indeed moves with the crack of the throttle and is linear.

[jackmilas]

Originally Posted by Bluevitz-rs

I forget. Did you confirm your TPS is around 10-12% at idle. And it indeed moves with the crack of the throttle and is linear.

My default (throttle off) position is 11.8% and the engine cutoff happens at around 17%. Gentler or harder acceleration doesn't produce the problem.
Disconnecting TPS doesn't make any change to the issue.

[Bluevitz-rs]

And if you hold it there it just keeps surging

[jackmilas]

Here is something that points me towards VVTi problem:

Originally Posted by Yaris Hilton

VVT-i is disabled when the "COOL!" light on your dash is on.

[Bluevitz-rs]

Ok then simply unplug the VVTi on the engine and see if the problem goes away.

[jackmilas]

Originally Posted by Bluevitz-rs

Ok then simply unplug the VVTi on the engine and see if the problem goes away.

I disconnected Oil Control Valve which I understand controls the VVT system. The problem is completely gone.
Since the the VVTi is controlled hydraulically using engine's oil my guess of incorrect oil might not be entirely missed. I am going to drive it without VVTi for a while and I will investigate what oil might be suitable.

[Bluevitz-rs]

If that's the case, there's a chance there's a clog in the passages of the VVTi oil flow to the variable cam. You can probably pull the valve cover and the cam cap over the pull side of the cams to expose the the oil passages. With the oil control valve out, you can blow it out with compressed air and some brake cleaner.

But before that you'd have to check the signal from the cam sensor to make sure it isn't false.

[sarap]

wow so much knowledge gained between the last two pages in this thread !
OP hope everything works out for you !