Strange fuel problem?

[Blueknightrick]

Hi All

I wondered if anyone could help me with a problem on my wife's 2006 1.3 Yaris, which has got me baffled. The problem started a few months ago, she told me the car was jerky to drive when you were keeping at a constant speed (i.e. going through a 30 mph area).  It is ok when decelerating or accelerating.  

I checked for Trouble codes and there was none. I investigated and thought it was the Mass Airflow Sensor, but a new one didn't make any difference.  I then tested all the other sensors and items that the workshop manual said to check and all tested within tolerances.  We then realised that it was only bad when it has a full tank of fuel.  When it gets below two thirds full it is ok, no problems at all.

My thoughts are now that it must be delivering fuel pressure that is too high when the tank is full, causing too much fuel to be injected when it the fuel system is trying to put exactly the right amount of fuel to keep it a constant speed.  So perhaps there is a pressure regulator in the fuel pump which is in the tank, which could be a problem,  I believe that there is also a fuel pulsation damper on the end of the fuel line, could it be this?

Any help would be most appreciated as the wife doesn't like to drive the car on long journies.

Thanks

Richard

ca

[stantheman1]

I can't see it being too much fuel as the O2 sensors would have flagged a P0172 code. When were the spark plugs last changed? Is the air filter clean? What mileage?

[Blueknightrick]

The car has done a 100k Miles, spark plugs changes 5k ago and I have checked them again and ok.  Air filter just changed.

Thanks

  

[stantheman1]

Have you confirmed the fault, does it happen when you drive the car?

[Blueknightrick]

Yes, the car still has a fault but only when you fill it up with more than two thirds of a tank of petrol.  Below a half tank it drives completely normally, hence the reason I was starting to think it is a problem with fuel supply pressure

.

[Stivino]

The pressure should be the same regardless of how much fuel is in the tank. 

[flash22]

52 minutes ago, Stivino said:

The pressure should be the same regardless of how much fuel is in the tank. 

You are on the right track, The tank pressure is controlled by the evap system there is a purge solenoid and also a canister vent (part of the canister), the op can check the in tank pressure with a scan tool

the vent solenoid maybe stuck closed

[Blueknightrick]

"the vent solenoid maybe stuck closed"

Is that the purge VSV? as I have tested that and it works ok.

[flash22]

The purge is in the engine bay, also check the breather pipe from the filler to the tank

the carbon canister could also be clogged (pipe work comes from the pump to the filter then goes to the neck

 

[Toyota 86]

Did you find the problem...?