8 replies to this topic

[podmak]

So I am approaching a red light, and I down shift, and down shif, and down shift to a really low speed.

 

When the car is in 1st gear, doing about 10km/h, and then I depress the throttle pedal, just a tiny bit to feed the engne a little bit, and let go of the throttle immediately, the car starts to "jump" forward in short weak bursts - when my foot is off the pedal. As if the fuel delivery system that's supposed to prevent engine from stalling was not delivering fuel properly.

 

And it keeps doing that until I accelerate normally.

 

There's no other problem, it starts normally (even the cold start in the morning is fine) and drives fine. No random rpm bursts or fluctuation when idle.

 

I hope this makes sense;

 

Can anyone suggest what the problem might be?

 

I've had the fuel pump changed recently, just the fuel pump, nothing else. And I don't remember if this was happening before. I've had problems starting her up in the morning, and replacing the fuel pump got rid of that.

[Robert]

They all seem to do it a bit

Group n engine and gearbox mounts help a bit
Check your diff mounts also

Unless it's just begun and was fine before
Maybe the ecu has leaned some bad habits in that case do an ecu reset

[duncanm]

yeh -- happens to all of us. Some drive train lash from worn bits, an old dirty IACV and whatever else leads to an unstable feedback loop where the ECU is tweaking the IACV to maintain your idle speed.

 

Have we all forgotten bunny hopping? 

[podmak]

Fair enough, thanks guys

[Robbks]

Also remember that when decelerating with your foot off the throttle the ECU turns OFF the fuel injectors if the rpm's are above a certain RPM (somewhere between 1100 and 1500rpm depending on model)
once the rev's drop below that, the injectors begin to cycle again.
IACV cleanliness and adjustment is critical to having this system work smoothly.
but if you happen to touch the throttle right at the point the injectors come back on, it gets a big dose of fuel and will lurch forward.

It's an easy test to see when they come back on.

drive in 1st on the flat, up to about 2500rpm and let your foot off.
let it roll along by itself and you'll feel the car take off at the point the injectors start cycling.

Given you haven't told us any vehicle details at all,
i'll take a blind stab in the dark that it's got the MY99 WRX Style "plunger" IACV mounted to the throttle body
remove the stepper motor and clean the plunger's knob, shaft and the housing thoroughly with carbie cleaner,

 

Replace, reset the ECU and see how it goes. It will take a while for the idle to settle as it learns the airflow characteristics again.

 

 

 

[podmak]

Also remember ...
 

 

Thank you very, very much.(!!!)

[Robbks]

Fixed..?
better?

[podmak]

Fixed..?
better?

 

We'll see. I'll post here once I clean it. Still didn't have a chance... I just wanted to appreciate your input, because I find it very constructive and educative. So thank you for that.

[podmak]

Ehm, with all this time passed I still didn't do anything about it;

Then why post you ask? (And honestly, that's a good question)