Missing under load

[Murdoch]

Have a problem i don't know what is causing it.
My car seems to miss or hesitate sometimes when under load at low revs?
It seems to happen consistently if i am in 5th gear going down hill say on the freeway, where you just cruise down with no accelartion. When i get to the bottom doing about 80 and keep it in 5th and accelate, it "misses", until th erevs build up again.

I have change:
plugs
leads
cleaned TB
new fuel filter
cleaned Air filter
cleaned MAF.
New CAS

Does anyone know if these syetems could be a buggered fuel pump? The car has 170k on the clock and i don't think it has ever been changed. I don't know what else it could be, its giving me the shits.
The car performs well otherwise.

Any help appreciated.

 

[Murdoch]

c'mon someone must know!! ****!

 

[fitzy2005]

holden did a diagnostic test (computer check) and also check my fuel pressure and reset base idle and did somethign to my iacv (they reckon it was locked and wasnt moving freely) all for 44 bux

 

[kazz]

Get your coil pack tested at an auto elec's and it could also be your fuel pump. Have had a lot of commodore fuel pumps replaced in the last 5 months, in the workshop, so 2 things to look at. Hope it helps

 

[Centurion]

Take your car to anywhere that has an injectorcleaning machine, get them cleaned and it will make it feel like a new car again.

 

[fitzy2005]

an auto electrician will charge heaps, take to holden and tell them what you told us, then the will diagnose it for 44 dollars, if no one else here can suggest anything.
Holden fixed my rough idle/stalling problem that ive been trying to fix for months now and it was done in a few hours and only cost 44 bux.

 

[Murdoch]

Thanks for the options guys. It is quite annoying at times.
It feels like more of a fuel problem to me because it only misses/ heitates once then runs fine. I thought maybe the fuel pressure is not high enough just for a short period of time when under half to full acceleration.
It doesn't happen under light acceleration.
.

 

[Don Sutton]

Thanks for the options guys. It is quite annoying at times.
It feels like more of a fuel problem to me because it only misses/ heitates once then runs fine. I thought maybe the fuel pressure is not high enough just for a short period of time when under half to full acceleration.
It doesn't happen under light acceleration.
.

If it hesitates or misses once it can be something sticking or after having deaccelerated for a long period such as when going down a hill something could have fouled or a sensor is missreading the dynamic change. Things to consider:
1] The fuel pump - This would only cause a problem under prolonged loading ie when it has to deliver a lot of fuel for a long period.
2] Bare in mind that when you increase the throttle opening that not only does the injectors have to deliver more fuel but the ignition system has to deliver a higher voltage (more with a full cylinder charge).

My bet would be, given that you have replaced the spark plugs and leads, is the coil. As soon as you open the throttle the cylinders will be full requiring the highest voltage. As the engine revs climb the cylinders do not fully fill and volumetric efficiency falls off with higher rpm so the coil does not have to produce as high a voltage.

 

[Murdoch]

If it hesitates or misses once it can be something sticking or after having deaccelerated for a long period such as when going down a hill something could have fouled or a sensor is missreading the dynamic change. Things to consider:
1] The fuel pump - This would only cause a problem under prolonged loading ie when it has to deliver a lot of fuel for a long period.
2] Bare in mind that when you increase the throttle opening that not only does the injectors have to deliver more fuel but the ignition system has to deliver a higher voltage (more with a full cylinder charge).

My bet would be, given that you have replaced the spark plugs and leads, is the coil. As soon as you open the throttle the cylinders will be full requiring the highest voltage. As the engine revs climb the cylinders do not fully fill and volumetric efficiency falls off with higher rpm so the coil does not have to produce as high a voltage.

Thanks for your thoughts mate, makes sense.
I have changed plugs and leads not long ago.
I also swapped my coil with my brothers car and it still does it. Mine in his and his runs fine. Maybe my injectors could be playing up? I have flushed them recently with fuel addative but i don't know how affective this stuff really is.
I might have to just send it to holden for a check i think, but i hate doing this from bad past experiences where they have "fixed" it but when i got the car back , it was still there.

 

[Don Sutton]

My last paragraph should have indicated that the injectors follow the same pattern as the ignition voltage requirement.
As the engine revs rise the injection quantity per injection will fall off as does ignition voltage. The amount of fuel per hour will increase with engine revs but only because there are more injections per minute. Fuel quantity per injection bares a direct relationship to the volumetric efficiency disregarding engine temperature cold enrichment.