RPM drop during slight acceleration

[Kurtly91]

Hi
I have a vy series 2 2004 auto Commodore; when I'm up to speed around 70 -80 km just cruising then go to ease the accelerator I notice the RPM starts to increase but then immediately drops down then increases again ?

I've had the transmission changed , throttle body cleaned , 2 bottles of injector cleaner .

Anyone got any ideas?

 

[Skylarking]

Don’t know if the idle speed control mechanism is in use at low throttle openings while moving. If so, maybe you have a sticky IAC valve?

Why was the transmission changed and did this RPM issue on light throttle exist before that work was done?

 

[Kurtly91]

The transmission was changed to try to fix the RPM drop and a bit if a thud that went through the car when accelerating , but the transmission replacement never fixed the RPM drop , if i accelerate harder there's no RPM drop ..

To be clear as possible when easing the accelerator the RPM climbs a little then pretty well immediately drops then accelerates as normal (I do feel it through the car slightly) just not as much as before i got the transmission swapped .

 

[Kurtly91]

IAC valve is that the same as the throttle body ? If so that was something I forgot to mention .. I've had the throttle body cleaned

 

[Skylarking]

I believe the throttle body contains the TPS and IAC. Cleaning a throttle body usually entails getting the gunk off the main air passage and off throttle plate itself.

If the throttle body has lots of gunk in it, cleaning can cause idle issues as the ECU has slowly learnt to cope with the clagged intake. Cleaning can upset this stored steering system and the engine doesn’t run sweet. There are relearn procedures that aid the ECU to reset its base but it doesn’t sound like that is your problem.

So how thoroughly the throttle body was cleaned and whether the IAC and MAF were cleaned and whether an IAC relearn procedure was done is a question to ask of the person who cleaned it. And supposedly you shouldn’t use carbi cleaner containing MEK as they can damage the MAF. There also are specific MAF cleaners to avoid such potential damage.

 

[Trevor loves holden.]

Have any fault codes been logged? When the body throttle was cleaned was the IAC removed and cleaned? Is there any vacuum leaks after the MAF that will change the air mixture settings? A easy quick test to see if your iac is responding is when you go from P to R your rpm should jump up to 1100-1200 rpm's then back to around 800 rpm's, if its lagging then you should replace it. Even your TPS could be not sending your computer the correct voltage position change some times that can be fixed by cleaning the harness and making sure its fitted snug on the connecting pins. Your computer uses your O2 sensors, MAF, TPS, to adjust the setting to fuel and air ratio and adjusting your IAC to allow the air it needs at low revs to keep a steady rpm. Well I think that's right.

 

[Lex]

What motor are we talking about v8 v6?

 

[Trevor loves holden.]

 

[Skylarking]

Another question, when the auto was changed, did they also change the torque converter or reuse the original?

I ask only since I’m not sure whether it’s possible for the torque converter to play funny buggers with engine load (thus rpm) when you ease off the throttle. Maybe something to consider.

 

[Trevor loves holden.]

I also read somewhere about MAF problems having a effect on transmission altering the shift pattern, don't no if it applies to commodores or not.