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RB20 light throttle problems. Hunting/jerky.

rayj

By rayj,
in Engines & Forced Induction

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rayj Proficient

rayj

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Posted (edited)

I have this really annoying problem with my auto R32 Gts-t. It only happens when switching from light throttle to no throttle at low speeds. For example, driving in traffic or in a shopping center car park (just tapping the accelerator). As soon as I release throttle, it starts hunting and hesitating, gets all jerky, then snaps back into normal idling.

It only happens at light throttle, any ideas?

I've check the TPS. I'm getting proper voltage readings, approx. 0.5v closed and 4.0v WOT, and switches between throttle open and closed properly... Is it possible the TPS is at fault even if it's giving proper readings?

What else could cause this issue?

Thanks heaps.

Edited by rayj
R_34 Community Regular

R_34

Posted
Posted

I would have pointed the TPS but you checked it already. 

Isn't the tps for auto different than the manual one ? I think it should have an idle and WOT contact on the TPS as well.

Otherwise dirty maf sensor maybe.

GTSBoy Grand Master

GTSBoy

Posted
Posted

The variable voltage output on the RB20 TPS is only used by the transmission's CU.  The ECU knows nothing about it.  The ECU looks only at the closed throttle position switch.  The very best way to make sure that an RB20 has the TPS adjusted correctly is to use your hand on the bellcrank* to just crack the throttle open the tiniest amount.  What you want to happen is the switch to change state at almost no movement in the throttle at all.  You will hear the injectors double fire (which is what the RB20 ECU does when it sees the throttle switch change at idle).  Basically the sound of the injectors clicking just speeds up to double rate for something like 2 rotations of the engine (ie <1/2 a second).

The single most likely thing that is wrong with your engine is that the throttle body and/or the idle control valve are full of grunge and need a clean. The idle control valve being the most likely.  What is happening is that when the IACV is dirty the stepper motor on it struggles to make it move fast enough to change position.  When you are running with the throttle open, the IACV is too closed to run the engine at the right speed.  When you then close  the throttle the revs drop low while the IACV tries to open up.

You need to dismantle it and soak it in carby cleaner, give everything a good scrub and put it back together/on.

rayj Proficient

rayj

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  • Author
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Thanks for your help. The Throttle body was taken off the car, cleaned thoroughly, then mounted back on with new gasket and torqued to spec. Much the same the entire AAC unit, completely soaked and cleaned thoroughly.

The problem still exists

rayj Proficient

rayj

Posted
  • Author
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I've checked for vacuum leaks, cleaned the AFM and tried resetting the TPS again. Same problem continues...

What I've noticed with the TPS, if I test it with Consult without the engine running, everything looks good. Closed throttle is approx. 0.5v and throttle switch is closed. As soon as throttle is opened, throttle switch changes to open, and throttle position will gradually rise to approx. 4.0v WOT.

However, as soon as I start the car, the throttle switch will immediately move from closed to open even though the throttle hasn't been touched (it's closed).

So then I adjust the TPS accordingly while the car is running, so that throttle switch open/closed occurs at the right time, all looks good, but the problem still persists.

This doesn't seem right, the output should be the same no matter if in the ON position or engine running. Why is the closed throttle value changing between ON and engine running?

GTSBoy Grand Master

GTSBoy

Posted
Posted

The throttle will either be rotating a little because of air flowing across it, or possibly there is bearing slop and it is just moving in some other direction under the force of the flowing air which turns up as a change in the TPS.  Ultimately it doesn't matter what the TPS says when the engine is off.

Missileman Veteran

Missileman

Posted
Posted

to get the correct contact position for the throttle...

warm up engine, disconnect AAC valve connector, rev engine to 2000 then slowly ease off the peddle until the idle switch light illuminates to indicate throttle closed position, taking note of the revs the exact moment the closed position light comes on, this should be 950RPM +- 150RPM if it is not then adjust to suite. This why the tps voltage specifies 0.4 to 0.5 Volts.

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