Throttle hang problems

Thought I’d post something I discovered whist looking for the throttle hang on our Yaris which is worse than any other drive-by-wire throttle I have come across.  It is a normal feature on most petrol engined cars these days and you can normally drive around it.  You can’t eliminate it completely because it is programmed into the ECU.  I've noticed that some people barely notice but others (sometimes pretty experienced drivers like me) find it ruins the whole pleasure of driving.  I think this because it is worse on some cars than others and I think I have discovered why!  If you are not interested in the theory, just scroll down to the last bit!

 

We already know that the ECU keeps the revs up slightly as you lift off, this is not really a problem unless you are lifting off for a gear change.  The ECU ‘knows’ as soon as you press the clutch.  There are two switches operated by the clutch pedal.  The lower one stops the engine starting unless you have the clutch pedal fully down, the upper switch (and the critical one in this instance) is located on the clutch pedal box so that as soon the clutch pedal is touched the ECU ‘knows’ and changes the characteristic of the throttle perhaps allowing a fractionally quicker release of the revs.

 

I took the switch off to see if any kind of adjustment was possible but it’s a fixed thing on the Yaris.  It appears to be a straightforward normally-closed plunger type switch.  It was at this point that I noticed that the pedal had slightly more slack on it than normal.  It looks like the cause of this is that the clutch master cylinder rod and yoke is plastic but the clevis pin that connects this yoke to the clutch pedal is metal.  I think some wear can occur between the two leaving enough slack to delay very slightly the release of the clutch switch plunger.  I added two small packing washers under the switch and it completely transformed the way the car drove.  This is only really a good workaround, the real cure would be a new master cylinder and pin and possibly in a bad case, the pedal itself.

 

I should mention that the clutch switch has a locating peg so you should try the washers on the switch first (as in the pic) to make sure that the they are not to large enough to obstruct the peg and also (depending on how many washers your car needs) that they don’t stop the peg actually engaging in its corresponding hole in the pedal box.  If the washers are thicker than the length of the peg, you will have to turn the switch to a reasonable position close to where the peg would have located.  Make sure the switch wires are not under any stress.

 

In order to try this simple 15 min modification you will need to first remove the small plastic panel below the pedals.  This is just two Phillips screws and a squeeze of a small plastic clip, the panel then unhooks from the front.  The clutch switch is held on by a single 14mm nut.  You can leave the wires connected.  For the amount of wear on my (24,000 miles) car I found, I used two washers amounting to 2.5mm in thickness. 

 

It is now easy to drive around the very slight throttle hang which is probably now back to what it was when the car was new.

 

In effect, I have simply moved the switch very slightly so that clutch pedal switch can respond as originally intended when the clutch is pressed. 

 

During experimenting with the switch I discovered that it is normally-closed, so the circuit is broken by pressing the clutch.  I got some odd results by removing the switch and driving with switch plunger out (as if the clutch pedal was down all the time) and also driving with the plunger pressed in and taped.  I won’t bother to go into why this might be but I think the timing of the switch is more crucial than it seems, possibly including a calculation involving the moment of the release of the throttle.  Anyway, my fairly crude ‘washer method’ transformed it.  All at your own risk of course!

 

 

So to sum it up: 

 

1.  Take the lower panel off.

 

2.  Lightly press the pedal slack up and down whist watching the clutch master cylinder-to-pedal connection.

 

3.  If slack is present, remove the clutch switch and add a washer or perhaps two underneath it.  (You will have to experiment with what works best on yours)  In order to replace the switch you will need to press the clutch pedal down.  You could do with three hands at this point!  It is possible to hold the pedal down with one hand and hold the switch and get the nut started with the other but it does take good coordination.  If you decide to use a stick or any other device to hold the pedal down be very careful that it can’t slip!

 

4.  Enjoy a completely different drive!