I would test the TPS for a dead spot/bad connection. Finding a dead spot with a DMM is very difficult as they don't react very fast and you could pass it. Use an analog needle volt meter or better an oscilloscope.

I've had bucking/jerking from a bad ignition coil, but yours is at a very specific speed/throttle.

 

I don't have access to an oscilloscope, only a DMM, and couldn't find anything obviously wrong with the TPS so I bit the bullet and bought a new TPS, alas the problem is still there
Weirdly the check engine light came on for about 20 minutes while driving to work. However, no codes are reported when I plug in the OBD reader......

 

Weirdly the check engine light came on for about 20 minutes while driving to work. However, no codes are reported when I plug in the OBD reader......

Does your reader also do "Pending Codes"?

 

Is it an auto? Does the issue disappear when you manually select gears? Or maybe try putting overdrive off as a possible easy fix. I would also check the throttle and kickdown cable to see if they are loose/within spec. Other common issues are the ignition wires being due for replacement or oversensitive knock sensors. Good to also clean pcv and IACV valves.

 

Yes, it's an auto. Tried the overdrive off & manual gear shift but no effect. Throttle and kickdown cables are all good. New PCV valve fitted. (was the first thing I tried) and I've cleaned the IACV and EGR Valve. I wonder if its the plug wires, they could be the originals! who knows! They are branded toyota ............

 

If you have no codes or pending codes and the plug wires test OK I would double check the accelerator cables because there are a lot of points they can stick and cause odd idling/light acceleration.

Inside the drivers footwell make sure the spring bracket is tightened down and the cable is not loose and not stuck when you let off the accelerator, also lube the points where the cable travels through any guides, just don't get lubricant on the brake pedal cause it will drip if you spray too much lol, a rag is a good idea.

I'd also take out the wires out from the pulleys inside the black cover by the battery/vacuum tank and then clean the pulleys with a plastic bristled brush and rag, then lubricate with teflon spray and a rag. Same deal with the throttle body linkages, any tiny grit inside the pulleys OR springs can cause the throttle to idle weird when fully released because it doesn't take much to affect a few 100 rpm in travel, a piece of sand stuck in the spring/cable guide/pulley can cause unstable idling/light acceleration.

Adjust the throttle body linkage from the accelerator linkage so the throttle is 100% open with the cable semi taunt but not tight when the throttle pulley is fully opened, but not loose enough you can lift it fully out of the throttle body cable guide when the accelerator linkage is completely released, then put the throttle cable back in the pulleys and adjust the accelerator cable linkage inside that black plastic cover so the throttle cable stays at the same semi taunt state through the full pedal travel without ever being so loose that the accelerator pedal doesn't put tension on the cable, then adjust the throttle cable at the throttle body so the idle is 650rpm +100/-50 when the engine is up to temp and the adjusting nuts are all tightened.

When you're really light on the throttle the cable doesn't have very much tension in it and so any of 4-5 potential sticking points will cause it to hesitate until you apply more throttle to move the cable past the sticking points, and then it jumps past the point it was sticking or where the pulleys were sticking and will cause bucking/jerking because then it can start engine braking sometimes if you're accelerating really lightly like in traffic from a stop. ; which is exacerbated if you have worn engine mounts Cleaning and lubricating helps a lot.

You can watch this just by using your fingers to move the accelerator cable pulley back and forth slowly and see if the throttle cable sticks in the throttle pulley and/or cable guide or if the throttle cable moves/doesn't move in a way that doesn't correspond to the accelerator cable movements.

 

Many thanks plenty for me to check.

 

Oh also adjust the idle screw that sits facing downwards and stops the throttle opener pulley from closing the butterfly all the way; you need a 8mm wrench or a 8mm socket works as well if you have a ratchet that fits; This will prevent the throttle from ever closing more than this screw allows. Use penetrating oil first, and it might still break.

You can also adjust the little 8mm screw on the rear side of the throttle body that is visible when the throttle is open all the way using the pulley and your finger; Im not certain what the function is but it seems like it does the same thing by allowing a hard lower limit to be set for your idle; if the slider track is gummed up it can also cause it to stick before reaching the stopper and cause a weird idle; also if the slider is bent it will stick in the track.

There is a ton of little mechanical things that it could be; best to check everything.

 

Light bucking is how I’d describe a misfire. That’s probably what the codes were. First thing I’d do is replace the plug wires, because of their age. Get NGK wires from a reputable source. Not eBay or Amazon. Too many counterfeits there.

 

I found the problem, the EGR valve. I pulled the vacuum line off and blocked it off. Car idles perfectly and all hesitation, bucking, etc have disappeared. I removed and cleaned it but that has not helped so I think it's the diaphragm that it shot. Crazy expensive for a new EGR from Toyota here in the UK £300+ !!!! I will order one from RockAuto, even with the shipping to the UK it will be much much cheaper. Weirdly, no check engine light with the vacuum line disconnected and plugged. Shame I can't just run the car without it.............but I suspect it won't pass the annual emissions test.