48mm Throttle body mod - first impressions mixed. (K Series)

[thompster]

I've just carried out the DVA-style K series throttle body mod - removing the cast 'wedge' from the butterfly and soldering up the holes.  First impression is a noticeable difference, perhaps too noticeable though.

Touching the throttle gives a much faster but also much harsher response e.g. driving in traffic becomes a series of clunky surges. It might be a case of relearning to drive  so I'll persevere for now but tempted to change it back.  Given this mod is said to replicate most of the benefits of a 52mm throttle body, does that have the same effect or does it deliver better overall flow without such a concentrated affect on very slight opening?

[Wrightpayne]

If youve had it all in bits have you reset the throttle pot - ignition on press throttle pedal 5 times and ign off. Recalibrate's the pot so the ecu knows the butterfly position.

Ian

 

[thompster]

I did - but only the once so might have got it wrong and will try it again.  Behavior seems reasonable given I've taken away the thing that softens throttle response but quite surprised at just how much difference it's made.  I was looking at 52mm bodies this pm but if its the same I'll go back to 48mm (That's unless another throttle postion reset improves it)

[Wrightpayne]

I fitted a new 52mm body and the difference was instantly noticeable. I used to get a slight hesitation / flat spot when I changed up a gear and the new TB fixed this. Its fine in traffic though I do have a lightened flywheel which can cause the same.

i'd persevere and just double check everything - all vacuum tubes reconnected, no air leaks, butterfly moves freely and closes fully.

i have a spare plastic 48mm TB if you want to revert.

Ian 

[oilyhands]

If the de-wedged 48mm causes an issue then it will be worse with the 52mm as this has no wedge and is a larger diameter so the progression. will be even more aggressive. I haven't experienced this before on any of the engines I have carried this mod out on, so I would be tempted to look at your TPS to make sure it is doing what you expect.

Oily

[revilla]

I had a very sharp progression from idle to drive on one of my engines with a 52mm TB. A real on/of step as you started to open the throttle that made it quite hard to drive in slow traffic. I tried adjusting the idle stop screw on the TB. This is basically what I found:

• If you close the screw too far, the idle dies away after you blip the throttle and it stumbles towards stalling before the IACV manages to catch it and restore the idle speed.
• If you open the screw too far, the idle tends to float high and the IACV cannot close enough to bring it back down under some conditions.
• There is a range in between the two where the idle behaves well. It returns smoothly to idle without floating or overshooting noticeably.
• Over this range however, the progression from the overrun to drive varies quite noticeably. If you have the screw open more than necessary, even though the idle behaves fine, you get the really sharp transition.
• I think what happens is:
  • The TPS is calibrated to recognise when the throttle is on the idle stop*. In order to get the ECU to switch from overrun strategy to drive strategy you have to open the throttle by a certain amount. This is probably equivalent to moving from Site 0 to Site 1 on a mappable ECU.
  • If the idle screw is open a bit too far, the throttle has then moved by the same amount from a more open starting point and is therefore physically further open when this transition occurs. This leads to a relatively large step in power when switching strategy.

On mine, closing the throttle stop screw a little bit made a big difference, to the point where I was able to dial out the transition step entirely after a couple of goes. Now there is absolutely no perceptible switch as you start to put your foot down from coasting, just a smooth progression to power. It makes a big difference to driveability.Two words of warning:

• It's a very sensitive adjustment. Mark the original position and count the turns so you know you can go back to where it was if goes wrong. On mine the whole usable range was well under half a turn.
• To do it properly you need to reset the TPS calibration after each adjustment before seeing what effect it has had.

* If your engine is EU3 / MEMS3, you can calibrate the ECU to recognise the limits of the TPS travel as follows: Ignition switch fully on but don't start, throttle smoothly to fully open and then back to fully closed five times, ignition off. Then ignition back on to start the engine. I'm not 100% sure that this applies to EU2 as well or if there is an alternative procedure.

Incidentally, if your car is MEMS3 and you have an OBDII reader that shows live data, you can check the TPS calibration; at idle it should show the TPS at 3.92% (10/255 so I guess it digitises it in 8 bits). As you tweak the throttle stop screw this will change, but should return to 3.92% when you recalibrate.

[Wrightpayne]

Slight thread hijack

Since I had the engine out over the winter, having had the supersport cams timed in, I've been having engine stall when warm as I pull up to a junction with the clutch depressed. TPS reset has improved the situation but not cured it.

Should I try adjusting the idle stop to increase opening?

Regards

Ian

[Sim]

Yes !

[revilla]

Sounds like it Ian. Mark it so you can go back if it doesn't help.

[thompster]

Thanks Oily and all.  Revilla's description is pretty spot on for what I'm getting. IIRC the butterfly is fully closing at idle and the stop screw stays a turn or two off the stop.  It never stalls though so I've not thought anything of it.  I'll have a play at the weekend and see if I can smooth the transition that way.  My OBD reader does live data so I should be able to see the TPS changing - neat!

[revilla]

If the throttle stop isn't sitting on the adjustment screw that doesn't sound right at all.

If the butterfly is closing fully, I don't think the IACV would normally be able to rescue the idle. Are you sure it is really closing fully? On mine I can just slip a 0.07mm feeler gauge past the butterfly at the bottom at idle ... it's a tiny opening but that's why the adjustment is so critical.

If the idle behaviour is about right with the throttle butterfly completely closed I would suspect an air leak around the inlet manifold somewhere letting air in bypassing the throttle, which might explain the driveability issue. Have a good look at all the vacuum pipes (probably just one to the fuel pressure regulator) to make sure it is in place and not cracked. Also check any unused vacuum ports to make sure they have good condition rubber blanking caps (on my engine there's one unused port on the TB and another on the back of the manifold). Then there's the rubber pipe to the IACV etc. and also probably a blanked off brake servo port on the side of the manifold.

If you hold one end of a rubber pipe near your ear and move the other end around the places on the manifold which may leak (gaskets, joints) you may hear a hissing at idle.

[thompster]

Fixed it, details below for the record.

Took the car out yesterday and had the same on/off throttle response and also a fast idle, dropping back after a while that looked like an air leak.  I'd already googled throttle-stop screw adjustment and found loads of old posts including from Oily saying that the butterfly should close completely at idle.  Apparently on an EU3 car the IACV does all the work and manually overriding it with the stop screw just confuses it. Anyway, I took the throttle body off and compared it to some pictures and found I had the butterfly the wrong way around.  There i an embossed '6' on it which I had facing out and it should have been facing in.  I removed and refitted the butterfly, let it centre itself before nipping up and lo and behold the car is driving fine again.

Footnote - seen Ian's post about stalling.  I had that same problem when I fitted and timed my Supersport cams.  IIRC I'd set them at the upper limits of Oily's figures so I reduced the exhaust cam lift slightly and the stalling went away and the idle improved.

Footnote 2 - Thanks Revilla for OBD tips.  I also get 3.9% for the TPS at idle, rising to 78% fully open. Expected to get 100% but never mind.

[revilla]

Glad you've fixed it.

Odd what you say about the throttle valve closing completely though. On my VVC engines (3 of them!), if you allow the throttle to close completely you get a tendency to stall as the idle drops really low initially before the IACV tries to catch it, and when a friend's car developed similar tendencies I remember standing on Oily's driveway discussing it with him and he advised us to open the idle stop screw a bit which should cure it. We did, and it did. When you buy a brand new 52mm TB, the idle stop screw is set to leave about 0.07mm opening below the butterfly and then sealed with locking paint and a plastic cap to prevent further adjustment.Things may be different with the 48mm TB.

I think the full range sweep of the TPS is larger than the roughly 90° swing of the throttle butterfly which I guess is why it never reads 100%.

[oilyhands]

Generally speaking you only need to crack the throttle open a bit when the idle MAP signal is disrupted, E.G. When you have uprated or retimed cams that have increased overlap, this is because the idle MAP signal doesn't settle fast enough when coming off the throttle, the ECU doesn't know that the engine should be idling so the IACV is not deployed quickly enough and the engine stumbles. Cracking the throttle openness a little provides a sufficient volume of air past the throttle plate to allow the engine to idle regardless of the IACV position. If you go too far then the engine flares whe the throttle is closed so there is a balance to be had.

Oily

[thompster]

Thanks Oily, that makes sense and explains why my throttle was set as it was.  When I reduced my SS cam  overlap i could just have given the idle some help. I'll remember that for when I next time them (and to refit the butterfly the right way round :-)