Idle control, technical help please!

[Julian Thompson]

On close inspection, I note that the input into the cylinder head from the IACV valve is actually very close to the cylinder head on the PTP throttle bodies. Forgetting transient throttle issues caused by the distance of the butterflies from the ports, with this new information I can see no reason why my car ought not run as well as any other at idle or, more specifically, when cold.

 

I would like to ask some fundamendal questions that might help me overcome my cold start problem. (It wasn't possible for Dave Walker to do the cold start bit the other day because, um, the engine was hot!)

 

1) The throttle stop screw clearly alters the "closed" position of the butterflies, and will impact the TPS reading at 0 throttle, too, thus altering the fuelling at this point as well as the airflow. I would like to know if a sound course of action might be to close this screw totally, allowing 0 air through the butterflies with the throttle shut. I could then bleed air in under control using the IACV - and maybe obtain a better idle since the air would not be subject to port resonance?

 

2) A "Choke" on an old car presumably covers the air intakes a bit in order to make the mixture effectively richer for cold start? If so, why would I need the IACV to move at all? Surely I could close the screw as above, determine the correct opening for the IACV at working temperature and then richen up the mixture at low coolant temps to create a "choke" effect - or do I need lots more air at cold start too?

 

3) So in simple terms, I know I need more fuel for cold start - so why ought I need more air because surely this will make the mixture leaner again, so then I'd need EVEN more fuel etc.....?

 

I know I'm a bit confuzzled - appreciate any help from you experts!

 

Cheers

 

Jue

 

 

 

Edited by - Julian Thompson on 15 Sep 2002 17:36:09

[Jason Plato]

Julian ,

I realy wouldnt bother with the IVAC at all - it appears to be a lot more trouble than its worth ??

The only time I have to hold the throttle on my engine is if the ambient air temp is say below about 10C , other wise it cranks with slight bit of throttle and then it settles to a controlled idle by itself once started . n the winter I have to hold the throttle for about 20seconds and then it's happy to look after itself as a smooth 1000rpm .

When warm or hot it starts without any throttle tickling . This was all achieved by playing with the Emerald setting over the course of about 6 cold starts .

 

Dave

[Julian Thompson]

So are we saying that extra air is needed at lower temperatures along with more fuel? This would account for your having to hold your foot on the pedal a bit?

[Julian Thompson]

You rascals!

 

33 reads and only 1 response!

 

Get on it!

[oilyhands]

Julian,

 

When an engine is cold, the charge burns more slowly, and often incompletely, this causes the idle to drop substantially since the effective pressure on the piston is much lower. You need more fuel to compensate for condensation on the cold surfaces and poor atomisation, you need more air to allow the idle to reach a self sustaining level. The best compromise is to allow the butterfly to pass enough air to maintain the idle when hot and allow the IACV to modulate sufficient extra air when cold. That way the air gets to go past the injector...

 

Oily

[Julian Thompson]

Thanks Dave!

 

Makes sense. So would the optimum be IACV totally shut at warm idle, air adjusted on the throttle screw, then open IACV at cold only ( I know Peter C talked about setting the IACV to a working opening then unplugging it - but how can that work if we know that an engine needs more air at cold?)

[Julian Thompson]

oooh. Forgot to mention that I had a look down the open throttles last night and actually, I'm pretty sure that the IACV air bleed holes are before the injectors. Does this alter your opinion Dave?

[Peter Carmichael]

I am with Dave J. You absolutely do not need an IACV for absolutely rock solid idling and easy starting. It is added complication and the juggling of the extra settings is likely to mean that you never get it right.

 

[oilyhands]

I should add that I'm with Dave J and Peter, my own engine doesnt need an IACV , nor have many of the conversions I've done (other than plenum based), with the right combination of advance and fuelling compensations you can set the throttle plates to give enough air when cold and then have the advance drop back when hot to slow the idle. Automatic idle control using advance should allow this. However the reasons for having an IACV are as stated above.

 

Oily

[Julian Thompson]

And for Dave and Peter's next trick......

 

We'll introduce Mr Ignition Advance

 

Doh!

 

I seeeeeeee. So - I bung up the IACV holes, wind on some throttle screw to open the butterflies a touch to give good warm idle running and use more advance when cold to speed the idle up, yes?

 

(FWIW, PTP said today that they recommend totally closing the butterflies and using IACV for ALL idle air. Interestingly, too, I plugged the MEMS in - which was always awesome on cold start - but which in fairness would absolutely NOT idle, ever , and that immediately motored the IACV out to a massive air bleed. I had to crank the Emerald IACV figure to 100 - max - to get anywhere near the level of opening.)