PWM for car choke?

[Dan2]

hey guys, i have an automatic choke on my car (bimetallic spring) and if i set it so the choke works - it never kicks out completely (ie fast idle is always on)

so i got a generic choke to replace the old one. only problem is that the generic choke kicks back in shortly after i turn the car off - which makes the car rev like mad and makes me look like an idiot at the traffic lights!!!

so this is my plan - i want to hook up a 555 timer to the bimetallic spring so it keeps voltage on it for a while after i turn the car off, so then the choke should only come on when the motor gets cold again.

i am also thinking of pulsing 12V onto the spring (say 50% duty cycle to put the choke "half on"). How do you think the spring will react??

[john blackburn]

An easier solution as used by the manufactures would be to make yourself a water jacket for the choke using the heat in the engine coolant to keep fast idle under control.

Maybe copper pipe?

John

[Dan2]

i cant see that as being an easier solution.

the problem is that the choke works on the ignition switch - when the ignition is on, 12volts is suplied to the choke and the coil unwinds (therefore slowly releasing the choke)

by the time i get to the shop, the choke has just released -> i turn the car off -> there's no power to the spring -> it coils up again just enough to kick the fast idle back in-> and the car revvs like mad 'cos the motor is still warm.

so i was thinking of using a timer that says on for about 10 mins after the car is turned off, so then when i do leave it for a few hours, the choke will kick in as normal. the resistance of the spring is pretty high so im sure i could just use a small transister to do the swithing. i would also need to make it so the timer switches on only when the ignition is off.

i just wonder if i could pulse 12v onto the spring to make it move back a little bit (if i need to)

[john blackburn]

The problem is that the bi metalic strip cools faster than the engine on very short trips so by using the coolant jacket heat is held around the choke unit thus keeping the mechanism warm. It may be worth checking the adjustment of the choke unit also. Loosen the choke unit and turn it slightly (only a few degrees) in the "off" direction to put a bit of tension in the mechanism.

I cant offer any suggestion about the 555 but if the bi metalic is in good order it shouldn't need any power on it after key off.

John

[Dan2]

so i put the old (original) choke back and set it so its works nicely, and found out that it is only short trips that i have this problem..... but then yesterday it was an unusually warm day ( 'cos we're going into winter now) and the choke didn't kick out at all - the motor was idling at almost 3000rpm. this makes no sense to me cos the spring (when warm) should move way past the point where the choke kicks out (about 30 degrees more) so a few degrees in the ambient temperature should have no effect to the turning off of the choke.

i think when i get some time i am just gonna put the generic spring in and make a timer that keeps 12v on it for about 10mins after the car is turned off and not worry about pulsing it..... and pray it works cos its giving me tons of grief

[john blackburn]

What make of car is it you are working on?

Is it a twin choke carburettor (twin venturi/barrel) It's possible that the second throttle butterfly isn't closing fully to its stop. They are normally, but not always vacuum operated so if the linkage dries out and becomes stiff the return spring wont fully close it.

Opel cars were bad for it when using the Pierburg carb. The second throttle butterfly had a plastic ball joint connecting it to the operating vacuum unit which if left unlubricated by bad maintainance would become sticky when hot.

It would drive fine until you drove it hard and opened the second throttle then the butterfly wouldn't fully close again when the vacuum dropped.

The air passing through the second throttle draws fuel in with it giving a high idle speed even with the first butterfly at the idle position. It also gives an unadjustable CO% though I guess you dont have access to a CO meter.

Try lubricating all the linkages and working them back and forth a bit and retry it..

Also start the engine and lightly spray WD40 or similar around the bottom of the carb area to check for air leaks. If you have a leak anywhere the solvent will be pulled in and cause your engine RPM to increase.

Give it a go and let me know how you get on.

John

[Dan2]

thanks John for the advice.

the other day i took the bi metallic spring off completely and ran the car. the butterfly drops down (fully open) very easily and i checked to see if it was sticking and it wasn't. the fast idle also disengages as soon as you tap the accelerator - so its not sticking either. i am pretty sure its the spring giving me grief.

right now i have the generic one hooked up to a power supply in my room. at 10v it draws 600mA and as it warms up the current drops a little (after about 10 mins now its reading 520mA) so if i use a small signal transistor (2n2222) as a switch it should be fine, right?

this spring has moved about 90 degrees in 10 mins, and seems to stop there. i know it only has to move about 45 degrees to turn the choke from fully on to fully off. the problem with this one is it moves back very quickly, i am going to test it now to see exactly how long it takes to return to its "cold" position. then i will build a timer to (kind of) compensate for this.


i drive a 96 nissan sentra, and spoke to 2 people with the same model and they have never had a problem with their choke. i guess i got a dud


oh yes there is one more thing - a vacuum on the carb pulls the choke butterfly open when i start the car (so even if i set the butterfly completely closed, as soon as i start the car it is half open) but the vacuum diaphram cant be broken or else it wouldn't pull - it just doesn't make sense that it opens the butterfly when the choke should be on.

[Dan2]

well its been 20 mins and the spring is almost back to its original position - thats much too fast. the engine would still be nice and toasty after 20 mins.

i have a spare pic lying around, maybe i will use that to do all the timing.

[john blackburn]

Hi Dan2.

I've been to see my Japanese car guru and he says..........

Check around your carb area on the inlet manifold. You are looking for two narrow coolant pipes that take hot coolant to the manifold and back away. They are rubber hoses that go onto metal stubs and they block up with rust and sediment. The effect is that the area which should be at coolant temperature to keep the heat into the bimetal is cold so it gives the problem you describe of re applying the choke too soon.

If you check them and they are as clean as a whistle with crystal clear coolant flowing freely from them then all is not lost. In that case the car needs a new thermostat as it has stuck open giving the effect of over cooling the engine and once again lowering the temperature of the heat mask water jacket area. I wouldn't have thought over cooling too likely in your climate, not like in the UK we have 47 words for rain here.

Hope this helps matey, let us know how you get on with it.

John

[john blackburn]

PS....

Any problems take a photo and we will see what's what.

John