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LPG tuning and repair help needed (pics and videos) 

 

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Posted: Tue May 16, 2006 8:37 am 
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rushed wrote:
So what do you think finaly fixed your problem.
As i have the same if not very similer problem, aka very rich and very low economy, on gas.


bozz wrote:
it seems the problem is cured for now because of the weakened spring in the mixer.


The mixer seems to control mixtures by allowing lpg pressure to push against a plunger that opens a diaphram allowing air through. The more air that the engine wants means the more gas that can flow which means the more the plunger pushes the diaphram out, allowing more air through.

On the rear of the diaphram there is a spring that pushes the diaphram closed. I believe this spring controls mixtures based on a fixed gas pressure entering the mixer. I suspect something has gone wrong with the converter to cause excess gas pressure. So I weakened the spring (flattened a turn in the middle) to reduce the pressure on the diaphram which means it will open easier, allowing more air through for the amount of gas that is entering.

All of this could be wrong on how it worked for me but like i said above, the symptom is cured however I dont think I found the problem.
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Posted: Tue May 16, 2006 9:31 pm 
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I don't know if this is for all diaphragm mixers, but the Impco ones work on vacuum.

Does your mixer (on the valve attached to the diaphragm) have little holes that allow air to enter and escape the spring side of the diaphragm?

If it does, thats the passage that "senses" how much vacuum is in the manifold.

The more vacuum (or the more air that is removed from the holes and the otherwise sealed diaghragm), the volume inside the spring-side decreases, opening that valve, to proportionally let in vapor.

 

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LSD High, 3.23, Black wheels, other shiny bits.

EB II Sedan <--- R.I.P. (Rest in pieces - gone to the wreckers for $50...... bargain!)

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Posted: Tue May 16, 2006 9:49 pm 
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Im not sure I understand that theory - It does have 4 tiny holes, perhaps under 1mm in size on the base of the diaphram but I thought they were there so they were there to allow the diaphram to move so there wasn't a compression lock behind it. I spent ages looking at it and I couldn't figure out how vacuum caused the diaphram to open....
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Posted: Wed May 17, 2006 9:08 am 
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If you were to apply a vacuum to all 4 of those holes, the trapped air inside gets removed, the diaghragm then collapses, and pulls on the valve to open it. If you were to drill a hole on the other side of the base metal, the diaghragm wouldn't move.

 

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EF II Sedan
Ex Chaser, Dual Fuel (Impco LPG)
LSD High, 3.23, Black wheels, other shiny bits.

EB II Sedan <--- R.I.P. (Rest in pieces - gone to the wreckers for $50...... bargain!)

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Posted: Wed May 17, 2006 10:02 am 
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Bozz at light throttle what ia the cruise air/fuel ratio mixturereading -

at 60km/h pulling say - 16 inHg vacumm and
at 100km/h pulling say -16 inHG vacumm .


Bozz at wide open throttle -

1. 2000rpm ? what is the air/fuel ratio?
2. 3000rpm ? what is the air/fuel ratio?
3. 4000rpm ? what is the air/fuel ratio?

if u don't know the readings i suggest see ur closest dyno shop and this will confirm whether ur car is running extremely rich on LPgas and while its on the chassis dyno have the high speed gas adjustment made.

for light throttle cruise a LPgas air/fuel ratio of 15.4:1 is ideal and at wide open throttle at 4500 rpm a ratio of about 13.0:1 mixture strength would be fine but when testing at these high engine speed the dyno operator should be able to adjust the best reading for ur car where it makes max horsepower at the leanest setting without deternation.

have fun

cheers

 

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Posted: Wed May 17, 2006 7:47 pm 
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FPV_GTp -

I have no idea what the stoich ratio of LPG to air is meant to be or if its the same as petrol at 14.7:1 but would I be correct in saying the tune is fine when the o2 sensor is happily going from 0-1v while the duty cycle of the mixture valve is around 50%, going from 40% to 60% extremes while revving it from 2000-4000RPM at no load?

The economy has DRAMATICALLY improved, I got just over 110km by the time the first LED (out of 4) on the fuel gauge extinguished, I was getting considerably less than 80km previously. I'm up to 200km on the tank and it still hasn't extinguished the second LED, by now it used to be indicating half a tank. I'll know more in a few days and I cbf driving to the city to use a mates dyno just to prove that everything is right under load if the KM travelled return to what they used to be.
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Posted: Wed May 17, 2006 11:04 pm 
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Just curious - whats the resolution of your duty cycle meter?

If the solenoid valve is only changing +/- 10% from 50%, thats great, but it could also mean your meter is not capable of measuring duty cycle that precise, and the gas computer is really holding the solenoid at 50% due to a possibly faulty EGO sensor?

What does the meter say when the car is started from absolute cold? The ego shouldn't be hot, and the gas computer should sense this, and be holding the duty cycle at 50% till it warms up and sees cross counts.

 

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EF II Sedan
Ex Chaser, Dual Fuel (Impco LPG)
LSD High, 3.23, Black wheels, other shiny bits.

EB II Sedan <--- R.I.P. (Rest in pieces - gone to the wreckers for $50...... bargain!)

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Posted: Wed May 17, 2006 11:56 pm 
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My duty cycle meter is a dual trace CRO and the resolution is zero hertz to 40MHz :)

Check the petrol video on the first post, it shows the oxy sensor working perfectly - after I modified the mixer, the oxy sensor is now responding identically on LPG as it does on petrol and the signal on the solenoid is directly responding to the EGO input. When the oxy sensor changes state, you see visually see the duty cycle change direction and start building/decaying. I'm confident the system is working correctly under no load free revving, it's operation is exactly what a closed loop petrol system does, which I am very familiar with.

Once I get off my a*** and put an LED on the oxy sensor (driven by a mosfet so it doesn't load the high impedance output of the oxy sensor) and one on the mixture valve, then I'll be sure its working correctly.

Or I'll just do nothing if I get good economy out of this tank of LPG, until something else goes wrong.
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Posted: Thu May 18, 2006 12:00 am 
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Sounds like you've nailed it then :)

I wish I could be bothered to remove my air filter and change it - thats probably all thats wrong with mine! heh

 

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EF II Sedan
Ex Chaser, Dual Fuel (Impco LPG)
LSD High, 3.23, Black wheels, other shiny bits.

EB II Sedan <--- R.I.P. (Rest in pieces - gone to the wreckers for $50...... bargain!)

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Posted: Thu May 18, 2006 12:04 am 
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Bozz wrote:
I have no idea what the stoich ratio of LPG to air is meant to be


15.5-15.7

 

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Posted: Thu May 18, 2006 8:35 am 
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Thanks for all the help!

Cheers
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