Classic Audi Community General Chat » Air fuel Ratio

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Old 13-12-20, 11:35 AM   #1
Colin Aitchison
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Default Air fuel Ratio

Its been a while since my last post and as you can see from the title this could be a big can of worms.
After our run to Oban via Glen Coe I noticed at the next cold start the Ur quattro was idling a little rough in the warm up phase, it did start normally and reversing out the garage was as normal as it could get but when I was locking the garage doors and the car was idling behind me this rough cycling idle started, so went for a drive, the car was not happy, after a couple of miles I had to return home, yes the car was idling better but I could still hear this rough cycling idle in the background noise.
On further inspection found cam box oil leak, and two exhaust manifold nuts missing that turned out to be three, anyway after removing the fuel injection system and inlet manifold to get access to the exhaust manifold and refitting thought my problems were over hahaha, no idle still the same, after a month or so of swapping bit from the other Ur and installing an air fuel ratio gauge and lambda probe in exhaust the light at the end of the tunnel is getting brighter, the original warm up regulator (WUR) starts the engine and runs as it should but as it turns out not very well, my spare WUR is so much better smoother running and much more low down torque, cold engine fuel pressure 1.2bar, hot 3.6bar. I did fit a new set of 5 fuel injectors and refitted my original WUR but found running back to what it was. The aux. air valve did close in warm up phase but did not stop any air flow into the inlet manifold, after getting it open with removing the 4 alloy rivet tops with a wood chisels and breaking off the crimped edge on the lower side found all looked ok and rebuilt with new rubber O ring and some grease, getting it resealed was tricky, liquid slilcon gasket seal between the lower and upper half's and drilled the 3mm alloy rivet legs with a 1.6mm drill and tapped out to M2 thread and secured with M2 x 8mm long machine screws, the rebuilt valve when closed now stops more than 95% of the air flow between open and closed, I don't know how efficient a new valve would be but I can now adjust the idle speed of the engine with the idle bleed screw at the front of the inlet manifold.
The chart below I have made up after many hours internet research, in one chart it combines Lambda, air fuel ratio and Co, while I have tried to make it 100% accurate I think it should only be used as a guideline.
As you can see, Lambda on the left, air fuel ratio across the bottom and Co on the right, the green line is Co, Air fuel ratio (AFR) of 14.7 is where you want to be and from my experience over the pasted month good luck getting their, Ur quattro now idles at 0.95 to 0.97 lambda, cruising at 50-60mph lambda 1.05 to 1.09, on full boosted foot on the floor 0.82 or a little lower sometimes, the cruising lambda would seem on first glance to be a little lean but with more internet research found this to be the kind of norm.
Anyway now moving onward and upwards and in a better place with the car, these old cars are getting on in years now and every component in the engine bay will have some sort of ware on it.
I do think I had 2 or 3 engine components that were just pasted their sell by date and 1 or 2 sealing problems that merged into this rough cycling idle, what I can tell you is if you have rough erratic or cycling idle my thought would be to look for a lean mixture, too much air at the wrong time getting in to the engine.
All comments are welcome negative or positive on the chart below.

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Old 13-12-20, 12:33 PM   #2
John.
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Does this mean the engine's not coming out?!
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Old 13-12-20, 12:51 PM   #3
Colin Aitchison
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Hi John,
At this moment it is staying where it is, but I do have a WR short block on the engine stand to strip and assess over the holidays.
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Old 13-12-20, 10:53 PM   #4
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So what CO are you actually seeing at idle ?

Lambda isn't relevant at WOT as it gets ignored
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Old 14-12-20, 05:24 AM   #5
Colin Aitchison
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Hi Mike, forgive me for not explaining the chart in the first post, I was trying to draw a comparison between Co, lambda and air fuel ratio, let me explain, what ever value have you run across or up to the green line and read off the corresponding value, for example- if you have a Co of 3%, lambda will be around 0.92 with an air fuel ratio of about 13.5 to 1.
In the Chart below I have added a pink band for wide open throttle (WOT), yellow for idle and the thin blue line at the bottom is for cruise, the advised idle Co for my car is 1%+/- 0.2 and over the past few years at MOT time it has never been below 3% Co with HC of 160 ish this is a pass due to the limits being 4.5% Co and 1200 HC, the car never idled or ran well with the Co lower than 3%, now after finding out that the WUR is the root of my running problems I have a idle Co of around 1.5%.
I too have seen that lambda is to be ignored at WOT and in the past I would have agreed with that statement but if your full throttle enrichment system is not working correctly and enrichment dose not happen damage to the engine will happen.
From my observations on using an air fuel ratio gauge for the first time as you open the throttle their is a clear step at around 0.89 lambda on acceleration as the throttle opens past 55 degrees and the enrichment device is triggered lambda drops to 0.82.

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