924Board.org

[clawless]

Hi All,

I am working with a low miles 1980 924 turbo, and am having an issue with the o2 sensor system. The details are below but my question is do I need it hooked up for any thing other than emissions?

I have read this post (http://www.924board.org/viewtopic.php?t=20275&postdays=0&postorder=asc&highlight=sensor&start=0) and see that it will go to 50% cycle but my gauge reads nothing except under load. My cat is hollowed out, boost set to 10 psi


Disconnected:
My A/F gauge read nothing (completely lean?) during normal driving, except when I punch it or it is under load, then it reads rich like it should. It runs and starts great. The gauge is connected under the car before the sheilded connector.

Connected:
The idle fluctuates about 100-200 rpm. When I start it hot, it barley runs till I rev it up for several seconds, cold start is normal until the thermo-time switch(?) cuts out and it dies or dips very low until it warms up. The A/F gauge swings wildly (like it should) however its very "slow" like almost 2 seconds from bottom of lean to top of stoich and the idle fluctuates with it. I have replaced the o2 sensor and no change. I am guessing the frequency valve is not working right or the o2 computer is shot? It did this before I connected the A/F gauge.


Thanks!

Clawless

1980 931 41000 miles

[Lizard]

the O2 system or otherwise known as the lambda system on these cars was a patch job at best, the euros never had it.

however first off you cannot run BOTH the Lambda box AND the A/F guage off the same O2 sensor it will NOT work.

what I would recommend that you do to correct your issue is disconnect the O2 sensor from the brain (silver box under steering wheel) and just leave the A/F gauge hooked up, that should help your idle, however you may need to have the injectors cleaned/replaced as well as a full tune up.
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3 928s,

[Smoothie]

My lambda system still works fine with both the gauge and the lambda box working from the same sensor, so low priority on seperating them.
What I'm seeing from your symptoms (extra lean with lambda disconnected and slow fluctuations with it connected) is a too-lean air:fuel mixture. It can be caused by vacuum leaks, so make real sure they're all eliminated first. Timing can also affect the burn and the lean-ness/richness that the sensor sees, so verify both valve timing (check that all timing marks line up at TDC) and ignition timing. If still no reading at idle with lambda box disconnected from the o2 sensor, adjust the air:fuel mix richer (clockwise) a tiny bit at a time until the bottom half of your a:f gauge lights are lit (this with engine warmed and lambda box disconnected from the o2 sensor and make a note of where the setting was before you touch it). This should give you an a:f reading at idle and eliminate the fluctuating idle speed when the lambda box is connected to the sensor.
Then if you still have a problem with low idle at cold start, there's a procedure floating around here somewhere for adjusting the 931 AAV (auxiliary air valve), but cross that when you get to it.

Of course the whole reason for the lambda system is to fine-tune the a:f for max catalytic converter efficiency, so with no cat, you don't need to have the lambda box connected to the sensor - could just disconnect it and let the freq valve default to 50% running. I just leave mine connected because it's in tune and everything works.
_________________
"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[clawless]

Thanks Smoothie, Lizard, I will track down any vacuum leaks, with all the bad plastic on this thing I would not be surprised. The belt was just changed when I had the head re-done, less that 2000 miles ago, it runs the same so I am assuming the valve timing is good and the ignition is right...but then again you know what they say about assuming.

So I have looked into how to adjust the air:fuel mixture, Haynes is not specific on the Lambda unit, they refer me to a pic of the N/A. How? Straight blade screw recessed between fuel dist and air intake? That one has been messed with. I see posts with a 3mm screw?

Runs great with the O2 Sensor disconnected. No idle fluctuation, smooth running.
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Clawless

'80 931 M471, Audi Throttle Body, 11PSI for now
'97 Volvo 850R (The other woman)

'85.5 944 SOLD - 175,000 mi most reliable car EVER
'79 924NA RIP

[Smoothie]

If you have a straight blade screw between the fuel dist and airbox, it's sealing the access hole to the real 3mm hex adjustment screw. You'd remove that cover screw, then go in with a 3mm allen wrench (hex key, whatever..)

What I think is happening is your mix is way lean due to vacuum leaks and/or being out of adjustment - then the lambda system is operating at an extreme end of its' range and is trying (evidenced by the idle speed fluctuations and slow mixture change), but it's too far out of range and can't regulate the mix to where it wants it. When it's correct and operating in closed-loop mode, there'll be no idle speed flux and the gauge will show a quicker bouncing up-down between a little above stoich and down to lean.
_________________
"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[clawless]

so...I am gonna go with its bad when I remove the screw and put my 3mm allen wrench in the hole and all I hit is the sensor plate arm? The screw is about 3/4 inches long and is and screws down tight...? no allen in sight...
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Clawless

'80 931 M471, Audi Throttle Body, 11PSI for now
'97 Volvo 850R (The other woman)

'85.5 944 SOLD - 175,000 mi most reliable car EVER
'79 924NA RIP

[Smoothie]

The 3mm hex screw is in the sensor plates' pivot arm. You have to feel around for it with the allen wrench. Don't drop the wrench.
If/when adusting, don't put any downward pressure on it - that'll move the arm and affect the mixture.
_________________
"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[Lizard]

I would recommend that you have the air fuel ratio set at a shop with gas sniffer, and the O2 disconnected.
I know that on Kenodogs car if the O2 sensor was connected the idle would fluctuate all over the place and as soon as we disconnected it ran like a champ. this was after he had it set at a shop. but after doing some research on O2 sensors I found that it is not a good idea to be splittling the voltage that comes off the O2 sensor, ie you should not run more than 1 device off of it.
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3 928s,

[clawless]

this will wait till friday when I can see, checking this out late, in the dark with my crappy flashlight is not the way to go.

i'll let you know...THANKS!
_________________
Clawless

'80 931 M471, Audi Throttle Body, 11PSI for now
'97 Volvo 850R (The other woman)

'85.5 944 SOLD - 175,000 mi most reliable car EVER
'79 924NA RIP

[clawless]

So I checked for vacuum leaks, with water, then carb cleaner, then starter fluid. Nothing. So I disconnected the O2 sensor, and began adjusting the mixture screw. Smoothie you weren’t kidding about how sensitive that screw is. Damn. I‘m almost with Lizard, take it to a shop, but I like doing stuff myself so until it does not run at all I’ll keep trying. In any event, after screwing (ha ha) with it I managed to get my A/F gauge reading at idle and while driving, although I could not get it to just light the bottom half. The lowest I could get it was just over the middle of stoich. About two or three LED’s short of rich. Which is only a little less that what it does under load, I get two yellow and one green. I guess I can mess with it more. I am down to moving the screw on feel alone.

Could I leave it? Is that too rich for daily driving? I don’t want to load up the plugs…or waste gas.

BTW I hooked O2 sensor back up and it still fluctuates…


Thanks!
_________________
Clawless

'80 931 M471, Audi Throttle Body, 11PSI for now
'97 Volvo 850R (The other woman)

'85.5 944 SOLD - 175,000 mi most reliable car EVER
'79 924NA RIP

[Lizard]

if will be fine to run abit rich to get it to a shop, but have the shop tune it without the O2 connected.
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3 928s,

[TonyBray]

My car is an 81 Turbo. I have read the posts about disconnecting the lambda circuit under the steering wheel. What do I disconnect? My car has a rough idle and I want to see if this is the culprit.

--Tony
P.S. Happy Thanksgiving all, especially thankful for all the giving (of advice and information) you all have been doing!
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67 912 2.0 Type 4
94 968 Cabriolet
82 924 Turbo

[Lizard]

I would not disconnect it under the dash,
if you want to find out if this is your problem go under the car and disconnect the O2 sensor there will be a plug which clips into the lower crossmember, just hope a PO hasnt cut this out.
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3 928s,

[TonyBray]

I thought about that at first but I wasn't sure if the disconnect under the dash turned off the whole circuit or not. If unplugging the o2 sensor itself will do the same, I will try it first.

Thanks!

--Tony
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67 912 2.0 Type 4
94 968 Cabriolet
82 924 Turbo

[Smoothie]

You wouldn't want to unplug the box under the dash and shut off the whole lambda system because the frequency valve is an integral part of the lambda fuel distributors' operation. When the o2 sensor alone is disconnected under the car, the lambda box defaults to 50% operation of the frequency valve (it opens-closes it at a rate that results in it being opened 50% of the time), so the fuel distributor's operating in the middle of its' range and regulates the air:fuel mix to the correct spec without need for any adjustment (assuming the air:fuel mix was already set correctly).
If you shut the whole lambda circuit off, the frequency valve would be inactivated (0% duty cyle) and the mix would go way lean. (The frequency valve bleeds off fuel pressure from the fuel dists' lower chambers, increasing flow to the injectors and enriching the mix, so with no freq valve operation it goes lean.)
Long story short - as far as I know, there is no easy way to unpug only the o2 sensor from under the dash, so yes, unplug under the car instead.
_________________
"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox