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L-jet idle related questions

 
Old 04-09-2019, 10:15 PM
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Default L-jet idle related questions

Hi guys,
I have a question related to setting my idle mixture. I follow the WSM and use a wideband O2 sensor to read AFR on a gauge, and a narrow band standard O2 sensor to tell the DME what is the lambda. The WSM tells me to disconnect my (narrow band) O2 sensor and make adjustment. As soon as I disconnect my O2 sensor, my AFR drops from 14.7 to 10.9 at 750 RPM. I think this is telling me that I am not in open loop at idle. I.e., my DME thinks it's getting 0.0 volts from the O2 sensor and tries to correct by going full rich. This full rich condition is immune from adjustment of the bypass screw on the AFM.
I'm thinking the DME does not know I'm in idle. My idle switch makes a click when the idle is closed. I will check the 35-pin connector pin no. 2 to pin no. 18 continuity with throttle closed tomorrow. Is there something else I should be checking. The car behaves appropriately with O2 sensor connected, meaning good AFR's from idle up to full throttle.
I think I can run with the idle switch working or not, but is that what tells the DME to go into an open loop idle map? Maybe closed loop at idle is just fine, I don't know and maybe not worth worrying about. I find if I don't follow factory advice, it usually comes back to bite me. That's why I'm asking.
Thanks very much,
Dave

Last edited by [email protected]; 04-09-2019 at 11:42 PM. Reason: ohms pin 2 to pin 18, not ground
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Old 04-10-2019, 02:34 PM
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The 35-pin connector is definitely receiving info from the idle switch. I should say I replaced my AFM about a year ago with a rebuilt unit from 928 Intl. The car has been running well, so maybe I'll just ignore the AFR's with the O2 sensor disconnected.
Thanks,
Dave
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Old 04-11-2019, 01:55 AM
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You have 2 throttle position sensors with AFC. WOT and idle. Use your ohmmeter to check both sensors at the 35-pin L-jet connector, and check both sensors at WOT and at rest. Make sure the entire circuit is working by design.
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Old 04-12-2019, 12:26 PM
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Thanks for the input. I just double and triple checked my throttle switches idle and WOT at the 35-pin connector, and they behave appropriately. I checked the Temp II sensor as well, and it seems to be in spec at warmed up temp; I will check it again when the engine cools of to 68 F.
With the O2 sensor disconnected, my AFR on the WB is 10.9, whether I adjust the AFM bypass screw to 0 turns out or 10 turns out. When I go to closed loop, I run at 14.5-15.0 at idle. That's with the idle speed at 750, in Park. At this idle speed, closed loop, when I shift into gear, the idle speed first drops a little, then the AFR goes up to 16-17, then the engine dies. In open loop with the O2 sensor disconnected , when I shift into gear the AFR goes from 10.9 to 12.6 and the idle speed goes from 750 to 700, and the engine continues to run. I am using the simulated NB signal from my one of my two WB O2 sensors, and monitoring AFR on the other WB sensor.
So to keep the engine from dying, if I want to run closed loop, I have to set the idle speed around 1100 RPM in Park, which drops to 800 RPM when put into gear.
I had a similar thread about a year ago, regarding engine dying when put into gear, where I thought I had solved the problem with a new AFM. ( Basically after much experimentation with the AFM spring settings, I gave up and ordered a rebuilt from 928 Intl.) Sorry for overlap of threads, I did not know they would converge when I started this one.

https://rennlist.com/forums/928-foru...into-gear.html

Thanks,
Dave

Last edited by [email protected]; 04-12-2019 at 12:54 PM.
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Old 04-12-2019, 03:10 PM
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Hi guys,
I have some more data:
ECU p/n: 928.618.106.04 280 002 104
AFM p/n: 0 280 203015 050
Both of theses are consistent with the 1984 MY of my car.
I measured the AFM resistance values at the 35-pin connector and all were within spec (WSM 24-20) for AFM manufacture date > 042, EXCEPT the air temp sensor was 1.8Kohms should be 4-6Kohms. I will recheck this tomorrow since the engine may still be slightly warm, ie > 68 F. My understanding is that air temp has only small effect on AFR compared to engine temp, but I may be wrong. This was a rebuilt AFM, so maybe the air temp sensor was out of spec. OTOH if air temp is important, maybe the DME thinks I have more air than I really do, and is adding fuel in open loop.
Thanks again,
Dave
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Old 04-12-2019, 11:03 PM
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Hi guys,
I remeasured my air temp and water temp sensors at 68 F. tonight. The water temp (Temp II) sensor was right in spec at 2.53K ohms (normal 2-3K ohms). The air temp (Temp I) inside the AFM was low at 2.76K ohms (normal 4-8K ohms). Since this is low, the AFM thinks the air is hotter, ie less dense. It should be reducing fuel, not increasing, so I don't think this is my problem. All air flow and temp sensor are in spec except this one.
I think I'm going to take Mark Kibort's advice and run closed loop, and let the O2 sensor and Lambda control circuit correct my mixture. I'll leave the idle speed set at 1100 RPM's.
Thanks,
Dave

Last edited by [email protected]; 04-13-2019 at 12:30 AM.
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Old 04-14-2019, 11:39 AM
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Originally Posted by Kiln_Red View Post
You have 2 throttle position sensors with AFC. WOT and idle. Use your ohmmeter to check both sensors at the 35-pin L-jet connector, and check both sensors at WOT and at rest. Make sure the entire circuit is working by design.
Thanks for responding. I came across your thread from 2014, where your car was also running rich, post no. 22:

https://rennlist.com/forums/928-foru...-and-dies.html

Your resistance values at the 35-pin are almost identical to mine, including slightly low for the Temp I sensor in the AFM. What did you finally find to be your problem?
Dave
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Old 04-17-2019, 08:06 PM
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Originally Posted by [email protected] View Post
Thanks for responding. I came across your thread from 2014, where your car was also running rich, post no. 22:

https://rennlist.com/forums/928-foru...-and-dies.html

Your resistance values at the 35-pin are almost identical to mine, including slightly low for the Temp I sensor in the AFM. What did you finally find to be your problem?
Dave
It was corrosion at the ignition ballast resistors. My car was running poorly almost all of the time though.
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Old 04-17-2019, 08:08 PM
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Thanks, Kiln. I appreciate the feedback.
Dave
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