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Old 06-11-2014, 10:23 AM   #1
billr
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Default '94 DOHC no closed-loop

Just before taking it for Calif. emissions testing I put a Snappy MT2500 on it to check for flags and such. It isn't going closed loop, at least while idling in neutral (5 spd). I got the ECT up to 200F. What are the conditions for going "closed-loop"? Obviously, the O2 isn't toggling, stays at .96V, and the fuel trim isn't changing. The engine runs fine, but I'm guessing passing emissions will be more of a challenge if it won't go closed-loop.
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Old 06-11-2014, 12:33 PM   #2
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Default Re: '94 DOHC no closed-loop

Basically hot O2 sensor. Temperature is from ECTS not gauge sensor. The ECTS will enable A/F ratio control based on temperature but O2 switching is what sets C/L. Disconnect O2 and see if you have 0.45v bias. If so I would try the spare sensor.

You could try this to see if the O2 sensor is even capable of switching. http://www.youtube.com/watch?v=stFhLSHsOGg
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Old 06-11-2014, 01:20 PM   #3
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Default Re: '94 DOHC no closed-loop

Somebody else advised that many pre-OBD2 cars stay open-loop at idle. I had my wife take me for a short drive and it went closed-loop as I left my driveway, with the O2 sensor and fuel trim responding. So... I took for smog test and it failed on NOX. I'll start a new thread about that.
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Old 06-11-2014, 04:34 PM   #4
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Default Re: '94 DOHC no closed-loop

This has truly become ugly. There seems to be much confusion about what gear it is supposed to be in while testing. Early this morning it failed at 15 mph, but passed (easily) at 25 mph. When I took it back and pointed out that it seemed to be in the wrong gear at 15 mph they agreed and did a re-test. At that point it passed at 15 mph (easily), but failed at 25 mph! On the second test the 15 mph rpm was as expected, but the 25 mph rpm was high. Exact opposite of earlier in the morning. From old records it looks like the two tests (15 and 25 mph) were done before in 2nd and 3rd. The first test today was 1st and 3rd, the second in 2nd and 2nd??? Let me regroup, collect my thoughts and data, and start a new thread.
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Old 06-11-2014, 05:25 PM   #5
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Default Re: '94 DOHC no closed-loop

Best bet is 2nd and 3rd after a good highway run to get the CAT hot.
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