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Question about Lamda Oxygen Sensor

My car (87 930) was running and idling prefectly and then for no aparent reason, starting idling very low and "hunting". I know that this indicates a rich condition, but am not sure what might have caused it (nothing is obvious).

When I unplug the lamda oxygen sensor, the thing idles perfectly. Would this indicate that the oxygen sensor is bad or that, since unpluged, it simply is not reading the rich condition anymore?

Several years ago I had the same problem prior to an engine rebuild. I replaced the oxygen sensor at the time, but along with all the other new parts, can not be sure that the new sensor was the "fix" at that time . . .

Any ideas?

Ron

Old 01-23-2013, 12:16 PM
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By unplugging a known working O2 sensor, the computer (and frequency valve) would respond by reducing fuel to the engine (because it would read as zero oxygen being present and too rich). Sounds to me like your Lambda circuits are working correctly. If not working correctly, then by unplugging the O2 you shouldn't see any change at all and the idle would still be crappy. If the sensor were bad, then even with it plugged in the car would be running in 'open loop' as though it were unplugged and unless you had tuned it to run open loop, it would run crappy plugged or unplugged.

How's your rear fuel pump relay? Perhaps you've lost the voltage to the heating element in the WUR and it's reverting to cold start control pressures and thus rich. You unplug a perfectly good O2 sensor, the Lambda computer says (****, I'm not smelling any air in this exhaust, I must be rich, I better lean her out).

NOTE: the O2 sensor produces millivolts in response to the oxygen content. Higher voltage = higher AFR (lean reading). Unplug it = zero voltage = lambda computer seeing false rich condition and adjusting to lean her out.
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Last edited by mark houghton; 01-23-2013 at 01:35 PM..
Old 01-23-2013, 01:31 PM
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Thanks Mark.

So I changed the relay for a new one from my spares box. No change, so I pulled the relay with the engine running and the idle was instantly smooth (but a little too low). I then put the relay back in, with engine still running, and the idle was perfect for about 30-40 seconds and then went back to low and hunting. I did this several times with the same results. This was all with the O2 sensor plugged back in . . .

I tried wiggling the relay in its socket to see if there might be a bad connection, but no change.

I wonder what is going on?

I guess next I need to see if the WUR is getting power?

Last edited by Ronnie's.930; 01-23-2013 at 02:25 PM..
Old 01-23-2013, 02:15 PM
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Well, the socket that plugs into the WUR is showing 12.4v when engine running (13.4v measured at battery) - no change in idle if plugged in or not.


I've repeated that strange relay deal that I mentioned above many times now:

Low hunting idle - pull rear pump relay and idle is smooth but a bit low - push relay back in and idle is now perfect (smooth and at about 1000-1100 RPM - right were I have it set) for about 30-40seconds and then goes back to low and hunting).

What do you think, Mark?

JFairman - are you reading this?

Last edited by Ronnie's.930; 01-23-2013 at 02:57 PM..
Old 01-23-2013, 02:52 PM
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Weirdness. First, unplugging the WUR won't result in an immediate change in cp but if you left it unplugged it should begin to go rich in a minute or two. But it looks like you've got good voltage to it, so let's focus elsewhere.

I'm starting to think fuel pump problem, like it' s drawing too many amps. It runs ok initially until it heats up/slows down/whatever and starts drawing amps. Pull the relay, the pump settles down/cools down, plug it back in and she runs fine for 30 seconds then starts to act up again. I can't see how this is related to the relay, especially since you've changed to a new relay and how this symptom is so repeatable.

Head scratcher at this point.
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Old 01-23-2013, 03:11 PM
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One other thing that might be worth mentioning is when the O2 sensor is unpluged the car idles perfectly - pull the rear fuel pump relay and the idle drops about 200 RPM but is still smooth - put the relay back in and idle raises about 200 RPM and is perfect. In other words, the realy deal is much different with the O2 sensor unpluged 'v' pluged . . .
Old 01-23-2013, 03:25 PM
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Just FYI, pulling the relay for the rear fuel pump (Fuel Pump Relay II) disconnects power to the Warm-up regulator, throttle bypass valve, and the O2 Sensor.

Might want to arm yourself with the workshop manuals. Check voltage from the O2 sensor. Should be between 0.1V and 0.95V.

Check pulsewidth (need a meter that can do that) at the rubber diagnostic plug by the CDI box in the back. Should be 45 to 55% between Pin A and ground.
Old 01-23-2013, 03:39 PM
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Thanks for the info, Speedy. Where do I check the O2 voltage?
Old 01-23-2013, 04:10 PM
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You check the voltage the narrow band lambda O2 sensor is generating at the wires coming from it. Thats easy with a generic low cost 2 wire O2 sensor that has no heater element in it but the one in your car has a heater element to get it up to operating temp quickly so you have to figure out which 2 wires to check.

When you say the idle is hunting, do you mean oscillating up and down steadily with dark colored rich exhaust full of stinky unburnt hydrocarbons ?
Mechanical air flow meter fuel injection like CIS and L-jetronic are both good at that when running too rich at idle.

Do you need the lambda system to pass emission testing in Texas?

If not, do your car and yourself a huge favor and get rid of it.
Then adjust the 6 fuel head allen head final fuel trim screws next to the injector line banjo's to compensate for fixed lower chamber pressure without the pulse valve returning that lower chamber control pressure fuel in varying amounts to the fuel tank. Thats tricky if you've never done it and takes an injector flow test to get it right so it's best to send the fuel head to to CIS flowtech and have Larry do it.

The black relay next to the yellow relay on an '87 supplies the heater element in the O2 sensor with 12 volts.
Old 01-23-2013, 04:37 PM
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Ronnie, do you have access to CIS fuel pressure gauges? I'd be curious to know what your cold and warm pressures are, and what happens to them when you pull the relay and it starts hunting.
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Old 01-23-2013, 04:52 PM
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Quote:
Originally Posted by gsxrken View Post
Ronnie, do you have access to CIS fuel pressure gauges? I'd be curious to know what your cold and warm pressures are, and what happens to them when you pull the relay and it starts hunting.
I've always wondered what people mean when they say CIS fuel pressure gauges?

Are they thinking of dual AC manifold gauges you use to check the suction low side and discharge high side of automotive air conditioning or something?

It's one single 0-100 psi fuel pressure gauge with a ball valve in the line for checking CIS... not gauges as in more than one gauge.

A wideband AFR gauge would be more helpful to diagnose the problem you're having.
Old 01-23-2013, 05:04 PM
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Hello Jim

No emissions testing for my year car in Texas so will have to look into what you suggest.

Yes, the idle is oscillating up and down and the exaust is very dark/rich. Perhaps I just need to adjust the idle mixture screw? I have thought about doing that already, but have been hesitant as the problem started without me making any adjustments.

I just went out and messed with the black relay you mentioned - I pulled it out, cleaned the pins and put it back in - taking the relay in and out with the engine running has a similar effect as to removing and installing the fuel pump relay #2 but not as dramatic - the idle drops a bit when removed and then increases and runs smoothly when replaced but only for 30 seconds or so.

Last edited by Ronnie's.930; 01-23-2013 at 07:03 PM..
Old 01-23-2013, 05:06 PM
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I don't have the CIS gauges but can get some at some pont if need be.

I also have a wideband AFR setup but it is not installed - looks like I will finialy get around to doing that!
Old 01-23-2013, 05:10 PM
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Quote:
Originally Posted by JFairman View Post

The black relay next to the yellow relay on an '87 supplies the heater element in the O2 sensor with 12 volts.
Exactly....beat me to it! And I'm not sure that that might also send power to the Lambda computer under the seat.
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Old 01-23-2013, 06:52 PM
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Too bad you're not closer. I have a pressure kit and an LM-1 wideband handheld setup. The fuel pressure gauge is a necessity for these cars eventually. Here's the one I have- Tool Aid 33865 C.I.S. K-Jetronic Fuel Injection Tester with Case - Amazon.com
Others will chime in on where to find it far cheaper. They come up on CL and eBay often enough.
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Old 01-23-2013, 06:55 PM
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Quote:
Originally Posted by Ronnie's.930 View Post
Thanks for the info, Speedy. Where do I check the O2 voltage?
First, get the car up to normal oil temperature by going up on the freeway and stomping a Mustang. (That last part is not per the manual).

Disconnect the O2 sensor plug connection on the left-hand side at the rear of the
engine compartment.

On the SENSOR end of the plug, check the voltage between the terminal with the Green wire and engine ground. It should be in the range of 0.1V to 0.95V. If it is running rich, it should be somewhere around the 0.8V to 0.95V range.
Old 01-23-2013, 07:27 PM
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Quote:
Originally Posted by Speedy Squirrel View Post
First, get the car up to normal oil temperature by going up on the freeway and stomping a Mustang. (That last part is not per the manual).

Disconnect the O2 sensor plug connection on the left-hand side at the rear of the
engine compartment.

On the SENSOR end of the plug, check the voltage between the terminal with the Green wire and engine ground. It should be in the range of 0.1V to 0.95V. If it is running rich, it should be somewhere around the 0.8V to 0.95V range.
I stand corrected....Speedy is right re: the voltage. I think I said just the opposite in post #2. The higher the voltage, the richer the exhaust mixture. I used to have a chart somewhere that compared O2 sensor output voltages to AFR's...will have to look for it.
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Old 01-24-2013, 05:01 AM
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Originally Posted by mark houghton View Post
Exactly....beat me to it! And I'm not sure that that might also send power to the Lambda computer under the seat.
Yeah, maybe it does. I've never bothered to try and figure that out.

There is no emission testing in Florida so I removed all that stuff and sold the k-jetronic lambda ECU from under the driver seat that still worked as it should.
I still have the rest of the lambda struff and should try and sell it.
Old 01-24-2013, 06:02 AM
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Quote:
Originally Posted by Speedy Squirrel View Post
First, get the car up to normal oil temperature by going up on the freeway and stomping a Mustang. (That last part is not per the manual).

Disconnect the O2 sensor plug connection on the left-hand side at the rear of the
engine compartment.

On the SENSOR end of the plug, check the voltage between the terminal with the Green wire and engine ground. It should be in the range of 0.1V to 0.95V. If it is running rich, it should be somewhere around the 0.8V to 0.95V range.
Ok and thanks. I will do this sometime today. And excuse my ignorance, but what is this going to tell me (for instance if the voltage numbers are within spec or if they are in the rich range)? Am I simply trying to verify that the engine is indeed running rich, as it seems to be, or am I testing to see if the O2 sensor is working? I'm a bit confused about this for some reason . . .

Thanks for your help, everyone!
Old 01-24-2013, 06:05 AM
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And you know what - I just remembered that I've got that well known Bosh fuel injection book somewhere (recently moved) - perhaps I should find it and start reading!


Last edited by Ronnie's.930; 01-24-2013 at 06:27 AM..
Old 01-24-2013, 06:09 AM
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