Author Topic: My mixture ratio feedback is mostly open loop.  (Read 1925 times)

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Offline Boz

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My mixture ratio feedback is mostly open loop.
« on: Jun 2, 2012, 11:22PM »
 Hi all, I'm new here, I bought a 92 Pintara Executive in January with only 81000 km on the clock. You beauty!  ;D

I've looked extensively through these forums and have found the advice very informative. I'm so glad this resource is here. I'm sure I'd be lost without it!

Now, I've had a problem with the car running rich and burning a hole in my pocket. The ECM reported a code 13 (oxygen sensor), so I went ahead and bought a suitable replacement from Repco. This was before I had read extensively through these forums and the UA Corsair fsm. When driving in field service mode, after the car warms up, I'm assuming the mixture ratio feedback from the O2 sensor to be in a closed loop most of the time. Is this correct? Because I'm finding that the ECM light is flashing rapidly (open loop) all of the time except when I'm decelerating, in which case the light stays solidly on, indicating it's now in a closed loop but running rich.

I'm going to test the water temperature sensor tomorrow but I can't see it as being the culprit. If it was reporting the engine temp as under 60 degrees, I'd expect the system to run in open loop all of the time and not going into closed loop every time the car is slowing.

From the fsm.
The mixture ratio feedback sensor does not operate (i.e. open loop) under the following conditions:
when starting the engine
when the exhaust gas is cold
when the engine speed is above 3800 rpm
when the engine speed is below 3800 rpm but the engine load is above a predetermined rate
when engine is idling for more than approx 20 seconds
when throttle position changes faster than a pre-determined rate (i.e. acceleration / deceleration)

The only thing I can see that is consistent with what I'm seeing (closed loop only when decelerating) is that the ECM is receiving feedback that the engine is under too much load, even when idling. It senses the load is acceptable when decelerating because the engine is being pushed along by the car's momentum rather than the engine pulling the car along or simply overcoming frictions from within the engine, from the belts and the torque converter. Does this sound reasonable?

The million dollar question is, how does the ECM know how much load the engine is under? Does it determine it from one or more sensors and which one(s)? I may have to resort to taking the car to an auto electrician if it all becomes to difficult to determine with a multimeter.  ::)

Thanks in advance to anyone who responds. Cheers!

Offline Boz

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Re: My mixture ratio feedback is mostly open loop.
« Reply #1 on: Jun 2, 2012, 11:26PM »
By the way, if I ever figure this out. I'll be sure to post back and let others know. There's so many helpful threads here but a lot of them don't have a conclusion, leaving you to wonder, how did they fix their problem?!  :)

Offline SSS

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Re: My mixture ratio feedback is mostly open loop.
« Reply #2 on: Jun 2, 2012, 11:49PM »
The million dollar question is, how does the ECM know how much load the engine is under? Does it determine it from one or more sensors and which one(s)?

Load is primarily determined by the air flow meter (AFM, or MAF) in addition to the throttle position sensor (TPS).

Check that the voltage from the TPS is approx 0.45-0.5 V when fully closed, and greater than 4.5 V when open; do this as a starting point.

Offline Boz

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Re: My mixture ratio feedback is mostly open loop.
« Reply #3 on: Jun 2, 2012, 11:56PM »
Thanks heaps ;D I'll test them out tomorrow if it isn't too rainy.

Offline Boz

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Re: My mixture ratio feedback is mostly open loop.
« Reply #4 on: Jun 5, 2012, 01:17AM »
I tested the TPS, the Corsair fsm says it should be between 0.3 and 4 volts. I got 0.53 closed and 3.44 open. I'm able to reset the ECM error codes so full throttle must be registering ok, even though it's not as close to 4v as I would have thought. The 'TPS supply' sat at 4.89v which is within spec. I checked the TPS ground for continuity and any voltages. Continuity good and no volts, so no shorts or issues there...

I tested the AFM and it seems to be responsive, the wiring to the ECM tested good also. I don't know if the AFM is worn and maybe reporting less flow than there actually is but a quick jab of the throttle makes the voltage dip quickly to 0.3v, then jump up to 0.7 and slowly return to 0.5. It stays pretty much right on 0.5 when idling. I probably only pushed the peddle a third of the way. I wouldn't wan't to over rev an unloaded engine. EDIT: These voltages are from the O2 sensor. The post below has the readings of my AFM.

Does the ECM use the hall sensor in the dizzy cap to determine load at all? I know it's central to how the ECM operates. I figure if there's any issues there, the whole system would run like crap.

I'm going to try a forced burn off on the AFM and a ECM reset tomorrow and see if that does anything. Do you know if the AFM will get damaged doing a burn off more than once within a few minutes? It might need a few to get it clean. I'll try it once and take it out and see how it looks before attempting it again. I hear you can get some kind of spray to help clean it. Not sure if it's effective but it'd be cheaper than a replacement! I'll retest the AFM after the burn off and I'll see how it responds while driving as well (rather than sitting on the driveway).

I'll figure this all out eventually but I'm learning lots in the process. I guess the knowledge is worth the headache in the end.  ;D
« Last Edit: Jun 7, 2012, 08:09PM by Boz »

Offline Boz

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Re: My mixture ratio feedback is mostly open loop.
« Reply #5 on: Jun 7, 2012, 06:34PM »
I just realised the voltages I listed for the AFM in my last post, were the voltages I got from the O2 sensor. I actually got 2.3v idling (cold) and up to 3.3v with a quick rev (about 1/3 throttle). There was no drop in voltage before the jump to 3.3v either, just straight from 2.3 to 3.3.

The UA fsm says "Ignition 'ON': engine stationary = 1.6v" and "engine running = 2 to 4.5v". When the engine warmed up fully and was idling in neutral the voltage is 2.10, barley above 2v. I'm not sure what engine stationary means, I assume ignition on but not started? In which case I'm getting 1.3v not the 1.6 the fsm states. It looks like the base resistance may be off or could the cold ambient temperature reduce the base reading by 0.3v? I'm wondering what the adjustment screw on the AFM, beside the electrical connector is for? Is it a potentiometer to adjust the base resistance (no airflow)? I don't want to touch it unless I know exactly what it does.

I inspected the AFM and the thin wire looks very shiny clean in the sun. I also did a forced burn-off and observed the wire go bright orange for 1 second. It physically looks to be in working order.

I appreciate anyone's advice on this. If anyone has a CA20E engine and can tell me what voltage they get from their AFM with the ignition on but the engine off, I'd greatly appreciate it. However, only tell me your voltage if you are getting good fuel economy 8-12L/100KM depending how/where you drive.

Cheers!
« Last Edit: Jun 7, 2012, 09:48PM by Boz »

Offline Boz

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Re: My mixture ratio feedback is mostly open loop.
« Reply #6 on: Jun 20, 2012, 07:03PM »
I managed to snag a used airflow metre for $10 off eBay and it looks like it solved the problem.

The replacement AFM had the same base voltage of 1.3v (engine off, ignition on) as the old one but it seems more responsive. It easily hits 3.6v+ when driving while the old one would struggle to hit 3.4-3.5v with high engine revs.

So if anyone has a seemingly unsolvable fuel economy issue it might just be a degraded AFM. Your AFM might appear to be functional and in physically good condition but has lost it's sensitivity.

Now I just have to fill her up and see how much more milage I get.

Offline Budgie

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Re: My mixture ratio feedback is mostly open loop.
« Reply #7 on: Jun 20, 2012, 09:55PM »
Good work mate. Give us an update once you've run a tank through it.
dont piss me off with your pillarless shit captain snappy wrist

Offline Boz

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Re: My mixture ratio feedback is mostly open loop.
« Reply #8 on: Jul 14, 2012, 08:37PM »
There's definitely an improvment. I was averaging 16L/100km when I bought the car. The last fill before the fixes did a terrible 18.7L/100km, but the fill I did today got me 13.4L/100km. It could still do a little better but it's definitely a jump in the right direction. Fresh tyres might help as mine are quite worn and my brakes are almost out. The front discs also need machining and the pads may be dragging slightly (just a hunch). I wouldn't be surprised if the drive belts are too tight, the previous owner had the car serviced and the mechanic checked the belts and tyre pressure. When I checked the tyres they were 300kPa!! They over inflated the tyres, so maybe they over tightened the belts too.

Between tyres, brakes and a tuneup hopefully I can get 12L/100km or better.