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Old 12-15-2002, 12:04 AM   #1
toobluewrx
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Lightbulb Part throttle hesitation mod....

I think I've found an inexpensive fix to minimize the hesitation that occurs under part throttle & light boost on some WRXs. My mods are uppipe, downpipe, muffler, and TXS Stage 2 Unichip.

Thinking that the hesitation is caused by the fuel mixture being slightly too lean while in closed loop control, I researched how to make the mixture richer. In closed loop mode, the ECU uses feedback from the front O2 sensor to control the air/fuel ratio. So if I could alter the signal from this sensor to indicate the A/F ratio was leaner than actual, the ECU would add fuel to compensate.

The front O2 sensor produces a milliamp signal proportional to the A/F ratio. The signal changes polarity as the A/F ratio changes from lean to rich. By externally adding a milliamp signal of the right polarity, I could shift the A/F ratio being reported by the sensor.

The sensor has four wires, two for a heater element, and two for the A/F signal. I used a 10K resistor between the +12V lead for the heater element and the + connection of the A/F signal. This provides ~1 milliamp of signal in the lean direction. The ECU compensates for this during closed loop by adding more fuel.

The connector for the front O2 sensor is located beneath the cover for the boost control solenoid. There is a vertical stack of three connectors here. The top connector is for the O2 sensor. On the female side of this plug, using a pair of needle nose pliers, push the resistor leads about an inch into the top two holes where the wires exit. With the key switch on, use a voltmeter to verify you have 12 volts on one resistor lead, and about 3.8 volts on the other.

Here is a simple diagram of the connector with the resistor installed:
Code:
   /\/\/\
  |     |
+-|-----|-+
| O     O |
|         |
| O     O |
+---------+
This mod has virtually eliminated the hesitation. I've had it in for about two weeks. The only downside that I've noticed is a drop of about 1 mpg in mileage.

Those of you with hesitation may want to give this a try. Post your results.

AB
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Old 12-15-2002, 03:19 PM   #2
nhluhr
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so you're telling me the current through the O2 sensor rises as the sensor reads leaner and leaner? And adding the 10kohm resistor across the two signal leads in effect presents LESS resistance to the entire circuit (resistors in parallel do not add more resistance)...

is that right?
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Old 12-15-2002, 04:08 PM   #3
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same confution as nhlur. Please elaborate a little more.
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Old 12-16-2002, 12:02 AM   #4
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nhluhr & inthememof911 - The A/F sensor's output is a current signal. This current is produced by an electrochemical reaction occuring in the sensor (like a battery) when the sensing element is heated to ~700 degrees C (by the heater circuit). The sensor produces zero current at an A/F ratio of 14.5, an increasing positive current for lean mixtures (>14.5), and an increasing negative current for rich mixtures (<14.5).

For the A/F sensor to work, the ECU must provide a bias voltage across the sensor of ~0.3V. It does this by holding one side of the sensor at 3.8V, and the other side at 3.5V. What I am doing is using the +12V supply (through the 10K resistor) to provide a current source to sum with the current signal produced by the A/F sensor.

The circuit that measures this current in the ECU thinks the current comes strictly from the A/F sensor. With the engine running, the battery voltage is about 14V and the + sensor terminal is about 3.8V (so there is ~ 10V across the 10k resistor which means there will be ~ 1 milliamp flowing). This current sums into the signal from the A/F sensor.

Use this link to learn more about this type of A/F sensor:
http://jon.in.ct.home.att.net/SAE970843.PDF
Credit to "Jon [in CT]" (who posts here frequently) for the link.

There is also evidence that the pressure in the exhaust manifold changes the calibration of the A/F signal. See this page: http://forums.nasioc.com/forums/show...hreadid=269143

Maybe this is the real reason we're having this poor driveability at low boost in closed loop. We've changed our exhaust system (uppipe, catless downpipe, low restriction muffler) which has significantly reduced the exhaust back pressure. Since the ECU can't measure the back-pressure, it must have look up tables to correct the signal from the A/F sensor. Since the back pressure with the revised exhaust is now less, the ECU is calculating an incorrect A/F ratio (too lean).

Comments?
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Old 12-16-2002, 12:53 AM   #5
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it seems to me it would be possible to have a pressure sensor in the exhaust near the O2 sensor and you would then have a small circuit that scaled the O2 signal, as necessary, with respect to the pressure in the exhaust at that point.
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Old 12-16-2002, 01:12 AM   #6
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Wouldn't an ECU reflash take care of this? My understanding of reflashing the ECU is the tuner can alter settings to adjust for modifications done to your engine/exhaust.

_0wen_
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Old 12-20-2002, 11:05 PM   #7
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Anyone else tried the resistor mod yet?
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Old 12-20-2002, 11:17 PM   #8
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Are you talking about poor closed loop performance during boost or non-boost conditions?
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Old 12-20-2002, 11:28 PM   #9
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recursian
Quote:
Are you talking about poor closed loop performance during boost or non-boost conditions?
During boost (20-50% throttle & 2-7psi boost).
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Old 12-20-2002, 11:42 PM   #10
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That's what I thought. It should be illegal for a person to make a turbo car that runs closed loop during boost. It's all politics. I'm concerned that the heater power isn't regulated. Also, if your mod is working, you are sacrificing fuel economy during the majority of the time you are driving. If you took a 300 mile trip on the highway and never ran boost, you would have blown some fuel. The best thing to do is make the computer go open loop. You can make your own circuit to do this as I did on my Honda or you can buy the Vishnu EMI Adapter. Yes, I talk about the EMI Adapter a lot, but it's because the thing works. Most people don't understand the importance of it. Anyway, the other thing you may consider is making your additional current flow only when under boost. You can get a 1 psi Hobbs switch and wire a relay or make a simple circuit with a comparator on the MAP sensor circuit. I would get a 5 volt regulator, comparator and trim pot and do it that way. Oh yeah, you might need an analog switch or cheap multiplexer too.
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Old 12-21-2002, 01:02 AM   #11
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Quote:
Originally posted by recursian
You can make your own circuit to do this as I did on my Honda or you can buy the Vishnu EMI Adapter. Yes, I talk about the EMI Adapter a lot, but it's because the thing works. Most people don't understand the importance of it.
Judging by the demand we're seeing, I think enough people do Certainly keeps us and our fabrication house busy. In fact, I just spent two hours this evening programming 75 of them! Time to go to sleep...

Cheers,
shiv
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Old 12-31-2002, 01:56 AM   #12
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so has anybody else tried this? is it safe?
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Old 12-31-2002, 07:08 AM   #13
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So Shiv, you're flashing the Pic microcontroller? Are you doing the programming too? Are you open to requests to modify the code if we come up with viable additions?

Chris
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Old 12-31-2002, 08:15 AM   #14
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I've been getting some bucking/hesitation at part throttle and full boost with my MBC. I'm thinking this might help a lot. Only thing is, if I'm running rich at WOT already, won't this just make it worse?

-IggDawg
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Old 12-31-2002, 06:23 PM   #15
toobluewrx
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Quote:
I've been getting some bucking/hesitation at part throttle and full boost with my MBC. I'm thinking this might help a lot. Only thing is, if I'm running rich at WOT already, won't this just make it worse?
-IggDawg
IggDawg,

This mod only comes into play during closed loop operation (<50% throttle and < 4300 RPM). It will have no effect during WOT.

Go to Radio Shack and buy a pack of 5 resistors for $1. Try it and let us know if it works for you. This simple mod has made a night and day difference in the drivability of my car!

AB
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Old 01-01-2003, 09:25 AM   #16
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Quote:
Originally posted by toobluewrx

IggDawg,

This mod only comes into play during closed loop operation (<50% throttle and < 4300 RPM). It will have no effect during WOT.

Go to Radio Shack and buy a pack of 5 resistors for $1. Try it and let us know if it works for you. This simple mod has made a night and day difference in the drivability of my car!

AB
As soon as I read "colsed loop" a big smile smeared across my face . I completely forgot that whole 02 sensor/closed loop thing. I skimmed 79 cents off petty cash yesterday when I was out getting lunch for everyone and snagged the resistors. that's the only question I had before I try this . I'll let you all know how it goes.

-IggDawg
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Old 01-01-2003, 10:24 AM   #17
Lit4
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Im no genius about this kinda thing so I'm still having second thoughts. There isn't anything that could go wrong after doing this mod?
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Old 01-01-2003, 12:16 PM   #18
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Anyone else tried this??
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Old 01-01-2003, 01:01 PM   #19
Chuck H
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I don't know enough about the Subaru ECU to really judge, but this doesn't seem like a very good idea. What you're doing is fooling the ECU so that it doesn't know the stoichometric a/f ratio (14.7:1) any more. That's going to affect a lot of other stuff, besides a stumble at it comes into boost at part throttle. You will be making the car run too rich the whole time it's in closed loop mode, even when not at boost. It will use more fuel, and probably damage the catalytic converters over time by dumping unburned gas into the exhaust.

I also don't know anything about Shiv's auxiliary electronics, but I would think that a circuit to trick the ECU into open loop mode at lower RPM would be a better solution.
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Old 01-01-2003, 06:12 PM   #20
sajohnson
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toobluewrx's mod seems fine to me. Other than the excess fuel consumtion, the price is right.

Ideally though, if you have the cash, an ECU reflash or aftermarket device from Vishnu or turboXS would be the way to go.
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Old 01-02-2003, 10:54 AM   #21
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toobluewrx.. On the connector there are 4 wires, two black, one white, one blue. Which ones did you use?

Did you try any other value resistors than 10K? If so what were the effects of more or less resistance. Other than making it richer or leaner... What I'm gettin at is using your method how much richer should it really be so it does not hesitate?

Also, Is the O2 sensor heated electrically ALL the time? Or only during the first few mins of operation? If only a few mins then the 12V would terminate...............

Last edited by AndyHidley; 01-02-2003 at 11:00 AM.
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Old 01-02-2003, 11:10 AM   #22
Lit4
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Any1 with an EGT gauge do this and see a change?
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Old 01-02-2003, 11:17 AM   #23
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I've got a legacy turbo, not a WRX. as a result, my front O2 sensor has 3 wires, not 4. any idea which ones I need to bridge? I went to try this out and saw I had a different setup. I'm getting a little tired of only being able to drive at 0-20% throttle or 80-100% throttle.

-IggDawg
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Old 01-02-2003, 09:43 PM   #24
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Quote:
toobluewrx.. On the connector there are 4 wires, two black, one white, one blue. Which ones did you use?

Did you try any other value resistors than 10K? If so what were the effects of more or less resistance. Other than making it richer or leaner... What I'm gettin at is using your method how much richer should it really be so it does not hesitate?

Also, Is the O2 sensor heated electrically ALL the time? Or only during the first few mins of operation? If only a few mins then the 12V would terminate...............
AndyHidley,

I used the top black & white wires (actually the wires on the female connector that mate to the top black & white wires). The two black wires go to the heater, and the white and blue are the sensor output.

I started with a 100K resistor, and kept decreasing the value until I got a noticeable effect. I'm currently using a ~8K resistor (two 3.9K in series). I would estimate that with this resistor, the A/F ratio is ~ 13.5 to 1.

It seems that one side of the heater always has 12V applied (with the key switch on). The other side is being switched to ground (probably varying the duty cycle to control the temperature).

AB
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Old 01-03-2003, 02:06 AM   #25
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Is there any significance of a 10k over an 8k? Why did you change
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