Hi,

Calling all experts! I have a 94 wrx

I am still trying to find out why my car is using too much fuel. My understanding of the fuel system is as follows:-
(correct me if im wrong) On cruise the ECU uses the lambda sensor to adjust the fueling (closed loop). Next When the throttle position sensor senses a change in throttle then it goes into open loop, At this point the ECU piles in the fuel and uses the MAP sensor and the MAF sensor to set the fuel.

Is this the case?

If so, at what point (angle of throttle) does the throttle position sensor tell the ECU to go into open loop mode. is it 40 degrees?

Does the TPS only measure either open throttle/closed throttle or does it read from 0%, 1,2,3,4,5, to 100% open etc, and relay this info to the ECU.

Now, what I am seing from my Dawes AFR meter is, whenever I start to accelerate, no mater how gently (or no mater what small amount of throttle I use & while driving off boost with my boost gauge never going above minus figures) The meter shows full rich, wich idicates to me that it has gone into open loop mode and started to pile the fuel in.

Should my car be going into open loop so soon?

Thanks

Mark

 

Sounds like a Lambda sensor fault to me.

 

Have already had a new one with no differance, fuel has been like this since I got the car over a year ago.

Cheers

also fuel has got a bit worse since I fitted a downpipe and induction kit but thats to be expected.

[Edited by SPARKY WRX - 8/18/2003 12:08:11 PM]

 

come on guys I need to know what % throttle the ECU goes into open loop??

I have to get to the bottom of this?

Cheers

 

I think it'll be based on load rather than throttle position, although throttle position will have a bearing on it. But it's rather ECU dependant & I don't know that ECU...
The 'piling the fuel in' should also vary considerably. Do you know what voltages the Dawes AFM changes its readings on? The closed loop fluctuations will swing about wildly, so won't give any useful reading other than saying 'closed loop'. If the Dawes is set to read 'full rich' at 0.8v for example, it will likely read 'full rich' whenever you try to accelerate at all, but this would be correct
Could be the MAF, I suppose, if it's reading off (and therefore instructing the ECU to go to open loop when it should be closed), but then I don't know the ECU

 

Thanks nom,

The dawes device have four LED'S, red, yellow, green then blue form lean to rich. in 0.5v increments.

Red = 0.75-0.9v
Yellow = 0.9-0.95v
green = 0.95-1v
blue = 1v to limit of sensor.

under normal driving when i touch the accelerator it runs straight to blue. but at cruise I get no lights but with the red flashing now and again. It's difficult to keep of blue in cruise, if i have to go up hill for instance, any slight axtra load or throttle and it goes to blue.

I have a HKS induction kit if that makes any differance because of the extra air flow im getting. Maybe the ECU is just trying to accomodate for the extra air provided bt the induction.

What do you think

Cheers

[Edited by SPARKY WRX - 8/19/2003 10:08:51 AM]

 

It all looks a bit high to me!
I think that full load should be 0.93v or thereabouts, but obviously it does rather depend on the sensor.
Flicking red at cruise would be right, I think.
It will switch very quickly to 'rich', but it should hit some stage in the middle...
My only thought at the moment would be to check that the lambda sensor is reading correctly, and the AFR meter is reading correctly. I didn't think a lambda probe could go over 1v in the first place, which has me a little stuck!

 

Thanks nom,

Yeah I agree that the lambda probe shouldnt go over 1v, I will have to check my sensor and gauge. For arguments sake lets say the lambda is working properly and the AFR is reading correctly and showing max 1v. What could be causing the fuel too be so rich on open loop.

Could it be the pressure sensor, pressure exchange soleniod MAF (you have already mentioned) or something else.

I was under the impression that if the MAF was not reading properly then I would run lean rarther than rich.

Also, would it be possible to buy a wide band lambda sensor and fit that to a spare hole I have in my down pipe to monitor with more accuracy or are they expensive.

Thanks

Mark

 

Nom,

I fitted an Apexi AVC-R boost controller last night and the TPS voltages seam to be fine so maybe ill look into the MAF a bit further.

Thanks

Mark

 

Not sure about the Scooby setup, but a lot of modern lambda sensors are narrow band. That is, they read either 0.2V (or lower) or 0.9V (or higher) - ie, 'lean' or 'rich' - no in between.

In closed loop mode, the ECU increases fuel slighlty until the sensor shows 'rich'. Then it decreases fuel until it shows 'lean'. It repeats this many times a second, so the fuelling oscillates very close to ideal all the time.

Hence a simple LED bargraph reading 0V-1V won't show anything useful from a narrow band lambda sensor.

Wide band ones are more expensive, but do have a more linear output.

Rich.

 

Cheers,

I am prety shure that the scooby sensor is a narow band sensor and does read from 0-1v according to the subaru workshop manual.

 

It is a narrowband sensor, but it can be useful in the range 0.80 to 0.95 to give you a rough idea of how rich you are. It is only rough but still useful.

Most Scoobies on full load as standard run between 0.89 and 0.93.

 

John,

My AFR is reading Blue on WOT wich is at least 1v, I am seeing this as soon as I give any slight throttle, what do you think could be the problem. Is there any thing that could be causing my car to go into open loop too soon. And when should my car go into open loop.

Thanks

Mark