Anybody familiar with OBD-II?

[umcamara]

I rarely ask for any help on here. I mainly try to help if I can, and give the odd person a hard time.

I've been trying to figure out this problem for some time now. '96 MTX RS. Not putting this in 4-cyl or v6 section because it could relate to both.

I've got no CEL right now, engine runs smooth (no misfiring), do regular maintenance and tune-ups, new O2 sensors. Problem is my fuel economy has dropped significantly (from 7 L/100 km to about 10 L/100 km). Essentially stock exhaust (warm-up cat but no main cat), black soot collects on bumper above tail-pipe.

I use ScanXL software on my laptop with my OBD-II reader, and all sensors seem to be operating normally (Coolant temp heats up from ambient temp to full operating temp, O2 sensor voltages constantly fluctuating, TPS values reading as they should). For awhile I was having a rather large negative long term fuel trim, meaning it was running rich, which I knew because of the black soot on the bumper. I pulled out my injectors the other day and "cleaned" them and the mating surfaces. I no longer get that large negative long term fuel trim, but my mileage still sucks.

One thing I noticed today is that the car runs in closed-loop mode as it should once coolant is up around 60 degrees Celsius, but when cruising on the highway at any speed above 110 km/hr (68 Miles/hr) it switches over to Open Loop-Drive and starts dumping fuel. It won't let me type anymore. Any input so far??

[Daninski]

This isn't really relevant for closed loop but, for open loop. Try removing your neg on the battery, hold the brake peddle down for 15 secs then rehook the battery neg. This clears the ECU memory. Start the car and let it run or better yet drive it for 15mins and let the ECU 'relearn'. This may help if there isn't another reason (vac leak, etc)
My ZE always had fuel trim because of the bigger injectors 220 vers 180 in the K8.

A transmission problem can cause poor gas milage as well.

[umcamara]

I'll definitely try that. Thanks! I can clear trouble codes through my laptop though, which I believe completely resets the ECU because all the long term trims, etc. reset to zero, and it warns you that all values will be reset to zero.

I know my clutch is slipping a bit, but once I'm in gear and it bites, the tranny is pretty solid.

My main concern is why it's going into Open Loop-Drive mode when I'm cruising. I know it should go into open loop at WOT, but I'm far from WOT when cruising at 110 km/h in fifth gear, reving at about 3500 RPM. If I downshift into fourth, it will enter Open Loop-Drive at about 100 km/h, reving at closer to 4000 RPM. As soon as I cut the speed down past the values I mentioned, it jumps right back into closed loop mode.

My TPS is adjusted according to factory spec as well, so it definitely doesn't think I'm at WOT when I'm not.

[Ryan]

Another thing to check in our corner of the world is the rear calipers. The like to seize on. Just jack up the rear and try spinning both wheels. Then crank the e brake, and let it down right away, and go try again.

This doesn't speak to your loop issue, its just another thing to try.

[umcamara]

Another thing to check in our corner of the world is the rear calipers.

Unfortunately it's an RS and I haven't done my rear disk swap yet. But thanks anyways bud! I should be able to bring those things out to you tomorrow morning/afternoonish...

[Custommx3]

Im suprised your in open look @ cruising. Generlaly open loop occurs @ WOT or under acceleration.

O2 sensors good?

[umcamara]

Im suprised your in open look @ cruising. Generlaly open loop occurs @ WOT or under acceleration.

O2 sensors good?

Both O2 sensors are about 2 years old.

[Daninski]

Just in case you decide to trouble shoot your O2 sensors. Try removing and cleaning them and check that the wires are good and the O2 sensor isn't actually broken. Cheers

[umcamara]

I'll check that, but I think my O2 sensors are working properly. They both constantly fluctuate in value from <0.2 to >0.8 volts. I replaced them not even 2 years ago with exact OEM replacements, and have only put about 20,000 km's on the car since.

AFAIK, the ECU uses O2 sensors to adjust air/fuel ratio in closed loop, and fuel maps to adjust air/fuel ratio in open loop. The exact fuel system status I'm getting is OL-Drive (Open Loop-Drive). Which differs from OL-Fault, and just OL.

Hmmm...

[Daninski]

As your Attorney I advise you to contact your OBD2 tool seller and ask them what that means, 'OL-Drive (Open Loop-Drive)' or visit their web site and see if they have a fax section. I used the tool from OBDpros, they have a forum so your answers might be there as well. http://www.obdpros.com/product_info.php?products_id=133" onclick="window.open(this.href);return false;

Edit. I like this site. http://mr2.com/TEXT/O2_Sensor.html" onclick="window.open(this.href);return false;

[umcamara]

Thanks for the replies so far. I do know that at WOT and under agressive acceleration or deceleration the ECU goes into open loop-drive mode and uses fuel maps to adjust the air/fuel ratio.

My problem is that at anything greater than 110 km/h in fifth gear, with the TPS only reading about 17% throttle and reving at about 3200 RPM's, the ECU is going into open loop-drive. Once it enters this mode, the O2 sensors readings no longer display. However, it wouldn't make any sense to me that it's the O2 sensors at fault, and that they stop showing readings at 110 km/h. I think something else is causing it to go into open loop-drive mode.

One thing that I just noticed is that just before, or just as it's entering open loop-drive, the MAF reading reports a large increase in air flow rate (all other readings stay consistent aside from the lack of O2 readings). This increase in airflow rate isn't really warranted because I'm not stepping on the throttle much more.

Perhaps it's time to replace the MAF sensor. I'll try the other one that I have, but I think I have blindly tried it in the past with the same fuel consumption issues. The procedure in the online manual for testing the MAF involves hooking up some equipment to the ECU, and I don't quite understand what it is.

Thanks for listening...

[umcamara]

Okay, I've been doing some interweb reading on open loop mode, and it seems that many different cars will enter open loop even when cruising if:

-A certain vehicle speed is reached
-A certain engine speed is reached
-Absolute throttle position is high enough

Now I'm really confused as to why it would do this. When I'm crusing at 109 km/h, the software tells me that the engine is burning about 5 L/hour of fuel. As soon as I hit 111 km/h or so, it jumps into open loop mode, and the software reports that nearly 9 or 10 L/hour of fuel are being burnt.

It's nuts that one km/h would make such a huge difference in fuel consumption!!

[Ryan]

Get bigger tires and call it a day.

Or, drive 109 everywhere.

[umcamara]

Get bigger tires and call it a day.

Or, drive 109 everywhere.

Meh... not the answer I'm looking for. Get back to work engi-nerd...

[Ryan]

Come on, isn't there some engineering saying about not reinventing the wheel, and KISS and all that?

Have you considered that the magic number is 110, and that its designed to do that? It would be an incentive not to speed, maybe Mazda got a government grant for doing it.