Running rich at hwy speeds

[uvbnbit]

'98 6.5 Roe, B & B's, headers, not cats, 3.55's, VEC 3. I can't seem to lean out the AFR. At hwy speeds (60-85) in the 1500-2250 the LC-1 is showing 10.9-12.5. I've pulled the rear O2's out of pipe (still connected though), reset PCM, pulled 35-40% fuel out on fuel % tables between those RPM's and still no change. It actually seems worse. What am I missing? bad rear O2's? Logs seem to be consistent with the WBO gauge. Thanks to those I've talked to about this for your time/help. Any other ideas?

 

[uvbnbit]

bump.
Now that there's a few more of ya'll (yes I said ya'll) awake.

 

[uvbnbit]

Here's kinda what I'm talking about. (hope this makes sense to someone)

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This log is while in neutral. On the road its worse.

 

[Sean Roe]

During idle and cruise conditions, your stock PCM is in adaptive mode (closed loop) and adjusting the AF by changing the injector pulse time. If the upstream sensor voltage is lower than it should be, the PCM adds fuel to get the voltage up. If the sensors are older (more than about 3 years) or carboned up some, they will produce a lower voltage than actual. I've seen cars come in with lean mixture codes but actually be so rich that your eyes would water.
Most likely, your upstream O2's should be replaced and your wideband is reading correct. Disconnect the PCM when changing the sensors to clear out all the bad learn adaptives. We have new factory upstream (primary) sensors in stock for $72.56 each. I've tried both OE and aftermarket and like the OE ones better for the $5 more they cost.

Regards,
Sean

 

[uvbnbit]

Thanks! I thought it might have something to do with O2 signals to PCM. I didn't want to change them until necessary. Should I change the downstream, leave them out of ****, or put back in the *********?

 

[Steve 00RT/10]

If your OEM O2s are in the 3rd tube instead of sensing all five.......that is very likely your problem. Happened to me.

Steve

 

[uvbnbit]

They're in the bend, right in front of side sill (DY's instruction thread)

 

[Steve 00RT/10]

That's the right place. ...So just to check....your WB sensor and OEM O2 sensor are within a couple inches of each other?

 

[uvbnbit]

That's the right place. ...So just to check....your WB sensor and OEM O2 sensor are within a couple inches of each other?

From what I remember, yes. Maybe 2-4 inches?

 

[EllowViper]

Put an ODB-II data reader on (via your laptop) and see if you have faulted into Open Loop. I had something trigger my PCM to go into a "limp home" mode that dumped tons of gas into the engine as a fail-safe. It should have been in closed loop but something triggered it to go into open loop all the time. Really weird. I think it was an 02 sensor but I can't remember right now (getting old). For what its worth, I run from 14.5-15.2 AFR at those engine RPMs/Load. I also have rear 02 sims.

 

[uvbnbit]

Put an ODB-II data reader on (via your laptop) and see if you have faulted into Open Loop. I had something trigger my PCM to go into a "limp home" mode that dumped tons of gas into the engine as a fail-safe. It should have been in closed loop but something triggered it to go into open loop all the time. Really weird. I think it was an 02 sensor but I can't remember right now (getting old). For what its worth, I run from 14.5-15.2 AFR at those engine RPMs/Load. I also have rear 02 sims.

I'm far from "knowledgable" with this, but if it were in Open Loop, I would think I'd see a difference in the AF. I reduced the fuel% from 111 to 75 (at 2000 rpm) and no change. Wouldn't it at least changed some if it were in open loop? (btw, the front O2's are 11 yrs old/32000 miles.)

 

[Steve 00RT/10]

That is open loop adjustments you made. It would not affect closed loop operation. I log from the 2000-2200 range in 3rd gear...up to 5400 or so. No need to go higher. If you are hitting it in this area as well....even though you removed fuel...it could be hard to see the change because that's the transition area. Normally though, it spikes to lean when you floor it....and then goes a tad rich before flattening out.

11yrs/32K miles......put new in. I'm replacing mine with new QFs at 3yrs/30K miles this spring.

Steve

 

[uvbnbit]

Ahhhhhh! It's all starting to sink in and make a little more sense now. I think ya'll (yes, I said ya'll again), might have just helped me solve my A/F issue. Thanks all..... uh... I mean ya'll!