Off idle hesitation

Engine is a 3400 from (for the sake of conversation) a 2001 chevy Malibu. 

It has an off idle hesitation. Also, when i lift to shift, it bogs getting back on it.

Check engine light on for oxygen sensor.  Haven't figured it out yet (upstream. Downstream tuned out of pcm)

Smoke test shows no vacuum leaks. Neither did soap or either testing. 

Fuel rail pressure at 43 psi at pump prime.

All sensors appear normal with live data on my cheap obd2 scanner. Except for the o2 sensor staying at .450 volts.

Im stumped.

What do i look at next?

Figure out why the O2 sensor is dead, then after you fix that, figure out whether the ecu is smart enough to learn and compensate.  Depending on your definition of hesitation, it is running lean.  Or rich, but that would stink and blow black out of the tailpipe.  Or a misfire, but that doesn't fit into my definition of hesitation.

That would be MAF, correct?  The MAF sensors are certainly not without fault on that vintage of GM.  You need to get the O2 sensor figured, so you can start to diagnose from the fuel trims.

How old is your gas?  When i ls swapped the datsun, i had year old gas in it and i was trying like mad to tune out an off idle stumble.  It was fixed by reverting all the fuel tables to stock and putting fresh gas in.

Will a dead oxygen sensor stick at 450? Or shoild i be going further upstream? I verified 12v on the pink, and .5 on the signal from the ecm....

A good O2 sensor should show fluctuating voltage, not static. Why not just throw a new sensor in it?

 want to make sure it's actually bad before I throw money at the problem

Static output voltage sure sounds bad to me. Should be flipping rapidly between ~.250 and .750, give or take.

Dusterbd13-michael said:

Will a dead oxygen sensor stick at 450? Or shoild i be going further upstream? I verified 12v on the pink, and .5 on the signal from the ecm....

An unplugged O2 will "read" .450 because of a bias voltage internally in the PCM.   

You have yourself a bad connection somewhere.

In reply to Knurled. :

Crap.

Yay, more underdash wiring work....

If you are reading .450 at the sensor with it plugged in, the wiring is likely good to the sensor, but the sensor itself is dead.  Quick and dirty wiring check if you have a scan tool hooked up is to take a AAA cell or similar, ground the flat end of the battery and then touch the unplugged O2 wire to the positive end.  Should read 1.5 v on the scan tool.

Knurled. said:

Dusterbd13-michael said:

Will a dead oxygen sensor stick at 450? Or shoild i be going further upstream? I verified 12v on the pink, and .5 on the signal from the ecm....

An unplugged O2 will "read" .450 because of a bias voltage internally in the PCM.   

 

You have yourself a bad connection somewhere.

 

 

That's what I was going to post had Knurled not done that. 

BTW, that's not exactly universal- not sure when the circuitry changed to have a dead sensor be .45- as it used to be 0.  But sometime around 2000, Bosch had everyone change the on board curcuits.

ok, so im stumped here. 

i chased all the oxygen sensor wiring. no notable resistance, and compklete continuity between correct pin (per ecm pinout) and correct wire on oxygen sensor. i found one bad connection, and fixed it. same behavior.  i even tried swapping a brand new oxygen sensor, and a different used. 

what im seeing on my scantool (open loop by the way, and that may have a bearing on this) is the oxygen sensor voltage is fluctuating between .450 and .455 volts. no fuel trims as they stay at 0.0 (again, open loop. i assume thats what it means by fuel status OL)

the MAF doesn't change a lot. .01 to .08 in neutral at the same test parameters with varying throttle positions and the engine running. i see variations in MAP, TPS, IAT, and CLT readings that seem normal. the MAF reading and o2 are odd to me, but may not actually be odd in reality. 

also, the off idle hesitation/bog behavior has not changed, but the engine is breaking up under load as of my last test drive post fixing a vacuum leak i found. 

ideas? tests? consolation?

So, no matter what you do, the O2 sensor is stuck at .450 or so?  No change if you coast, no change if you are full throttle?  Three different sensors do the same thing?

That's unpossible.  Scan tool, wiring or ecu is busted.

Have not tested under load or fully warmed up yet.

Vaccum leak? I had a stupid vaccum leak that i finally figured out....loose spark plug that was only loose enough when the engine heated up to leak.

That was annoying

Ok, did a smoke test again, since i questioned my last results due to leaving the open air cleaner attached. And i was right to question it.

Fiund a half dozen more leaks, and now the only place letteing smoke out is the oil fill cap. 

Im going to clean the maf while its on the workbench, and try a trst drive and see what i get on the cheap code reader and left asscheak diagnostic device.

Tge o2 is baffling though. Could it be just because its in open loop? Could the guy that tuned this pcm turned it off entirely?

So a cold, but otherwise ok, O2 sensor will also return a dead voltage value. 

And it will warm enough just driving it around to see if you even get a signal.   If you do, the issue is in the heater circuit.  

So i let coolant get to 210f. Get an open loop fault. Never webt to closed loop. 

Wont rev above 3k underload regardless of gear and throttle position. 

Hesitation still present under quick throttle opening, but not under gentle. Kind of like a bad accelerator pump in a carb.

Where to next?

Never exceeds 3K?  I have run into similar issues where the car seemed stuck running on the limp tables.  What I did was, for the ECU I was using, researched to find all the sensors involved in shifting off the tables and into closed loop.  Then I tested the warm operating values on those.  If they were nominal, I swapped in another stock ECU.

I am wondering about your MAF voltage in particular.  The voltages you said above should be rising to at least 1.5-2.5v once you get to 3K rpms, I think? 

I have a motor right now where I chased my tail for a year trying to fix a flatline stumble that showed up around 3.8k rpm at hard throttle.  I accidentally destroyed the MAF sawing vent holes in the hood (d'oh).  $45 later for a new stock MAF and all was cured.

In reply to JBasham :

Your post just led me to try the old obd1 gm trick of unhooking the maf to see if the problem goes away.

Doesnt idle worth a damn, but it revs quick and free. Like it did before this started. Damned quick, too. I think it may have more power now. Hesitation SEEMS gone, but need a little more testine to confirm.

Still open loop with a seemingly dead oxygen sensor though.

What next?

That would make me suspect a bad MAF

Dusterbd13-michael said:

So i let coolant get to 210f. Get an open loop fault. Never webt to closed loop. 

Wont rev above 3k underload regardless of gear and throttle position. 

Hesitation still present under quick throttle opening, but not under gentle. Kind of like a bad accelerator pump in a carb.

Where to next?

Now that you've driven it, what are the remaining codes?  Including the specific open loop fault.

Sounds like multiple things are broken.

In reply to alfadriver :

It stayed open loop with the same symptoms on the oxygen sensor circuit. I'm getting a generic code for no signal from bank 1 sensor 1 oxygen sensor

And you've checked the wiring from the sensor back to the ECU?  

Possible that your module totally died.

In reply to alfadriver :

Correct. No open circuits. 

12v on the switched power on the heater, no measurable resistance to ground for the heater. .450 v on both the sensor and return legs.