Misfire and O2 Sensor condition.

[Kurt Johnson]

Hello all. 

I have an interesting one here.  98 Tahoe 5.7.  New engine, new distributor, new injectors, AC delco cap rotor wires and plugs.

Fuel pressure good.  Full throttle acceleration good.  (with MAF unplugged)  read on!

Vacuum signal at idle 20-21Hg, PCV good, EGR good, proven with blockoff plate not to be an issue.

No exhaust restriction or clogged cats.  Gauge tested, and physically tested engine pipe off the vehicle.

The driver side CAT 250 degrees inlet, 320 degrees outlet.  Passenger, 250 degrees inlet, 350 degress outlet.

I have a Snapon scanner and have scope tested all sensor outputs to the PCM and tested all injector resistance.

All wiring harness checks are done.  Power, ground, continuity from all sensor connectors to PCM including the O2 wiring.  All good.

No codes other than random misfire.  Misfire data, cylinder 1 and 4 pretty much together in unison constantly. 

Long term fuel trim on driver bank 1, 14%, bank 2, 3%, short term  fuel trim normal for both sides.

Bank 2 upstream and downstream O2 sensors operating normally. 

Bank 1 upstream reading lean, to normal, but a bit lean, while the downstream bank 1 O2 sensor is stuck at .700 or so.  It changes a tiny bit.  Unplugged, the PCM shows default .450 volts.

So,  the PCM fuel trim loop is setting a long term high percentage fuel trim to bank 1, while the upstream sensor is going rich/lean, a little on the lean side, and the downstream O2 is pretty much pinned rich.   I swapped the passenger bank 2 downstream O2 to driver bank 1.  Same condition.  The swapped downstream O2 is the only O2 that is not new, and shows normal operation when on bank 2.

Now, it gets more interesting. 

When I unplug the MAF sensor, the misfires disappear and the engine runs and drives quite well.  NEVER misfires, NONE.

The fuel trim numbers and O2 numbers stay the same.  High on bank 1.  Bank 1 downstream, pretty much stuck at around .700

I swapped in a new MAF sensor, and the same symptom.  Misfire with regular occurrence on both cylinder 1 and 4.  Unplug new MAF, misfires disappear.

 

All things considered, I want to blame the PCM, but thought I would throw this out here for peer review.

Or replace the CATs and all fixed?  BUT: How can the downstream O2 on bank 1 really be as rich as it says, even though the fuel trims are high on bank 1 trying to correct for what the PCM believes is a lean condition?  The exhaust isn't going to get any richer, unless the CAT is so bad it is polluting the O2 sensor?

My mind goes to confused PCM here.

 

 

Kurt

 

 

[davester]

unplugging the maf switches the ecm to open-loop mode, so it doesn't use sensors to run the engine, just default tables (at least, that's my understanding).

 

Is your exhaust possibly leaking?  On my truck, it had the usual broken bolts on the exhaust manifolds, and would let air into the exhaust making the O2 sensors read lean, so the pcm was just dumping fuel into the engine to try to compensate.   Maybe check and/or replace exhaust gaskets at the head and down pipe?

[asilverblazer]

Upstream O2 sensors control how the engine runs. The downstream O2 sensors determine condition of catalyst. The upstream sensor is controlling the fuel trims. The downstream may eventually throw a catalyst system efficiency code, but nothing related to drivability.  

 

Cylinders 1 and 4 are on different banks so fuel trim of bank 1 causing misfires on bank 2 would be odd. I would also confirm that the P0300 is in fact described as random, sometimes it is not described as 'random' but as a confirmation that misfires are happening (in conjuction with P0301 and P0304 in your case). This might rule out or confirm the misfires is happening ONLY on cylinders 1 and 4 or MOSTLY on cylinders 1 and 4. The ONLY unique items to check on those cylinders would be plugs, wires, cap, valves, compression. For a mostly condition that opens the amount of items to check up further to include injectors, vacuum leaks, etc.

 

In summary, PCM's don't get confused so I think you need to keep digging elsewhere. Try disconnecting the battery for a while to reset the PCM, might give you a warm fuzzy feeling about it.

[Kurt Johnson]

Indeed, asilverblazer; keep digging.  Outstanding advice.

I had felt all along that the PCM replacement was a novice diagnosis.  My head was spinning on this one.

Since I posted, I gave the truck back to the owner running on the MAP, to get his front end aligned while I pondered the situation.

When I first put the engine in, not wanting to have to go back in after all the accessories were installed, I started the engine with no coolant or water pump, and I didn't hook up the canister purge hose.   Plugged the purge solenoid instead.  It ran great, and I took some video at that time for my own sanity and to share with the owner.

I didn't think about this first start condition until long after becoming frustrated with the misfires.

Thinking about what was replaced, and what was in common and what could be wrong, I considered the cylinders misfiring.  1 and 4 are close in the order.   Hmmm...

Maybe the water crossover in the intake that was reused is leaking just enough for cylinder 1 and 4 to misfire as it gulps a bit of coolant, and that explains the lean o2 readings on bank 1?  Maybe the purge solenoid that was plugged on the first startup is stuck open and is making the intake charge too rich? 

Really the only 2 things I could think of aside from the PCM based on the first start success.

I went to his garage and tried the easy check first and plugged the purge solenoid with a vacuum plug.

The engine runs perfect.

I posted the solution, as I have found many times, people reach out, and the thread ends with no solution, only the symptom.

Thanks guys for being a sounding board and keeping me sane!!

NEVER GIVE UP!!!

 

Kurt