'96 C1500 4.3/4L60E - Misfire after Trans Swap

[AboveTheLogic]

Just got finished swapping in a rebuilt transmission and the engine is running/idling rough, is very low on power, and threw the following codes on my Actron CP9185:

-P0151 O2S Cat Low Volts 2/1
-P0300 Random/Multiple Cylinder Misfire Detected

Today I went back under there and inspected the O2 sensor wiring and reseated the Bank 2 Sensor 1 connector, then cleared codes and started it up. Misfire is noticeable at idle in park, with no codes thrown yet. This makes me think the O2 sensor is not actually the problem, but the code is a symptom of another problem.

The truck sat in the garage with the battery disconnected for about 2 months while I removed/replaced the transmission and had it rebuilt. Engine ran fine with some mild shuddering when accelerating off idle when cold before all this.

This makes me think something got jostled during the swap, but I can't think or what it is. It did take me some time to get the dipstick lined up which involved pushing a prybar up from behind the car to move the wiring looks back there aside.

On my to-do list already is a swap of the fuel filter. I bought this truck 6 months ago and never did that. The gas tank was nearly empty when I test drove it yesterday and it took 21 gallons of fuel to fill back up - which is odd since I don't remember running it quite that low.

I think my next steps will be to replace the fuel filter and monitor the fuel pressure levels for low pressure, or I guess if the pressure doesn't hold after shutting the engine off it could be leaky injectors. I'm hoping my scanner will show the fuel pressure readings, connecting a gauge is sounds like the opposite of fun.

Truck has about 91,000 miles. Not sure if it matters, but the trans would not shift past 2nd gear which led me to pulling it out.

Any ideas/comments are welcome, thank you.

 

[AboveTheLogic]

This is interesting. Scan tool doesn’t show fuel pressure, but does show O2 sensor voltage.

That one is lower than the other three, and it is literally right next to another one.

Maybe that’s the problem.

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[AboveTheLogic]

Another observation - under light throttle the engine has almost no power, but at somewhere around 70-80% throttle it wakes up suddenly and depending on the situation will downshift and take off.

Almost like the ECU is in some sort of limp mode that it ignores at a certain throttle position.

 

[Schurkey]

Just got finished swapping in a rebuilt transmission and the engine is running/idling rough, is very low on power, and threw the following codes on my Actron CP9185:

-P0151 O2S Cat Low Volts 2/1
-P0300 Random/Multiple Cylinder Misfire Detected

Misfire causes false-lean signal from an O2 sensor.

Is this scan tool capable of showing individual-cylinder misfire codes? How about individual-cylinder misfire COUNTS?

Misfire is noticeable at idle in park, with no codes thrown yet. This makes me think the O2 sensor is not actually the problem, but the code is a symptom of another problem.

Likely. Not guaranteed, but very likely.

OTOH, how old are the O2 sensors? Old sensors get goofy, and often they drift to the lean side of the scale.

The truck sat in the garage with the battery disconnected for about 2 months while I removed/replaced the transmission and had it rebuilt. Engine ran fine with some mild shuddering when accelerating off idle when cold before all this.

This makes me think something got jostled during the swap, but I can't think or what it is.

First Guess: Engine slouches to the rear when the transmission is gone--no transmission or transfer case mount to hold the back-end of the engine in place. Cracks the distributor cap. On a Vortec, the whole plastic distributor is at risk.

On my to-do list already is a swap of the fuel filter. I bought this truck 6 months ago and never did that. The gas tank was nearly empty when I test drove it yesterday and it took 21 gallons of fuel to fill back up - which is odd since I don't remember running it quite that low.

Short-bed or long-bed? A long-bed pickup would have a 30-something gallon tank.

I think my next steps will be to replace the fuel filter and monitor the fuel pressure levels for low pressure, or I guess if the pressure doesn't hold after shutting the engine off it could be leaky injectors.

Reasonable. Loss of fuel pressure could also be a failed fuel pump (check valve) or failed pressure regulator.

I'm hoping my scanner will show the fuel pressure readings, connecting a gauge is sounds like the opposite of fun.

Get prepared to have no fun. There is no fuel pressure sensor. You could look at short- and long-term fuel trims to see if the computer is adding fuel due to low fuel pressure, though.

This is interesting. Scan tool doesn’t show fuel pressure,

See? I told you.

but does show O2 sensor voltage.

That one is lower than the other three, and it is literally right next to another one.

Maybe that’s the problem.

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Not sure what they mean by O2 sensors for Bank One, sensor one, sensor two, and sensor three; while Bank Two has only sensor one.

 

[Erik the Awful]

I'd guess Bank 2 is your downstream O2 sensor. It just checks to ensure the catalytic converter is functioning. It's not your problem.

 

[AboveTheLogic]

Is this scan tool capable of showing individual-cylinder misfire codes? How about individual-cylinder misfire COUNTS?

It does not appear so - which is disappointing since it seems to be a higher end scanner.

OTOH, how old are the O2 sensors? Old sensors get goofy, and often they drift to the lean side of the scale.

They all seem original. I did replace the Bank 2 Sensor 1 O2 sensor and it doesn't seem to have made a difference. There goes $60. Maybe its due for all new sensors anyways but I'm not trying to throw more money at it just yet.

First Guess: Engine slouches to the rear when the transmission is gone--no transmission or transfer case mount to hold the back-end of the engine in place. Cracks the distributor cap. On a Vortec, the whole plastic distributor is at risk.

This is VERY plausible, thank you for the suggestion. I will check the distributor cap. Considering the O2 sensor throwing the code only picks up exhaust from the passenger side, this makes the most sense as the problem seems to be on that side. Would not really be the case if it was a fuel pressure or delivery issue which is what I was getting ready to check next. Would also not be isolated to one side if it were a crank sensor or a crank relearn or any of the other things I read about.

Short-bed or long-bed? A long-bed pickup would have a 30-something gallon tank.

Regular Cab, Short Bed. Just the way I like 'em.

Reasonable. Loss of fuel pressure could also be a failed fuel pump (check valve) or failed pressure regulator.

This is my next step. I have a replacement fuel filter on its way right now too.

Not sure what they mean by O2 sensors for Bank One, sensor one, sensor two, and sensor three; while Bank Two has only sensor one.

I'd guess Bank 2 is your downstream O2 sensor. It just checks to ensure the catalytic converter is functioning. It's not your problem.

I am reasonably confident it goes like this:

Bank 1 Sensor 1 - Drivers side pipe near the manifold.
Bank 1 Sensor 2 - Before the catalytic converter.
Bank 1 Sensor 3 - After the catalytic converter.
Bank 2 Sensor 1 - Passenger side pipe near the manifold (which is RIGHT next to Bank 1 Sensor 2).

Bank 2 Sensor 1 is the sensor that threw the code and I replaced.

 

[AboveTheLogic]

I have updates.

This is the fuel pressure at idle.This is before installing the replacement fuel filter I ordered. With the ignition off, I saw it creep down to about 50psi after about 5 minutes.

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The pressure fluctuated between 60-70 psi while I ran the engine in drive with my foot on the brake, revving the engine a bit and feeling it misfire. I don't think fuel pressure is the problem.

Here are two photos of the distributor cap. It does not look cracked or damaged at all. Just old. Keep in mind, this problem really started after the trans swap.

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I'll replace that because it needs to be replaced, but would be more confident that its the problem if I saw a crack.

Here is what the ground under the tailpipe looked like after 10 minutes of testing. I'm guessing that's fuel and it's running rich. I tried to light it on fire, it didn't light.

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Lastly, here is a video of me running through all the options of my scan tool while idling and the misfire happening...

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[Erik the Awful]

I am reasonably confident it goes like this:

Bank 1 Sensor 1 - Drivers side pipe near the manifold.
Bank 1 Sensor 2 - Before the catalytic converter.
Bank 1 Sensor 3 - After the catalytic converter.
Bank 2 Sensor 1 - Passenger side pipe near the manifold (which is RIGHT next to Bank 1 Sensor 2).

Bank 2 Sensor 1 is the sensor that threw the code and I replaced.

It's been a few years since I had a Vortec GMT400, but I know that's not right. You need to dive into the manual.

 

[AboveTheLogic]

I was going off a diagram I found online for a 4.3 Vortec S10, couldn’t find one specifically for a C/K1500 4.3.

Just ordered a used chitons manual and will take a closer look.

I hope you’re right and I have it wrong and it’s a matter of replacing a different O2 sensor.

 

[AboveTheLogic]

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I suppose just looking at it this way, it seems 1,1 and 2,1 would be the other way around