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Author Topic: LT1 Lean issue  (Read 109 times)
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Jon
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« on: June 08, 2018, 07:40:23 PM »

So I've been having an issue on the 96 for a few weeks.  I suppose I'll back up a bit and tell the story from the beginning:
Last November I had the Check Engine light which revealed a P0157 (Bank 2 Sensor 2 Low O2 Sensor Voltage).  I cleared the codes and it never came back until May.
What I noticed was my fuel economy is getting better, when I'm driving I have low power in the 0%-20% Throttle position but when I go up to 40% and high Throttle it surges to life.  Since most of my drive is highway it's not really causing a problem but I want to fix it.
I hooked up a scan tool today and found it has both P0137 and P0157 (Bank 1 and Bank 2 Rear sensor codes) and the O2 sensor readings are too low (0mV for Bank 1 and 15mV for Bank 2).  When the exhaust is cold the sensor slowly goes down to 0 so I know the circuit isn't bad.
The Fuel trim was 0% on Bank 1 and 3% on Bank 2.
I didn't see any problems with TPS reading, it seemed smooth all the way through pedal travel.
The Service manual points to a couple options:
1. Leaky exhaust (some small leaks where I put the new pipes on,  but that's downstream of the sensor and I don't see it affecting the driveability).
2. Sensor Wire,  but again, it shouldn't affect driveability and both sides are low.
3. Lean injectors, but it would be a coincidence if both sides have lean injectors
4. Fuel pump pressure.  This one seems to be able to explain the driveability and the low voltage on both sides,  but I thought a low fuel pressure condition would cause worse driveability at higher RPM where I'm seeing plenty of power.
5. Bad Sensor,  which is strange for both sides to go, and I don't think these sensors are affecting fuel trim so it doesn't explain the driveability issue.

I checked the misfire data and even when I have it under load and partial throttle (standing on brake pedal in gear with a slightly raised RPM) it feels like it's stumbling but all cylinders are reporting no misfires using the scan tool.
Any thoughts?   I haven't been able to find anyone around here with a fuel pressure gauge and I haven't gone out and bought one yet.
« Last Edit: June 08, 2018, 07:42:45 PM by Jon » Logged

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madcaddyman
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« Reply #1 on: June 09, 2018, 08:20:33 AM »

If it was a LO5 I'd say timing but being an LT1 it's unlikely. I've seen something like this before but I can't remember if it was a GM or Ford, but it was a pick-up & a new crank censor corrected the issue. Maybe FPR, low rpm its restricted but high rpm the diaphragm opens up? Sorry I can't be more help.

Rene
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EXCRUISERGUY
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« Reply #2 on: June 09, 2018, 10:39:22 AM »

Have you checked the temperature sensor at the water pump?

GJ
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KevinW
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« Reply #3 on: June 09, 2018, 12:13:09 PM »

Are those fuel trims Block Learn or Integrator values?  These parameters may not be reliable if the system isn't entering closed-loop, which requires a number of conditions to be met, one of which is an oxygen sensor voltage above a certain value within a range of throttle positions.

Downstream exhaust leaks will cause a lean-bias in the sensor readings due to reversion.

Fuel system delivery is probably fine if it runs strongly at high throttle positions.
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Jon
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« Reply #4 on: June 09, 2018, 12:42:52 PM »

The system was in closed loop while I was monitoring the values.
I did a full clean of the MAF today and it seems to be driving better, I didn't feel any hesitation. I'll need to do a few drives before I'm confident the driveability is good.
I rechecked the values and the O2 readings are 8mV for Bank 1 and ~30mV Bank 2.
It's still low but it has gone up.
The 8mV for Bank 1 was not fluctuating at all while Bank 2 was bouncing around a little bit.
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68 Buick Wildcat Custom Convertible 455 4bbl (Wild Kitty)
85 Cadillac Fleetwood Brougham (Chester)
92 Buick Roadmaster Wagon (Peach)
96 Buick Roadmaster Wagon Collector's Edition (Rocky)
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« Reply #5 on: June 09, 2018, 04:42:28 PM »

Are those Sensor 1 or Sensor 2 readings?  Post-cat O2 voltages are just used to monitor catalyst efficiency, not for closed-loop fuel control.

A dirty MAF would result in B/L values well above 128 at part throttle while in C/L.
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Jon
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« Reply #6 on: June 09, 2018, 07:22:42 PM »

The next time I get a chance to work on it I'll try and seal the exhaust leaks.
It's the post cat sensors so they are not controlling the A/F ratio.
I've got exhaust leaks on both sides so hopefully this issue was a mixture of exhaust leaks and dirty MAF.
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68 Buick Wildcat Custom Convertible 455 4bbl (Wild Kitty)
85 Cadillac Fleetwood Brougham (Chester)
92 Buick Roadmaster Wagon (Peach)
96 Buick Roadmaster Wagon Collector's Edition (Rocky)
GM Engineering
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