High idle miss fire

Chebby k3500
the last TBI, 1988 the stills runs, id be happy it ran at al,
causes well many (yes I know EFI well back to 80s,)
fails lean at idle hot and at 2500 hot, in PARK no load? on engine;
no closed loop at 2500 rpm? we test at 3 speeds and even moving fast easy cruise.

• the O2, only works hot engine. if no heater inside the O2 it wakes up on hot engine only..
• if 02 will not swing, around 0.45vdc that means it's not in closed loop. and have vast causes but for lean only I post here.
• Exhaust leaks near the front 02 will cause rich burn not lean.
• so what causes lean, so lean the 02 command authority is OUT OF RANGE lean,<< EFI tech words,
• fuel pressure too low. (spec too low) FPR bad or weak pump or clogged filters (in part)
• Fuel bad. or this nasty E10 fuel with 10% ethonal wreck fuel line devices and fails , as in all plastic parts end to end fuel.
• Injectors partially clogged, some of theses old TBI relics have idle injector and if clogged part way goes lean.
• Lean this the ECU does not know when injectors clogged or fuel pressure is wrong. ever, it assumes all that is correct and fails hard when wrong, Fuel pressure at idle must be in spec and at 2500 rpm and while driving fast..

learn first that the 02 is only very find trim of fuel raters
it basically compensates for MAP reading bit off.
or fuel pressure a tad off, not way off
the MAP sensor is bad will make it run lean sure. or rich.
make sure the MAP vacuum hose is new and not cracked
and sucks on both ends, a vacuum there, 20"inches HG. vacuum at idle.
Old maps love to fail, Id bet none this old are good.
it has baro diaphragm inside the MAP that when old gets stiff
and or cracks, ( one of the worst things to happen.)
MAP pin 3 power is 5vdc keyon and pin 1 0v ground
MAP pin 2 is output, and key on , not started. goes like this.
the map can be tested 3 ways pin2 back probed, Ive done many.

1. keyon, not running, 1.7v (par)
2. at idle to 4vdc,

MAP is a baro sensor in shop terms is measures the state of vacuum (Hg") in the engines plenum that area just below the TB.
the key on and idle readings are key test.
this magic vacuum is engine load, as vacuum drops the PCM
add fuel matching the exact vacuum read and is a linear device.
here is my old JEEP Map from 1988 I made then.
what you GM part does I am not sure,yet but will look it up later
20 is idle, 23 is using the engine as Jake brake, down a hill
4.5v or 0 vacuum is wot, WIDE OPEN THROTTLE.
SOME MAKERS OF CAR USE REVERE BARO MAP SENSOR.
IF you like Bar or KPa, we can convert that easy.


not how linear it is, and must be. or it is bad.
the vacuum hose fails first, 10+ old;. most do. and yours is 3 times that.some hoses collapse inside. ouch.

Usually its too much fuel.
Is the check engine light on ?

You have to correct the cylinder misfires first

You may have vacuum leaks somewhere in the
intake or exhaust

You really need to look at data with a scan tool

You start with Mode $06 & cylinder misfire count ,if that
even works well on your motor

Then you look at fuel trim & add short & long together,
to get an idea if it is running rich,which would be a lean
condition from a vacuum leak or rich from leaking injectors

All the data is only good if you haven't cleared it or disconnected
the battery. If you did, then you have to drive until all I/M Monitors
run & then look at data

You look at O2 sensor data & even scope them

You made no mention of checking for codes
No mention of a cylinder leak down test for valve leaks
Exhaust Converter age or condition !
Fuel Pressure !

how about a PO133 ,,,should be the first in system

Schematic
The Oxygen Sensor 1 (O2S-1) is an electrical source that responds to oxygen content in the exhaust manifold. When the sensor reaches approximately 316°C (600°F), it produces a voltage based on the difference in oxygen between the atmosphere and exhaust gas. The PCM sends a bias voltage (391-491 mV) on the signal line which is pulled up through high resistance. When the O2S-1 is cold, it produces no voltage and has extremely high internal resistance. The internal resistance of the sensor is much greater than the resistance of the bias pull-up resistor. However, when the sensor heats up, it produces voltage that overrides the bias voltage. This voltage is read by the PCM to determine a rich/lean O2S-1 signal used to adjust injector pulse width. Under normal conditions, low sensor voltage means high oxygen content/lean air-fuel mixture and vice versa. Normal sensor readings will fluctuate between 10 mV and 999 mV DTC P0133 sets when the O2S-1 signal rich to lean or lean to rich average response time is too slow.

DTC PARAMETERS
DTC P0133 will set if the average response time of the LEAN to RICH switches is greater than 125 ms or the average response time of the RICH to LEAN switches is greater than 156 ms for 100 seconds when:
^ Engine speed is between 1500 and 3200 RPM
^ Loop status is closed
^ Calculated air flow is greater than 7 gm/s
^ Commanded air/fuel ratio is 14.7 to 1
^ No cam, CKP, ECT, EGR, EVAP purge solenoid, fuel trim, IAT, MAP misfire, system voltage or TP sensor DTCs have been set.

DTC P0133 diagnostic runs continuously once the once the above conditions have been met.

DTC P0133 is a type B DTC.

DIAGNOSTIC AIDS
Possible causes of DTC P0133:
^ An intermittent connection or corrosion in the O2S-1 harness connector can set this DTC. Use Scan tool to monitor O2S-1 voltage with engine running at normal operating temperature while wiggling the signal and ground wire. Make sure the sensor is tight.
^ The most probable cause for DTC P0133 is contamination. Check for obvious contamination (oil, fuel or engine coolant) by removing sensor. Identify and correct the cause of the contamination if contaminated. Replace the O2S-1.
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should be in the 750-1000 range, just an adjustment on the idle probably

Do you have the idle speed turned up to keep it from dying? Because it is high. Check engine light on? Have you checked for vacuum leaks at the gasket under the throttle body? How does the car run under acceleration? You might have a worn distributor shaft.

Before we both knock ourselves out with fuel injector testing. Hook up a Digital voltmeter (DVM) to upstream O2 sensor and lets see how she is burning. Remember positive lead of DVM to signal wire and negative lead to ground. Anything above 550 mV is rich anything below 350mV is lean. Be mindful that an exhaust leak the size of a pin hole in this region can cause a rough idle. A healthy reading should bounce between 550mV and 350 mV.
Next to test is O2 is working block off air in take making mixture rich and see what O2 says about it . Then pull off a vacuum hose (a lean condition) and see if DVM drops to 200to 300mV's.
Keep me posted.

Did you clear the code first after replacing and did you replace the spark plugs. If the plugs are fouled the cylinders arnt firing and the excess air is going over the 02 sensor and since it PCM cant command enough fuel it stays lean.

check for vacuum leaks especially around the intake manifold gasket. Although the engine appears to be running rich, it could also be misfiring due to being too lean in one cylinder. When the cylinder doesn't fire the charge, it goes into the exhaust completely unburned and looks like a rich running condition due to the unburned fuel. If it is a vacuum leak causing the problem, the engine will run fine at road speeds but misfire at idle and not pass smog.

check timing and set idle to 650rpm