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1990 isuzu trooper ls
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Discussion Starter · #1 ·
Hello All,
Like the title says, my trooper is bogging down at low rpm and high load. I have replaced the O2 sensor, which made the issues less severe but its definitely still there. Im guessing that it just had a really rich condition making it more obvious before replacing the O2. There is also a CEL that comes on but resets when i turn the car off. This CEL comes on during extended highway speed driving. So I am imagining that the issues is something related to ignition or fueling. I can find any codes for my OBD 1 but I do have a video of it incase anyone knows them by heart…


Sorry about the camera work…


Thanks,
Leo
 

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Leo, unless the ECM is completely bonkers, it looks like maybe it's going into Diagnostic Mode while you're driving, and this is called "Field Service" mode. Description below, maybe that'll help:

FIELD SERVICE MODE:
If the diagnostic terminal is grounded with the engine running, the system will enter the "Field Service" mode.

In this mode, the MIL "SERVICE ENGINE SOON" light will indicate whether the system is in OPEN or CLOSED LOOP operation.

If the system is in OPEN LOOP operation, the light will flash approximately two and one-half times per second.

CLOSED LOOP operation is indicated by the light flashing approximately once per second. Also, in "CLOSED LOOP" the light will stay "OFF" most of the time if the system is running lean. It will stay "ON" most of the time if the system is running rich.

While the system is in the "Field Service" mode, new codes cannot be stored in the control module and the "CLOSED LOOP" timer is bypassed.


Maybe some of the wiring in the center console is messed-up or connected/disconnected when it shouldn't be. Note that there are 2 ways to put the ECM in Diagnostic Mode: 1) to jumper the pins in the ALDL plug and 2) to connect the (2) white wires with blue strips (does the same thing as shorting the pins in the ALDL plug).

So if the ECM is inadvertently in Diagnostic Mode and you start the engine, it'll then be in Field Test mode and the flashes have no relevance as far as codes are concerned, they're only indicating Open/Closed Loop and Rich/Lean.

When the ECM is in this mode the engine won't be running very well, either. Which is why I'm inclined to think there's a wiring/connection issue.

See the attached pic; the ALDL wires are shown as disconnected from each other. Also shown is the plastic inline connector for the ESC (Electronic Spark Control) circuit, shown connected. One wire running down the index finger is hard to see but it's there. Note that you'd disconnect this wire when checking ign base timing. But it's normally connected to enable spark control.

So I'd check the wiring and ALDL connector in the center console and make sure everything is sat.

Here's a reference list of Isuzu Trouble codes, any code annotated with a {5} is specifically for the GM TBI system.


Hope that helps..........ed

Hand Gesture Finger Thumb Nail
 

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A bit more info, I found a Corvette site with a good discussion on Field Service Mode and how to interpret the check engine light when in FSM:

Interpreting a Flashing MIL Lamp

If the engine controller is in open-loop mode, the MIL will flash extremely rapidly, about two-and-a-half times per second. If the engine controller is in closed-loop mode, it will flash about once per second.

When the engine is first started, the system goes into open-loop operation. In open loop, the engine controller will ignore the signal from most of the engine sensors and calculate the air/fuel ratio based on inputs from the coolant and MAP sensors, along with a pre-programmed memory.

The system will stay in open loop until the following conditions are met:

  1. The O2 sensor has varying voltage output, showing that it is hot enough to operate properly (approximately 600 degrees Fahrenheit).
  2. The coolant sensor is above a specified temperature (approximately 104 degrees).
  3. A specific amount of time has elapsed after starting the engine.
When these conditions are met, the system will go into closed-loop operation. In closed loop, the ECM will calculate the air/fuel ratio based on a variety of sensors and keep it as close to 14.7:1 as possible.

When in closed loop, if the MIL is off more than it is on, the engine is running lean. If the opposite is true, this indicates that the engine is running rich. These displays are based on readings taken by the oxygen sensor.

In a lean condition, the engine controller will command the block learn/integrator to enrich the fuel mixture above the 128-count baseline. In a rich condition, the block learn/integrator will lean out the fuel mixture below the 128 baseline. (This number can only be monitored using a scanner.)

This 128 baseline number on the ’82 through the ’95 Corvettes comes from monitoring the Block Learn Multiplier (BLM). The BLM is the long-term fuel adjustment that the engine controller “learns” in order to keep the air/fuel ratio within acceptable parameters. The integrator works using the same idea, only for short-term adjustments.

The BLM value can range from 0 through 255, with a value of 128 (the midway point) being ideal. Changes below 118 or above 138 usually indicate a problem.

There is another mode that can be accessed through the ALDL that can be helpful in rudimentary diagnostics. It’s known as the Backup Fuel Mode, and it can be entered by inserting a 3.9K resistor between terminals A and B of the ALDL connector pinholes.

Backup Fuel Mode—more commonly referred to as “Limp Home” mode—forces the engine controller to rely on set fuel calculations in the engine-controller PROM, instead of the learned inputs in active RAM. Backup Fuel Mode is usually set at around 12:1 or richer. If your engine seems to run better in this mode, there is likely a sensor that is reporting incorrect data to the engine controller.


You could use the FSM to your advantage by taking a test drive on a long, straight road. Such that the engine is fully warmed and hopefully it goes Closed Loop as well. The flashes will tell you what's going on and maybe will lead to a solution.

Take note of that last section above, if you can find a 3900-ohm resistor and insert in the 2 pins that you short together to achieve Backup Fuel Mode, you can drive it and see if it runs better. If it does, that points to a problem with a sensor.
 

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1990 isuzu trooper ls
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Discussion Starter · #4 ·
Thanks for your extensive information! What makes you think that it is it field service mode? Just to clarify I did connect the obd1 wires to get the CEL to flash like this. I then disconnected them again so my wires look like your picture regularly. The CEL comes on after cruising but doesnt stay on after turning the car off, so I left the truck on after a drive to get that read out in the video.
Could it be an ignition problem?
Leo
 

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1990 isuzu trooper ls
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Discussion Starter · #5 ·
A bit more info, I found a Corvette site with a good discussion on Field Service Mode and how to interpret the check engine light when in FSM:

Interpreting a Flashing MIL Lamp

If the engine controller is in open-loop mode, the MIL will flash extremely rapidly, about two-and-a-half times per second. If the engine controller is in closed-loop mode, it will flash about once per second.

When the engine is first started, the system goes into open-loop operation. In open loop, the engine controller will ignore the signal from most of the engine sensors and calculate the air/fuel ratio based on inputs from the coolant and MAP sensors, along with a pre-programmed memory.

The system will stay in open loop until the following conditions are met:


  1. The O2 sensor has varying voltage output, showing that it is hot enough to operate properly (approximately 600 degrees Fahrenheit).
  2. The coolant sensor is above a specified temperature (approximately 104 degrees).
  3. A specific amount of time has elapsed after starting the engine.
When these conditions are met, the system will go into closed-loop operation. In closed loop, the ECM will calculate the air/fuel ratio based on a variety of sensors and keep it as close to 14.7:1 as possible.

When in closed loop, if the MIL is off more than it is on, the engine is running lean. If the opposite is true, this indicates that the engine is running rich. These displays are based on readings taken by the oxygen sensor.

In a lean condition, the engine controller will command the block learn/integrator to enrich the fuel mixture above the 128-count baseline. In a rich condition, the block learn/integrator will lean out the fuel mixture below the 128 baseline. (This number can only be monitored using a scanner.)

This 128 baseline number on the ’82 through the ’95 Corvettes comes from monitoring the Block Learn Multiplier (BLM). The BLM is the long-term fuel adjustment that the engine controller “learns” in order to keep the air/fuel ratio within acceptable parameters. The integrator works using the same idea, only for short-term adjustments.

The BLM value can range from 0 through 255, with a value of 128 (the midway point) being ideal. Changes below 118 or above 138 usually indicate a problem.

There is another mode that can be accessed through the ALDL that can be helpful in rudimentary diagnostics. It’s known as the Backup Fuel Mode, and it can be entered by inserting a 3.9K resistor between terminals A and B of the ALDL connector pinholes.

Backup Fuel Mode—more commonly referred to as “Limp Home” mode—forces the engine controller to rely on set fuel calculations in the engine-controller PROM, instead of the learned inputs in active RAM. Backup Fuel Mode is usually set at around 12:1 or richer. If your engine seems to run better in this mode, there is likely a sensor that is reporting incorrect data to the engine controller.


You could use the FSM to your advantage by taking a test drive on a long, straight road. Such that the engine is fully warmed and hopefully it goes Closed Loop as well. The flashes will tell you what's going on and maybe will lead to a solution.

Take note of that last section above, if you can find a 3900-ohm resistor and insert in the 2 pins that you short together to achieve Backup Fuel Mode, you can drive it and see if it runs better. If it does, that points to a problem with a sensor.
Or is it a possibility that the 2.8 just feels like it bogging down constantly? Haha. I mean thats a joke obviously, but I really dont have a reference point in what a healthy engine should preform like.
 

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Just to clarify I did connect the obd1 wires to get the CEL to flash like this. I then disconnected them again so my wires look like your picture regularly. T Leo
OK, well it is relevant to know that the wires were deliberately connected. If you drive it in Diagnostic Mode, that is what you're gonna get, "Field Test Mode". You normally wouldn't run it this way.

Question, can you can the Check Engine light to flash trouble codes with the ALDL wires connected/ALDL Plug's pins jumpered?

If it does flash as it's supposed to with the engine "ON" but not running, then the ECM is OK. If it doesn't flash the ECM may be bad.

A normal trouble code is gonna be (2) digits, repeated twice for each code. It's normal to get code 12 (one flash, pause, flash-flash) in Diagnostic Mode with key "ON" and the engine not running.

Any other stored codes will flash in succession, three times per each code.

Let me know if you do have any more trouble codes stored............ed

p.s. if you're getting a code while driving (i.e. the light comes on steady), even though that may go away when you turn the key off and then on again, it should have set a trouble code in the ECM. You should be able to go into Diagnostic Mode and read that code.
 

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Discussion Starter · #7 ·
OK, well it is relevant to know that the wires were deliberately connected. If you drive it in Diagnostic Mode, that is what you're gonna get, "Field Test Mode". You normally wouldn't run it this way.

Question, can you can the Check Engine light to flash trouble codes with the ALDL wires connected/ALDL Plug's pins jumpered?

If it does flash as it's supposed to with the engine "ON" but not running, then the ECM is OK. If it doesn't flash the ECM may be bad.

A normal trouble code is gonna be (2) digits, repeated twice for each code. It's normal to get code 12 (one flash, pause, flash-flash) in Diagnostic Mode with key "ON" and the engine not running.

Any other stored codes will flash in succession, twice-per.

Let me know if you do have any more trouble codes stored............ed

p.s. if you're getting a code while driving (i.e. the light comes on steady), even though that may go away when you turn the key off and then on again, it should have set a trouble code in the ECM. You should be able to go into Diagnostic Mode and read that code.
Oh, my mistake! Ok so the codes I pulled are 13, 32, and 45.
 

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The above table lists multiple models and attributes for various codes, it's either noted as applicable to V6, applicable to all if nothing else is noted, or you'll see this: {5} which means it's specifically for the GM TBI V6.

OK, here we go:

Code 13: Oxygen sensor or circuit Since you already replaced the sensor, either the old code is lodged in the ECM and hasn't cleared yet, or you have another problem with the circuit. You can clear codes by disconnecting the battery ground cable, then turn and leave the key "ON" for at least 30 seconds. If any codes come back after a time of operation, then that code is "hard". If the code stays clear, then you're good on that one.

Code 32: Exhaust Gas Recirculation (EGR) system failure Could be a stuck EGR valve, plugged EGR passages, bad EGR vacuum control solenoid, leaking or mis-routed vacuum lines, or an electrical control circuit issue. Common to see EGR valve either stuck or leaking. Pull the valve, check passages, put vacuum on the valve diaphragm to see if it opens, etc.

BTW I bet this is the Check Engine light that's coming on after a long drive then goes off. Mine will do the same thing, and I think it's because the ECM is tuned to a 2.8's exhaust emissions and when it's "sampling" EGR after a long driving period, it's not seeing the results it's expecting. But IDK. I just turn the danged key off then on and it turns the light off for a while! Other than that it doesn't affect how the engine runs at all. Go Figure. Maybe yours is a sticky EGR, you never know.

Code 45: Oxygen sensor (rich condition indicated) Maybe residual code from sensor replacement? Or other problem causing rich running I suppose.

Maybe the thing to do is clear all codes and then take it for a drive and see what (if any) "hard" codes get logged by the ECM.
 
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