1988 k2500 goes into "limp mode" when accelerating going uphill

[TheKOOK]

I have a 1988 k2500 tbi that has been giving me some grief lately. The truck starts and idles very well and drives like normal for a few miles then starts acting up. For example the other day I drove the truck to work which is about seven miles from my house. The started fine in the morning and I drove it to work with no issues. however when driving back home from work about half a mile from my house the truck falls flat on its face, no power and when i go to accelerate it starts popping like its back firing im only able to go about 20-30 mph max and i noticed right when i lost power the SES light came on. so when I got home I checked for codes and I'm getting a code 44 and code 51. I know code 44 is a lean code and code 51 is PROM/MEM-CEL error. I had code 51 when I first bought the truck back in April of this year so I replaced the ecm and prom with another one out of the same exact truck as mine, I made sure the numbers on the ecms matched. The part that is stumping me is if I pull over and turn off the key when this issue happens and then immediately start the truck back up the light goes away and the truck runs normal until several miles then it happens again. Also Id like to point out this issues seems to only happen when im going uphill and accelerating but not every time I accelerate up a hill, its a very intermittent issue.

 

[movietvet]

What is the fuel pressure when this is happening? Possible clogged catalytic converter? Condition and age of tune related components?

 

[TheKOOK]

The spark plugs are all new, new spark plug wires, new distributor cap and rotor. I haven't checked what the pressure is when the issue happens but i did test it while it was idling and after the fuel filter and it was at 12 psi. Ill have to check for a clogged converter.

 

[docstoy]

Pull vacuum line off of egr valve and see if any change, also inspect O2 sensor see if tip of sensor is white, could be silicone contamination on O2 sensor, but as movievet stated could also be exhaust or fuel relayed

 

[evilunclegrimace]

Change the fuel filter.

 

[1998_K1500_Sub]

I'm still wondering about the ECU, why that persistent code 51 that you've had with two different ECUs.

Is the ECU receiving stable power? I wonder. A weak or poor connection to the ECU, either ground or power, might trigger a code 51...

and, if that was the case, the problem would persist even if you swapped ECUs (which you did).



If you connect a scan tool to your ECU, will it report a voltage measurement in the stats? I know in later ECUs (e.g., 1995) this stat is present.

Why is it dependent on load (uphill drive)? Or is that just a coincidence? An uphill drive might trigger "power enrichment", so more fuel...

I'll keep thinking.


(edit) Thoughts:

The injectors are actuated by grounding them at the ECU. A weak or poor ECU ground connection might trigger both a "lean" condition (injector not fully opening / unable to deliver larger amounts of fuel) and potentially an ECU code 51.

Are your LT fuel trims showing any odd numbers?

Lack of exhaust gas in the EGR system, when it's engaged, would cause a lean condition too (too much oxygen present).

This one's an enigma.

 

[TheKOOK]

Change the fuel filter.

The fuel filter is three weeks old

 

[TheKOOK]

I'm still wondering about the ECU, why that persistent code 51 that you've had with two different ECUs.

Is the ECU receiving stable power? I wonder. A weak or poor connection to the ECU, either ground or power, might trigger a code 51...

and, if that was the case, the problem would persist even if you swapped ECUs (which you did).



If you connect a scan tool to your ECU, will it report a voltage measurement in the stats? I know in later ECUs (e.g., 1995) this stat is present.

Why is it dependent on load (uphill drive)? Or is that just a coincidence? An uphill drive might trigger "power enrichment", so more fuel...

I'll keep thinking.


(edit) Thoughts:

The injectors are actuated by grounding them at the ECU. A weak or poor ECU ground connection might trigger both a "lean" condition (injector not fully opening / unable to deliver larger amounts of fuel) and potentially an ECU code 51.

Are your LT fuel trims showing any odd numbers?

Lack of exhaust gas in the EGR system, when it's engaged, would cause a lean condition too (too much oxygen present).

This one's an enigma.

I was going to check all the ecu grounds and wires this weekend. I’ve also heard that a bad ignition switch may fail to send power to the ecu and my ignition switch is aftermarket so I’m going to check that too

 

[1998_K1500_Sub]

I was going to check all the ecu grounds and wires this weekend. I’ve also heard that a bad ignition switch may fail to send power to the ecu and my ignition switch is aftermarket so I’m going to check that too

I like your approach. That code 51 on the ECU (both of them) is telling me that you’ve got a problem someplace else, likely electrical, not in the ECU, and that external problem is causing duress for ECU and so it’s reporting code 51.

The coincident code for the lean condition may be a byproduct of whatever is causing the code 51 in the ECU.

Remember, the ECU can run on low voltage… it functions when the starter’s engaged, so that’s evidence that it can function on low voltage.

But the ECU is likely intolerant of an intermittent loss of power, e.g., due to an intermittent bad ground or power supply.

I can see how an intermittent loss of power might put the ECU’s processor into reset mode, e.g., where it tries to reread the PROM, and because the voltage is erratic, the PROM read fails to pass the checksum … so the ECU thinks there’s a PROM problem and set code 51.

 

[TheKOOK]

I like your approach. That code 51 on the ECU (both of them) is telling me that you’ve got a problem someplace else, likely electrical, not in the ECU, and that external problem is causing duress for ECU and so it’s reporting code 51.

The coincident code for the lean condition may be a byproduct of whatever is causing the code 51 in the ECU.

Remember, the ECU can run on low voltage… it functions when the starter’s engaged, so that’s evidence that it can function on low voltage.

But the ECU is likely intolerant of an intermittent loss of power, e.g., due to an intermittent bad ground or power supply.

I can see how an intermittent loss of power might put the ECU’s processor into reset mode, e.g., where it tries to reread the PROM, and because the voltage is erratic, the PROM read fails to pass the checksum … so the ECU thinks there’s a PROM problem and set code 51.

I’ll report back what I find. I’m also going check the ecu itself and make sure the prom is seated correctly and the pins are in good shape.