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

[1998_K1500_Sub]

I wonder if there's a way, while a scan tool is connected, to ascertain if the ECU resets / goes through re-initialization. I've never had to look for this occurrence and I'm not certain how to go about it.

Perhaps there's a persistent stat that the ECU increments very time it (re)inits, that one could query with a scan tool.

Even an "uptime" stat would help.

@Road Trip, you might find this thread interesting too, if you haven't seen it already.

 

[PlayingWithTBI]

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.

^^^This. I'd clean and confirm all grounds, especially the ones on the T-Stat bolt(s). Those go to the ECM. Make sure you have the ground going from the back of the passenger's side head to the firewall.

Then, if that's not the problem, put a scanner on it. You have to find the Code 51 issue before going anywhere else.

 

[PlayingWithTBI]

Also, when you turn the KOEO does the SES light come for a moment, go off, then come on steady? If it doesn't you definitely have an issue with your ECM/BCC (chip) communications. Whether it's hardware, wiring, or software is unknown.

 

[Road Trip]

I wonder if there's a way, while a scan tool is connected, to ascertain if the ECU resets / goes through re-initialization. I've never had to look for this occurrence and I'm not certain how to go about it.

I've been wondering the same thing. The first system I learned to troubleshoot at the microcode
level was a PDP-11/70, and as the power came up & ACLO was negated, it always started running
at microaddress 0200. (Bit 7 set, all others clear in Octal) In the microcode flows this was
identified as 'ZAP 200'.

Back when I was in the first level of support, within seconds I could figure out what was ailing the
CPU that the onsite guy couldn't get running no matter how many boards he had thrown at the
central processor. He was thinking hung machine = bad CPU with a logic failure.

When I would see it stuck at ZAP 200 I knew that it was a good CPU waiting for the signal from
the power supplies that there was enough power for it to safely start logical operation. I would
immediately start heading towards the power supplies feeding the CPU, instead of the CPU itself.

****

Apologies for the historical detour, but your statement about a place to look where we could
check for the # of inits that's occurred since the initial Key On would be extremely informative.
I understand that the ECM/PCM/VCM is a dedicated realtime system, and there's no onboard TOY
clock, so I shouldn't expect there to be a detailed timestamped logfile that I can access...but I sure
wish there was. :0)

Even so, I wish that there was some sort of white paper/SAE paper where the basic overall
HW/SW architecture behind the TBI ECM/PCM and Black Box VCM was available. (And especially
the next-gen 0411.) By knowing this stuff, we could make sense of weird symptoms by figuring
out that something hung at a higher priority was casting long shadows on lower priority processes
that were essential to successful everyday operation. This level of knowledge helps to make sense
out of otherwise counterintuitive system behavior. (!)

So, yeah, if any of the long-time GM ECU spelunkers in here can point me in the direction of this level of
documentation I would be much obliged. For I spent an entire career proving to myself time & again that
it takes a long time to fix something that's not broken. (ie: Troubleshooting success really depends upon
translating detailed symptom observations into a good problem definition at the outset...and the tighter
the better. :0)

PS: There has to be a 'so many millisecond' window where the power has to be perfect in order to not get
a nonsensical result from the POST (Power On Self-Test) routine. Otherwise, you are quickly led down the
troubleshooting rabbit hole. This is one of the holes in the "I'll have the single computer test itself on the
way up" strategy. (Works fine 99% of the time, but not if there's flaky power being fed to it at the exact
wrong time.) EDIT: Some systems only test once during power up, while others will continue this level of
confidence testing periodically inbetween injector calculations and spark plug firings. It all depends upon
how many extra CPU cycles are available vs what's needed to keep the engine running. (!)

Good stuff. I look forward to a successful resolution that we can all learn from.

Cheers --

 

[TheKOOK]

Also, when you turn the KOEO does the SES light come for a moment, go off, then come on steady? If it doesn't you definitely have an issue with your ECM/BCC (chip) communications. Whether it's hardware, wiring, or software is unknown.

The SES light comes on when the key is turned to the accessory position then turns off when the truck is started. I only see the SES light when the truck loses power and goes into the alleged limp mode. However if the truck is turned off and then started again the SES light will not be there

 

[TheKOOK]

Here’s a question when I installed the ecm is there a step where it needs to be updated or initialized so it can communicate with my truck or does it do that on its own once it’s installed?

 

[Road Trip]

The SES light comes on when the key is turned to the accessory position then turns off when the truck is started. I only see the SES light when the truck loses power and goes into the alleged limp mode. However if the truck is turned off and then started again the SES light will not be there

The first part is expected behavior and denotes good communication between the ECM and the SES light.
(And also if you are seeing the flashing 12 when Pin A & B are jumped together in the ALDL.)

Here’s a question when I installed the ecm is there a step where it needs to be updated or initialized so it can communicate with my truck or does it do that on its own once it’s installed?

Assuming good power, grounds, & pin seating, a previously functional ECM of our vintage should take off and
run with no further ado.

****

By the way, have you taken a look at the coloring of your spark plugs? Does the color of the porcelain confirm
the Lean code? Are they glazed & possibly misfiring under load? (Misfires will cause a 'False Lean' to be reported.)

Thinking about your symptoms and the fact that you have already checked fuel pressure, I'm wondering about
your O2 sensor? Has it been recently replaced, or does it harken back to the assembly line? (Nearly all POs will
give the O2 sensor a good leaving alone.) I'm basically echoing the thoughts in reply #4.

Currently this issue is occurring with the O2 sensor connected and (I'm assuming) that we are in Closed Loop mode.
I would be interested to find out if you were to temporarily disconnect the O2 sensor would the problem clear up,
remain unchanged, or get worse?

If the engine were to clear up while the disconnected O2 sensor is forcing Open Loop operation, then we know that you have a
mechanically good engine, good fuel delivery & ignition system, and the engine runs well from the factory calibration
in the ECM stored tables, and that the root cause is that the good powerplant is a victim of a bad feedback loop. (O2 sensor)

****

Whether or not your engine will run properly Open Loop vs Closed Loop will tell us a lot.

Give this a whirl and let us know what you discover.

 

[TheKOOK]

The first part is expected behavior and denotes good communication between the ECM and the SES light.
(And also if you are seeing the flashing 12 when Pin A & B are jumped together in the ALDL.)




Assuming good power, grounds, & pin seating, a previously functional ECM of our vintage should take off and
run with no further ado.

****

By the way, have you taken a look at the coloring of your spark plugs? Does the color of the porcelain confirm
the Lean code? Are they glazed & possibly misfiring under load? (Misfires will cause a 'False Lean' to be reported.)

Thinking about your symptoms and the fact that you have already checked fuel pressure, I'm wondering about
your O2 sensor? Has it been recently replaced, or does it harken back to the assembly line? (Nearly all POs will
give the O2 sensor a good leaving alone.) I'm basically echoing the thoughts in reply #4.

Currently this issue is occurring with the O2 sensor connected and (I'm assuming) that we are in Closed Loop mode.
I would be interested to find out if you were to temporarily disconnect the O2 sensor would the problem clear up,
remain unchanged, or get worse?

If the engine were to clear up while the disconnected O2 sensor is forcing Open Loop operation, then we know that you have a
mechanically good engine, good fuel delivery & ignition system, and the engine runs well from the factory calibration
in the ECM stored tables, and that the root cause is that the good powerplant is a victim of a bad feedback loop. (O2 sensor)

****

Whether or not your engine will run properly Open Loop vs Closed Loop will tell us a lot.

Give this a whirl and let us know what you discover.

Will do! Tomorrow is my day off so I'm going to spend a good part of the day going through everything, ill check back with what I find.

 

[mountie]

On my ( since sold) '88 C2500, 5.7, My fuel pump started to fail... it was intermittent at first.
( you mention, " as you are driving uphill" )..... I remember, when the pump was going bad, .. And the symptom was happening, The less I had my foot on the gas, the better it ran. When I went a tad faster, my truck had the issue like a 'limp mode'....
Changing the fuel pump was the fix.

 

[dhworkin]

Mounty said exactly what my 95 trucks fuel pump symptoms were.

When I tune with the emulator sometimes I forget to save changes before I disconnect the laptop and it goes into limp mode. It will stay in limp mode until the battery is disconnected for a few seconds.
So you might not be going into limp mode.