92 GMC troubleshooting

[Anchor]

Good Morning; I'm real new here and realize I ask a lot of questions but I am pretty needy in the troubleshooting department. New to me about 2 months ago a 1992 GMC Regular Cab short bed K 1500. I love the truck and intend it to be my driver. They still look as good as ever to me and I may even paint this one. I guess I could drive newer stuff but it doesn't appeal to me. I recently got a Snapon MT2500 since I have a couple other 400 series trucks and can use on all. It quickly helped me find a bad cat on my 1990 K1500 regular cab long bed daily driver. I got the Brick scanner because I get mad at myself if I resort to the parts cannon. I should add that the 92 GMC is also a 5.7. The brick helped me find a bad map sensor and temperature sensor. Once I put those in along with 195 t-stat I could get closed loop at warm idle. Codes 15-34-42-&54. I have replaced the temp sensor, map, and fuel pump relay as well as a bad fuel pump. I have not replaced the ignition module yet but have a new one in the shop. Could that account for the bit of roughness I still have? Also, I think the ecm is original. At 32 years is it safe to assume it is time to change ecm? Prom is 5841. Any suggestions on where to get ecm?
I'll be honest, I have enough hours in this that I am starting to run things together and repeat procedures. An owner, once removed from me had had the engine rebuilt. I don't think it has been on the road since. You can tell by the assembly that they had some sense and everything looks pretty well done. All the brackets are done right, some nice paint on thing, just looks like care was used. Some grounds still need to be addressed, the one on passenger side firewall broken off at other end. Will get to that. I put this in my first post at the forum but engine has comp cams hyd. roller, (specs unknown), and guides in valley for same as well as roller tip rockers. Also a Holley 502-9 tbi. Other than that stock heads etc. Factory exhaust. Cam is not wild. No significant loping at least. I have put in new plugs, ohmed out the new plug wires, adjusted valves, timed, etc. etc. Will list numbers pulled with scanner yesterday at 190 degree temp. tps-0.76, closed loop, 02-325 to600, block learn-133, Integrator-127 to 130, IAC-84, 02 crosscounts-7-20, map V.-1.88, Map Hg-13.3 These numbers at about 1100 rpm idle. Some of these numbers I understand but some are mumbo jumbo to me. By the way, I fought with this thing for a long time before doing what all the people on forum said and bought a fuel pressure tester and found major problem. PS 175# in all cylinders
Any ideas appreciated and I will pay attention to them. If I knew everything I wouldn't be throwing myself on my sword like this. Thank you all.

 

[GoToGuy]

Why would you change / replace something that is fully operational, has no service life limit, has no fault indicators?
EGR function? Distributor guts in good shape? Does Holley operate on same specs as OE TBI? Correct model for truck year?

 

[Anchor]

Why would you change / replace something that is fully operational, has no service life limit, has no fault indicators?
EGR function? Distributor guts in good shape? Does Holley operate on same specs as OE TBI? Correct model for truck year?

Thank you for reply. I guess mean why would I change ecm? That's why I'm asking. I really don't know if ecm is a serice item or not. Distributor is an unknown but I have a module to put in. Is it likely that distributor with 233,000 miles is getting sloppy? Are there good aftermarket distributors available? I seem to read on here nothing but horror stories about aftermarket distributors. It did throw a 42 code. EGR checked out ok with a mighty vac. As far as the Holley tbi from my web searching it was a direct oem change out. Any suggestions as far as distributor source if I need one? I need to find a diagram for my ecm wiring and prove out the grounds. I guess that my reason for questioning the ecm is every flow chart for troubleshooting leads to, finally, the ecm. I appreciate your feedback. I will ultimately find the problem. (I think).

 

[Schurkey]

Within the last three weeks I learned that the Holley throttle bodies have to have the MAP sensor connected to a vacuum nipple on the FRONT of the throttle body, not on the rear.

Verify that FIRST. Makes a major difference in the operation of the MAP sensor.
www.gmt400.com/threads/tuning-for-a-rich-305ci-sbc.65061/page-2

https://www.gmt400.com/threads/tuning-for-a-rich-305ci-sbc.65061/page-2

 

[Road Trip]

...I guess that my reason for questioning the ecm is every flow chart for troubleshooting leads to, finally, the ecm.

Greetings Anchor,

For years & years changing the ECM** was an act of desperation, the Hail Mary of
troubleshooting. It was a high visibility/low probability piggy bank buster. Especially
for the first 20-25 years or so, hearing that an ECM was what it took to finally fix a
balky vehicle was almost unheard of...even if it was at the very end of most every
troubleshooting flowchart. (EDIT: On the rare occasion that the ECU required
replacement, usually it was on a vehicle that had been involved in serious collision
repair and precautions to protect the ECU from heat or electrical surge in the shop
weren't followed.)

In English, the tech writer would add in the ECM at the end of every troubleshooting
tree for completeness, but in terms of probability of being the root cause of the
problem, it was at the very bottom of the list.

****

Today, however, aging ECMs are, on occasion, finally starting to become a bit flaky.

The primary reason this is happening is because of the electrolytic capacitors that
underpin so much of the power that supports the ECM's operation had a design
lifespan of ~20 years. (for the good stuff.)

What happens is that they eventually leak or dry up, changing their value, and no longer
properly support the voltage regulators that are responsible for holding the +5v reference
voltages steady. (By the way, these +5v reference voltages are essential to the proper
operation of the algorithms that do all the realtime decision making when it comes to
making the engine run as clean as possible while simultaneously meeting the driveability
expectations of the owner.)

Keep in mind that environmental variables come into play. ECMs that have spent their
entire life in Canada and lived in a engine bay where all the heat is generated by a 4.3L
V6 (and always had an empty bed to pull around) will no doubt have electrolytic capacitors
that are still in reasonably good shape.

At the other extreme, a hard-working 5.7 or 7.4 down in TX, NM, or AZ with the A/C always
on and a poorly maintained cooling system have subjected the ECM to extreme underhood
temps, shortening their lifespan. As a direct consequence, the capacitors are much more likely to be
dried out. When these capacitors near their end of life, the ECM first becomes 'flaky', sometimes
for quite awhile, before it quits completely.

****

But instead of fear-mongering, I instead would recommend that it's OK to rely upon the
Self-Test routines that the ECM runs on every power up during the initialization cycle.

If you read through the P0xxx codes, you will see several having to do with ECM memory
testing, etc. In other words, if the ECM will communicate with the code reader/scan tool
and it isn't reporting errors detected by itself on itself, then it's pretty safe to say that
issues are being caused by the electronics forced to sit atop a hot, vibrating powerplant
(ie: sensors) ...instead of the electronics sitting in a heat-sinked enclosure over on the
driver's side fenderwell.

Of course the above is valid only when the powertrain is completely original as
well as the ECM is a perfect match. Of course, a lot of mischief can be done
under the hood in the 32 years since your truck left the assembly line, so
if the PO threw a Treasure Yard computer at a problem without dotting the
i's & crossing the t's, then your mileage may vary.

****

I apologize for the lengthy explanation, but this whole issue of electrolytic capacitors
being the weak link in the long-term reliability in all these embedded computer systems
is a real issue. Reliable computers can only be so when they are fed Reliable power, and
if you Google electrolytic capacitors you will see that this affects a lot more than just
the GMT400 community. (Electrolytic capacitor Rabbit Hole jumping off point)

Last but not least, I decided to respond because you have indicated that you are
interested in learning...and anyone who is interested in learning is well worth
helping them do just that.

Hope this helps to answer your questions about the ECM.

Cheers --

**ECM = Engine Control Module. ECU = Engine Control Unit. (Different name
for the same thing.) Confusion started when the engine computer started
taking on additional functions: PCM = Powertrain Control Module (Diesel) and
VCM = Vehicle Control Module (gas) These new names started showing up roughly
around the '96 timeframe. And 'Black Box' is a slang term that is used also.

Wish the whole nomenclature thing wasn't so confusing, especially if you are
relatively new to the computer-controlled engine game.

 

[Anchor]

Within the last three weeks I learned that the Holley throttle bodies have to have the MAP sensor connected to a vacuum nipple on the FRONT of the throttle body, not on the rear.

Verify that FIRST. Makes a major difference in the operation of the MAP sensor.
www.gmt400.com/threads/tuning-for-a-rich-305ci-sbc.65061/page-2

https://www.gmt400.com/threads/tuning-for-a-rich-305ci-sbc.65061/page-2

Thank you for that. I had seen it in my endless quest of the web. It didn't seem to make a difference but I still have the hoses I made up to try it again now that it is running better.

 

[Anchor]

Greetings Anchor,

For years & years changing the ECM** was an act of desperation, the Hail Mary of
troubleshooting. It was a high visibility/low probability piggy bank buster. Especially
for the first 20-25 years or so, hearing that an ECM was what it took to finally fix a
balky vehicle was almost unheard of...even if it was at the very end of most every
troubleshooting flowchart.

In English, the tech writer would add in the ECM at the end of every troubleshooting
tree for completeness, but in terms of probability of being the root cause of the
problem, it was at the very bottom of the list.

****

Today, however, ECMs are, on occasion, finally starting to become a bit flaky.

The primary reason this is happening is because of the electrolytic capacitors that
underpin so much of the power that supports the ECM's operation had a design
lifespan of ~20 years. (for the good stuff.)

What happens is that they eventually leak or dry up, changing their value, and no longer
supporting the voltage regulators that are responsible for holding the +5v reference
voltages steady. (By the way, these +5v reference voltages are essential to the proper
operation of the algorithms that do all the realtime decision making when it comes to
making the engine run as clean as possible while simultaneously meeting the driveability
expectations of the owner.)

Keep in mind that environmental variables come into play. ECMs that have spent their
entire life in Canada and lived in a engine bay where all the heat is generated by a 4.3L
V6 (and always had an empty bed to pull around) will no doubt have electrolytic capacitors
that are still in reasonably good shape.

At the other extreme, a hard-working 5.7 or 7.4 down in TX, NM, or AZ with the A/C always
on and a poorly maintained cooling system have subjected the ECM to extreme underhood
temps, shortening their lifespan. As a direct consequence, the capacitors are much more likely to be
dried out. When these capacitors near their end of life, the ECM becomes 'flaky', sometimes
for quite awhile, before it quits completely.

****

But instead of fear-mongering, I instead would recommend that it's OK to rely upon the
Self-Test routines that the ECM runs on every power up during the initialization cycle.

If you read through the P0xxx codes, you will see several having to do with ECM memory
testing, etc. In other words, if the ECM will communicate with the code reader/scan tool
and it isn't reporting errors detected by itself on itself, then it's pretty safe to say that
issues are being caused by the electronics forced to sit atop of a hot, vibrating powerplant
(ie: sensors) ...instead of the electronics sitting in a heat-sinked enclosure over on the
driver's side fenderwell.

Of course the above is valid only when the powertrain is completely original as
well as the ECM is a perfect match. Of course, a lot of mischief can be done
under the hood in the 32 years since your truck left the assembly line, so
if the PO threw a Treasure Yard computer at a problem without dotting the
i's & crossing the t's, then your mileage may vary.

****

I apologize for the lengthy explanation, but this whole issue of electrolytic capacitors
being the weak link in the long-term reliability in all these embedded computer systems
is a real issue. Reliable computers can only be so when they are fed Reliable power, and
if you Google electrolytic capacitors you will see that this affects a lot more than just
the GMT400 community.

Last but not least, I decided to respond because you have indicated that you are
interested in learning...and anyone who is interested in learning is well worth
helping them do just that.

Hope this helps to answer your questions about the ECM.

Cheers --

**ECM = Engine Control Module. ECU = Engine Control Unit. (Different name
for the same thing.) Confusion started when the engine computer started
taking on additional functions: PCM = Powertrain Control Module (Diesel) and
VCM = Vehicle Control Module (gas) These new names started showing up roughly
around the '96 timeframe. And 'Black Box' is a slang term that is used also.

Wish the whole nomenclature thing wasn't so confusing, especially if you are
relatively new to the computer-controlled engine game.

 

[Anchor]

Thank you Roadtrip; I'm not Worthy! That took a little time and I very much appreciate it. I'm looking for a map of the ecm wiring and when I find it I will prove out those grounds. If I understand you correctly, typically I should get some kind of a code or indication if ecm is dying. I won't rule it out but will continue to look elsewhere for the moment. I have been looking for my degree wheel and I have dial indicator out to map the camshaft. I fear that the cam, lifters, and springs that were installed by others may require different prom card or chip or whatever it is. Thanks again.

 

[Anchor]

Thank you Surekey

Thank you Schurkey for that. I had seen it in my endless quest of the web. It didn't seem to make a difference but I still have the hoses I made up to try it again now that it is running better.

for that. I had seen it in my endless quest of the web. It didn't seem to make a difference but I still have the hoses I made up to try it again now that it is running better.

 

[Road Trip]

Thank you Roadtrip; I'm not Worthy! That took a little time and I very much appreciate it. I'm looking for a map of the ecm wiring and when I find it I will prove out those grounds. If I understand you correctly, typically I should get some kind of a code or indication if ecm is dying. I won't rule it out but will continue to look elsewhere for the moment. I have been looking for my degree wheel and I have dial indicator out to map the camshaft. I fear that the cam, lifters, and springs that were installed by others may require different prom card or chip or whatever it is. Thanks again.

My pleasure, Anchor. I was fortunate to have solid mentors at critical junctures in
my career...and now it's my turn to pass along their guidance/pay it forward.

****

Alright. Your search for a map of the ECM wiring is over. Over HERE is a collection of
the Factory Service Manuals. You are in luck, for it looks like all the '92 Manuals are in
the collection. You should download these so that you can learn all about the GMT400
systems, documented by the people who designed and supported them.

To give you a quick taste of what you will find, here's a couple of pages from the
Electrical Wiring & Diagnostic manual, focusing on the ECM inputs & grounding:

You must be registered for see images attach

And here's looking at the Ground Distribution diagram -- essentially looking at the information above from the other side of the circuit:

You must be registered for see images attach

Finally, I've attached the Title Page of the manual I pulled the above from -- this just shows
you that the diagrams I'm sharing match the year of your vehicle.

****

I encourage you to take full advantage of these valuable technical resources,
for when it comes to all things having to do with mobility, the following is true:

"There is nothing more expensive than a lack of knowledge."

Best of luck in your journey towards GMT400 self-sufficiency. And if you find
something in the manuals that you don't understand, just ask in here
and no doubt a member will help you sort it out in short order.

Cheers --