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Need Help for my 1981 (again) ECU-I Have Snap On MT2500

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Need Help for my 1981 (again) ECU-I Have Snap On MT2500

 
Old 09-24-2018, 12:13 PM
  #1  
Hamerdown
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Default Need Help for my 1981 (again) ECU-I Have Snap On MT2500

And just like that, my 1981 is 'idling' like crap again.
I do know...the 81 ECM only manages Engine at idle, and that seems to be where the problem is.
I'm trying to avoid throwing parts at this Car like; re-manufactured ECM, carb-rebuild, carb parts like TPS, MCS etc, possible distributor and so on.
I have borrowed a Snap-On MT2500 Scanner...but no instructions or any idea how to use it or what menu's or what parameter values/readings are acceptable.
If someone out there with expert knowledge of the 1981 ECM and the Snap-On MT2500 Scanner would be willing to call me at your convince to (via phone real time) walk me through all the Scanners menu's and to dig deep into what the Scanner can tell me would be greatly appreciated.
If someone actually has the Snap-On Scanner and an 81 Vette it may be even better to go through all the steps in real-time with your Scanner also hooked-up.
Please PM me or call 610-304-4729
(SE-Pa eastern timezone)
Thanks, Ray
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Old 09-25-2018, 07:34 AM
  #2  
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The Snap-On scanner did not like my car. It caused a CEL and made the car run rough. I use an OTC scanner now. I highly suggest you buy a copy of the dealers manual, and go thru all their procedures.
Everything but 1% you need to know is in there. Good luck.
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Old 09-25-2018, 09:59 AM
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BTW, The ECM manages the engine all the time, NOT just at idle.
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Old 09-25-2018, 10:00 AM
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Originally Posted by Big2Bird View Post
The Snap-On scanner did not like my car. It caused a CEL and made the car run rough. I use an OTC scanner now. I highly suggest you buy a copy of the dealers manual, and go thru all their procedures.
Everything but 1% you need to know is in there. Good luck.
Anything 'mechanical' (and I mean anything) I can handle however, when it comes to electronics and lets say going through a service manual electronics 'diagnostics tree' I'm lost...to me it might as well be written in a different language and that is exactly what's in the GM Service Manual.

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Old 09-25-2018, 12:32 PM
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check for vacuum leaks first. evena small crack can cause what ur describing, the 81 is a first year cpu model and 82 changed the cpu so its unique in what it does.

all the cpu does is control the e4me at idle, later it did more but on 81 is only idle.

if you cant find any loose or cracked vac lines try doing a vac test to see if system is holding steady vacuum at idle, if it inst it can be something related to headlight or charcoal canister vac connection. etc

pull the trouble codes from the obd link under the ashtray this can point to a specific issue, map sensor iac etc.

hers 2 very good links to get you started

http://www.neilkline.com/Tech%20Tips/obdcodes.htm


http://www.early3rdgen.com/forum/viewtopic.php?t=25
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Old 09-25-2018, 12:43 PM
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also I forgot to add, you don't need the scanner to troubleshoot.
a dwell meter and vac guage are essential though,
a tip for connecting the dwell meter, one goes to a good ground, usually the engine to body ground strap, the other lead goes to the diagnostic connection in the engine bay, it is usually green and on the pass side. just look for you wire bundle going along the pss valve cover, halfway theres should be a loose connection that doesn't go anywhere, this is the diagnostic link that you connect the dwel meter to.
heres a good low cost dwell meter
https://shop.advanceautoparts.com/p/...B&gclsrc=aw.ds

Last edited by sambrand; 09-25-2018 at 12:44 PM.
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Old 09-25-2018, 02:48 PM
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Again, the ECM controls the engine at all speeds. EVEN AT WOT, the ECM signals the carb for full enrichment, and controls the spark advance. It also controls T/C lock up, EGR, function, A.I.R.function,

If you "think" the ECM only controls the E4ME at idle, then your misinformed, or still do not understand how it works.
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Old 09-25-2018, 03:44 PM
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Originally Posted by Big2Bird View Post
Again, the ECM controls the engine at all speeds. EVEN AT WOT, the ECM signals the carb for full enrichment, and controls the spark advance. It also controls T/C lock up, EGR, function, A.I.R.function,

If you "think" the ECM only controls the E4ME at idle, then your misinformed, or still do not understand how it works.
no you are wrong as for the 81 year it only controls the idle mixture this was done for emissions starting in 82 the ecm began takin on other duties in addition. please do the research find out your wrong then go from there instead of digging in on your position. I own an 81 and just finished replacing the after market holwy that was installed because the cel was constantly illuminated, I have been through all the diagnostics and troubleshooting to get a rebuilt e4me re tuned in the 81.

Last edited by sambrand; 09-25-2018 at 03:47 PM.
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Old 09-25-2018, 07:06 PM
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Originally Posted by sambrand View Post
no you are wrong as for the 81 year it only controls the idle mixture this was done for emissions starting in 82 the ecm began takin on other duties in addition. please do the research find out your wrong then go from there instead of digging in on your position. I own an 81 and just finished replacing the after market holwy that was installed because the cel was constantly illuminated, I have been through all the diagnostics and troubleshooting to get a rebuilt e4me re tuned in the 81.
fix it your damn self.
bigbird is right.
i have owned an 81 since new,
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Old 09-25-2018, 07:16 PM
  #10  
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Personally I would throw the carb and the ECM into the river. But this guy apparently feels a little bit different. http://early3rdgen.com/forum/viewtopic.php?t=25. And it sounds like he knows what he's talking about. As far as what it controls, you're both right. It controls idle and part throttle mixture.

Last edited by derekderek; 09-25-2018 at 07:17 PM.
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Old 09-25-2018, 07:27 PM
  #11  
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saying the computer only controls idle is wrong.
not partially correct.
at idle the ecu has very limited control.
carry on
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Old 09-26-2018, 01:44 PM
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Originally Posted by sambrand View Post
check for vacuum leaks first. evena small crack can cause what ur describing, the 81 is a first year cpu model and 82 changed the cpu so its unique in what it does.

all the cpu does is control the e4me at idle, later it did more but on 81 is only idle.

if you cant find any loose or cracked vac lines try doing a vac test to see if system is holding steady vacuum at idle, if it inst it can be something related to headlight or charcoal canister vac connection. etc

pull the trouble codes from the obd link under the ashtray this can point to a specific issue, map sensor iac etc.

hers 2 very good links to get you started

http://www.neilkline.com/Tech%20Tips/obdcodes.htm


http://www.early3rdgen.com/forum/viewtopic.php?t=25
This same problem raised its ugly face last June...no CEL and no codes stored.
I did the hunt for vacuum leaks and found zero...over the decades of ownership (original owner) I have had a prior vacuum leaks; a few times the usual headlight actuator cone-seal went bad, and one time the little vacuum hose connecting to the Trans modulator.
Last June when the bad idle happened (and this is different vs the typical vacuum leak) it seems like the Timing retards and the RPM's drop way low...the engine labors down. As soon as I rev above 1000 RPM all is good.

Anyhow... last June a fellow on another forum (electronic engineer) called me and walked me through checking and testing MCS, TPS, O2-sensor and even to make sure the ECU was properly grounded and all were good.
In frustration...I pulled battery and removed Computer/ECU cleaned-up connections, reinstalled and the next day it started and idles perfectly! I thought 'maybe' connections/plugs to ECU were just corroded.

Last June I did borrow (and still have) the Snap-On scanner others told me I needed...at that time engine was running good, a forum member was kind enough to call me and walk me through the Snap-On scanner menu's and steps.
Again...the engine was running good...the scanner showed all readings were where they should be, in the middle of all parameters. (but I didn't take notes)

All was good until...I started the car last week, once again I disconnected the battery, re-connected and it idled bad but as I was driving it seemed to clear-up and was ok.
I stopped to fill the gas tank, started the engine and BAM...it went back to idling bad again...and that is where I am at again.

At this point and because I still have the Snap-Onn scanner I would like to use it (while engine is acting-up) to see what the scanner shows...but I just don't remember how to properly use the scanner or what to look for as far as what readings may or may not be good or bad and that is WHY I was requesting someone that has an 81 and knowledge of the Snap-On scanner to give me a call and do a real-time step by step walk through. 1981 ECU is a stand alone unit, different from all prior and later.

For some reason...I feel the ECU is acting-up. It does seem the problem is being controlled or, not properly controlled by the ECU...but I can't be sure and don't want to start throwing costly parts at it.

BTW...I still have my old Dwell meter from way back in 1975 when I had my BB 69 SS Chevelle...a long time ago I did use it on my Vette (green terminal) to adjust the E4ME carb after I got a rebuild kit from Cliff's High Performance Quadrajets.

Anything mechanical I can easily handle...even build a house from foundation to chimney, but when it comes to electronics and diagnosing those problems, I hit a wall.

Last edited by Hamerdown; 09-26-2018 at 01:49 PM.
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Old 09-26-2018, 01:49 PM
  #13  
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To the OP. Okay, I started reading this thread and had to chime in. I have owned my 1981 for 4 years. Rebuilt all the mechanical's from front to rear myself. The E4ME is not difficult to set up, but there is a learning curve. I originally took mine to a local carb rebuild shop and ended up back there 12 times. The issue, they did not take into consideration this is an electronic carb and requires to be set up on the car after rebuild with the ECM connected, and not like a normal Rochester. The carb shop finally gave up and returned my money. I read up and asked a lot of questions on the forum and elsewhere to finally understand how to rebuild, set up on the bench and then on the car. The ECM controls the following: Air/Fuel/dancing needles while idling or when at cruising speeds. It also controls the lock up torque converter and timing. If in doubt, look at the forum notes from Cliffs High Performance https://cliffshighperformance.com/si...php?topic=58.0
When I had issues I talked to Cliff and he steered me in the right direction, review other forum threads under Reaper19.

Items I would recommend picking up and testing:
- 1981 Shop Manual - there is a great section on troubleshooting carb and ECM as well as adjusting all settings
- Mightyvac hand pump
- Vacuum gauge
- Monitor 85 Scan tool - bought mine on EBAY for 40 bucks. The monitor will read all settings in real time
- Set everything from timing to carb adjustments to factory first. These are a good starting point and tweak from there
- Check plugs, distributor cap, plugs, ignition system, EGR. Ignition system must be in good shape
- Check for Vacuum leaks, this requires repairs first. If there are any, you will not be able to adjust carb or ask the ECM to do it's job properly.
- If carb has been rebuilt, again, all settings. lean and rich stops must be bench set, IAB, mixture screws etc.
- Is TPS and Lean Mixture Solenoid working properly and adjusted?

Another thread on the E4ME
https://www.corvetteforum.com/forums...djustment.html


Good Luck!

Maybe a sticky can be created by the forum on this subject, troubleshooting and setting up an E4ME. This issue seems to get a lot of questions and threads.
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Old 09-26-2018, 06:42 PM
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Fellas, I thank you for the replies...I wish you wouldn't argue between each other.
If you want to read the rest below...thank you
.
I have the feeling my issue may not be with the carb itself but maybe, the ECM/U (whatever it should be called) controlling it AND also retarding the distributor timing at idle and how the engine drops RPM at idle, carb goes rich (heavy exhaust smell) and the engine just labors down...like it is in a 'limp-home mode'.
If I disconnect the battery for 15 minutes-reconnect and drive the car the computer seems like it retrained the engine to run good again, but turn it off/ re-start and it's right back to square one or sometimes and for whatever reason, it just slides back to idling bad again
This is why I was hoping someone that knows the Snap-On scanner like the back of their hand and the 1981 ECU, carb, distributor etc could via phone walk me through a diagnoses in real time while my car is idling poorly...it may show a faulty something...or maybe not?
Everyone just responds with "get the GM manual and figure it out for yourself"...more now than ever I wish well over a decade ago I would have removed the ECU, computer controlled carb, distributor and all related computer carb parts and just have an old school tuneable engine...something I can EASILY do.

BTW, back in late 2013... I did remove the differential rebuilt that, removed trailing arms and rebuilt those and rear axles/bearings, new rear brake hoses etc, new master cylinder, removed the TC350c trans and rebuilt that, dropped the oil pan for a new gasket, replaced the original engine mounts-(pita) new water pump, all new AC components, replaced the entire exhaust system, deleted the catalytic converter,-air pump, blocked-off the EGR and the engine ran great with no CEL!
(many years prior I pulled and rebuilt the heads, replaced the cam and lifters-I know my way around a engine)
I did all that... in a small single 9 foot wide garage over the course of my winter without heat...boy was it tough, (the narrow garage part was tough)...I would slide out from under the car and shimmy up the garage wall.

So and again... when it comes to 'mechanical work' I have the skills for all that plus so much more but...when it comes to electronics, understanding or diagnosing I just hit a wall.

I visited a local well know shop... that decades ago had a reputation for the earlier Corvettes (still newer Vetts) and they also work on exotic sports cars $$$.
The owner that I know said; "Ray, we haven't touched a computer controlled C3 in over 25 years, maybe even 30. We ourselves would almost have to retrain someone to dig into it and it is going to tie-up an engine bay, time, and to where' we would lose money on other quicker money making jobs, it's just not worth our time."
Would the shop do a 4-wheel alignment on my 1981 or mechanical repairs (to an extent) absolutely.

The ONLY reason... I did not remove all the computer controlled components (wiring included) decades ago was because I wanted to keep the car original and a few owners encouraged me to do so.
The reality..., the 1981 Corvette is not and may never be (in my lifetime) a collectable like a 1967-427 or some of the anniversary models.
My 81 is what it is, a Vette that handles nicely, great line, still get the looks/comments/thumbs-up and (when running right) fun to drive.
Maybe... my energy should be focused on doing what I wanted to do 25 years ago...but to do it 'correctly' I would need direction and there's the TH350c converter lock-up

Last edited by Hamerdown; 09-26-2018 at 06:52 PM.
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Old 09-26-2018, 06:55 PM
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That would really be best. Those ecms were a crappy design trying to get the thing to pass emissions because they couldn't get a fuel injection system working well enough yet. There's a reason they only ran it for about a year. and some of the stuff that the manufacturers did with the emissions back in the day are the reason emission stuff is warrantied for a decade now. Don't even bother taking out the ECM. Just get an HEI distributor with a vacuum can on the front and a qjet from Lars or go get an Edelbrock. You will be glad you did. Either that or get a rebuilt ECM and cross your fingers. 47 year old electronics stuff. all right, Lars doesn't actually sell carbs. You have to scare one up and send it to him to get it put right. And it will be right.

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Old 09-26-2018, 07:51 PM
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Originally Posted by Hamerdown View Post
Fellas, I thank you for the replies...I wish you wouldn't argue between each other.
If you want to read the rest below...thank you
.
I have the feeling my issue may not be with the carb itself but maybe, the ECM/U (whatever it should be called) controlling it AND also retarding the distributor timing at idle and how the engine drops RPM at idle, carb goes rich (heavy exhaust smell) and the engine just labors down...like it is in a 'limp-home mode'.
If I disconnect the battery for 15 minutes-reconnect and drive the car the computer seems like it retrained the engine to run good again, but turn it off/ re-start and it's right back to square one or sometimes and for whatever reason, it just slides back to idling bad again
This is why I was hoping someone that knows the Snap-On scanner like the back of their hand and the 1981 ECU, carb, distributor etc could via phone walk me through a diagnoses in real time while my car is idling poorly...it may show a faulty something...or maybe not?
Everyone just responds with "get the GM manual and figure it out for yourself"...more now than ever I wish well over a decade ago I would have removed the ECU, computer controlled carb, distributor and all related computer carb parts and just have an old school tuneable engine...something I can EASILY do.

BTW, back in late 2013... I did remove the differential rebuilt that, removed trailing arms and rebuilt those and rear axles/bearings, new rear brake hoses etc, new master cylinder, removed the TC350c trans and rebuilt that, dropped the oil pan for a new gasket, replaced the original engine mounts-(pita) new water pump, all new AC components, replaced the entire exhaust system, deleted the catalytic converter,-air pump, blocked-off the EGR and the engine ran great with no CEL!
(many years prior I pulled and rebuilt the heads, replaced the cam and lifters-I know my way around a engine)
I did all that... in a small single 9 foot wide garage over the course of my winter without heat...boy was it tough, (the narrow garage part was tough)...I would slide out from under the car and shimmy up the garage wall.

So and again... when it comes to 'mechanical work' I have the skills for all that plus so much more but...when it comes to electronics, understanding or diagnosing I just hit a wall.

I visited a local well know shop... that decades ago had a reputation for the earlier Corvettes (still newer Vetts) and they also work on exotic sports cars $$$.
The owner that I know said; "Ray, we haven't touched a computer controlled C3 in over 25 years, maybe even 30. We ourselves would almost have to retrain someone to dig into it and it is going to tie-up an engine bay, time, and to where' we would lose money on other quicker money making jobs, it's just not worth our time."
Would the shop do a 4-wheel alignment on my 1981 or mechanical repairs (to an extent) absolutely.

The ONLY reason... I did not remove all the computer controlled components (wiring included) decades ago was because I wanted to keep the car original and a few owners encouraged me to do so.
The reality..., the 1981 Corvette is not and may never be (in my lifetime) a collectable like a 1967-427 or some of the anniversary models.
My 81 is what it is, a Vette that handles nicely, great line, still get the looks/comments/thumbs-up and (when running right) fun to drive.
Maybe... my energy should be focused on doing what I wanted to do 25 years ago...but to do it 'correctly' I would need direction and there's the TH350c converter lock-up
I don't think the fellas are arguing, these carbs and the ECM's have been mysterious over the years and as you have seen there are a lot of opinions. :-) Hopefully you are able to find the issue. Once you do get the CCC dialed in it runs great.

Good luck, stick with it.

Last edited by Reaper19; 09-26-2018 at 07:52 PM.
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Old 09-26-2018, 09:53 PM
  #17  
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Originally Posted by derekderek View Post
That would really be best. Those ecms or a crappy design trying to get the thing to pass emissions because they couldn't get a fuel injection system working well enough yet. There's a reason they only ran it for about a year. and some of the stuff that the manufacturers did with the emissions back in the day are the reason emission stuff is warrantied for a decade now. Don't even bother taking out the ECM. Just get an HEI distributor with a vacuum can on the front and a qjet from Lars or go get an Edelbrock. You will be glad you did. Either that or get a rebuilt ECM and cross your fingers. 47 year old electronics stuff. all right, Lars doesn't actually sell carbs. You have to scare one up and send it to him to get it put right. And it will be right.
I have to agree with this. In Ca. I am stuck with stock per law. Comply or park it.
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Old 09-26-2018, 11:10 PM
  #18  
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There are a bunch of reman ecm's on eBay. 85-110 bucks. Mostly Cardone rebuilts.
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Old 09-27-2018, 04:52 AM
  #19  
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Unfortunately I can't help you with the scanner. But from what you are describing it sounds like an electronic problem. Sensors typically work or don't work, they don't normally have intermittent faults like you are seeing. A mechanical problem also doesn't fit. I can only think of two things that could cause what you are seeing, a faulty ECU(or ecm) or EPROM. (The EPROM is a chip that plugs into the ECU and must go in the correct way). I would guess the ECU has the fault. Maybe someone close may have a spare you can borrow to see if that helps?
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Old 09-27-2018, 10:32 AM
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Originally Posted by derekderek View Post
There are a bunch of reman ecm's on eBay. 85-110 bucks. Mostly Cardone rebuilts.
To clarify, Cardone units are cleaned and tested. If they fail, they are tossed. If they pass, they assume it will last another year.
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