'81, inconsistent spark firing
Racer
Joined: Oct 2001
Posts: 474
Likes: 64
From: Portage La Prairie Manitoba
I don't remember if the 81 had weights in the distributor...… your problem sounds timing related.
If it has weights check if they are stuck.Set initial timing to 12 degrees btdc….. leave distributor wiring unplugged and test drive.
It may set a code but if your miss is gone off the line accelerating..... it is likely the ecm or sticking weights in distributor
If it has weights check if they are stuck.Set initial timing to 12 degrees btdc….. leave distributor wiring unplugged and test drive.
It may set a code but if your miss is gone off the line accelerating..... it is likely the ecm or sticking weights in distributor
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
I don't remember if the 81 had weights in the distributor...… your problem sounds timing related.
If it has weights check if they are stuck.Set initial timing to 12 degrees btdc….. leave distributor wiring unplugged and test drive.
It may set a code but if your miss is gone off the line accelerating..... it is likely the ecm or sticking weights in distributor
If it has weights check if they are stuck.Set initial timing to 12 degrees btdc….. leave distributor wiring unplugged and test drive.
It may set a code but if your miss is gone off the line accelerating..... it is likely the ecm or sticking weights in distributor
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Starting to pull my hair out on this one. I spent some time last night making sure that all the vacuum hose connections were good. Idle is fine, WOT is fine, high speed running is fine. It's that off idle stumble. Wants to stall out coming off every traffic light. Also during mild acceleration it feels like one or two cylinders aren't firing.
Double checked the accelerator pump- both jets are squirting.
Checking the timing. I did advance initial from 6 to 8. Connecting the ECU adds about 20. Blipping the throttle does not retard the timing as I expect with a vacuum advance distributor. Post 20 is my vacuum sensor readings which seem close enough.
Rich mixture ignites sooner, so retard the timing to burn the accelerator pump gas. This isn't happening. But I'm not hearing any knock.
Double checked the accelerator pump- both jets are squirting.
Checking the timing. I did advance initial from 6 to 8. Connecting the ECU adds about 20. Blipping the throttle does not retard the timing as I expect with a vacuum advance distributor. Post 20 is my vacuum sensor readings which seem close enough.
Rich mixture ignites sooner, so retard the timing to burn the accelerator pump gas. This isn't happening. But I'm not hearing any knock.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Bob Szabo - aka "Carburetor Bob" Fuel Curve Engineering near Mill Spring NC (southwest of Rutherfordton NC)
just met him yesterday ... he was at an acquaintance's home & dialing in a Holley on a shoebox.
828.894.8941
https://www.enginebuildermag.com/author/bob-szabo/
https://racecarbook.com/about/the-author/
just met him yesterday ... he was at an acquaintance's home & dialing in a Holley on a shoebox.
828.894.8941
https://www.enginebuildermag.com/author/bob-szabo/
https://racecarbook.com/about/the-author/
Last edited by jackson; May 22, 2020 at 01:17 PM.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Are the fuel pump hoses molded or just hose?
Wrong hoses kink, causing this condition.
If your keeping the car, drop the tank and change the sender. The sock gets brittle and clogs. Cracked hoses on top of the tank also suck air.
How old is the fuel pump? Is it correct with return?
Sounds like fuel starvation, but with an 81.........
Wrong hoses kink, causing this condition.
If your keeping the car, drop the tank and change the sender. The sock gets brittle and clogs. Cracked hoses on top of the tank also suck air.
How old is the fuel pump? Is it correct with return?
Sounds like fuel starvation, but with an 81.........
Burning Brakes
Joined: Jan 2003
Posts: 961
Likes: 275
From: Frederick MD
What does your throttle position sensor input look like? Is it consistent or notchy as you vary the position back and forth? If it jumps around or is not consistent in its read outs it’s gotta go.
While I say that it almost sounds like a timing thing IE timing is not being added at higher RPMs. Definitely could be a result of the TPS sensor.
Additionally if your gonna keep the OEM carb/dizzy/ECM for now I would look to see if there is a way to read real time data inputs and commanded outputs from the ECM.
Your sensors might be reading properly with a multimeter but how do you know if the driver isn’t burned out in the ECM? Yeah it should set a code but remember this is also very early in GMs implementation of computers in their cars.
Never worked on something of that vintage with a factory ECM but it holds the same as working on newer computer controlled stuff. Can you validate that the ECM is actually seeing the inputs properly? Testing it at the sensor(s) is only 1/2 the battle.....
So I guess I’m offering a suggestion as well as a question as well. Is there a reasonably priced scanner that can read real time data off the ECM? If there is I’d be all over it because without firing the parts cannon at it your gonna have a hard time nailing it down.
While I say that it almost sounds like a timing thing IE timing is not being added at higher RPMs. Definitely could be a result of the TPS sensor.
Additionally if your gonna keep the OEM carb/dizzy/ECM for now I would look to see if there is a way to read real time data inputs and commanded outputs from the ECM.
Your sensors might be reading properly with a multimeter but how do you know if the driver isn’t burned out in the ECM? Yeah it should set a code but remember this is also very early in GMs implementation of computers in their cars.
Never worked on something of that vintage with a factory ECM but it holds the same as working on newer computer controlled stuff. Can you validate that the ECM is actually seeing the inputs properly? Testing it at the sensor(s) is only 1/2 the battle.....
So I guess I’m offering a suggestion as well as a question as well. Is there a reasonably priced scanner that can read real time data off the ECM? If there is I’d be all over it because without firing the parts cannon at it your gonna have a hard time nailing it down.
Last edited by kossuth; May 23, 2020 at 08:28 PM.
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Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
E Bay. Ave $50. Need 81 specific adapter.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
you say the ecm is communicating with all components. do you see timing change with rpm when timing connector is connected? it should advance from base timing at idle to 30-something as you rev it up, just like weights and springs. i would not waste time on this system. it is designed to get the car thru emissions. and it wasn't very good at it when new.
Last edited by derekderek; May 24, 2020 at 11:56 AM.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Are the fuel pump hoses molded or just hose?
Wrong hoses kink, causing this condition.
If your keeping the car, drop the tank and change the sender. The sock gets brittle and clogs. Cracked hoses on top of the tank also suck air.
How old is the fuel pump? Is it correct with return?
Sounds like fuel starvation, but with an 81.........
Wrong hoses kink, causing this condition.
If your keeping the car, drop the tank and change the sender. The sock gets brittle and clogs. Cracked hoses on top of the tank also suck air.
How old is the fuel pump? Is it correct with return?
Sounds like fuel starvation, but with an 81.........
Fuel pump is also original. Seems to be working fine but I will test for fuel pressure after I do simpler tests first.
I have no idea what that is.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
What does your throttle position sensor input look like? Is it consistent or notchy as you vary the position back and forth? If it jumps around or is not consistent in its read outs it’s gotta go.
While I say that it almost sounds like a timing thing IE timing is not being added at higher RPMs. Definitely could be a result of the TPS sensor.
Additionally if your gonna keep the OEM carb/dizzy/ECM for now I would look to see if there is a way to read real time data inputs and commanded outputs from the ECM.
Your sensors might be reading properly with a multimeter but how do you know if the driver isn’t burned out in the ECM? Yeah it should set a code but remember this is also very early in GMs implementation of computers in their cars.
While I say that it almost sounds like a timing thing IE timing is not being added at higher RPMs. Definitely could be a result of the TPS sensor.
Additionally if your gonna keep the OEM carb/dizzy/ECM for now I would look to see if there is a way to read real time data inputs and commanded outputs from the ECM.
Your sensors might be reading properly with a multimeter but how do you know if the driver isn’t burned out in the ECM? Yeah it should set a code but remember this is also very early in GMs implementation of computers in their cars.
Never worked on something of that vintage with a factory ECM but it holds the same as working on newer computer controlled stuff. Can you validate that the ECM is actually seeing the inputs properly? Testing it at the sensor(s) is only 1/2 the battle.....
So I guess I’m offering a suggestion as well as a question as well. Is there a reasonably priced scanner that can read real time data off the ECM? If there is I’d be all over it because without firing the parts cannon at it your gonna have a hard time nailing it down.
So I guess I’m offering a suggestion as well as a question as well. Is there a reasonably priced scanner that can read real time data off the ECM? If there is I’d be all over it because without firing the parts cannon at it your gonna have a hard time nailing it down.
I think you are on the right track with regard to sensors working but not talking to the ecm. Will test for continuity with the ECM connector. I have the scanner pictured and will see what that reads out, if anything.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Tonight I connected the Monitor 85 tool with the following results, numbers are test modes, unusual in bold.
2. trouble codes- none
3. Prom ID 1881
4. loop- closed / R/L status, switches between L04, R04
5. O2 voltage- varies between .65 and .87
6. air cleaner divert- on / Air switch solenoid- off
7. EGR solenoid on / canister purge- off
8. P-N switch- off (does not change when in gear) / AC clutch off (does not change when engaged)
9. RPM= TP sensor volts- varies as follows:
11. RPM /MPH- working and correct
13. MPH / Torque converter lockup- working as expected
15. Manifold pressure= volts - varies as follows
17. Coolant temp- 097/ Idle air position- 17
19. RPM/ Mixture control dwell
2. trouble codes- none
3. Prom ID 1881
4. loop- closed / R/L status, switches between L04, R04
5. O2 voltage- varies between .65 and .87
6. air cleaner divert- on / Air switch solenoid- off
7. EGR solenoid on / canister purge- off
8. P-N switch- off (does not change when in gear) / AC clutch off (does not change when engaged)
9. RPM= TP sensor volts- varies as follows:
- 775=0.66
- 1300=0.90
- 1800=1.00
- 2400=1.16
11. RPM /MPH- working and correct
13. MPH / Torque converter lockup- working as expected
15. Manifold pressure= volts - varies as follows
- 34=1.34
- 43=1.82
- 48=2.08
- 55=2.44
- 65=3.00
- 70=3.26
- 81=3.86
- 96=4.34
17. Coolant temp- 097/ Idle air position- 17
19. RPM/ Mixture control dwell
- idles, cruises at 30
- part throttle 25, 20 range
- WOT to 05
- Does not go higher than 30.
Last edited by Yadkin; May 26, 2020 at 09:49 PM.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Safety Car
Joined: Jul 2005
Posts: 3,697
Likes: 833
From: WI
Is the purge canister still installed? Are the vacuum switches on the top operating correctly ...or leaking (ripped diaphragms)?
Have you checked the spark (with one of those spark tools) to see if it's a good strong spark?
There is a pick-up assembly on the lower part of the distributor. And thin, frail wires down there which can fray and cause problems. Only way to get at them is to remove the distributor, then remove the gear and pull the shaft 'up'. I had a freeky ignition problem with a classic pickup of mine and the star was corroded (to the point of breaking) and the pick-up wires frayed down to the smallest strand. It took me quite a while to zero in on that problem. I don't remember how I got there, but I was thinking bad spark too.
Take it out on a dark night and pop the hood while running, look for sparks jumping between the wires. I've seen some vehicles that looked like a xmas light show under the hood. Make sure the cap/rotor are all good too.
.
Have you checked the spark (with one of those spark tools) to see if it's a good strong spark?
There is a pick-up assembly on the lower part of the distributor. And thin, frail wires down there which can fray and cause problems. Only way to get at them is to remove the distributor, then remove the gear and pull the shaft 'up'. I had a freeky ignition problem with a classic pickup of mine and the star was corroded (to the point of breaking) and the pick-up wires frayed down to the smallest strand. It took me quite a while to zero in on that problem. I don't remember how I got there, but I was thinking bad spark too.
Take it out on a dark night and pop the hood while running, look for sparks jumping between the wires. I've seen some vehicles that looked like a xmas light show under the hood. Make sure the cap/rotor are all good too.
.
Last edited by Mark G; May 26, 2020 at 10:44 PM.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Have you checked the spark (with one of those spark tools) to see if it's a good strong spark?
There is a pick-up assembly on the lower part of the distributor. And thin, frail wires down there which can fray and cause problems. Only way to get at them is to remove the distributor, then remove the gear and pull the shaft 'up'. I had a freeky ignition problem with a classic pickup of mine and the star was corroded (to the point of breaking) and the pick-up wires frayed down to the smallest strand. It took me quite a while to zero in on that problem. I don't remember how I got there, but I was thinking bad spark too.
Take it out on a dark night and pop the hood while running, look for sparks jumping between the wires. I've seen some vehicles that looked like a xmas light show under the hood. Make sure the cap/rotor are all good too. .
Last edited by Yadkin; May 28, 2020 at 10:49 PM.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
I found a missing nipple on the coolant temperature switch that controls the vapor canister purge. Some sly guy put it back that way. I jumped the vacuum line past that and it didn't make a difference.
I checked continuity from each wire on these devices to the terminal of the ECU- all OK:
Plugged everything back in and checked the timing. The ECU no longer advances, I no longer hear the mixture control solenoid clicking. I must have a bad connection between the ECU and one or both of its connectors. Too late for further diagnosis. It will have to stay until next week unless I can find tomorrow (doubtful).
I checked continuity from each wire on these devices to the terminal of the ECU- all OK:
- Coolant
- O2
- Dizzy
- Vac sensor
- Throttle position sensor
Plugged everything back in and checked the timing. The ECU no longer advances, I no longer hear the mixture control solenoid clicking. I must have a bad connection between the ECU and one or both of its connectors. Too late for further diagnosis. It will have to stay until next week unless I can find tomorrow (doubtful).
Safety Car
Joined: Jul 2005
Posts: 3,697
Likes: 833
From: WI
Getting back to the basics for a moment ...I know this might sound stupid, but did you set your base timing with the dist wire unhooked and the rest of the instructions per pg 6D-35 in the manual (that's the pg in my manual)?
Secondly, did you get a chance to put a compression tester on the cylinders to see what it looks like? So we can rule out that? It's unlikely, but always possible a rocker slipped off, or there's a bent push rod (or flat cam lobe). I had a CCC pickup once with a flat cam lobe back in the day which was tricky to figure out.
Do you see fuel squirt out nice and strong from the accelerator pump?
What was the reason you replaced the distributor? Did this issue occur before you replaced it?
Hang in there ....we'll help you zero in on things. I've driven several CCC carb'd cars to well over 200k miles and they all ran great with few problems (better than old-school carb'd cars). Who knows, the problem may ultimately not have anything to do with the computer or sensors. People tend to out of the chute blame the computer and sensors on these older cars. Sometimes it is, lot of times it isn't. I see that on the Jeep forums all the time.
.
Secondly, did you get a chance to put a compression tester on the cylinders to see what it looks like? So we can rule out that? It's unlikely, but always possible a rocker slipped off, or there's a bent push rod (or flat cam lobe). I had a CCC pickup once with a flat cam lobe back in the day which was tricky to figure out.
Do you see fuel squirt out nice and strong from the accelerator pump?
What was the reason you replaced the distributor? Did this issue occur before you replaced it?
Hang in there ....we'll help you zero in on things. I've driven several CCC carb'd cars to well over 200k miles and they all ran great with few problems (better than old-school carb'd cars). Who knows, the problem may ultimately not have anything to do with the computer or sensors. People tend to out of the chute blame the computer and sensors on these older cars. Sometimes it is, lot of times it isn't. I see that on the Jeep forums all the time.
.
Last edited by Mark G; May 29, 2020 at 12:03 AM.