Low dwell problem (ECM, PROM M/C solenoid) what to do next?

[Peer81]

Hello everbody,

This time I really don't know it anymore so I hope somebody could give me some pointers or a hind into the right direction.

Here it goes. I made a vid of the problem that is much easier then pages of text but this is the problem.
The problem is the solenoid inside the carb goes into full rich (stops working) after a few minutes. What do you see in the vid. In the left lower corner is the wide band module. In the left exhaust is the wide band sensor that is in the top of the screen, right exhaust is the lower one. The sensor in the right exhaust will be replaced to disregard the weird signal is it displaying. The wirde band module gives a narrow band output signal from the left sensor to the ECM so the ECM knows to rich up or lean out the mixture. You can see that the mixture is about 13.3:1 (ideal is 14.5:1) so a little on te rich side. (after this vid I turned the mixture screws in a little and the signal was perfect around 14.5:1). A new right side sensor is on it's way.
Next to it is the dwell meter. You can see that it is around 30 degrees (6 scale)
Then the carb, I used the rich adjust tool to see if the plunger is still moving in and out of the solenoid.

During the first 50 seconds everything is normal. Mixture is a little rich at 13.3:1 and the dwell is around 30 degrees. Weird thing is that the dwell is almost fixed and is not hunting! The tool is jumper around on the solenoid plunger.
After 50 seconds you can see the plunger is not slowing down but it decreases in hight till it is just standing still. The dwell meter has moved from 30 degrees to about 5 degrees and the wide band shows a mixture of 12.3:1 (rich).

https://www.youtube.com/watch?v=SfyEp7ub0RQ

What did I do in the mean time. First I thought that the solenoid was busted so I changed it for another one. Today I thought the solenoid worked when cold but failed when hot so I changed it for a new one. Damn, no change!
Put in a new ECM, no change. Cor showed me that I first had to check a few thing before putting in the new ECM (which I didn't). It was measuring resistance on a few solenoids in the system. One solenoid showed to low resistance, but as I had Cor his measurements he also had to low a resistance on the same solenoid so I think that isn't the problem and the new ECM isn't f*cked up from the start.
Next I changed the PROM for a new one (hyper tech as the OEM one was in back order for 90 days or so). Put in the new PROM no change..
When I disconnect the M/C solenoid from the ECM the dwell meter is reading nothing and the needle is ad the left corner, when it is at 5 degrees it is at the right corner so I don't think the problem is in the wiring somewhere.

What did I do after the vid?
I disconnected the ignition connector from the system. It showed as a trouble code and the dwell meter was pinned at 0 degrees in the right corner. But when I reconnected the ignition connector the solenoid came to live ones again, only for a few minutes but the dwell meter was right back a 30 degrees! After that it just stopped as before. I tried the same with other connectors and it did the same with the TPS connector only for a shorter period of time.

Also when I turn the throtlle by hand the solenoid comes to live but just for a few seconds (also dwell is going back up to 20 degrees or so) The problem is also there without the wide band module (sensors) only with a narrow band sensor.

The problem is that the ECM THINKS there is a vacuum leak or something so it is correcting for a lean condition with a rich mixture.
Or the ECM doesn't know what to do with the narrow band simulation the wide band module is giving it. When I don't activate the narrow band option this ECM doesn't throw a troublecode as the previous ECM did

I just find it weird that the solenoid comes to live when the ECM switches back from open loop to closed loop (solved the troublecode) but after a few minutes stops again.

If somebody could give me a point to look for you're more then welcome!

Greetings Peter

 

[GerryLP]

Hello everbody,

This time I really don't know it anymore so I hope somebody could give me some pointers or a hind into the right direction.

Here it goes. I made a vid of the problem that is much easier then pages of text but this is the problem.
The problem is the solenoid inside the carb goes into full rich (stops working) after a few minutes. What do you see in the vid. In the left lower corner is the wide band module. In the left exhaust is the wide band sensor that is in the top of the screen, right exhaust is the lower one. The sensor in the right exhaust will be replaced to disregard the weird signal is it displaying. The wirde band module gives a narrow band output signal from the left sensor to the ECM so the ECM knows to rich up or lean out the mixture. You can see that the mixture is about 13.3:1 (ideal is 14.5:1) so a little on te rich side. (after this vid I turned the mixture screws in a little and the signal was perfect around 14.5:1). A new right side sensor is on it's way.
Next to it is the dwell meter. You can see that it is around 30 degrees (6 scale)
Then the carb, I used the rich adjust tool to see if the plunger is still moving in and out of the solenoid.

During the first 50 seconds everything is normal. Mixture is a little rich at 13.3:1 and the dwell is around 30 degrees. Weird thing is that the dwell is almost fixed and is not hunting! The tool is jumper around on the solenoid plunger.
After 50 seconds you can see the plunger is not slowing down but it decreases in hight till it is just standing still. The dwell meter has moved from 30 degrees to about 5 degrees and the wide band shows a mixture of 12.3:1 (rich).

https://www.youtube.com/watch?v=SfyEp7ub0RQ

What did I do in the mean time. First I thought that the solenoid was busted so I changed it for another one. Today I thought the solenoid worked when cold but failed when hot so I changed it for a new one. Damn, no change!
Put in a new ECM, no change. Cor showed me that I first had to check a few thing before putting in the new ECM (which I didn't). It was measuring resistance on a few solenoids in the system. One solenoid showed to low resistance, but as I had Cor his measurements he also had to low a resistance on the same solenoid so I think that isn't the problem and the new ECM isn't f*cked up from the start.
Next I changed the PROM for a new one (hyper tech as the OEM one was in back order for 90 days or so). Put in the new PROM no change..
When I disconnect the M/C solenoid from the ECM the dwell meter is reading nothing and the needle is ad the left corner, when it is at 5 degrees it is at the right corner so I don't think the problem is in the wiring somewhere.

What did I do after the vid?
I disconnected the ignition connector from the system. It showed as a trouble code and the dwell meter was pinned at 0 degrees in the right corner. But when I reconnected the ignition connector the solenoid came to live ones again, only for a few minutes but the dwell meter was right back a 30 degrees! After that it just stopped as before. I tried the same with other connectors and it did the same with the TPS connector only for a shorter period of time.

Also when I turn the throtlle by hand the solenoid comes to live but just for a few seconds (also dwell is going back up to 20 degrees or so) The problem is also there without the wide band module (sensors) only with a narrow band sensor.

The problem is that the ECM THINKS there is a vacuum leak or something so it is correcting for a lean condition with a rich mixture.
Or the ECM doesn't know what to do with the narrow band simulation the wide band module is giving it. When I don't activate the narrow band option this ECM doesn't throw a troublecode as the previous ECM did

I just find it weird that the solenoid comes to live when the ECM switches back from open loop to closed loop (solved the troublecode) but after a few minutes stops again.

If somebody could give me a point to look for you're more then welcome!

Greetings Peter

Peter,

First of all, she is looking sweet!

Secondly, you are much better at the electronics than i would ever be, but I wonder if it would still doing the problem if you were to disconnect/swap the O2 sensor to stock one and only one O2 sensor as in stock. My premise is that if you are still using the stock computer, but using a bit more advanced components, if the computer may be overwhelmed and sort of gives up (get home mode kicks in), and that would be perhaps why the problem is happening?

So I recommend you go back to square one on the components and use the stock ones, and then, if the problem is still the same, then perhaps it could illuminate the next course of action....or perhaps along the way, a wire was placed or left on the wrong spot while adapting the new components? Anyway. It is easy to lose one self's way while dealing with a problem because we are too close to it, and what helps is to stand back, and start over.

 

[Antz81]

Have to agree with Gerry here. Confirm that the engine runs fine with the stock set-up. After that try the modified sensor system.

As another thought though, what O2 sensors are you now using and what signal is getting sent back to the ECM?
The original would have been a Zirconia type. But if your combined signal is being sent back to mimic a Titania type sensor this would confuse the computer.

 

[Peer81]

Thank you both. I changed the OEM sensor a few years ago for a heated one. Its a bosch (same as C4 ZR1 uses, don't know if this is zirconia oe titania type?) The wide band is a bosch LSU 4.2 sensor.

But going back to basics (stock) is always a good idea! Ashame I didn't think of that!

I'll let you know when I have done the switch!

Greetings Peter

 

[Antz81]

Thank you both. I changed the OEM sensor a few years ago for a heated one. Its a bosch (same as C4 ZR1 uses, don't know if this is zirconia oe titania type?) The wide band is a bosch LSU 4.2 sensor.

But going back to basics (stock) is always a good idea! Ashame I didn't think of that!

I'll let you know when I have done the switch!

Greetings Peter

The c4 zr1 uses a heated zirconia sensor. so if you still had it you could connect it instead if a stock sensor if its easier.

 

[Peer81]

Yep I still have it and the wiring is already adapted to be a plug and play with the zr1 sensor.
The thing that i think about right now. The OEM sensor is a two wire with a high ref input to the ecm and a low ref input. But the low ref sensor is a isolated ground return connected. The 3 wire zr1 sensor has one return hi ref inlut signal for the ecm and 2 wires for the heated element. If i remember correctly i took 12v from the spark input and ground element is connected to the isolated ground. This sensor uses the cars ground (exhaust) as sensor ground (not the isolated ground like the 2 wire). So I'm thinking. Maybe connecting the heater element to low ref ecm ground isn't such a good idea. Maybe using car ground instead of low ref ecm ground for the oxygen sensor ground also not a good idea?
As I look at it now maybe a 4 wire sensor would me a better idea..

Greetings Peter

 

[Antz81]

I don't think which ground you use for the element is going to make a difference. All grounds are connected together anyway.
As for going to a 4 wire sensor so the low ref sensor is connected, may be a good idea. But on saying that, I'm currently using a single wire sensor without any issues.

 

[Peer81]

I'll measure the resistance in the isolated wire to the ECM and by the body ground to the ECM tomorrow, but as you say I also think it doesn't make a difference. Thanks for the info

Greetings Peter

 

[Peer81]

I found the problem and solved it, thanks to Gerry and Antz!
Changed the wide band back to the heated narrowband and everything is working perfectly!
Thanks for thinking with me

Greetings Peter

 

[GerryLP]

Hey.

Maybe it means that you won't need the aftermarket prom :L, and so you can send it to me. Lmao!!!...jk

 

[dougelam]

What is your carb number? 170???
I will tell you how to calibrate it to get its full potential.
and not to be picky but your distributor is off a tooth, coil should be even and not turned like it is. Just a cosmetic thing for me.

 

[Peer81]

If you could do that, great! The number is: 17081218 (E4ME Rochester).

I was thinking about this.
First setting idle rpm in drive when hot.
Checking A/F when driving at constant speed (hot) and adjust.
Checking WOT and adjusting sec. valve opening and rods.
Checking acc pump (and sec. valve opening) when for accelerating from speed a to speed b
Adjusting cold start choke duration and choke valve opening (rich / lean)

And when the carb is setup right, dailing back and forward the ignition timing to see which setting gives the best performance. At this moment it's at 17 degrees advance (with ECM disconnected). So with a timer and some 1/4 mile space I can see which timing gives me the best performance.

@Gerry, I was thinking the same thing! I was also thinking maybe my old ECM wasn't that bad after all! Only weird thing is that Cor checked my ECM and OEM Prom on his 81 and that gave the same weird fluctuations and on mine.

But it's easy to switch PROM's and I have a dail back timing light so I could see what is the difference between the spark table OEM and Hypertech (something Cris has done a few years earlier).

Greetings Peter

 

[dougelam]

Sorry about the wait, I haven't worked on a carb since the 90's I had to dig out all the stuff I have to repair them. It takes a couple of special tools though and they are getting hard to find. I remember loaning some of mine out a while back and they never got returned. Do you have any or access to them?

Found a set on E-Bay
Thexton GM C 3 GM CCC TBI Fuel System Adjustment Kit Thexton 2500 Nice | eBay
There is a little bag in the back of the picture that has what you need; that the angle gauge

Some preliminary thoughts,
What spark plugs are you running?
Is everything still stock or close to it?
Canister purge system still intact?
Does it have a ISC? (idle speed control)
Has your carburetor been apart that you know of?

 

[dougelam]

All I wanted to add is how important the little spacer is when setting the mc solenoid.
Also the relationship between the linkages are important as you already know.
And by looking up your carb number the mc dwell is adjusted by the idle air bleed screw and not the idle mixture screws.
And once you like all the settings and the way it runs warm you can then adjust the choke pull off for optimum cold running.