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 wide 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 the 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 height 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 throttle 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

[ejowrench]

Make year and model might help. What is a wide band doing on a electric Q-jet? Do you mean o2 sensor? Any other modifications you would like to share? Sorry I have more questions than answers.....

[Peer81]

Sorry I forgot to mension that. It's a E4ME q-jet on a '81 Corvette. The wide band is there to setup the carb the right way, idle WOT etc etc. It simulates a narrowband signal the goes to the ECM input. As i still have the narrowband sensor I can swap it without a problem. Other mods: The airpump it empty, vacuumlines to egr are plugged and the egr has a steel plate under it. Changed the narrow band O2 sensor for a heated sensor and I can swap it for a wide band. New coolant temp sensor in front of the intake. True dual exhaust with x-pipe but OEM exhaust manifolds. Well that's it i think.

Greetings Peter

[ejowrench]

Bear with me Peer81, just trying to wrap my head around this problem. Question: Are you substituting the Original o2 sensor with a wide band, or is the Original [one wire] o2 sensor still present and hooked up to the ECM, unmolested? [no other wires tapped into the o2 signal wire?]

What code   are/ is setting?

[jjr]

 This system does have a tendency to go to open loop with extended idling. It's because
the O2 sensor cools too much.

 In "open" loop the ECM does not use the O2 sensor to control the air/fuel mixture. The
manual says the ECM sends a "calibrated" signal to the M/C solenoid based on the MAP,
coolant temp, and TPS position.

 The ECM also sends full rich commands during engine start and WOT.

 I've chased a few of these 1981 CCC issues before, I now connect to the ECM and look at
what all the sensors are doing and mode status. 1981 stands alone, as 1982 and later had
a good number of changes. I humorously call the '81 CCC System... OBD 0.5

 Joe

[Peer81]

I think OBD 0.5 is a good discription of what this system is! 
On the L81 corvette forum people are also pointing at the wide band as it could be the problem. No the original oxygen sensor is not in there it's the wide band (Bosch LSU 4.2 sensor) and the Fast module sends a simulated narrowband output to the ECM. I also changed the original oxygen sensor in the past to a heated sensor (widebands are also heated) so no more in and out of closed loop at idle cooldown. But I'll first switch back to the original oxygen sensor and only use the wideband to monitor on the passengers side. Maybe the ECM gets an overload on oxygen sensor input so it just goes into full rich mode. Only one way to find out
I'll let you know!

Greetings Peter

[jjr]

 Peter I own and admin a automotive resto site that deals
with the Pontiac 301 and 301Turbo engines.

 Lots of '81 owners have CCC headaches, and I have done a good deal
of research on the 81 CCC. I also have a lot of 81 CCC materials posted
there.

 You "might" need the original O2 sensor back in there as the 81 ECM is not
too tolerant. Also,I would highly suggest the use of an original Delco sensor
as I have found the Bosch replacements to NOT be very consistent.
 Some are great from day one, others die a few days after installation.

 What is ironic is that Bosch pioneered the tech for the thing.

 But to be sure, a 1981 CCC system CAN be made to work just as the
general intended, even on a Vette... just kidding... <grin>

 Joe
 

[Peer81]

Hello Joe,

Great to hear other people also dig deep into the first ecm systems. I also have the OEM 2 wire oxygen sensor laying around so I'll reconnect that tomorrow.
The first question. The OEM oxygen sensor is a two wire system. First goes to the high ref? oxygen sensor input on the ecm.
Second goes to the low ref? oxygen sensor input on the ecm, it appears to also be connected to ground. So what does the low ref input do as it is connected to ground? Or am I reading the wiring diagram in the wrong way?
I'll also check the low ref to ground connection tomorrow.

Greetings Peter

[ejowrench]

As far as I know, the ground or reference low, simply completes the circuit. Some o2 sensors are grounded through the exhaust [one wire], grounded by the ECM, or some other ground source [engine , chassis]. Think of the sensor as a small battery, generating voltage to the ECM.

You mentioned you had hedders, and they can be a problem for o2 sensors, as you probably know. Placement of the o2 sensor is critical on some applications. Just because the hedder manufacturer put the o2 sensor bung in this spot doesn't mean it is the right spot.

Where's the best spot for the sensor? In the factory exhaust manifold! Sorry, I know you didn't want to hear that. I think if you locate it as close to the engine as possible, with the pigtail up, you can get it to work.

One more thing, some o2 sensors use reference ambient air through the sensor wiring, and some through the sensor itself. So splicing some harnesses can be a problem.

[Peer81]

Thanks for the tip but no the exhaust manifolds are still stock and the O2 sensor is still at the original place. There is no splicing going around in the signal wiring so that isn't a problem. And I also think connecting the low ref wire to isolated ground or body ground shouldn't make a difference if the ground signal is good and strong. I'll let you know tomorrow what I found out

Groeten Peter

[jjr]

 None of my materials expand on "low reference", it is my belief that it's an attempt to
ensure a good ground. Here is a snippet from a 1981 Pontiac service manual about the
sensor.



 And I also believe that the 1981 Corvette calls for the exact same O2 sensor as the Pontiacs.

 Joe

[jjr]

 In a non-gm chart of the system, it is shown as simply a ground...



 Joe

[jjr]

 Well the "calibrated" signal when warm and in open loop is 10 times a second.

 I think you may have a bad wire somewhere now...

 Also, I just finished a dwell adjustment thread for the 1981 Qjet.

 Assuming the bench settings got you in the ball park, here would be how to trim
the carb...





 Joe

[Peer81]

Thanks for all the info Joe!
I'll read it tomorrow to fine tune the carb.

BTW. I found the problem and solved it!
I changed the wide band back to the heated narrowband and everything is working perfectly!
Thanks everybody for thinking with me

Greetings Peter

[jjr]

 Ok, here's how the general educated it's line people, maybe this
will help also...









 Joe