See attached file.

 

The lasted update on my quest to pass state emissions test. After a very busy week I finally was able to work on my truck. The engine was HOT when I did a KOER test and this time a 42 code came up. The engine was must have to cool the last time. After receiving the 42 code we checked the voltage from the C02 sensor, at idle .74 volts at around 2000 rpms .86 volts. Next we moved to the Feedback Control Solenoid(FCS) we unplugged the connector applying 12 volts directly to the FCS and did not hear any clicking. We then bench checked the solenoid and heard a faint click, having to lean into it to hear the click. Then I sprayed some carburetor cleaner into the top part of the solenoid the applied voltage to activate. After reinstalling we checked the incoming voltage with a 5 volt reading at idle and a 13.4 reading at 2000 rpms. Next we searched for vacuum leaks and found a large one at the T at the base of the carburetor. There seams to be an improvement in engine performance but ran out of time to redo the KOER test. I am interested in your thoughts and suggestions. Thaqnks

 

"2000 rpms .86 volts." Is the .86v making it back to the ECA pin 29?

If not, then the ECA will interpret a lower voltage as a lean condition, and make output adjustments to Rich up the Fuel/Air mixture.

 

Fixing that big vacuum leak may be all it needs.

 

At idle, while monitoring the voltage at O2 sensor manually apply 12 volts to the FCS.
The O2 sensor voltage should drop to .3-.4 and the engine should run noticeably rough.

If this happens then the computer has a chance at controlling the mixture, if all the other needed parameters are meet.

If this doesn't happen remove the FCS again. The lower "can" is removable. The 2 wires terminate in the bottom of the can, there is a matching plug / socket wiring connection that could have failed due to corrosion and or bad connection.

Good luck
Jim

 

Pin is getting 8.6 volts so we moved on to your suggestion of monitoring to O2 and applying 12 volts to FCA. No change in engine performance or O2 readings. After removing the lower can of the FCA wire checked out OK however spring at bottom seems to be badly worn not sticking out of the plunger. Can a spring be purchased and where, does the entire FCA need replacing and from where.


Thanks
Frank

 

I am not sure if you can buy an FCA but a lot of members on this site have converted there 1985-86 inline six to a Duraspark Ignition System and used an older style Carb. So there should be a lot of these Carbs on garage shelves just collecting dust.

So it this point I would consider starting a different thread titled ....
Wanted, feedback carb parts
or something like that.

I think you will find the members helpful, and also you are one of the few that is trying to fix a feedback Carb.

Jim

 

Thank you for your suggestion Jim, the thought had floated through my mind.


Frank

 

Jim, I found a spring that worked and tested the solenoid before and after assembly and it worked, I heard a clack when engaged. Upon reinstallation on carb I ran a new KOEO test with no codes, then a KOER test with three codes; 13,42, & 58. I decided to perform the test you suggested of watching voltage of O2 while applying voltage to FCS. Ran engine at 2000rpm for 2min. to insure it was hot, volt reading of 8.7 from O2. Returned to idle and applied voltage to FCS, at first no changed detected and O2 stayed around 6.0v. After paying real close attention, confirmed by my eagled eared wife, the rpm's increased by a couple hundred, no tach hooked up.
Is it time to order the new FCS or could it be something else? I am researching the 13 and 58 codes next.

 

Code 13, you must have a little servo motor of some kind on the throttle. When the computer tried to lower the idle it couldn't, that is where this code came from.

Code 42, system rich code, you already know about that. From your voltage tests, the oxygen sensor is working and can be believed, and the computer is working correctly too. It that correct?

Code 58, do you have a microswitch on the throttle also?

 

It appears from my research that both code 13 & 58 have to do with idle issues.


Code 42 is system rich, new O2, new engine temp sensor and refurbished(I think) FCS with voltage coming from computer. Is there something else I should be checking or doing?

 

I just did the same test on my truck, here what I got...

Fully warm engine, at Idle 1100-1200 RPM in park.
FCS energized . little to no change in O2 read out, about .8 - .85 VDC. Dropped about .05 when energized.

Repeated same test with it in drive, brake on, 700 RPM. When energized O2 went from .85 VDC to .25 VDC. Engine still ran smooth at .25 Volt, I thought it would run rough at .25 volts.

The Feedback Carb has 2 circuits, cruise and Idle. I did test the cruise many years ago by unplugging the right headlight plug and connection jumper wires to the FCS. So when I pulled the light switch ON the FCS would energize. At 55 MPH , turning on the headlights would cause a sudden and abrupt loss of power, almost like taking your foot off the gas.

I would retest at a lower idle.

Jim

 

Jim, that is neat information, regarding your experiment of the FCS. Regarding the loss of power [lean?], when functioning as designed, doesn't this get turned on and off very quickly rather than staying on all the time?

 

That's the way the GM system worked. I had one of those quadrajet carbs with the solenoid on it, and it sounded like a lifter ticking all the time. And I had to get a dwell meter and hook it up to the solenoid to get the system calibrated in. I think it had a stop on it you adjusted. It actually pull needles up and down inside the main jets.

 

Yes by design it does cycle on-off. It is called "crossover" , every time the O2 voltage crosses .45 volts it toggles the FCS.

https://www.discountconverter.com/ca...rter-&News=248

It was my first quick and dirty cruise test to see how lean I could force it. At the time everything I had read on this site said a feedback Carb was WAY RICH and would cause poor gas mile at cruise speeds.

So based on what I had read, I designed and built a variable frequency and pulse width generator which could replace the missing computer supplied voltage signal to the feedback solenoid (FCS) on the carburetor. The idea was to be able to dial in the mixture at cruise based on the O2 sensor.

I then hooked up a voltmeter in the cab to monitor the O2 sensor. It always read lean on the freeway. So I replaced the O2 sensor, still had low O2 readouts. So you can't really trust what you read on the internet.

While everyone seems to think the feedback carburetor runs very rich at cruise it really doesn't. The Carb is "ALL IN" at speeds over 60 MPH. I never did use the pulse width generator because at around 60-65 mph the engine goes lean anyway. About 55 MPH the O2 readout was .6-7 volts, as you speed up the numbers drop to zero. Letting your foot off the gas makes the numbers jump up. This is also why I never replaced the feedback Carb.

Jim