Digital Tuning Vapor recovery cycle explained

[Mcmahst]

Another issue on my 73 Mustang with a transplanted 87 TBird 5.0. As part of the swap, I decided to retain the emission control systems. One is the vapor recovery system. It uses a charcoal canister with draws (?) gasoline vapors from the fuel tank and via an electrical solenoid, using engine vacuum, draws the collected vapors into the intake.

My question is, when is it suppose to do that? I ask because I’ve noticed at idle, when I’ve first started the cold engine, that the solenoid is activated. That system is reducing the idle vacuum 2 inches. If I plug the vacuum draw, it runs a bit better; not a lot. It’s not running bad. I was just wondering if that is a sign of other trouble with the EEC-IV sysyem.

I have Probst’s book on the 80-87 EEC-IV book, and he mentions when the purge cycle usually occurs, but no mention or diagnosis of what to do if it is purging at an odd time/condition. Any ideas what to do or what this situation may indicate?

 

[Mustang5L5]

It's complex......

And here's the actual programming logic for A9x family ECUs. It's incomplete, as i don't have the doc handy that tells what the various setpoints are. For instance coolant tamp. I had it...but it's on a different laptop that won't boot. You might be able to make some assumptions to conditions.

 

[Mcmahst]

Since mine is Speed Density, I wonder if that affects the cycle. What I know is, it’s purging at idle, on a cold engine. That doesn’t make sense to me. While I tested the purge solenoid, I’m wondering if the valve is leaking vacuum? I’ll test it again (it’s been 6 months since I verified it was a working solenoid).

Thank you for the excellent write up on the programming. Very nice.

 

[General karthief]

Another consideration is the fuel system on efi cars are sealed for the most part, my guess is you have a vented fuel tank (73 tank/vented cap).
With SD efi systems a vacuum leak, even small, will effect idle/cruise modes. Just cap the lines, I would delete the canister/purge valve but that's just me. I'm kinda 'shade tree' you know?

 

[Mustang5L5]

Btw I didn’t write that. It’s a ford doc i got my hands on

 

[Mcmahst]

This retrofit of the vapor recovery system, using the 73’s canister modified to work with the vacuum purge valve, isn’t an invention of mine. I got the idea from someone who has great builder skills; he did the same on a 72 some years ago. The car is garage kept in Az, and I’m trying everything I can to reduce that unburnt fuel odor in the garage.

I’m gonna retest the purge valve to make sure it’s not leaking. I’ll go from there. Thank you for the insights.

 

[General karthief]

I haven't dealt with a 70's era stang in too long, don't remember them having a charcoal canister. That's good,
Can you pull codes?

 

[Mcmahst]

Ok, so I retested the purge valve and interestingly enough, it passed, but the results were not expected. So, I connected my vacuum pump on the side of the valve I had connected to the manifold vacuum. Pulled the vacuum and saw that it slowly leaked off. Rats, I thoughts. So, just for grins, I did the same test on the other side of the valve and it passed with flying colors. So, I reversed the valve, ran it and the idle vacuum pickup 2”. Who knew it has an orientation?

 

[General karthief]

I thought about that but dismissed it, figured it was either open or closed, not directional

 

[Mustang5L5]

It is, but sometimes valves like this work better in one direction than the other. Has to do with the manner in which the valve closes. Picture a cone into a seat. When you pull a vac in one direction, the pressure on one side can help open the valve. But if you go the other direction, the pressure differential helps seal it better.

 

[Mcmahst]

Yep, I was surprised at the results of the test, but once I thought it I pver, it made sense. Is that true also for the TAD and TAB solenoids?

 

[Mustang5L5]

. Is that true also for the TAD and TAB solenoids?

I don't really know. It's been a while since i've had them in my hand so i can't picture what type of solenoid they are. Usually when solenoids are directional, there is a small arrow molded into the body somewhere....usually.

 

[Mcmahst]

I’m going to test them sometime this week; I’ll report back!