OBD 2 codes and operation OF

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Douglas Friedberg

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Never know enough about OBD2 system, But i keep getting evap error codes large small leaks, i went through the system found bad purge sol. replaced it then replaced rear vent solenoid and checked the lines, replaced gas cap twice, and then i thought i would play around with trying to fool the system and i disconnected rear vent sol. connector. to see if leak would go away or not come back and found that while disconnected i got sol code right away, ok understandable but leak code for evap system never came back. So disabling vent sol will that stop OBD2 from coming on or is this program depended on not coming up with evap leak detection if vent sol is diconnected or did i just prove that rear vent solenoid is doing the leaking and is bad after 2 months of being new. ?????? Someone has been through system before and maybe tank was dropped for fuel pump because i see all new hose clamps. So if what i did proves nothing then i guess next step is smoke test and proably come to find out O ring was screwed up when fuel pump was done. All the hoses are in great shape as well.
During the time after replacing purge and vent sol. system did self pass for awhile and then came back again. Even cleaned up gas tank area where gas cap seal sits.
Any thoughts here.
 

Bill 1960

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Leaving a plug to a component off won’t stop the codes, because it’s always operating the valves and reading the pressure sensors to see if the tank pressure behaves as it should.

If I understood your post correctly it worked for some time after replacing the two valves, then the error returned. Which makes me ask were these OEM parts or aftermarket?

There are some excellent videos on YouTube about testing evaporative emissions systems using a bidirectional scanner, which IMO should be done prior to chasing a leak with smoke. The testing with the scanner will tell you if each valve operates properly and if the entire system holds pressure or in actuality has a leaking valve or leak elsewhere.

The Tech2 scanner is used by enthusiasts here; dealers or any shop worth doing business with will have a capable scanner.

When the purge valve on mine failed it behaved similarly to yours. Sometimes it was working for enough cycles the code would clear. Then it would leak again and I’d get the code back.
 

Big Mama

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I can’t help much with the scan too but If I remember right the rig has to have several ignition cycles to set the code after you clear it. I had the same code and I’d clear it and it wouldn’t come back for a few days. I also tried a new gas cap. Once under it (read I’m lazy) my vent hose was split in half. Got the kit to replace it and problem solved.
 
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Douglas Friedberg

Douglas Friedberg

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Leaving a plug to a component off won’t stop the codes, because it’s always operating the valves and reading the pressure sensors to see if the tank pressure behaves as it should.

If I understood your post correctly it worked for some time after replacing the two valves, then the error returned. Which makes me ask were these OEM parts or aftermarket?

There are some excellent videos on YouTube about testing evaporative emissions systems using a bidirectional scanner, which IMO should be done prior to chasing a leak with smoke. The testing with the scanner will tell you if each valve operates properly and if the entire system holds pressure or in actuality has a leaking valve or leak elsewhere.

The Tech2 scanner is used by enthusiasts here; dealers or any shop worth doing business with will have a capable scanner.

When the purge valve on mine failed it behaved similarly to yours. Sometimes it was working for enough cycles the code would clear. Then it would leak again and I’d get the code back.

I used ac delco for purge valve. And auto zone stuff in back.
While I did disconnect vent solenoid in rear it never came back with leak codes but just solenoid code understandable. However once I plugged rear sol back in sol code went away as did dash one indicator and next day small leak came back on
No tech 2 scanner no way to cycle vent solenoid so that’s why I did it manually. Because what promted me to do that was solenoid was very hot. Meaning to me on to long for operation and I feel shouldn’t get that hot. For something that should just cycle. Would also tell me something
Is fishy with that circuit or it’s operation. Now I did discover that purge solenoid up front was replaced with a China version and I don’t like there crap and I found that when I took it apart to look at it China used a cheap steel retainer to hold coil in place and it let loose allowing coil to fall out of place. The retainer is not heat treated spring steel and can not hold coil in place under vibrations and road bumps.
I guess I’ll get a AC delco for the back as well and get rid of auto zones junk. While ac delco maybe made somewhere else people have to realize Delco has strick guide lines on there manufacturing proceedings over anything generic made somewhere else.
thanks for the info
Douglas.
 
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Douglas Friedberg

Douglas Friedberg

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The evap system did cycle into system ok and ready fir about a month. While it did come up with code again of leaks I did play around with what set off the code sooner as farc as gas tank half or full doesn’t make a diff.
While Iam on fuel matter. I come to find out Yukon is 2006 flex according to vin. Now did that mean gas cap should be yellow ? I bought
Both thinking that vent relief might be diff. But didn’t help the matter. It’s a Cal vehicle in the Midwest. Midwest sure eat it up that’s for sure.
 
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Douglas Friedberg

Douglas Friedberg

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I can’t help much with the scan too but If I remember right the rig has to have several ignition cycles to set the code after you clear it. I had the same code and I’d clear it and it wouldn’t come back for a few days. I also tried a new gas cap. Once under it (read I’m lazy) my vent hose was split in half. Got the kit to replace it and problem solved.

I can understand that and thanks for info. First thing I looked for was hoses. Nice that there not using so much rubber anymore but that was the first thing I always look for. Someone had the tank down it looks like to me all new hose clamps and some hoses.
 
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Douglas Friedberg

Douglas Friedberg

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End result of evap code was fuel pump rott. The pressure side rotted in half springing a fuel leak dropped the tank found original pump 240,000 miles and return side back feeds evap system was rotted in half as well. Many dollars later and fuel line replacement and repairs system is now sealed and evap code bye bye. Almost had to buy new mounting ring.
I also rebuilt rear end while I had it on lift as well. G 80 was not in trouble just all the bearings. There’s a few tricks you have to know about taking one apart with 240,000 miles on it and with inside the tube ABS sensors inside the tubes.
 

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To be clear on the vent valve, it stays open when the ignition is off but is mostly closed during routine driving. It can be commanded closed during testing using a bidirectional scan tool or by applying 12v to the terminals on the valve.

The ECU closes and opens it during the evap tests, generally commanded after each cold start, but there are many parameters to meet before it will even test. In order to complete a drive cycle for the evap, the fuel tank level needs to be between 1/4 and 3/4 full. The first time it fails it will push the "tighten fuel cap" message and will set a pending code. If it fails the next test it will set the CEL.

Essentially the evap test closes the evap system and applies vacuum (through the purge valve on the engine) to the system -- through the evap canister and all the way to the fuel cap -- to see if the fuel pressure sensor indicates a vacuum condition. This is to see if the evap system can be closed/sealed. The system remains closed for a time and if the vacuum decay (the time it takes to get back to atmospheric pressure) is long enough, indicating a sealed system, it passes the evap test. If the system returns quickly to atmospheric pressure, a leak is indicated.
 
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Big Mama

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I have brembo brakes in the front will they fit the 22x10 -19 wheel?

End result of evap code was fuel pump rott. The pressure side rotted in half springing a fuel leak dropped the tank found original pump 240,000 miles and return side back feeds evap system was rotted in half as well. Many dollars later and fuel line replacement and repairs system is now sealed and evap code bye bye. Almost had to buy new mounting ring.
I also rebuilt rear end while I had it on lift as well. G 80 was not in trouble just all the bearings. There’s a few tricks you have to know about taking one apart with 240,000 miles on it and with inside the tube ABS sensors inside the tubes.
Thanks for closing the loop on what you did. Hopefully it’ll help the next guy.
 
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Douglas Friedberg

Douglas Friedberg

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To be clear on the vent valve, it stays open when the ignition is off but is mostly closed during routine driving. It can be commanded closed during testing using a bidirectional scan tool or by applying 12v to the terminals on the valve.

The ECU closes and opens it during the evap tests, generally commanded after each cold start, but there are many parameters to meet before it will even test. In order to complete a drive cycle for the evap, the fuel tank level needs to be between 1/4 and 3/4 full. The first time it fails it will push the "tighten fuel cap" message and will set a pending code. If it fails the next test it will set the CEL.

Essentially the evap test closes the evap system and applies vacuum (through the purge valve on the engine) to the system -- through the evap canister and all th way to the fuel cap -- to see if the fuel pressure sensor indicates a vacuum condition. This is to see if the evap system can be closed/sealed. The system remains closed for a time and if the vacuum decay (the time it takes to get back to atmospheric pressure) is long enough, indicating a sealed system, it passes the evap test. If the system returns quickly to atmospheric pressure, a leak is indicated.
Both connection tubes 5/16. And 3/8 steel output lines of pump where rotted and cracked almost in half.
 

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