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Old 01-22-2011, 08:18 PM   #1
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does the ecm have to be in closed loop for canister purge?

my fuel tank is backing up with pressure, always has, but it's worse now, I am waiting for a new purge valve, mine is intermittently faulty.

if the purge solenoid is faulty would it throw a code?, mine has not.

I removed the purge valve for the time being and ran a direct line from the tank to the canister without the vacuum operated value,to see if pressure can escape into the charcoal canister. while waiting for the new valve.

and to relief the pressure, I guess if the hissing coming from opening the gas cap goes away, then the solenoid works right?

Last edited by RBob; 01-23-2011 at 09:41 AM. Reason: Corrected subject line (to closed loop)
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Old 01-23-2011, 09:47 AM   #2
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Re: does the ecm have to be in closed loop for canister purge?

Typically yes, CCP takes place during closed loop operation. This allows the ECM to correct for the added fuel or air.

Note that on f-body's it is normal for the tank to build pressure. There is a safety release valve near the tank that will relieve pressure once it is above the CCP vapor management valve (VMV) release pressure. This same valve will also allow air into the tank to replace the fuel that has been removed.

The CCP canister is open to the atmosphere, it can not be pressurized. Once the tank pressure is above the VMV pressure the vapor will go into the canister. The charcoal will hold it until the ECM purges it.

For excess pressure to build in the tank both the VMV and tank safety valve would need to be bad. Although the CCP vent line at the tank has a restriction. If this plugs then the tank pressure will be slightly higher as it will depend upon the safety valve to relive pressure.

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Old 01-23-2011, 12:52 PM   #3
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Re: does the ecm have to be in closed loop for canister purge?

when you say typically, you mean can purge while in open loop, how do guys running open loop address this issue?

for the life of me last year can only get a perfect tune in open loop and frankly I like it.

otoh, I would mind purging as designed, I have driven many many miles 8k? or so in open loop, when I pulled the canister is was not filled with gas, when I pulled the line to it, it was dripping gas, I blew on that line and I cold feel the air going into the gas tank, although a just a little.

I used a clear motorcycle hose to bridge the line to the canister without the valve, while running, the line fills with gas, (wow) and streams towards the canister.

if my open loop driving do not purge wouldn't the can fill up with gas, and I should smell gas all the time? I took it off, it appears to be in good condition and not full of liquid.

I also bypassed the solenoid and the car ran rich(wideband) and ran like crap at idle, something I read says the system only purges in cruise, just wanted to check,

I then pull the vacuum line atop the can's valve and the car ran as normal with the solenoid bypassed, so the valve atop the can does it job.

and so does the solenoid, since Haynes repair manual says the solenoid shuts flow when energized.

there is a flag in my bin file for request "closed loop for can purge", if was not check off, is this the "flag" that allows purge in open loop, or am I wishing?
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Old 01-24-2011, 07:44 AM   #4
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Re: does the ecm have to be in closed loop for canister purge?

Some calibrations/firmware have an option switch to allow open loop purging. So they can be set up to purge in open loop.

Purging is done while cruising. Also note that the purge solenoids come in both NO and NC configurations. So a NO solenoid that has the electrical connector up-plugged is always purging.

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Old 01-24-2011, 07:51 AM   #5
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Re: does the ecm have to be in closed loop for canister purge?

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There is a flag in my bin file for request "closed loop for can purge", if was not check off, is this the "flag" that allows purge in open loop, or am I wishing?
That should be labeled as "set for open loop purge." Just to be sure this is mask $6E, address $14, bit 5.

Setting that bit allows purge in open loop.

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Old 01-24-2011, 07:54 AM   #6
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Re: does the ecm have to be in closed loop for canister purge?

I would say yes, I did not know I had a CCP with an internal short. And as such Id have no trouble driving around the block before the new ECM would start to smoke when the CCP was turned on and shorted out the ECM board.

did that twice before resting my hand on the screaming hot CCP valve n figured it out.
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Old 01-24-2011, 11:56 AM   #7
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Re: does the ecm have to be in closed loop for canister purge?

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Originally Posted by RBob View Post
That should be labeled as "set for open loop purge." Just to be sure this is mask $6E, address $14, bit 5.

Setting that bit allows purge in open loop.

RBob.
it is a 6E mask, it went over my head with the "$15 bit 5 address" comment

it says "request for close loop purge" it's the other way around the default is is purges in open loop I believe...so withing 2 minutes time it the ecm seem a cruise condition, it purges... am I wrong?
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Old 01-24-2011, 12:00 PM   #8
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Re: does the ecm have to be in closed loop for canister purge?

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I would say yes, I did not know I had a CCP with an internal short. And as such Id have no trouble driving around the block before the new ECM would start to smoke when the CCP was turned on and shorted out the ECM board.

did that twice before resting my hand on the screaming hot CCP valve n figured it out.
you'd figure it would throw a code and not fry the ecm, that is weird...
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Old 01-24-2011, 02:52 PM   #9
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Re: does the ecm have to be in closed loop for canister purge?

the older computer systems don't check the solenoids, they also don't check to see if the purge systems is working or not, so there are no codes set for those things.
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Old 01-24-2011, 03:01 PM   #10
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Re: does the ecm have to be in closed loop for canister purge?

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the older computer systems don't check the solenoids, they also don't check to see if the purge systems is working or not, so there are no codes set for those things.
how old is old? there is a flag to turn on purge, don't they test it while you are getting a smog test?
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Old 01-24-2011, 05:12 PM   #11
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Re: does the ecm have to be in closed loop for canister purge?

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you'd figure it would throw a code and not fry the ecm, that is weird...
Yep, it was a ral head scratcher, I was ready to send the car through a wood chipper.

Needless to say I no longer have a CCP, not gonna risk that again, Ill free vent.
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Old 01-24-2011, 08:26 PM   #12
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Re: does the ecm have to be in closed loop for canister purge?

the flag is just for the computer to run the purge routine. but there aren't any purge diagnostics that the ECM runs like there are for the EGR system.

i don't remember for certain, but i believe some systems started checking the solenoids in 94 by turning the solenoids on and then off and watching for either a change in voltage on the ECM side of the solenoids, or for the inductive spike produced when the solenoids are turned off.
i know for certain that starting in 96 some solenoids were checked and would set codes if the computer didn't see the right reaction when cycling the solenoids.
just because the solenoids pass the computers' test doesn't mean they are good, just that the coil tests ok. they could test good but still be bad because they are stuck open or closed.

im not sure just how the state of California does their emissions test, but i know its much more strict than what Texas uses.
here, pre OBD2 cars that are 25 years old or less receive a visual check and then a tail pipe test that is ran on a dyno.
if the inspector doesn't see any thing missing or disconnected and tail pipe emissions are within limits, the car passes its' emissions test.

cooked computers because of shorted solenoids or wiring are somewhat common with the older systems. its why rebuilt computers come with a list to write down measured specs for each solenoid and relay when tested at the computer connectors.
the newer OBD2 systems can see shorted solenoids and will disable any it sees as a problem to protect the computer. they also set a code and turn on the MIL.
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Old 01-24-2011, 08:51 PM   #13
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Re: does the ecm have to be in closed loop for canister purge?

I can say I went in circles to find it, regular checks didn't but with any ECM short, the quickest solution is to post a pic of what section burned out to narrow it down. Some reason I didn't do that to begin with.

On top of the the ECM the car came with was already fried out in that section but still ran great for 10 years, then didn't one day, and new ECMs fried out also but didn't still run great.
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Old 01-24-2011, 09:34 PM   #14
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Re: does the ecm have to be in closed loop for canister purge?

i always open the computer and look for any signs of over heating before i condemn the computer.
sometimes shorted solenoids or relays don't show up with regular resistance tests. i always test for correct resistance first and then do a current test on them before replacing the computer. if i find a hot spot in the computer, when possible i trace it back to the pins. then closely check the component that part of the computer drives. if the part checks ok, i open the harness up. even if everything checks ok, i still replace the part the computer drives where it cooked at before i put the new computer in.
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Old 01-29-2011, 09:47 AM   #15
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Re: does the ecm have to be in closed loop for canister purge?

I am having this same issue with my newly acquired 89 IROC 5.7. I am getting positive pressure in the gas tank after only a short drive. The check valve in the rear near the tank is releasing the pressure causing a bad fuel vapor smell when parked after a drive. Stinking up my garage!

I pulled the evap line from the char canister under the hood and I CANNOT blow air backwards towards the tank. I pulled the rubber line from the hard line at the tank and I CANNOT blow air towards the tank there either- but the hard line going to the engine compartment is open.

Do I have a clog in that evap hard line on top of the tank?

I see reference to their being some sort of engineered restriction on that line- but this seems to be a full clog.

Time to drop the tank? I hate the thought of that just to check on this issue. Seems the tank was dropped recently (last few years). There is visual evidence that someone had the tank straps off...I assume to replace the fuel pump. Maybe they crushed the evap line?

thoughts?

thanks- Jason
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Old 01-29-2011, 09:53 AM   #16
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Re: does the ecm have to be in closed loop for canister purge?

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thoughts?

thanks- Jason

I felt the same thing on my other carb'ed car, vent line didn't seem to vent for the size, changed allot of stuff, then dropped the tank to find out once I broke that shiled away from the vent hole on the under side of the sending unit, the vent line is big, but the hole for it that vents the tank is tiny. Making it feel clogged when ya blow air into it.
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Old 01-29-2011, 10:40 AM   #17
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Re: does the ecm have to be in closed loop for canister purge?

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I felt the same thing on my other carb'ed car, vent line didn't seem to vent for the size, changed allot of stuff, then dropped the tank to find out once I broke that shiled away from the vent hole on the under side of the sending unit, the vent line is big, but the hole for it that vents the tank is tiny. Making it feel clogged when ya blow air into it.

So you are thinking I dont have a clog- the vent hole/line is just tiny. I wonder why then- my tank is building pressure? It should vent out to the front to the char canister- correct?
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Old 01-29-2011, 11:12 AM   #18
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Re: does the ecm have to be in closed loop for canister purge?

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So you are thinking I dont have a clog- the vent hole/line is just tiny. I wonder why then- my tank is building pressure? It should vent out to the front to the char canister- correct?
unless its rusty only raw fuel could get in there, mine was rusted out, I replace it and then thought maybe something had crawled in it and died. But turned out not as such. Just a tiny hole on the sending unit side. [probably to keep raw fuel out]

could just leave your gas cap lose 4 now
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