-   Jeep CJ Forum (
-   -   purge signal constant when cto is open? (

thompsoj22 06-18-2013 06:09 AM

purge signal constant when cto is open?
just trying to understand how the evap canistor can be effective if it only purges when the cto valve/switch opens?. so i shut the engine off on a hot evening and the fuel tank is creating alot of fumes/pressure which the charcoal canistor captures? then when i start in the morning it purges after the cto reaches temp and opens? and after that it is constantly purging?. the vac line running to it is rubber fuel line and when it gets hot it is collapsing from the suction, is this normal?. can someone explain the system? what prevents vacume from being applied to the fuel tank during operation?:confused:

Matt1981CJ7 06-18-2013 06:23 AM

The canister is designed to trap fuel vapors when the engine is not running.

The purge signal should be connected to ported vacuum thru the CTO, so it doesn't purge at idle or when the engine is cold.

Hope this helps,


John Strenk 06-18-2013 06:30 AM

603 Attachment(s)
Collapsed rubber hoses is not normal. Either wrong type or deteriorated

thompsoj22 06-19-2013 06:27 AM

thanks for the info
i was under the impression that the cto is a normally closed valve/switch that opens above a certain temp? does it also in some way limit vacume at idle?. the purge line Ts from the pcv circuit and is approx 3ft to the canistor. the heat softens the hose and it collapses. ill go to a hard plastic line and improve that issue. ill also throw a new CTO at it just for kicks. it is the original so it cant hurt. thanks again for the help!:cheers2:

INowUnderstand 06-19-2013 08:47 AM

The purge line connects to the pvc line via t is on manifold vaccum. It sucks during idle then drops off some during acceleration. Ports For this are below idle plate. The signal line connects to the cto which is then connected ported vacuum. This vacuum is 0 at idle then increases as you accelerate. Ports for this are above the throttle plate.

When engine warm and you accelerate ported signal opens diagram valve on canister and allows manifold vaccum to suck out vapor via purge line.

Doesn't have to be a hard line. Just a proper good vacuum line.

See this website for lots of help. Its like my jeep bible.

Specifically on canister

On vacuum set up


D-BO 09-08-2013 02:18 PM

So I installed the new canister as described above and I have some questions:
I drilled out the hole for the Carb Vent. Does it matter if I switch the Carb vent with the fuel vent? It is easier and closer to hook them opposite as to what is described in the write up.

Also and most importantly, when I disconnect the line to the PCV while idling the Weber 38 runs wayyyyyy better. Is that wrong? Is that an indicator that the CTO valve is not working properly?

Matt1981CJ7 09-08-2013 04:26 PM


I assume you retro-fitted a mid-90's Grand cherokee canister, correct? If so, I think the carb vent should go to the larger of the vent nipples on the canister, as the write up states.

Disconnecting the purge line that T's into the PCV, without plugging the line, introduces a major vacuum leak. If your engine runs better with the leak, I suspect your air/fuel mixture is too rich, which is normal for the stock jetting on a Weber 38 at your elevation. If you haven't already, I strongly recommend re-jetting your carb.

I live at 6000 feet, and I found that 135 mains, F7 emulsion tubes, and 170 air correctors was the winning combination for my 38.

Hope this helps,


D-BO 09-08-2013 04:44 PM

Hey Matt-
Yeah I am up in Mead which is North of you but around 5000 ft. I usually go wheelin' in and out of 7000 or 8000 ft.

I bought the Weber 38 Jet kit but it didn't come with any of those numbers that you have. Which is freaking great considering it was 80 bucks.

However, you are running a 360 V8 so that has to change things considerably right? I don't know maybe not but you are right it is definitely running way rich. I followed the set up and tune thread and no matter what I do my engine won't run unless the fast idle screw is turned in greater than 1/2 turn. Meaning I can't even start the tuning process correctly. I have tried all the idle screw sizes in the kit and no dice. I guess I will order your combo of mains tubes and corrector, but what idle's are you using?

Also are you not using the Pulsed air injection system? I am asking that because I was thinking of removing that connection from the PVC to the Pulsed Air injection system.

Matt1981CJ7 09-08-2013 04:52 PM


I ran the Weber 38 on my 258 before I swapped in my 360. I loved that carb on the 258. Below is a short video I took after re-jetting and tuning.

The rule of thumb is one main jet smaller for every 2000 feet above sea level. I ran the standard idles jets, whatever they were. Had I kept that engine in my CJ, I probably would have experimented with smaller idles.

Contact Tom at Redline, and let him know I referred you. He will hook you up with the proper jetting kit.

Good luck,


nichols077 09-08-2013 05:10 PM

Reading this makes me reconsider my vap can installation. Howell, Hei, no EGR. The signal is Tee'd into the vacuum advance (doesn't suck at idle). Purge goes to pcv valve, carb port left undrilled.

Since I do not have any EGR components, the signal will suck when cold from what I have just learned. Will everything function properly the way I have it hooked up?

walkerhoundvm 09-08-2013 05:16 PM

nichols - you don't have a CTO is what I'm reading? Before I had to reinstall my original intake, CTO, and EGR, I had it hooked up exactly like you do. It passed emissions no problem. Next year around, they started looking closer at things and realized I had a Howell kit and not an original carb. It failed because they also recognized that I didn't have an EGR, etc.

Bottom line - yes, that system will function just as well as you have it. It'll compensate for cold draw on the canister. I'm guessing you don't have emissions where you are, so it won't matter from that standpoint, either.

nichols077 09-08-2013 05:20 PM

Thanks. I did a little more research after I posted and I realize I don't understand the pcv manifold vacuum. I connected the vap can purge to the rear hole in the valve cover since the front hole was already in use for the throttle body. Can someone give me information on why this is incorrect and how it should be revised? Thanks!

Matt1981CJ7 09-08-2013 05:38 PM

1 Attachment(s)

I think this is the basic vacuum layout you should have with the Howell kit.

Good luck,


nichols077 09-08-2013 05:44 PM

Yes I referred to that diagram but didn't think it would be an issue using the rear pcv port on the valve cover. I will tee it into the front port, was just curious on why the back port cant be used for this.

Matt1981CJ7 09-08-2013 05:59 PM

The "rear PCV port on the valve cover" is simply a air intake for the PCV system. There is no vacuum there.


The time now is 08:56 AM.

Powered by vBulletin Version 3.8.8
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.

User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.