Engine Idle issue - Possible Vac leak - canister purge valve?

R82148V

Active Member
May 26, 2020
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Hello,

I know this has been discussed all over this site with some really good detailed checklists to troubleshoot with too.

I have a possible vac leak. I've done base idle reset which works "ok" but I cant seem to get her under 950 - 1000 RPM. At full warmup it sits at 950 - 1000. Once you hit the throttle, it comes down to idle but then starts to rise up to 1300 to 1500 and a little hunting. I hit the throttle again, sometimes it takes 3 jabs but the idle will drop quickly back down to 950-1000 and stay there until you tap the gas and it starts all over again.

When I disconnect the IAC, idle does drop but I cant get it to 600-650 base rpms using the TB screw. Even when turned all the way out it hovers at 750. There is a small hole on the butterfly which is supposed to be there, but when I plug it up with my finger, idle goes down what it should.. This is telling me I have a vac leak somewhere, no? I have performed a process of elimination going around and capping off one thing at a time. Capped off the brake booster at the tree to eliminate the BB. This didn't make a difference. Hvac, same thing. Cruise Control, same thing. Fuel pressure reg is new and new line, New EGR valve since there was a little leak with the diaphragm. This helped a little. two new IAC's. tested TPS, all good. TAB and TAD are new with new lines.

KOEO codes - 11 1 11
KOER codes - 11 1 11
CYL balance test - 9

The last thing I needed to check was the canister purge valve. I disconnected the line from the upper intake and applied hand vacuum to it. It would not hold Vac!! Yessss!
I disconnected the valve from the canister and re applied vac. Still would not hold vac and this is the original on the car. Now if I lightly "tap" the valve while applying vac, it seems to be better at holding so I just picked up a new Motorcraft one since they are suppose to be the best .
I applied vac to the new one to test and it doesn't hold vac!! whaaaat! with a slight tap it then works. I checked the ohms and its right in line. I even plugged it in and did a KOEO to energize the valve thinking this would close it and nothing.

My question is - does the engine need to run to energize the valve and signal it to closet to hold vac at idle? I know these valves are suppose to stay closed at idle and WOT and only open at cruising speed.

Just thinking also, when performing base idle reset with the IAC unplugged, are you also supposed to disconnect the SPOUT? I ask because I did not do that and some internet searches say to. I dunno.....


Thank you for all your help and time.
 
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R82148V

Active Member
May 26, 2020
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Did you check your vac reading at idle?
I actually haven't done that yet, but will get back to you with the number. As it was running, I was disconnecting the brake booster connection to cap it off just for testing for leaks and the sound of the vacuum coming from that hose made me just as there was ALOT of vac.. I'll get an accurate reading though.
 
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R82148V

Active Member
May 26, 2020
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A vacuum leak should throw a lean code
Yeah that's the funny thing, no codes what so ever. No smoke out the tail, no gas smell, no overheating issues. Running a 2 core Champion rad w/ stock fan and 192 tstat. no pinging. bases timing is right on 10 btdc, idles nice and even but higher than it should and now has a that surge.
all new vac lines and I actually used stainless steel hose clamps on all ends. all new gaskets everywhere.

Going to tighten both 10 pins connectors and see if that makes a difference

im at a loss
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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Is the valve closed without power (normally closed) or open without power (normally open)? I figure it is normally closed (closed without power) so the canister is not venting to an engine that’s not running.
And is it getting good signal power?
Tapping on them both should not be needed.
I am going to assume you have a volt meter and know how to use it. See what voltage is at the terminals. And when.
 

R82148V

Active Member
May 26, 2020
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Is the valve closed without power (normally closed) or open without power (normally open)? I figure it is normally closed (closed without power) so the canister is not venting to an engine that’s not running.
And is it getting good signal power?
Tapping on them both should not be needed.
I am going to assume you have a volt meter and know how to use it. See what voltage is at the terminals. And when.
I would assume too that the valve would be closed when powered as to not allow fumes to enter the intake during idle or it would create a vacuum leak.

I applied 12 volts to both old and new valves and it doesn't matter. They both stay open and allow air to flow.

Just thinking out loud here. With engine running, when does the ECU "ground" the valve? and when grounded, does the valve open or close?
Perhaps the valve actually works in reverse, meaning when grounded, it closes the valve which would be most of the time. At cruising speeds the ground would be lifted and the valve would open..

I guess there is only one way to figure this out and eliminate a strange theory. :chin
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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I would assume too that the valve would be closed when powered as to not allow fumes to enter the intake during idle or it would create a vacuum leak.

I applied 12 volts to both old and new valves and it doesn't matter. They both stay open and allow air to flow.

Just thinking out loud here. With engine running, when does the ECU "ground" the valve? and when grounded, does the valve open or close?
Perhaps the valve actually works in reverse, meaning when grounded, it closes the valve which would be most of the time. At cruising speeds the ground would be lifted and the valve would open..

I guess there is only one way to figure this out and eliminate a strange theory. :chin
You have two bad valves. They should be closed either with or without power.
 

ReefBlueGT

Active Member
Oct 4, 2018
66
38
28
Hello,

I know this has been discussed all over this site with some really good detailed checklists to troubleshoot with too.

I have a possible vac leak. I've done base idle reset which works "ok" but I cant seem to get her under 950 - 1000 RPM. At full warmup it sits at 950 - 1000. Once you hit the throttle, it comes down to idle but then starts to rise up to 1300 to 1500 and a little hunting. I hit the throttle again, sometimes it takes 3 jabs but the idle will drop quickly back down to 950-1000 and stay there until you tap the gas and it starts all over again.

When I disconnect the IAC, idle does drop but I cant get it to 600-650 base rpms using the TB screw. Even when turned all the way out it hovers at 750. There is a small hole on the butterfly which is supposed to be there, but when I plug it up with my finger, idle goes down what it should.. This is telling me I have a vac leak somewhere, no? I have performed a process of elimination going around and capping off one thing at a time. Capped off the brake booster at the tree to eliminate the BB. This didn't make a difference. Hvac, same thing. Cruise Control, same thing. Fuel pressure reg is new and new line, New EGR valve since there was a little leak with the diaphragm. This helped a little. two new IAC's. tested TPS, all good. TAB and TAD are new with new lines.

KOEO codes - 11 1 11
KOER codes - 11 1 11
CYL balance test - 9

The last thing I needed to check was the canister purge valve. I disconnected the line from the upper intake and applied hand vacuum to it. It would not hold Vac!! Yessss!
I disconnected the valve from the canister and re applied vac. Still would not hold vac and this is the original on the car. Now if I lightly "tap" the valve while applying vac, it seems to be better at holding so I just picked up a new Motorcraft one since they are suppose to be the best .
I applied vac to the new one to test and it doesn't hold vac!! whaaaat! with a slight tap it then works. I checked the ohms and its right in line. I even plugged it in and did a KOEO to energize the valve thinking this would close it and nothing.

My question is - does the engine need to run to energize the valve and signal it to closet to hold vac at idle? I know these valves are suppose to stay closed at idle and WOT and only open at cruising speed.

Just thinking also, when performing base idle reset with the IAC unplugged, are you also supposed to disconnect the SPOUT? I ask because I did not do that and some internet searches say to. I dunno.....


Thank you for all your help and time.
+1 for removing the SPOUT and unplugging the IAC when resetting idle. You want to set the idle without the computer adding air or advancing engine timing. Speaking of which - double check your timing. I had mine set to around 12-14* BTDC, but recently pegged it back down to 10* BTDC. I didn't lose much power accelerating from a dead stop, and it helped smooth out my idle considerably, in addition to a new Motorcraft IAC and resetting the idle.

ALSO - I followed the procedure in this thread during my last reset, and it seemed to be a good method: https://www.stangnet.com/mustang-forums/threads/base-idle-reset.918550/post-9285313
 
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R82148V

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May 26, 2020
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You have two bad valves. They should be closed either with or without power.
ugh i was afraid you were going to say that. I took both old and new valves and tested with hand held vac. both would not hold. thought about it for a min and figured there has to be a plunger or something in the valve ports sticking so i squirted a little wd40 in it, jumped them right to the batt and wham! they both now hold vac until power and grounded which opens them. I then tested them over and over with every time holding vac until power and grounded. I know i have a new motorcraft valve but it had to been sitting there for a long time. don’t know if they use these ones anymore. anyway it seamed to work and the purge valve now holds vac. haven’t started the engine yet to check idle .
 
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R82148V

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May 26, 2020
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You haven't posted if this is a stock, unmolested, not swapped 5.0.
Sorry about that Gk. I read all your posts and learn an incredible amount from you. You are correct. This is a stock, unmolested, not swapped original 5.0 with only 72k original miles on it.I’m the second owner since 1995. picked her up a few years out of high school then life got in way. Just trying to bring her back to life the right way.
 

R82148V

Active Member
May 26, 2020
121
36
38
+1 for removing the SPOUT and unplugging the IAC when resetting idle. You want to set the idle without the computer adding air or advancing engine timing. Speaking of which - double check your timing. I had mine set to around 12-14* BTDC, but recently pegged it back down to 10* BTDC. I didn't lose much power accelerating from a dead stop, and it helped smooth out my idle considerably, in addition to a new Motorcraft IAC and resetting the idle.

ALSO - I followed the procedure in this thread during my last reset, and it seemed to be a good method: https://www.stangnet.com/mustang-forums/threads/base-idle-reset.918550/post-9285313
will absolutely try this brother
 

General karthief

wonder how much it would cost to ship you a pair
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Sorry about that Gk. I read all your posts and learn an incredible amount from you. You are correct. This is a stock, unmolested, not swapped original 5.0 with only 72k original miles on it.I’m the second owner since 1995. picked her up a few years out of high school then life got in way. Just trying to bring her back to life the right way.
you didn't learn nothin from me, I just regurgitate stuff others have posted to help members with problems and maybe a little googlefu thrown in just to make me sound like I know what I'm posting.
 

7991LXnSHO

wanna catch the space herp
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ugh i was afraid you were going to say that. I took both old and new valves and tested with hand held vac. both would not hold. thought about it for a min and figured there has to be a plunger or something in the valve ports sticking so i squirted a little wd40 in it, jumped them right to the batt and wham! they both now hold vac until power and grounded which opens them. I then tested them over and over with every time holding vac until power and grounded. I know i have a new motorcraft valve but it had to been sitting there for a long time. don’t know if they use these ones anymore. anyway it seamed to work and the purge valve now holds vac. haven’t started the engine yet to check idle .
Good job! That’s how I thought they were supposed to work. If you had not posted this, I was planning on heading out under the hood this weekend to make sure for you.
 
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R82148V

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May 26, 2020
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you didn't learn nothin from me, I just regurgitate stuff others have posted to help members with problems and maybe a little googlefu thrown in just to make me sound like I know what I'm posting.
hahah lol still. you make is easier then to find stuff. Here’s what i’ve done during covid….before and after.
 

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KRUISR

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+1 for removing the SPOUT and unplugging the IAC when resetting idle. You want to set the idle without the computer adding air or advancing engine timing. Speaking of which - double check your timing. I had mine set to around 12-14* BTDC, but recently pegged it back down to 10* BTDC. I didn't lose much power accelerating from a dead stop, and it helped smooth out my idle considerably, in addition to a new Motorcraft IAC and resetting the idle.

ALSO - I followed the procedure in this thread during my last reset, and it seemed to be a good method: https://www.stangnet.com/mustang-forums/threads/base-idle-reset.918550/post-9285313
I struggled with the getting idle consistent until I pulled the SPOUT connector as well as the IAC. I followed the thread linked above and found if simple and effective to follow. Good luck with the idle.
 
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