C6 Vacuum Modulator Line
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Cross-Country
Joined: Jun 2016
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C6 Vacuum Modulator Line
Truck is an '86 F250 with 351w. I just replaced the vacuum modulator but it still revs up high and clunks into gear. I looked over everything and I followed the vacuum hose/line up to the firewall. I found that there's NOTHING connected at the end so obviously I found my problem.
My question is what is supposed to be hooked up to this? I found a hose with nothing attached to it but I'm not sure if it's the right one. I couldn't feel any vacuum coming from it, shouldn't I be able to with the engine running? A pic or diagram of the proper setup for this line would be very much appreciated. Thank you!
My question is what is supposed to be hooked up to this? I found a hose with nothing attached to it but I'm not sure if it's the right one. I couldn't feel any vacuum coming from it, shouldn't I be able to with the engine running? A pic or diagram of the proper setup for this line would be very much appreciated. Thank you!
FTE Legend
Joined: Nov 2015
Posts: 30,906
Likes: 4,122
From: Angier, NC
The 1 you tried was it the white one to the right of the metal line?
The mod line should go to intake manifold vacuum so any fitting on the intake with 100% vacuum will work.
Most of the time there is a manifold vacuum tree on the back right of the manifold but not sure on Fords.
Dave ----
The mod line should go to intake manifold vacuum so any fitting on the intake with 100% vacuum will work.
Most of the time there is a manifold vacuum tree on the back right of the manifold but not sure on Fords.
Dave ----
Thread Starter
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Cross-Country
Joined: Jun 2016
Posts: 57
Likes: 1
The 1 you tried was it the white one to the right of the metal line?
The mod line should go to intake manifold vacuum so any fitting on the intake with 100% vacuum will work.
Most of the time there is a manifold vacuum tree on the back right of the manifold but not sure on Fords.
Dave ----
The mod line should go to intake manifold vacuum so any fitting on the intake with 100% vacuum will work.
Most of the time there is a manifold vacuum tree on the back right of the manifold but not sure on Fords.
Dave ----
Thread Starter
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Cross-Country
Joined: Jun 2016
Posts: 57
Likes: 1
Thank you for the pic, that's pretty close to how I set mine up.
The hose you circled green does not reach and it had that plastic fitting on there already. I noticed the blue circled nipple as well, I think this may be a swapped motor or something, lots of stuff missing or capped off. I didn't check for vacuum on that one but maybe that would be a better source otherwise it's just leaking? I'm guessing where I have it now is causing my issue.
The hose you circled green does not reach and it had that plastic fitting on there already. I noticed the blue circled nipple as well, I think this may be a swapped motor or something, lots of stuff missing or capped off. I didn't check for vacuum on that one but maybe that would be a better source otherwise it's just leaking? I'm guessing where I have it now is causing my issue.
Lead Driver
Joined: Apr 2007
Posts: 5,958
Likes: 105
From: Houston
Mine gets vacuum from a vacuum tree/log mounted on the firewall. one hose goes to the vacuum tree on the intake manifold and one of the hoses goes down to the modulator tube not sure where the third vacuum hose from the firewall tee goes but that is how my '82 is set up.
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FTE Legend
Joined: Nov 2015
Posts: 30,906
Likes: 4,122
From: Angier, NC
Green & blue do check for vacuum leaks on them.
Green no clue what it is or goes to.
Blue may go down to a heat riser valve between exh manifold & head pipe?
Red you know what that is.
Where you have it works just use the right size hose as you could have a vacuum leak.
Do you have a vacuum advance dist. and if so remove the hose and plug and take it for a drive to see what happens.
Could also be a carb issue but check the dist hose first.
Rusty, does your truck have AC? From what I have found about the fire wall tree it was used on AC trucks.
My AC parts truck has one but not my non-AC truck.
Dave ----
Green no clue what it is or goes to.
Blue may go down to a heat riser valve between exh manifold & head pipe?
Red you know what that is.
Thank you for the pic, that's pretty close to how I set mine up.
The hose you circled green does not reach and it had that plastic fitting on there already. I noticed the blue circled nipple as well, I think this may be a swapped motor or something, lots of stuff missing or capped off. I didn't check for vacuum on that one but maybe that would be a better source otherwise it's just leaking? I'm guessing where I have it now is causing my issue.
The hose you circled green does not reach and it had that plastic fitting on there already. I noticed the blue circled nipple as well, I think this may be a swapped motor or something, lots of stuff missing or capped off. I didn't check for vacuum on that one but maybe that would be a better source otherwise it's just leaking? I'm guessing where I have it now is causing my issue.
Do you have a vacuum advance dist. and if so remove the hose and plug and take it for a drive to see what happens.
Could also be a carb issue but check the dist hose first.
Mine gets vacuum from a vacuum tree/log mounted on the firewall. one hose goes to the vacuum tree on the intake manifold and one of the hoses goes down to the modulator tube not sure where the third vacuum hose from the firewall tee goes but that is how my '82 is set up.
My AC parts truck has one but not my non-AC truck.
Dave ----
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Lead Driver
Joined: Apr 2007
Posts: 5,958
Likes: 105
From: Houston
My truck has dealer installed A/C but not factory thus there is no blend doors for my A/C so there is no reason for vacuum to be needed. Mine the evaporator is mounted behind the dash and has a simple on off switch with a capillary tube that goes to the evaporator to allow you to control the temperature by controlling the cycling of the clutch on the compressor.
The 82 I put the 5.0 coyote in at work it has one as well but that one is a factory A/C truck with the vacuum controlled blend door motors.
Mine does how ever have factory cruise control though which it gets vacuum from the vacuum tree on the intake manifold the same place the brake booster gets it from. Maybe that's where the firewall mounted tree is used is to add extra ports when options like factory A/C or cruise control is mounted.
The 82 I put the 5.0 coyote in at work it has one as well but that one is a factory A/C truck with the vacuum controlled blend door motors.
Mine does how ever have factory cruise control though which it gets vacuum from the vacuum tree on the intake manifold the same place the brake booster gets it from. Maybe that's where the firewall mounted tree is used is to add extra ports when options like factory A/C or cruise control is mounted.
Posting Guru
Joined: Jul 2015
Posts: 2,239
Likes: 5
From: Northeast Ohio USA
I think i would check the green circled line for vacuum and if it has vacuum, move the modulator to that with a longer line of course. Take the hose/clamp off and plug that port or at minimum, like Fuzz mentioned....reduce the line size if you choose to retain that as the source for the modulator. I realize that hose was probably temporary while you sorted this out.
Check the blue circled tube for vacuum and cap it off if it does in fact have vacuum. Auto parts stores sell rubber caps in packages (Dorman HELP) with multiple sizes
(like the ones covering the screws at your firewall). It's also a good idea to buy some vacuum line in lengths or a small roll and start replacing "all" of the vacuum lines, one at a time. Mine were all dry rotted due to the truck spending it's life in South Carolina. Ford engines rely heavily on solid vacuum.
Check the blue circled tube for vacuum and cap it off if it does in fact have vacuum. Auto parts stores sell rubber caps in packages (Dorman HELP) with multiple sizes
(like the ones covering the screws at your firewall). It's also a good idea to buy some vacuum line in lengths or a small roll and start replacing "all" of the vacuum lines, one at a time. Mine were all dry rotted due to the truck spending it's life in South Carolina. Ford engines rely heavily on solid vacuum.
Thread Starter
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Cross-Country
Joined: Jun 2016
Posts: 57
Likes: 1
I think i would check the green circled line for vacuum and if it has vacuum, move the modulator to that with a longer line of course. Take the hose/clamp off and plug that port or at minimum, like Fuzz mentioned....reduce the line size if you choose to retain that as the source for the modulator. I realize that hose was probably temporary while you sorted this out.
Check the blue circled tube for vacuum and cap it off if it does in fact have vacuum. Auto parts stores sell rubber caps in packages (Dorman HELP) with multiple sizes
(like the ones covering the screws at your firewall). It's also a good idea to buy some vacuum line in lengths or a small roll and start replacing "all" of the vacuum lines, one at a time. Mine were all dry rotted due to the truck spending it's life in South Carolina. Ford engines rely heavily on solid vacuum.
Check the blue circled tube for vacuum and cap it off if it does in fact have vacuum. Auto parts stores sell rubber caps in packages (Dorman HELP) with multiple sizes
(like the ones covering the screws at your firewall). It's also a good idea to buy some vacuum line in lengths or a small roll and start replacing "all" of the vacuum lines, one at a time. Mine were all dry rotted due to the truck spending it's life in South Carolina. Ford engines rely heavily on solid vacuum.
Thanks again for the replies, this forum is a big help!
Cargo Master
Joined: Aug 2007
Posts: 2,558
Likes: 69
From: New Zealand
Prior to a manual conversion, the vacuum line (the steel tube running down the chassis to the modulator) on my '83 was 3/16" OD, if that is what you are asking........so you would need 3/16" ID hose.
The modulator hose fitting is 1/4" but AFAIK the lines were all 3/16"
The modulator hose fitting is 1/4" but AFAIK the lines were all 3/16"
Thread Starter
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Cross-Country
Joined: Jun 2016
Posts: 57
Likes: 1
Prior to a manual conversion, the vacuum line (the steel tube running down the chassis to the modulator) on my '83 was 3/16" OD, if that is what you are asking........so you would need 3/16" ID hose.
The modulator hose fitting is 1/4" but AFAIK the lines were all 3/16"
The modulator hose fitting is 1/4" but AFAIK the lines were all 3/16"
Thread Starter
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Cross-Country
Joined: Jun 2016
Posts: 57
Likes: 1
Sorry it took so long to update but yes it seems to be shifting now. It's smooth enough that I'm not even sure it's actually shifting but I think that's the way it's supposed to be. All I know is it doesn't speed up to 40 and clunk anymore so I'm assuming it's doing what it should. Maybe I'll invest in a tach. You guys were probably right about the hose, it must have been too big and was leaking. That or maybe it was one of those vacuum ports I plugged. Thanks again for all the help. Now onto all the other stuff that needs to be fixed...