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Chasing vacuum leaks. Thermactor the culprit?

thegreatjustino

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Red Head Grease Monkey
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Jan 23, 2002
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Stockton, CA
I have been having vacuum leak issues for months. Back in August I replaced the intake manifold gasket and that took care of the biggest one. However, I've still got some smaller ones. Spraying the vacuum tree on the back of the manifold with carb cleaner increased the RPMs so I took the tree out and put in a plug. Helped a little and no more RPM gain when I spray the base of the plug.

However, now when I spray the base of the thermactor (canister part in the first pic) I also get a steady increase in RPM. Does this canister generate vacuum? I'm not familiar enough with the smog parts to properly diagnose. In picture #2 you can see the #4 intake runner on the manifold. When I spray carb cleaner beyond that runner toward the canister, I get the steady RPM increase. So, it's either that canister or I might have a cracked manifold. There doesn't appear to be any other fittings in this area that could cause a leak.

What do you all think?
 

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DirtDonk

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Yeah, it's often hard to be sure it's the component you're spraying, or right behind or near it when the stuff gets sucked into a hidden leak.
Never seen one exactly like yours, so wondering what that steel line is fitted to the back? Is it a section of steel tube connected to rubber hose at the other end, and going to vacuum?

Generally the thermactor is just blowing air into the exhaust, but any components that are controlled by vacuum can have a vacuum leak. So maybe you can temporarily wrap something around the doohicky and then spray again to isolate just to the manifold?
Looks like that would be awkward, to put it mildly. Being in an odd and tight spot as it is.
So just a thought. If you can do it, that might be worth a try.

Paul
 
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thegreatjustino

thegreatjustino

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Well I removed it and no change. When driving the Bronco, it's got plenty of power, accelerates fine, but at idle it has a bad miss and often dies coming to a stop while driving.

Hooking up my vacuum gauge while idling, I've got a jumping needle between 14 and 19 inches of vacuum. Increasing the RPM smooths out the reading to around 20 inches of vacuum. Something's going on...
 

ared77

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A machine that injects smoke into a vac system is probably the best way to find leaks. But these are expensive, I believe. Does anyone know of a cheaper one made for shade-tree mechanics like me?
 

B RON CO

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Hi, If you have an automatic tranny you removed the vacuum hose that goes to the vacuum modulator and that one should always be hooked up. I don't think the thermactor could have anything to do with it if the vacuum is unhooked, but I would remove the belt from the pump for now. If you can't get anywhere with a carb adjustment ( smooth idle and high vacuum ) you have to find the vacuum leak. I hate to say it but the Ford intake gaskets have a way of getting out of shape when you drop the intake onto the block. I always used four long threaded rods to hold the gasket in place and guide the manifold down, then start the bolts before removing the rods. Sometimes the gasket is out of shape at the lifter valley and oil will be sucked into the manifold port. Good luck
 

gnpenning

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I have more questions than answers.
Have you tried unplugging all your vacuum sources and capping at the manifold? Then hooking them up one at a time and checking for leaks at the source and hose all the way to whatever it's hooked to.

Have you checked the carb base for leaks?

Triple checking the intake is not a bad idea.
 

tirewater

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Jan 28, 2011
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San Francisco Bay Area
Perhaps resetting the idle mixture screws will help out the idle issue. If you didn't reset them after fixing the other vacuum leaks then they'll need to be redone.

Off topic & out of curiosity, are your Thermactor ports on the side of the cylinder heads or the rear?
 
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thegreatjustino

thegreatjustino

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Stockton, CA
Head ports are on the sides. Because the heads are interchangeable side to side, the passenger side is in the front near the smog pump and the driver side is in the rear near the MC.

I messed with the carb and timing this weekend and the Bronco pops wheelies when driving, it's the idle that sucks. Vacuum gauge indicates a leak somewhere and the RPMs increasing with carb fluid also indicates a leak. :mad:
 
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