Vacuum issues

[LEB Ben]

Hey fellas...I have what I think is a vacuum issue in the 400 in my Bronco, not big...but annoying enough. It may even be cam related since I went bigger than stock. Anyway, on a fresh start up (hasn't run in a while) at high idle in park, no issues...but when I knock it down (and choke wide open, cuz it's 80+* all the time) and put it in gear, the thing idles really rough and doesn't really wanna run (might even have a small miss). I have to keep my foot on it a little to keep it running...and the brakes are spongy. However, after driving for a little a bit and I come to a stop, it idles perfectly and brakes are back to normal. But if I leave it sitting or idling, it progressively gets worse again until it is driven or starts pulling vacuum. Now I've double checked, triple checked and quadruple checked intake bolts, carb to intake and some of the vacuum lines.


So does anyone have any potential suggestions? I didn't think my cam was radical enough to cause an issue like this...is it? Is it possible a leak in the booster could cause all these issues? I was reading through my Ford manual and in the trouble shooting section, it said a bad PCV valve could cause these symptoms.

Thanks in advance.

 

[fatherdoug]

Did you check the vacuum tree at the back of the engine, Ben?

 

[LEB Ben]

Well it either has lines coming off of it, or I plugged the port, it's screwed in there well...beyond that, is there anything else to check on it? Does that tree need pipe thread?

 

[Mil1ion]

Get a can of carb cleaner and point the straw at vacuum connections looking for a leak.

On AT trucks the vacuum line to the Transmission modulator develops leaks the most.

Get rid of swollen hose or cut it back to fit tighter

 

[LEB Ben]

Thanks Dennis.

 

[fatherdoug]

Sounds like you covered that base. One of my ports on the bottom of the tree had an old rubber plug which was dry-rotted.

 

[73F100Shortbed]

A bad booster can cause vacuum issues. The pcv valve can because it is technically a calculated vacuum leak. There are some issues with aftermarket pcv valves causing issues like this but that was mostly on Dodge because they made one aftermarket pcv valve try to accomodate a bunch of different applications. I'm not saying it is your problem. Just thought I would throw that info out there that I learned a few years ago. I would do what Dennis suggested and see if you can find any obvious leaks. You can also plug off the booster line and see if it idles better. Sometimes the check valves on the booster go bad. But I would start by spray down everything first. I suggested this to my neighbor and he brushed it off. A month after replacing a bunch of parts he finally did this and found his vacuum leak

 

[LEB Ben]

Thanks for the input fellas. When spraying the lines with the carb cleaner...what am I looking for? Will it bubble? Dry up? Get sucked in?

 

[73F100Shortbed]

The idle will change if you found the leak.

 

[1985 Ford F-150]

Might also check the little elbow thing that goes into your booster if you pulled that off instead of the line it could be cracked or the rubber grommet around it could be garbage.

 

[LEB Ben]

Well now boys and girls...it appears I found a couple leaks. That carb cleaner trick was nifty. The intake tree end of the booster line appears to be an issue...around the base of the intake tree appears to be a problem...there seems to be about a 2" stretch of problem area where the intake meets the head...and finally there appears to be an issue at the carb end of the vac advance line.

 

[73F100Shortbed]

Glad you found the issue Ben. Did you need a pm to remind you later that you figured it out?

 

[Mil1ion]

The alternative to the carb cleaner is to use an unlit propane torch if you can afford it

 

[Jolly Green]

The alternative to the carb cleaner is to use an unlit propane torch if you can afford it

Last option is to use a lit propane torch and burn it down.

 

[LEB Ben]

Last option is to use a lit propane torch and burn it down.

I was contemplating that option about 3 weeks ago.