79 f250 supercab, 400 motor, c6 tranny. The past week when put into drive or reverse the truck wants to die. I have to power brake it at stops to keep it from during. The past 2 days I replaced the fuel filter which didn't seem dirty as I just replaced it 1 week prior, replaced the fuel pump because I saw it spritsing fuel so i figured bingo! That's it. Still doing it. I read proper timing for a 400 is 12-14, my uncle helped me get it to 14 and it actually sounds like it runs better when not trying to die, it was on 5 before that. We found I had a vacuum leak around my intake manifold because when spraying with carb clean around it looking for vacuum leaks it would jump the idle up and make it run better till it evaporated. So just now I was going to remove the carb and get a new gasket, but I figured since there was all sorts of other loose things on this truck when I got it I'll see if it's even tight.... sure enough I could tighten ever bolt down 3-4 turns... I sprayed with carb clean around it while running and it seems that solved that issue. I figure heck ya! That's got to be it! Nope! Still when shifting into drive or reverse it wants to die. But will idle up just fine when gas is given. Does anyone know what the heck is causing this! I'm lost as to what it could be. Thank you.

 

Do you have power brakes? Sounds like you may have a leaky brake booster. Do you hear any noises when stepping on the brake pedal?

Have you sprayed brake clean on all the vacuum lines? Sometimes old ones can crack and the cracks can be hard to see.

Does the truck smoke at all? Pull the vacuum line from the back of the intake that goes to the hard metal line running down the tranny. Is it wet inside? Maybe tranny modulator is junk?

Establish a baseline. What's your current idle RPM in PARK and DRIVE? What does your vacuum gauge look like? Is the needle steady or bouncing? What reading does it give?

 

Sounds like a classic vacuum leak. Spots which can leak which you may not hear would be inside the vacuum booster, the valve where the hose connects to the vacuum booster (sorry, brainfarting on the correct nomenclature) at the vacuum line to the tranny, at the other end of any hose connected to your vacuum tree.

 

Sorry I'm learning as I go as I don't have anyone to show me these things. Where is the vacuum tree located? I don't have a tack on the truck so I don't know rpm, I also don't have a vacuum gauge.... no noises when I press brake pedal though.

 

Look under the hood for something sticking out of your intake manifold with vacuum lines hooked to it - if you have one that is where I mean. It's just a splitter that allows multiple hoses to get manifold vacuum. Do a search in the forums for testing the vacuum booster, you do not need tools to test that. The other spots to test for leak you just spray with carb cleaner, just like you did at the base of the carb.

 

Ok great thank you! Just to be sure, i have a deal that is behind the card on the intake manifold that has a tree with multiple vac lines in it. Maybe I'll just replace all the vac lines that go to it? I just replace my oil cap with a new one, pcv valve and vent cap filter. The oil cap and pcv valve were all gunked up so I was hoping that would do it, but didnt.

 

Replacing all the vacuum lines is a great project but I'd suggest using some carb cleaner to find your immediate problem. Nothing guarantees it's a vacuum leak, and nothing is more frustrating than to replace a bunch of parts in the hope of solving a problem and then finding out you still have it. Diagnose first, then fix. Not the other way around.

 

Ok thank you for your help. I'll have to wait for my wife to get home because it idles fine in park so I'll need her to put it in reverse while I investigate

 

A vacuum leak will ordinarily cause a motor to idle pretty fast, to the point where people complain that they can't get the idle to stabilize at a low level. That sound like yours?

 

I can adjust the idle anywhere low or high when in park. It doesn't force it to idle higher than what I turn it to. When put in drive or reverse it tries to die unless I give it gas. Like idle is too low, it does increase it at all even when I adjust the idle higher it still tries to die

 

If you can get a satisfactory low idle that doesn't sound like a serious vacuum leak. I am not the best authority, but having found two maintenance issues (timing off 10 degrees and loose carb) I wonder whether the rest of the tune like plugs, wires, etc. may be off. It would not surprise me to find your general tune is deteriorated to where no one issue is killing you, but a stackup of plugs, cap, rotor, wires, etc could combine against you. A C6 has a lot of parasitic drag, and will load any engine down a bit. Try Dave's suggestions above, definitely.

 

I have new plugs in it and the wires seem good. Not damaged or corroded. The crazy thing is, before a week and a half ago when I got it started 2 months ago is has been running like a top.

 

If that's the case, I'm drawing a blank - sorry. Someone else will probably pop up with "oh, it's obviously XXX" and make both of us feel dumb.

 

What is this canister thing by the vacuum tree?

What is that canister. My vacuum tree is a little loose, like I can wiggle it back and forth easily by hand. That canister blocks the tree so I can turn the tree to tighten it. The canister has a nut below it but a regular wrench won't fit so I'll have to get one that has an angle to it?

 

The canister under the tree is the sender for the oil pressure

The tree is missing a hose-that'll be a guaranteed leak! The tree screws into the intake manifold and the threads should be sealed and tight.

The round disk above is EGR.

Also check the large diameter vacuum hose for the power brake booster. The hose connects with a check valve. I once blew one out in a backfire.