Need help locating vaccum leak?

[Alvin]

I'm pretty sure there is a vaccum leak causing my truck to stumble and sometimes die when I stop. The truck only stumbles when it warms up, and I am stopping at a light or stop sign, and I leave the tranny in drive. If I bump it into neutral, the engine recovers. So I'm thinking it's somewhere between the engine and the brake booster...but this being my first truck (and my first real V8 vehicle), I'm clueless. I've researched all the other stuff, but common engine malfunctions just never came to mind. So any help on isolating the location of the leak would be very helpful. Also the engine burns a lot of oil on startup and at idle. It's not the worst smoking I've ever seen, but it really makes my truck seem like an old beat up farm truck, and that's not the image I want!! I'm just looking for some opinions before I send it off to the mechanic. Would love to be able to fix it myself though.

I know I've said and asked a lot here, but I need help. I drive the truck everyday, and for now it is my only vehicle. It could be out of comission for at most two days, and it seems to rain every other day. Thanks in advance for any help. My dad doesn't get back from AZ until Tuesday, then hopefully he can help me fix it. But like I said would love to be able to do it myself.

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- Adam L. Vogel
University of Cincinnati
White 68 GMC short, fleet, 350/TH350, 67-8 chevy grille, 5-lugs, front power disc brakes, power steering, a/c (not complete yet)

My truck site
IM= newt0

[Longhorn Man]

Did it start when you played with the carb? It actualy sounds like a tuning problem to me. Timing, idle RPM, fuel/air mix, weak spark....
You aren't still running that POS triangle air filter are you? (was just lookin at your site) If you are, you'r better off WITHOUT a filter than with that thing on there. HUGE fire hazard. I won't even give mine away...not even to a Ford owner. (if ony it fit on imports though....)

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'69 G.M.C. 350/350. Trying to clean up the left over damage from the Dope-Smokin-Old-Man
I've been dubbed the Longhorn Freak/Fanatic/Expert, I just hope I can live up to it.
FINALLY got the HORNIAC...a '70 one ton Longhorn with a Pontiac 350/350 and lots of 'personality'. Check out The Longhorn Webite.
If you need a pic posted, E-mail me at longhornmail@yahoo.com
My name is Andy...not Randy...I'm in Ohio...Not Illinois...close enough?
Columbus Ohio

[Scrub]

You might try a bit of carb cleaner. If you get the right carb cleaner, it will make you engine speed up when you spray it in the leak. Spray it around areas that are suspect and see if the motor speeds up.
How are your ful filters.
What kind of gas are you running? I can't make anything run on 87 octane and I have found 76 stations have the best gas. I run 89 octane.

Good luck

[Alvin]

Yeah, I'm still running that air filter. It has caught on fire a few times, but nothing too bad. Hasn't done it in at least three days now I am going to change it soon...probably order a cheapo summit normal air filter set up just to get me by on the cheap side.

I'm not sure on the tuning problem, because it just started dying one day. It didn't do it the day before, but last sunday it just started doing it.

But I do need to check over everything on the engine this weekend. I kept saying I was going to check it out real good when I got it home, but then things got busy and all I could do was drive and get gas. Finally changed the oil, fuel filter, and cleaned out the carb filter on Tuesday. It helped for about a day. Then by Wed. night it had burned up half a quart of oil. Wednesday was an above average day mileage wise though.

Thanks though, I'm taking notes and will make sure to check everything out Sunday afternoon.


I am running 87 octane. I was thinking of trying the higher grades, definately worth a try. Thanks for the tip on finding the leak (if it's there).

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- Adam L. Vogel
University of Cincinnati
White 68 GMC short, fleet, 350/TH350, 67-8 chevy grille, 5-lugs, front power disc brakes, power steering, a/c (not complete yet)

My truck site
IM= newt0



[This message has been edited by Alvin (edited May 11, 2002).]

[Andy4639]

1St. check timing. If it's a auto. it should be around 700-900 and then when you drop it into gear it should drop to around 600.
2nd. If it's straight drive it should be 600-700, doesnt need any higher because you have clutch pushed in at lights are in neutral.
3rd. Check hold down on dizzy to make sure it hasn't moved the timing.
4th. You can use w/d 40 and spray all over the engne to find any vacuum leaks, just leasen for motor to rev up.
Hope this helps.

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Andy
71's Are for ever!!! or 67,68,69,70,72
71,factory tach,tilt for straight drive,gauges,ac,speed warning,clock,305 HO TPI with 700-R4, lowered 2" & 3" ,and 17" Chrome Stealth wheels, Pioneer reciever,100 watt amp, 6 disc changer, 2-10" subs (behind seat) 2-6x9 in doors, 2-3ways mounted under seat, and 2 tweaters on door panels

www.geocities.com/chevy71tpi/main.html

www.picturetrail.com/Andy4639

[Fast68Chevy]

plug in a vacuum gauge into a spot on the bottom of carb or on a port on the intake somewhere and check to see that its between 18-22 in of mercury
if its not then you have manifold vacuum lake, IE hose leaking or carb not tight or bad carb base gasket, or you need ot adjust timing, keep eye on the gauge as you turn the distributor with engine running, set it to where its 18-22 and then see how it runs, if engien hesitates when restarting when hot then turn the dizzy back clockwise a little til it doesnt do that anymore, if the vacuum is not at 18-22 after setting dizzy so it dosent do that anymore then you got another problem. in the carb or in the heads

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Randy- Lincoln, IL
randy@ccaonline.com 2177353230
*many GM parts bought, traded, or sold -am parting out: '81 Cutlass Supreme Brougham

current fleet: -71 C-10 350/CH465/3.07- basketcase, #1
-90 Cavalier 2.2L sky blue, a driver
-84 Z28 5.0L/700R4/3.73 daily driver
-84 Firebird/SE 2.8L/T-5/3.73/engine bad, project #2
-82 purple Trans Am, project #3
*Paint is overrated*
© RSS Enterprises 2000

[shortbed70]

Ive heard of people using propane torch(unlit) around the motor and see if idle pics up. Thats where your leak could be if it a hose or gasket. Never tried it though. Good luck finding your problem. As for the oil I would suggest something in the heads like valve seals. Also you might want to check the oil return holes on the heads, if they are pluged oil can squeeze by the seals causing this problem.

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1970 shortbed(project)
350/350 dropped 4/6 21/2" flowmasters. Dropping bigblock idea and building a stout smallblock...




[This message has been edited by shortbed70 (edited May 11, 2002).]