Can't find vaccum leak - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Tom · 10-19-2003, 10:33 PM

Recently my truck starting idling really bad, throttle lag, running rough under 1500rpm etc. I finally traced it down to a vaccum leak but I can't find it anywhere. There aren't many rubber hoses on my truck [vaccum advance, brakes, PCV] and they all look good. I tried spraying the intake manifold where it meets the heads and the carb with no change. Any other ideas where it could be? O yeah and this is a off/on kinda of thing. One day its good, the next its not.

lukecp · 10-19-2003, 11:15 PM

I would lean out the idle settings on your carb as far as possible until the truck will barley run, then check for leaks. The engine RPM will jump right up if you check with a can of carb cleaner, much more than if it was idling properly.

Frank-id · 10-19-2003, 11:25 PM

Vacuum leaks can be very difficult to find.
The intake manifold and carb base is quickly checked by squirting some starting fluid and watching for engine rpm to increase. Several other places to check for leaks are, power brake booster, plug line temporary,, check AC and heater vac lines, and the trans vac line to modulator.
Some times when modulator goes bad, the trans fluid will be drawn into engine and trans oil level will go lower.

Frank

Tom · 10-19-2003, 11:32 PM

Good ideas guys. I don't have to worry about the a/c, heater, or modulator leaking because I don't have any of those

*1FaastC10* · 10-19-2003, 11:49 PM

is it possible you've got an internal vac leak on the intake?

Tom · 10-20-2003, 12:20 PM

What do you mean jeremy? Internal as in? Cause I've sprayed the intake down with starter fluid and nothing has worked yet.....

*1FaastC10* · 10-20-2003, 12:26 PM

i've heard of the intake gasket letting go below the intake. so basically what you would have is a leak into the lifter valley. i'm not sure exactly how to check for that. what kind of gaskets are you using?

Tom · 10-20-2003, 12:43 PM

Good idea. Vortec's are known to blow gaskets according to some people. Im using fel-pro's that have been on for about a year. Looks like the trucks coming apart tonight

Blue Beard · 10-20-2003, 09:25 PM

Try using soapy water for leaks, I've found lots of leaks with this that I couldn't find other ways.

79Silv4x4 · 10-20-2003, 11:38 PM

Just out of morbid curiosity--how have you 'traced' it to a vacuum leak if you haven't yet found a vacuum leak??

*1FaastC10* · 10-21-2003, 03:07 AM

Tom, ditch the Fel-Pro gaskets, and get a set of GM gaskets. i'd bet that's 90% of your problem.

Blue Beard · 10-21-2003, 08:45 AM

I've heard the Fel-Pro were getting worse that before.

Also a lot of people not using the rubber gaskets on the front and back of the intake, and just using sealer.

Tom · 10-21-2003, 10:37 AM

I've been using fel-pro's on vortecs for years now, they've always done me good. Besides, the GM gaskets are what everyone complains about blowing out. I use a 1/4" bead of RTV for the ends.

So I pulled the top end off last night, replaced the nice looking intake gaskets, put everything back together, and of course it still is doing it.

79silv- my friend down the street thinks its a vaccum leak, and so do I. Hes been building street rods for 30+ years. Theres nobody I would trust more when it comes to cars save maybe David Vizard

I've already replaced and check the ignition and fuel system, so a vaccum leak is all I can think of. It dosen't have any problem once Im moving with some throttle, its just the slightly open throttle/ idle that I have problems.

86SILVERADO · 10-21-2003, 02:08 PM

Try the vacuum advance can. I would assume by now that you have replaced all your vacuum lines?

79BIG10 · 10-21-2003, 05:11 PM

Maybe it's a carb/intake gasket issue. That would cause a vacuum leak if that was the problem. Also what about the brake booster itself? Not the line coming from the carb but a bad or going out booster will cause a leak as well.

*1FaastC10* · 10-21-2003, 10:22 PM

GM has redesigned their intake gaskets since then. the problematic gaskets you're referring to are the early 96-99 gaskets. give them a try, they arent that much more expensive than the Fel-Pro gaskets you're using.

lukecp · 10-21-2003, 10:50 PM

Also check around the tranny modulator for a vaccum leak...

10-19-2003, 10:33 PM	#1
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,748	Can't find vaccum leak Recently my truck starting idling really bad, throttle lag, running rough under 1500rpm etc. I finally traced it down to a vaccum leak but I can't find it anywhere. There aren't many rubber hoses on my truck [vaccum advance, brakes, PCV] and they all look good. I tried spraying the intake manifold where it meets the heads and the carb with no change. Any other ideas where it could be? O yeah and this is a off/on kinda of thing. One day its good, the next its not. __________________ -78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me.

10-19-2003, 11:15 PM	#2
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	I would lean out the idle settings on your carb as far as possible until the truck will barley run, then check for leaks. The engine RPM will jump right up if you check with a can of carb cleaner, much more than if it was idling properly. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

10-19-2003, 11:25 PM	#3
Frank-id Registered User Join Date: Sep 2003 Location: Twin Falls, Idaho Posts: 73	Finding a vacuum leak Vacuum leaks can be very difficult to find. The intake manifold and carb base is quickly checked by squirting some starting fluid and watching for engine rpm to increase. Several other places to check for leaks are, power brake booster, plug line temporary,, check AC and heater vac lines, and the trans vac line to modulator. Some times when modulator goes bad, the trans fluid will be drawn into engine and trans oil level will go lower. Frank __________________ Chevy stepper 400sb 350 turbo Last edited by Frank-id; 10-19-2003 at 11:55 PM.

10-19-2003, 11:32 PM	#4
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,748	Good ideas guys. I don't have to worry about the a/c, heater, or modulator leaking because I don't have any of those __________________ -78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me.

10-20-2003, 09:25 PM	#9
Blue Beard Registered User Join Date: Jan 2003 Location: Washington State Posts: 8,831	Try using soapy water for leaks, I've found lots of leaks with this that I couldn't find other ways. __________________ 1970 Chev CST 2003 Harley Fatboy 1975 Chevrolet Step Van 1956 Chev Bel Air 1977 Blazer 2WD For Sale $3000.00 1978 Blazer 2WD For Sale $7000.00 1978 Silverado 2005 Monte Carlo

10-19-2003, 11:49 PM	#5
1FaastC10 Account Suspended Join Date: Oct 2002 Location: Elkhart, IN Posts: 6,399	is it possible you've got an internal vac leak on the intake?

10-20-2003, 12:20 PM	#6
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,748	What do you mean jeremy? Internal as in? Cause I've sprayed the intake down with starter fluid and nothing has worked yet.....

10-20-2003, 12:26 PM	#7
1FaastC10 Account Suspended Join Date: Oct 2002 Location: Elkhart, IN Posts: 6,399	i've heard of the intake gasket letting go below the intake. so basically what you would have is a leak into the lifter valley. i'm not sure exactly how to check for that. what kind of gaskets are you using?

10-20-2003, 12:43 PM	#8
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,748	Good idea. Vortec's are known to blow gaskets according to some people. Im using fel-pro's that have been on for about a year. Looks like the trucks coming apart tonight

10-20-2003, 11:38 PM	#10
79Silv4x4 Workaholic Join Date: Oct 2002 Location: Katy, TX Posts: 1,589	Just out of morbid curiosity--how have you 'traced' it to a vacuum leak if you haven't yet found a vacuum leak??

10-21-2003, 03:07 AM	#11
1FaastC10 Account Suspended Join Date: Oct 2002 Location: Elkhart, IN Posts: 6,399	Tom, ditch the Fel-Pro gaskets, and get a set of GM gaskets. i'd bet that's 90% of your problem.

10-21-2003, 08:45 AM	#12
Blue Beard Registered User Join Date: Jan 2003 Location: Washington State Posts: 8,831	I've heard the Fel-Pro were getting worse that before. Also a lot of people not using the rubber gaskets on the front and back of the intake, and just using sealer. __________________ 1970 Chev CST 2003 Harley Fatboy 1975 Chevrolet Step Van 1956 Chev Bel Air 1977 Blazer 2WD For Sale $3000.00 1978 Blazer 2WD For Sale $7000.00 1978 Silverado 2005 Monte Carlo

10-21-2003, 10:37 AM	#13
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,748	I've been using fel-pro's on vortecs for years now, they've always done me good. Besides, the GM gaskets are what everyone complains about blowing out. I use a 1/4" bead of RTV for the ends. So I pulled the top end off last night, replaced the nice looking intake gaskets, put everything back together, and of course it still is doing it. 79silv- my friend down the street thinks its a vaccum leak, and so do I. Hes been building street rods for 30+ years. Theres nobody I would trust more when it comes to cars save maybe David Vizard I've already replaced and check the ignition and fuel system, so a vaccum leak is all I can think of. It dosen't have any problem once Im moving with some throttle, its just the slightly open throttle/ idle that I have problems. __________________ -78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me.

10-21-2003, 02:08 PM	#14
86SILVERADO Registered User Join Date: Dec 2002 Location: Hills of Western Mass Posts: 626	Try the vacuum advance can. I would assume by now that you have replaced all your vacuum lines? __________________ 86 Silverado C10 back to 305 power! 67 C30 Dually Dump, with 350 transplant, Rockhauler 05 Duramax 3500 "If at first you don't succeed, try a bigger hammer" Member of the 1-Ton club!

10-21-2003, 05:11 PM	#15
79BIG10 I'm back with 2nd truck! Join Date: Feb 2002 Location: Texas Posts: 3,774	Maybe it's a carb/intake gasket issue. That would cause a vacuum leak if that was the problem. Also what about the brake booster itself? Not the line coming from the carb but a bad or going out booster will cause a leak as well. __________________ 1979 Chevrolet Bonanza Big10 "Tootsie Roll" 1985 Chevrolet Silverado (wife's) Member of the Southern Bowties Club "Don't underestimate how sexy a fat man who drinks to excess can be." Homer Simpson



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10-21-2003, 10:22 PM	#16
1FaastC10 Account Suspended Join Date: Oct 2002 Location: Elkhart, IN Posts: 6,399	GM has redesigned their intake gaskets since then. the problematic gaskets you're referring to are the early 96-99 gaskets. give them a try, they arent that much more expensive than the Fel-Pro gaskets you're using.

10-21-2003, 10:50 PM	#17
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Also check around the tranny modulator for a vaccum leak... __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

Register or Log In To remove these advertisements.