HELP.......Truck has major shudder. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Mccoleman830 · 07-27-2011, 05:02 PM

I fired up the truck today to discover that my 78 has a major shudder problem. Trying to diagnose the problem has left me clueless. It shudders extremely bad when in reverse, and all drive gears. When placed in park and neutral it still has a slight vibration. I tried to drive it down the street, but while a stop light you have to keep your foot on the gas pedal or it has a violent shake. Any advise where to look at would be great. 1978 GMC K-15 350/350. The only thing ive done lately was get a fill the tank with fuel. I thought it might be bad fuel, but switching tanks from right to left doesn't help at all.

Thanks.

Stepper562 · 07-27-2011, 05:40 PM

try the obvious and most simple thing first like checking how high/low your idle is in park, bad fuel filter if you have one, bad carb, tune up etc...little more ibfo is required in order to diagnose the problem.

ratrod67 · 07-27-2011, 06:31 PM

Sounds like U joint or carrier bearing. Maybe check your transmission mount also.

Eyegore · 07-27-2011, 06:35 PM

Quote:

Originally Posted by ratrod67

Sounds like U joint or carrier bearing. Maybe check your transmission mount also.

Add motor mounts also.

rwhit57 · 07-27-2011, 07:15 PM

Could be something as simple as a vacuum line loose. Check things out vacuum lines, plug wires, etc.

HTX86chevy · 07-27-2011, 07:21 PM

Quote:

Originally Posted by rwhit57

Could be something as simple as a vacuum line loose. Check things out vacuum lines, plug wires, etc.

x2............... i recently messed with the vacuum lines on mine and it started having the same problem youre having. Check Vacuum lines, i bet you'll solve your prob

68GMCCustom · 07-27-2011, 08:59 PM

are you sure its firing on all 8?

71swb4x4 · 07-27-2011, 09:07 PM

Since you have it at idle you can eliminate the transmission and back.
My "guess" would be that you are missing on a cylinder or more. I would check the distributor/wires/plugs first. Start with the simple things.

Mccoleman830 · 07-27-2011, 10:59 PM

Weird, I just happened to tightened the carb bolts to the intake manifold, and start the truck. After doing that the truck runs perfect. I drove around the block and by the time i got back home it started shuddering again. I'm thinking that maybe I need to re-check all bolts and nuts on the carb and intake. It has been 5-6 years since I've had the intake manifold off. Thanks everyone for your ideas and input.

71swb4x4 · 07-27-2011, 11:23 PM

Might be a vacuum leak if tightening the bolts helped.

Stepper562 · 07-28-2011, 06:49 AM

Best way to check for leaks around the manifold and carb is to spray around it while running. Listen for a change in idle it'll be an obvious change. Also +1 on the vaccum lines. Check for dry, cracked and disconnected lines. Replace if needed.
Posted via Mobile Device

71swb4x4 · 07-28-2011, 08:56 AM

Stepper is saying to spray something flammable, usually people spray carb cleaner.

motornut · 07-28-2011, 09:35 AM

check your fan
my 4 blade fan lost one took out my lines master cylinder to the valve on the front crossmember
vibrated after a lot but maybe it would too just before letting go
surprised me it could do that

Stepper562 · 07-28-2011, 10:01 AM

Quote:

Originally Posted by 71swb4x4

Stepper is saying to spray something flammable, usually people spray carb cleaner.

Yep sorry about that...carb cleaner is the best and cheapest way to go. Thx for clearing that up.
Posted via Mobile Device

amodedude1 · 07-28-2011, 11:18 AM

an UNLIT (lol) propane torch works too, when the propane gas finds a leak your idle speed will audibly change and will narrow down your search, somewhat safer than carb cleaner. Be very careful with carb cleaner, don't do it right after you drove since as they said, it's flammable.

rwhit57 · 07-28-2011, 02:28 PM

Sounds like it's time for a new carb gasket, some vacuum lines, and general tune-up. Snug the intake bolts down don't tighten them hard. Still sounds like vacuum leak to me.

irishman999 · 07-29-2011, 05:56 AM

Take a look at the harmonic balancer.

07-27-2011, 05:02 PM	#1
Mccoleman830 Registered User Join Date: Aug 2009 Location: Sacramento, CA. Posts: 57	HELP.......Truck has major shudder. I fired up the truck today to discover that my 78 has a major shudder problem. Trying to diagnose the problem has left me clueless. It shudders extremely bad when in reverse, and all drive gears. When placed in park and neutral it still has a slight vibration. I tried to drive it down the street, but while a stop light you have to keep your foot on the gas pedal or it has a violent shake. Any advise where to look at would be great. 1978 GMC K-15 350/350. The only thing ive done lately was get a fill the tank with fuel. I thought it might be bad fuel, but switching tanks from right to left doesn't help at all. Thanks. __________________ 1978 K-15, 350/350, 4.11's, 17" KMC's, 37's.

07-27-2011, 05:40 PM	#2
Stepper562 Registered User Join Date: Jun 2011 Location: Downey, ca Posts: 262	Re: HELP.......Truck has major shudder. try the obvious and most simple thing first like checking how high/low your idle is in park, bad fuel filter if you have one, bad carb, tune up etc...little more ibfo is required in order to diagnose the problem.

07-27-2011, 06:31 PM	#3
ratrod67 Registered User Join Date: Sep 2002 Location: DFW Texas Posts: 2,269	Re: HELP.......Truck has major shudder. Sounds like U joint or carrier bearing. Maybe check your transmission mount also.

07-27-2011, 07:15 PM	#5
rwhit57 Registered User Join Date: Jan 2011 Location: Tulsa,Okla Posts: 590	Re: HELP.......Truck has major shudder. Could be something as simple as a vacuum line loose. Check things out vacuum lines, plug wires, etc.

07-27-2011, 08:59 PM	#7
68GMCCustom Truck and auto performance nut Join Date: Feb 2007 Location: McKinney,Texas Posts: 3,848	Re: HELP.......Truck has major shudder. are you sure its firing on all 8? __________________ Kurt - '68 GMC short step - NIB '09 LY6 6.0L crate motor w/mods, NIB '12 crate 4L85e w/billet 3k stall Circle D, 3.73 posi 12 bolt, DynaTech f-swap headers, 3/4 drop, handling mods, etc. - my toy '72 Chevy LWB C-10 Highlander - 350/350 ps/pb/tilt/ac - not original but close '06 Chevy TrailBlazerSS - LS2/4L70e - little black hot rod SUV - my DD '18 Kia Sorento - wife's econo-driver '95 Chevy S10 - reg cab shortbed, LS, 4.3, auto... my '68's powertrain and chassis build -links broken A surprise phase - carb to efi -links broken

07-27-2011, 09:07 PM	#8
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: HELP.......Truck has major shudder. Since you have it at idle you can eliminate the transmission and back. My "guess" would be that you are missing on a cylinder or more. I would check the distributor/wires/plugs first. Start with the simple things. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

07-27-2011, 10:59 PM	#9
Mccoleman830 Registered User Join Date: Aug 2009 Location: Sacramento, CA. Posts: 57	Re: HELP.......Truck has major shudder. Weird, I just happened to tightened the carb bolts to the intake manifold, and start the truck. After doing that the truck runs perfect. I drove around the block and by the time i got back home it started shuddering again. I'm thinking that maybe I need to re-check all bolts and nuts on the carb and intake. It has been 5-6 years since I've had the intake manifold off. Thanks everyone for your ideas and input. __________________ 1978 K-15, 350/350, 4.11's, 17" KMC's, 37's.

07-27-2011, 11:23 PM	#10
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: HELP.......Truck has major shudder. Might be a vacuum leak if tightening the bolts helped. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

07-28-2011, 06:49 AM	#11
Stepper562 Registered User Join Date: Jun 2011 Location: Downey, ca Posts: 262	Re: HELP.......Truck has major shudder. Best way to check for leaks around the manifold and carb is to spray around it while running. Listen for a change in idle it'll be an obvious change. Also +1 on the vaccum lines. Check for dry, cracked and disconnected lines. Replace if needed. Posted via Mobile Device

07-28-2011, 08:56 AM	#12
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: HELP.......Truck has major shudder. Stepper is saying to spray something flammable, usually people spray carb cleaner. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.

07-28-2011, 09:35 AM	#13
motornut 78K & 79C Jimmys Join Date: Nov 2009 Location: Ottawa Ont CANADA Posts: 7,901	Re: HELP.......Truck has major shudder. check your fan my 4 blade fan lost one took out my lines master cylinder to the valve on the front crossmember vibrated after a lot but maybe it would too just before letting go surprised me it could do that __________________ John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350

07-28-2011, 11:18 AM	#15
amodedude1 Registered User Join Date: May 2010 Location: Phila, PA Posts: 51	Re: HELP.......Truck has major shudder. an UNLIT (lol) propane torch works too, when the propane gas finds a leak your idle speed will audibly change and will narrow down your search, somewhat safer than carb cleaner. Be very careful with carb cleaner, don't do it right after you drove since as they said, it's flammable.

07-28-2011, 02:28 PM	#16
rwhit57 Registered User Join Date: Jan 2011 Location: Tulsa,Okla Posts: 590	Re: HELP.......Truck has major shudder. Sounds like it's time for a new carb gasket, some vacuum lines, and general tune-up. Snug the intake bolts down don't tighten them hard. Still sounds like vacuum leak to me.

07-29-2011, 05:56 AM	#17
irishman999 Registered User Join Date: May 2010 Location: Reno Ne-VAH-da Posts: 594	Re: HELP.......Truck has major shudder. Take a look at the harmonic balancer.



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