Hard idle and rough shifting - The 1947 - Present Chevrolet & GMC Truck Message Board Network

85_Sierra_ Dually · 05-18-2019, 08:16 PM

Hey all, I have a 1976 C10. Chevrolet Performance 350, Stock th350 trans and everything is stock other than the engine. Problems all started today 5/18/19. Drove truck this morning and it was fine, parked it, ran errands and came back to wash it. I noticed it wasnt idling like normal so i let it cool and came back later. I then installed a heat insulator under the carburetor (been having heat soak issues). I went to drive the truck again, and boom. Truck was idling hard (not rough but seemed as it was just idling... hard is what ill say), going into gear it would almost break your neck it shifted so hard (both drive AND reverse), I went to test drive it to see if anything else was going on, and i noticed when first starting off, the truck would have a nice hesitation to it as well. I also noticed (only happened one time today and no times before) that it did not want to shift out of whatever gear it was in (2nd I believe). I have not checked timing or anything yet, a few plugs do have oil on the threads. ANY help would be GREATLY appreciated!

**truckster** · 05-18-2019, 08:30 PM

Look for the vacuum line that goes to the trans modulator. The modulator helps soften shifts, and a vacuum leak will cause rough idle.

85_Sierra_ Dually · 05-18-2019, 09:17 PM

Okay will check it tomorrow, as far as the hesitation, any ideas? The heat insulator raised the carburetor .32" and it doesnt seem to have done anything to the linkage, as far as moving the throttle (that i noticed). All of this started at the same time and i think its weird how the trans started breaking my neck and also started to idle (hard) and hesitate under initial acceleration.

RustyPile · 05-18-2019, 10:07 PM

Quote:

Originally Posted by 85_Sierra_ Dually

Okay will check it tomorrow, as far as the hesitation, any ideas? The heat insulator raised the carburetor .32" and it doesnt seem to have done anything to the linkage, as far as moving the throttle (that i noticed). All of this started at the same time and i think its weird how the trans started breaking my neck and also started to idle (hard) and hesitate under initial acceleration.

Not weird at all when you understand how it they all work together.. The modulator valve is a part that's attached to the transmission.. It basically controls the shifting.. It's powered by vacuum from the engine's intake manifold.. That vacuum is "piped" to the valve by a hose or some type of "hard line".. When this pipe develops a leak, the vacuum doesn't reach the modulator valve and the valve can't do it's job.. The leak also causes the engine to run lean (too much air for the amount of fuel being delivered by the carburetor), causing hesitation, rough idle, and possibly developing a miss..

**Steeveedee** · 05-18-2019, 10:13 PM

^ Yup.

Especially if the engine is able to draw transmission fluid into itself.

85_Sierra_ Dually · 05-18-2019, 10:17 PM

Okay thanks guys, for the help. Ill check all that tomorrow, the truck does need a transmission (gotta work for what I need haha), i just found it odd that it just happened all at once and i overlooked that possibility, thanks for your help, ill take a look at it tomorrow and let you guys know what i found

05-18-2019, 08:16 PM	#1
85_Sierra_ Dually Registered User Join Date: May 2015 Location: Raleigh Posts: 4	Hard idle and rough shifting Hey all, I have a 1976 C10. Chevrolet Performance 350, Stock th350 trans and everything is stock other than the engine. Problems all started today 5/18/19. Drove truck this morning and it was fine, parked it, ran errands and came back to wash it. I noticed it wasnt idling like normal so i let it cool and came back later. I then installed a heat insulator under the carburetor (been having heat soak issues). I went to drive the truck again, and boom. Truck was idling hard (not rough but seemed as it was just idling... hard is what ill say), going into gear it would almost break your neck it shifted so hard (both drive AND reverse), I went to test drive it to see if anything else was going on, and i noticed when first starting off, the truck would have a nice hesitation to it as well. I also noticed (only happened one time today and no times before) that it did not want to shift out of whatever gear it was in (2nd I believe). I have not checked timing or anything yet, a few plugs do have oil on the threads. ANY help would be GREATLY appreciated!

05-18-2019, 08:30 PM	#2
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,976	Re: Hard idle and rough shifting Look for the vacuum line that goes to the trans modulator. The modulator helps soften shifts, and a vacuum leak will cause rough idle. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson

05-18-2019, 09:17 PM	#3
85_Sierra_ Dually Registered User Join Date: May 2015 Location: Raleigh Posts: 4	Re: Hard idle and rough shifting Okay will check it tomorrow, as far as the hesitation, any ideas? The heat insulator raised the carburetor .32" and it doesnt seem to have done anything to the linkage, as far as moving the throttle (that i noticed). All of this started at the same time and i think its weird how the trans started breaking my neck and also started to idle (hard) and hesitate under initial acceleration.

05-18-2019, 10:13 PM	#5
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,676	Re: Hard idle and rough shifting ^ Yup. Especially if the engine is able to draw transmission fluid into itself. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

05-18-2019, 10:17 PM	#6
85_Sierra_ Dually Registered User Join Date: May 2015 Location: Raleigh Posts: 4	Re: Hard idle and rough shifting Okay thanks guys, for the help. Ill check all that tomorrow, the truck does need a transmission (gotta work for what I need haha), i just found it odd that it just happened all at once and i overlooked that possibility, thanks for your help, ill take a look at it tomorrow and let you guys know what i found



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