rough idle in gear

[osbornl1]

79 blazer 305 with a 2 brl carb on it. when in park or netreul it idles just fine when in gear with foot on brake it idles rough but has great throttle response and accelerates smoothly. what could the problem be.

thanks for the help once again
Larry

 

[dyeager535]

Foot off brake with say, ebrake on, still rough idle?

 

[75-K5]

I don't know too much about it, but maybe the torque converter isn't unlocking all the way? Does it try to lurch forward when you let off the brake just a little?

 

[osbornl1]

cant try ebrake because cable is currently broke but if i am sitting still in gear (no foot on brake) it still idles rough no it doesnt lurch forward when i let off brake.

Larry

 

[dyeager535]

Whats the engine RPM?

High idle is typically used to hide a vacuum leak, and right off idle/in gear is sometimes the only time you notice it.

 

[osbornl1]

i have been told it could be a vaccume leak. is there a way to check for these leaks. is there a tool? should i check my pvc valve first??

thanks again

 

[TSGB]

Do tell! /forums/images/graemlins/ears.gif

 

[dyeager535]

Multiple ways to tell, but the cleanest I've heard of is to use an un-ignited propane torch, just pass it around the intake, fittings, etc. If idle changes, you've got a vacuum leak there.

However, if your idle is turned up high, that needs to be brought down in the first place. Bad idle will result from a vacuum leak, but if the idle is set higher than normal, you won't notice the vacuum leak.

 

[m j]

what is the vacuum reading at idle?
that is your starting point for diagnoses

 

[Rolled]

[ QUOTE ]
i have been told it could be a vaccume leak. is there a way to check for these leaks. is there a tool? should i check my pvc valve first??

thanks again

[/ QUOTE ]

I just spray carb cleaner around carb and other vacuum areas. If the idle changs, there is a leak. It also keeps the top of the motor looking clean /forums/images/graemlins/thumb.gif