Engine Idle's High when warmed up

[wkeithwilson@gmail.com]

This is my first post, and I'm a newbie to the whole Chevy truck restoration.  I am pretty ignorant on engine repair, but wanting to learn.  I know much more about it today than I did 6 months ago...problem is, I knew nothing 6 months ago.

I have a 1985 Silverado my grand dad bought new off the lot.  It has 93,000 miles, but only about 25,000 on a rebuilt motor (it sat for about 3 years toward the end of his life).  Recently, I've been trying to learn how to fix "things" on the truck -- little body work/rust remove, replacing lights, etc.  Nothing much other than general maintenance stuff on the engine.  Here's what I've noticed and can't find what the issue might be.

The truck starts up fairly easily, hot or cold weather.  A couple pumps of the pedal, bam, it's running.  The idle as it warms up sounds normal.  However, when the truck gets warmed up, and/or I have driven it some, and I put it in park, I notice a very high idle.  In the past, I could pop the gas pedal and it would reduce back down to normal idle (I'm assuming that takes the auto choke off???).  Anyway, when I bump the pedal now, nothing happens.  It just idles high....and getting about 7-8 miles a gallon!  V8 engine (305).

Obviously, I can carry it to a mechanic, and might end up doing that.  But, I would really like to try to figure this out myself --- at least determine the problem even if I can't fix it.  The carburetor, BTW, is about 6-7 years old.  This is something that has started recently with the idle --- NOTE-- before I started this project, the truck sat for a few weeks at a time, without being started. 

Any responses to this post is GREATLY appreciated.

[thefarmboy21]

Sounds like you either have a vacuum leak or you have a problem with the "high idle" in the carburetor. (Which is what normally shuts off when you blip the gas pedal) You could also have a bad brake booster....it would rev up when you push the brake.

[bd]

This is my first post....  I have a 1985 Silverado my grand dad bought new off the lot.... 

The truck starts up fairly easily, hot or cold weather.  A couple pumps of the pedal, bam, it's running.  The idle as it warms up sounds normal.  However, when the truck gets warmed up, and/or I have driven it some, and I put it in park, I notice a very high idle.  In the past, I could pop the gas pedal and it would reduce back down to normal idle (I'm assuming that takes the auto choke off???).  Anyway, when I bump the pedal now, nothing happens.  It just idles high....and getting about 7-8 miles a gallon!  V8 engine (305)....

Welcome to the site from California!   

It's nice that you have your Granddad's truck.  Keep plugging away and soon you'll be expert at routine repairs.   

If you don't have any vacuum leaks, thoroughly wash all the external carburetor linkage with some Berrymans B-12 aerosol or equivalent, paying particular attention to the choke linkage and weighted fast idle cam on the passenger side.  Dirt and gum build up in and around the linkage causing it to stick.  With the choke plate and throttle held open (engine off) the 'cam' should rotate freely.

[wkeithwilson@gmail.com]

Thank you from Georgia for your reply!!

I've bought the Haynes manual and watched every YouTube video out there that I know of, and that idle cam is what I thought it was.  However, I am having problems identifying the dang thing.  Also, I don't know how to check for vacumn leaks.  I asked someone I know how to check for that, and he told me to spray starting fluid around the vacumn areas and if the motor revs up, there's a leak.  I bought starter fluid but I'm sort of afraid to do that.  I also bought carburetor cleaner but haven't used it either, yet, because I wanted to wait until I posted here.

[wkeithwilson@gmail.com]

Farmboy -- I don't believe it's the brake booster, but one of the other two things you mentioned in your post certainly sounds feasible.  Do you have any "shade tree mechanic" suggestions on how to test for those issues?  I promise, I haven't joined this forum just to find a lazy way out of figuring out issues.  I've tried and this one is really bothering me.

[bd]

DO NOT USE starting fluid to spray for vacuum leaks!!!  Starting fluid is far too volatile and combustible.  Use carburetor spray.  Even so, exercise caution not to spray directly at the distributor or electrical components, since carb spray is also very combustible and a fairly strong solvent.  Although it's a routine method, if you're not comfortable with the procedure, keep a fire extinguisher handy and know how to use it.

Initially, look for any vacuum hoses that may be missing, unplugged, spongy, split, swollen, etc.  Vacuum hoses should have a noticeably snug fit at their connection points.  Then use the plastic spray nozzle packaged with the carb spray to spray all around the base of the carburetor, along the left and right edges of the intake manifold, etc.  If there is a noticeable increase in idle speed, there is a leak. 

NOTE:  You will nearly always have a slight increase when you spray directly at the ends of the throttle shafts, because a small amount of air normally leaks past the shafts.  In addition, any mist that makes its way in through the top of the carburetor may affect the idle speed.

The "Fast Idle Cam" is item 25 in Figure 6C10-19 on page 6C10-14 (pg 912) of the 1984 Service Manual.  Don't be concerned about the year difference between the manual and your truck - the choke is the same.  Allow several minutes for download and save the file to your local hard drive for faster access in the future.  The cam pivots on a Teflon coated shaft (part of item 24).  Rarely, the Teflon will peel and ball up between the shaft and cam, causing the cam to stick.  More often, dirt becomes lodged between the two pieces.  Spray the choke linkage thoroughly while the engine is off and sponge up any carb spray accumulated on the manifold, before starting.