Recently when I have been coming to a stop my idle bounces up and down between 750 and 1250 RPMs. When I let my foot off the brake it surges higher and bounces up and down between 1250 and 1750 RPMs. This only happens when my truck is at normal operating temperature, and when it is happening I can detect a burnt coolant smell. I have checked my coolant levels and they are fine. This only happens about half the time I drive the truck, but always happens on longer trips. Any ideas as to what may be causing this?

Give this a read and see if it fits.

Bizarre Idle

I've adjusted the idle back to factory specs and the problem persists. The only difference between the problem I'm having and the one described in the article is that when I let off the brakes the problem doesn't go away, but rather resumes at about 500 RPM higher. Thanks for the article though!

Coolant level proper and no air pockets in the cooling system?
If that is OK, have you pressure tested the cooling system to check for leaks. Could be the ECU coolant temp sender is getting air bubbles on it, leading to false readings and the surging idle.

That would be my article. What I've seen cause the idle problem when your foot is NOT on the brake is two things; 1) coolant is low or there is a bubble at the temp sensor, 2) the air conditioning "idle up" is not adjusted right (set too high if I remember correctly). You said you checked the coolant level, but you also mentioned a burnt coolant smell. Have you been having to refill the coolant at all, and have you been able to find a coolant leak?

I often see this behavior after doing a full cooling system drain and fill (and more often on new engine installs) while trying to burp the cooling system. Often, only 50-75% of new coolant will go into the system upon a fresh refill, and the remaining 25-50% of coolant only goes in after thorough burping. It is during this burping that the idle will fluctuate updownupdownupdownupdown even when NOT stepping on the brake.

I recommend taking a closer look at your coolant level, checking the radiator and the overflow container. Check the overflow container when cold and hot (not one or the other).

My truck doesn't have A/C so that shouldn't be a problem. Before getting a coolant system pressure tested I think I'll burp my coolant system to make sure there are no air bubbles causing the surging idle. I've never burped the cooling system before, but from my understanding I just want the front of the truck slighty elevated and then run the engine for 15 minutes with the radiator cap off and the heater on MAX right? Should I drain some of the coolant out of the radiator first to avoid a big mess? Thanks for your help guys!

I had similar issues that were completely solved after changing to the late model (i.e. 1990 and later) throttle body/upper intake assembly. Noticeable increase in power too.

I highly recommend this mod, although the earlier design can be made to operate correctly, and does work well for many people.

I have been losing about a quart or so of coolant every 3K miles. Additionally I noticed a rushing sound in the heater core when I was running my heater this winter. These signs lead me to believe that I might have a blown HG, which I have been dreading, since my current living situation prevents me from tackling its replacement myself. How much should I expect it to cost for a shop to replace the HG?

I had some debris clogging my throttle body coolant passages (90 pickup 22re) that when cleaned out solved the idle problem I was having.

A pressure test or chemical test on the coolant (to check for exhaust gas contamination) will tell you if you have a HG leak into the cooling system. That was the case when I had a HG leak. Observed coolant loss and a rushing/gurgling sound from the heater core when the engine was revved. Replacement cost will depend on your shop's labor rates. Check around and see what you can find.