Silverado not running well

Asked by Sep 06, 2014 at 12:26 PM about the 1985 Chevrolet C10

Question type: Maintenance & Repair

Hi, ive got a problem with my 1985 silverado. it has a factory 305 and its not running to
well. i'm sixteen and feeling like im getting in over my head. there are several problems
with it. first, she runs really crappy under load when the engine is warm. second, when
you start her up whitish bluish smoke comes out the exhaust, not a lot but enough for
me to notice. third, she stays in high idle when i start her up for a few minutes, then it
will go down after she is warm and i rev it a couple of times. please comment if you
have any idea. i love this truck and dont want to have to sell her. thanks

1 Answer

730

High Idle at start up: Depending on your carburetor you may have either a temperature controlled choke, a vacuum controlled choke, or an electrical choke. regardless of which type, if the idle speed drops after the engine is running and you step on the gas peddle lightly a few times then all is working as it should be. The actual warm idle speed can be adjusted up or down with the stop screw located below the choke assembly. This actually should be done with a tachometer and set to 800 RPM on a warm 305. Note: a poorly timed distributor will affect your idle speed, also. Depending on the production period in '85 you have (I'm betting) a computer controlled ignition system. This is a very simplistic "computer" that adjusts the timing advance based on a vibration or "knock" sensor mounted to the engine block. After 30 years of use it's not surprising that this circuitry has become "flaky" A quick test is to block your wheels, apply full hard parking break, start the motor and put it in drive; attach a timing light to the #1 spark plug and find the timing mark on the front flywheel. If the mark seems to jump around randomly or move several degrees back and forth (wiggles a lot) - the Electronic Spark Control (ESC) or the sensor or both are bad. When the engine is under load and you give it more gas to accelerate the timing should delay or "retard" just slightly until the engine spins up. A bad ESC sometime ignores this condition and can cause heavy pinging, backfiring, engine "lugging", and knocking. Many mechanics of that period to today will tell you to replace the electronic control in an '82 thru '88 chevy with the "old fashioned" vacuum and mechanical advance system. If these are set up properly and you do not change your truck's load often, you will not have problems. I just performed this on an '85 305 suffering from your exact symptom. It runs great now at all speeds - even if it's loaded to max and gets much better gas mileage (went from 9 mpg to 16 cty/22 hwy mpg). Bluish smoke means oil is burning with the gas in the cylinder. This could come from worn rings, worn seals around the valve stems, a bad head gasket, or worst case, a cracked head. If the smoke appears white at any time it will be most likely one (or both) of the last two. Other signs of a cracked head MAY be water in your oil and/or exhaust gasses in your cooling system. Water contaminated oil will look like chocolate milk when it's up to temperature. There are chemical fixes to this, but I have never seen one last more than a few months. The real way to fix it is to pull the heads and replace what ever is bad, valve seals, gaskets or the heads themselves - and pray it is NOT a cracked block. Good luck, kid. Don't be afraid, you will learn much. This engine is not that complex. Just buy a good mechanic's manual for the year and make and follow it's procedures to the letter. Chilton's is a good example.

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