1986 K10

I just bought a 1986 K10 Silverado with a 305 V8. The truck is a beautiful single owner, owned by a certified Chevy mechanic. It has 120,000 miles, and after my first 100 or so miles of driving it started acting strange. Everytime I shut the engine off the engine trys to keep running, and it sounds exactly like a diesel. I have no idea how this is happening. I'm only 18, and I don't know a lot about cars, but my first thought was maybe somehow my gas had some diesel in it. So I ran through that tank, and filled up somewhere else, but it keeps doing it. It'll sputter for like 5 or 6 seconds before dying. When the engine is running it sounds just like a regular gas engine, but when I shut it off it sputters and sounds like a diesel. How is this possible?

i had a 86 with a 305 in it and i had to run premium fuel in it, try that and if that don't work you might have to adjust the timing.

Does it idle high ?? If the engine is idling to high it when you shut it off it will run on. It can also be caused by carbon buildup in the cylinders if it has been babied. Also check the timing.

Just got back from the shop. Supposedly a shaft in the carb is shot. Which is why it's not idling right, and it is dieseling. I'm taking it in tomorrow to have it fixed for $150.

I'm sure it was babied by it's only owner (75 year old guy), so do you guys think it has carbon build up? If so is there something I can run in the oil or through the fuel system to help clean that out?

I agree with your mechanic.

I repair carbs all of the time with worn throttle shafts.
When they are worn, they allow excessive air to be drawn in past the shaft. This leans out the mixture, and the "owner" usually turns up the idle to keep the engine from stalling.

Mine was worn when I bought my 86 C10... I had to machine the carb and install a bushing for the shaft. The cause is a return spring design which uses the throttle shaft as a pivot point... pulling on top & pulling on bottom during use. It will re-occur unless the spring is relocated to pull in-line with the throttle cable. Except, when it re-occurs, both the shaft AND the bushing will be worn... and then it cannot be repaired. again without welding up & grinding down the shaft. This problem is as old as dirt.

FYI.. I do this repair for $20 - $30 when the owner brings me the carb.

My 86 has the semi-electronic factory Q-Jet... dual level float bowl, controlled by a temp sensor, but no other electronics (e.g.- mixture control solenoid) on the carb. This carb is a PITA to work on... the air horn tends to warp and causes all sorts of problems since the air horn gasket completes several fuel and vaccum passages. It's also extremely expensive to replace, so be nice to it.

I gave up on mine and replaced it with a Holley... but I had to modify the emissions equipment to work properly to pass emissions... used a 1980 Corvette EGR valve and machined it to clear the carb, re-routed the vac lines etc.

It's being repaired as I type.

Thank you very much for the answers (especially half-fast) :).

yeah i hate it when mine diesels like that. my timing must be too far advanced. also, make sure your truck isn't overheating because that will cause dieseling too...they call it dieseling not because of the way it sounds, but because of the way it runs when it diesels. it basically is caused by when you turn off the ignition, the spark plugs cease firing and normally the engine dies. however, if the engine is hot enough and not properly timed, then the heat alone from the block will be enough to ignite the fuel without the spark plug firing. with fuel injection, that doesnt happen because the fuel supply is computer controlled and it just stops injecting fuel to the cylinders when you turn the ingition off, making dieseling impossible. carbs on the other hand, due to the venturi effect, will keep spitting fuel in there as long as there is vaccuum to suck the fuel in. thats why only carbureted cars diesel. its kinda bad for the engine i heard so if it happens make sure to throw it in gear and thats enough to make the motor die

The gasoline will be ignited by a hot-spot, which is almost always caused by carbon buildup.

I'd pour a small amount of tranny fluid in the top of the carb. Pour it very slowly while the engine is running. After 1/4 of a quart, shut the engine off and let it sit for about 5 minutes. Tranny fluid disolves carbon very well, and should get most of it out. You could also use Sea Foam, but it seems to me that tranny fluid works better for this aplication.

When mine was dieseling due to a sticking choke, I'd leave it in gear, turn it off, then put it into park. It only dieseled once or twice over about a week (as opposed to every time shuting it off in park), and would only fire a few times when it did.

Although everything mentioned earlier is true, I recall that on some M4ME Quadrajets(electric choke), there was an anti-dieseling solenoid that de-energized when you shut the engine off, closing the throttle plates completely, and preventing just such an occurence. Normal idle is set using the solenoid plunger when it's extended. Just a thought. One way to check if you do have one is to look on the throttle cable side of carb. The solenoid plunger presses against the throttle. You can check it's operation by disconnecting the wire while the vehicle's running, which would cause a pronounced drop in idle speed.