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Sudden very rough idle, engine shaking

G

GoGoGirl

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Some of you know my recent adventures here:

https://ck5.com/forums/threads/help-truck-won’t-start-gas-350.336174/

Yesterday I got the Blazer started and plowed my driveway.

Today all was going well until about ten minutes into my plowing. The idle started to sound weak like the engine was going to die. But it hasn’t died. This has never happened before. Everything sounds normal under throttle, but the idle is rough. It sounds slowed-down if that makes any sense. Even when pausing to re-angle my plow. The engine and whole vehicle is shaking now along with that bad idle sound.

I made a video, I’ll try to attach it here. You can’t tell how much everything is shaking but you can somewhat hear the noise.

I see no fluid leaks or unusual things under the hood. Sounds like it’s coming from the driver side exhaust manifold area. It seems (may be in my mind) the HVAC fan slows down a lot at idle as well. When I give it gas, when parked, the fan speeds back up.

Instant update! Uh ok this is weird! As I’m sitting in the Blazer and typing this, I was seeing how the fan responded to throttle. I gave the gas pedal a quick push and heard a pop like a tiny backfire from the engine compartment, and now it’s idling fine. I had been quickly pressing the gas pedal quite a few times but this didn’t happen before. Sounds like normal now. No shaking or odd sounds. No fire or other changes under the hood.

Any ideas?
 
Does it have a stock Quadrajet carb ?..
I've had a few that idled fine after a cold start,but would start slowing down and loading up ,threaten to stall,until the engine warmed up fully..then it ran perfect the rest of the time until it cooled off overnight again..I think the float needle valve would stick until the carb reached normal operating temperature..

Could have been the choke too,not opening enough after a cold start--there is a vacuum operated "can"called a choke pull off, that opens the choke slightly after a cold start ,so it wont flood the engine with raw fuel..if the pull off fails to work or doesn't open the choke butterfly the proper distance it'll run like crap till it gets warm enough for the choke thermostat to open it..

If its cold and damp up in VT today,you may have had carb icing,that is when the humidity in the air rushing into the carb freezes up on the throttle butterflies,which chokes the engine of air,and it'll idle ragged ,maybe even stall,until the engine heats up enough to prevent the ice from forming..the "pop" was likely a lean backfire ,that blew the ice out and allowed the carb to flow fuel & air properly again..often you can see the outside of the carb frosting up when this happens..
 
Low idle rpms? The blower motor symptom sounds like low voltage, alternator don't perform well at low rpms.
Why it changed while plowing could be number of reasons.
My first check would be timing, make sure your distributor is tight. With motor off reach back there grape the cap and twist. Mark the location of small wire block on driver's side for your own reference.
 
Thank you both.

It has a Q-Jet but don't know if it was stock to the engine. I have a Franken-truck, originally a 6.2 diesel, so engine/carb/driveline history is unknown.

I will have to take a closer look at the carb if that happens again.

Timing is something I've never had done or checked on this since I got it. My mechanic is an old guy but doesn't have a timing light/gun thing. I do know someone who can help with that, maybe it's time to bake him some cupcakes and drop by for a visit!
 
If your air cleaner lacks the hot air stove setup or the heat riser isn't working in the right side exhaust manifold (or was removed),then the carb will tend to ice up in weather like today,that has high humidity and the temps are below 50 degrees out..

Often these parts get deleted or left disconnected and this is what can result..while the engine will run fine after warming up without either of these items,it can negatively affect cold weather operation during the warm up period..you'll have to sit and wait for it to warm up,and may have to give it some throttle to prevent stalling until it warms up..

Some heat risers on later engines were vacuum operated instead of using a spring --when the valve closes,it forces the exhaust to go under a passage in the intake manifold under the carb,which heats it up and prevents carb icing and poor running at low speeds until the engine fully warms up..

chevy thermac air cleaner.png chevy heat riser valve.jpg
 
How often is the truck driven outside of plow season? If it has set a lot, and moisture got in, the fly weights may have gotten rusty/corroded. If they stuck in a slightly advanced spot would explain the running better with rpms and poorly at idle. If so, the pop or backfire may have broke it loose to run normal.

A low voltage situation could also make sense. Especially considering the previous electrical issues.
 
yes after more thought recheck all the recent connections maybe low voltage condition weakening spark
 
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