Truck Accelerating By Itself, Then Stalls Out

Yoox_II · Feb 21, 2016

I've had an idling problem for as long as I can remember. It will randomly not hold its idle, causing a stall unless I keep my foot slightly down on the gas pedal. It can happen one minute, then fix itself the minute after. Then, the idle sometimes goes so high that when I'm driving 60km/h for example, I take my foot off the gas pedal and it is still accelerating on its own. It's a scary issue. It sounds to me like something is sticking, which might explain the very high idle and random acceleration, but that does not explain the sudden low idle. Do you guys have any suggestions based on these symptoms?

Rustycowl69 · Feb 21, 2016

probably something electronics related. Some of the other members will likely have some advice on that stuff. Mechanically, it could be a vacuum leak. A real long shot, and I mean really rare, I once saw a situation with the throttle blades were literally falling out of a new Carter 500 4bbl. I don't know if they forgot the screws, or forgot to swedge them, but the throttle blades were sliding out of the slots in the primary throttle shaft, and the throttle/rpm was acting very similar to what you described.

Yoox_II · Feb 22, 2016

Thanks for the reply. If I'm not mistaken, it seems to happen more often when it's raining, snowing or humid outside. I've replaced distributor cap, rotor and even the entire distributor. Checked spark plug wires as well. I've also tried replacing that ceramic piece on the firewall, and also have a new voltage regulator. Also just redid a bunch of the wiring over the driver side fender in case, but there's no change.

sbarron · Feb 22, 2016

If we are talking 1984, this sounds like a carburetion issue to me.

Rustycowl69 · Feb 22, 2016

I'm sorry, I don't see that info on my mobile. I thought we were dealing with a pcm truck. It still could be a vacuum issue. And/or a carburetor issue like previous poster suggested. Possibly worn accelerator pump, missing accelerator check valve, failed charcoal canister. Could be alll of the above, could be some others, too, which would complicate diagnosis. Might help to know the engine, carb type, etc. Is there vacuum advance and/or mechanical advance on the distributor? Use a timing light and a mity-vac to con firm advance is working and returning to zero. Use timing light to confirm mech advance is working and returning to zero. Maybe advance return spring is busted.

Rustycowl69 · Feb 22, 2016

I'm sorry, I don't see that info on my mobile. I thought we were dealing with a pcm truck. It still could be a vacuum issue. And/or a carburetor issue like previous poster suggested. Possibly worn accelerator pump, missing accelerator check valve, failed charcoal canister. Could be alll of the above, could be some others, too, which would complicate diagnosis. Might help to know the engine, carb type, etc. Is there vacuum advance and/or on the distributor? Use a timing light and a mity-vac to con firm advance is working and returning to zero. Use timing light to confirm mech advance is working and returning to zero. Maybe advance return spring is busted.

Yoox_II · Feb 24, 2016

It's a 1984, 318 full carb'd motor, with a Holley 2280, a yes the distributor has a vacuum advance. I got it checked for vacuum leaks twice and the answer was, no vacuum leaks at all. I'm stil convinced it happens more in the wet. Today, I had to drive around the city for a couple hours getting stuff done. So when I got home it was completely warmed up and at running temp. It idles fine on its own, pretty high though, so then I took a spray bottle and started spraying different parts in the engine bay. The idle dipped considerably once I sprayed around the alternator. Does that make sense? My battery never dies, so it isn't an issue of my alternator not charging the battery, but the idle became sporadic once I sprayed around it. I would spray it, the idle would lower, causing considerable engine shaking due to almost stalling, but if I stopped spraying, the idle would slowly return back up to where it started (as if it was drying off or something). Any ideas? I will go outside tonight and look for a spark arching somewhere under the hood.

Joshwaa · Feb 24, 2016

Does your distributor have a vacuum advance? Also do your barrels in your carbs look clean if you open them up when it is not running. If you have build up around where they close that could also be an issue. Kinda sounds like you might have more than just one issue causing you problems. Also look at all the wires that go to your distributor and see if any are missing the insulation or if any connectors are loose. Also have you put a timing light on the truck. Also check the timing when you spray the water by the distributor.

crash_eco · Feb 24, 2016

Easy way to check for vacuum leaks. Use a small propane torch. DONT light it. Turn it on and slowely spray the propane gas over the fittings for any vacuum hoses. The leak will suck in the propane and your engine with speed up for a quick second. Don't keep feeding it propane though.

Yoox_II · Feb 24, 2016

It's raining hard tonight. I just went out and started up the truck, and of course it's running like crap. Sorry, not running at all, unless I place a brick on the gas pedal to make it idle. I placed the brick there and got to idle, pretty high around 1700rmp I'm guessing, and popped the hood and the engine SOUNDS wet! There's a horrid "spinning" or plasticy rattling sound, I think it's coming from one of the pullies. Everytime it gets wet and doesn't run good in the rain, it makes that sound. I also hear a ticking sound, but I'm not sure if it's the sound of a spark or not. But the thing is, I didn't see any spark arcing anywhere, so WHY does it run bad, religiously every single time it rains outside. I'm close to giving up. PS I've checked for vacuum leaks and got 2 different mechanics to check as well. I'm out of ideas

Rustycowl69 · Feb 24, 2016

so how much vacuum at, say 900rpm? Is the needle steady, or floaty. Is the vacuum advance plumbed into the correct port on the carb. The vacuum advance should see zero vacuum at idle. Then it should begin to see vacuum as you begin to open the throttle blades and expose the vacuum port to manifold vacuum. What is the initial spark advance at 850 rpm?

Truck Accelerating By Itself, Then Stalls Out

Yoox_II

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Rustycowl69

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Yoox_II

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sbarron

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Rustycowl69

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Rustycowl69

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Yoox_II

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Joshwaa

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crash_eco

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Yoox_II

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