Why does idle raise with hose off?

[19trax95]

I bought some seafoam and planned on running it through my truck. I looked up videos on how to do it so I went and let the truck warm up. Then I pulled the brake booster vacuum line off and the idle went up to 1200. It scared the heck out of me for a second because I thought it way keep going up haha. But then I just sat and listened to it idle for a minute and decided that there's nothing wrong so why even bother with the seafoam. But I poured a tiny amount down the line and there was no smoke which I figured since I poured so little in there. Then I put some in the gas tank.

But my question is, why does the idle raise when the vacuum line is disconnected. I noticed that if I put my finger over the line it can stop the engine (I found this out by accident, and I didn't let the engine stop I just let it slow) thanks for any answers.

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[Horns]

The idle is controlled by vacuum. When there is a leak the computer gets the wrong reading.

Try unplugging the MAP sensor, she'll go to WOT.

~Via Mobile \,,/

 

[19trax95]

I get weary when I hear an engine raise it's idle with out me expecting it. Reason being is I've have an old Detroit diesel run away on me before. Luckily my instructor was in the room when it happened and he threw rags into the turbo. That was scary ****

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[great white]

That's because diesels don't have throttle plates.

They are wide open all the time and are speed controlled by the amount of fuel allowed in to the combustion chamber. If something goes wrong int he injection pump and it goes full fuel, the engine will run away to max rpm until you choke the air supply. This is what happened when the rags were thrown into the turbo (probably destroyed the turbo also). Most shops that bread and butter on diesels have a couple pieces of plywood around (or something similar) to plop down on the intake mouth to strangle a runaway.

Otto cycle engines (IE: spark) have throttle plates that restrict air in to the engine. This was required to create a strong vacuum signal for carbureted designs as carburetors require a vacuum signal to work. They don't run away because when the throttle is closed, minimal air can get in. Your idle raised because you provided another way for air to get in to the engine around the throttle plates. But you can't provide enough air to a gasser around the throttle plates to make it run away. It will lean out and stall before that happens because of the way the fuel is controlled as opposed to a diesel.

The throttle plates are also what creates the vacuum on a gasser and the absence of them on a diesel is why they do not have any real usable vacuum for items like brakes. Diesels use systems like the hydraboost brakes and either engine driven or electric vacuum pumps to run vehicle systems that require vacuum.

 

[RHamill]

I bought some seafoam and planned on running it through my truck. I looked up videos on how to do it so I went and let the truck warm up. Then I pulled the brake booster vacuum line off and the idle went up to 1200. It scared the heck out of me for a second because I thought it way keep going up haha. But then I just sat and listened to it idle for a minute and decided that there's nothing wrong so why even bother with the seafoam. But I poured a tiny amount down the line and there was no smoke which I figured since I poured so little in there. Then I put some in the gas tank.

But my question is, why does the idle raise when the vacuum line is disconnected. I noticed that if I put my finger over the line it can stop the engine (I found this out by accident, and I didn't let the engine stop I just let it slow) thanks for any answers.

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Be careful with that sea foam, read the instructions carefully, Ive heard of people hydolocking their engines by pouring to much in (usually the wrong place), I usually get the engine up to normal operating temperature, pour it in until it dies then let it sit for 2 hours, then go out and crank until it fires (will take a bit of cranking), then let it get warm, then go out and drive it kind of hard (in a legal manner), until all the smoke is gone.

 

[19trax95]

Oh I know why that Detroit ran away like that. I was just freaking out and didn't have the piece of mind to stop the air going in. In fact I now carry a rag in my pocket now because of that. But I decided not to put the seafoam directly into the engine. I just put about half the can into the tank (which is full and 34gal so half a bottle is about right as per the instructions) I just was a little hesitant on putting it directly into the engine like that.

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[great white]

Oh I know why that Detroit ran away like that. I was just freaking out and didn't have the piece of mind to stop the air going in. In fact I now carry a rag in my pocket now because of that...

Rag in the turbo usually destroys it and potentially sends shrapnel down the manifolds. Better to have something flat you can hold over the intake to suffocate it (NOT YOUR HAND!). Stops it dead away and doesn't do any damage. Rag hitting the inducer at 150,000 rpm is a very bad thing....

 

[19trax95]

Yeah the turbo did get destroyed but at least no one got hurt. It's just easy to carry a rag than a cap which we have. But I can't easily carry. That was an old two stroke which we still have but don't run it anymore. Now we work on mostly new Cummins and macks. And those are in trucks so I not be able to throw the rag into the turbo on those. The only thing I could do is cover the air filter or disconnect the ecm to cut fuel. Unless it's running on oil.

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