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Question about rpm's?

I have a question. I have an automatic Hyundai that has recently started to give me some problems. Sometimes when I slow down and am about to stop, all of a sudden the rpm’s will jump up and the car will buck forward. This has happened just a few times over the past couple of months, then recently after “bucking” a few times it actually stalled out. This only happens when I’m going really slowly. My dad says this couldn’t possibly be related but I think it may also only happen when I have very little gas. Does that make sense? What’s going on?

It makes sense with very little gas. The pump is probably sucking some air and as the mixture leans out, the RPMs will rise.

But normally, I think you may have a small vacuum leak someplace.

I’d be inclined to look at the idle air controller and throttle position sensor.

What year is the car? What model? How many miles?

Gas tanks normally have a baffle in them to prevent fuel starvation from gas sloshing. However, if you let the level get too low, you could starve the pump. This is not good for a number of reasons. I would keep the tank at least half full for a while to see if the problem goes away.

If not, I’d ask if this problem seems to occur mostly when coasting down from highway speeds. Aside from the good suggestions already given, the torque converter lockup may be sticking on and reluctantly releasing just as you come to a complete stop.

I’m not inclined to blame the fuel system. As mentioned above, the IAC (idle air control) motor could be at fault, and so could other gadgets such as the MAP or MAF sensor. We also can’t rule out a vacuum leak either. Unfortunately I can’t think of any good way for an owner to test each possibility. When this problem advances to the troublesome stage, discuss it with a professional shop (not necessarily a Hyundai dealership).

Can you cause the engine to run rough at idle by: pressing and releasing the gas pedal? Again. Again. What happens? If the rpm isn’t always the same at idle (nice and smooth), the engine might have what we call, “Dirty intake”. Use an MAF cleaner, and a Carb/Throttle Body cleaner in the air intake, the idle air control (aic) valve passage, the throttle plate and bore. Let the cleaner soak a few minuets. Start the engine and let it run at a fast idle for a few minuets.