My Ford Focus is a 2000, and a lot of times it shudders violently when I'm idle or under 1000 RPM. Only once have I had it actually kill the engine. For now I've been keeping my foot on the gas and brake at the same time when I must idle in order to keep it at or above 1000 RPM. I recently replaced my battery and alternator and its been acting up ever since. What else could be wrong?

Hi, You need to clean out the Throttle body with cleaner. Also the idle air control.
Check also for vacuum leaks. Good luck

This is usually caused by having a vacuum hose come off or
connected to the wrong place. The diagram is under the hood.
Also the cold air bypass could be dirty and need cleaning. I use
carb cleaner in a spray can.
Other check air filter, plugs, wires.

Check out related help links to troubleshoot these problem:----Click the link below:---Idles Fine but RPM jumps in drive mode? http://technoanswers.blogspot.in/2012/07/idles-fine-but-rpm-jumps-in-drive-mode.html
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Cold start problem and vehicle shudder around 2500 to 3500 rpms
http://repairhelpcenter.blogspot.com/2012/10/cold-start-problem-and-vehicle-shudder.html

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http://whatisbyme.blogspot.in/2012/05/1990-toyota-4runner-engine-cuts-off.html

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1999 Toyota Camry: Erratic stalling and high engine RPM at idle?

http://whatisbyme.blogspot.in/2012/05/1999-toyota-camry-erratic-stalling-and.html

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1995 Toyota LS: Misfire at idle or low RPM http://whatisbyme.blogspot.in/2012/04/1995-toyota-ls-misfire-at-idle-or-low.html

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Lexus RX: Tapping noise under Valve Cover At Low RPM? http://technoanswers.blogspot.in/2012/04/lexus-rx-tapping-noise-under-valve.html

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These will help.
Thanks.

I realize that fixing yourself & saving money is the way to go

The problem is,that there are lots of hoses,we don't know as you
don't, what the real issues are or if hoses are even a problem

So if you have no back yard mechanic friends, you have to go
to a repair shop.

Very possibly a vacuum issue. Maybe a line that needs vacuum for the trans is on an inlet that effects the throttle. Not sure how old your vehicle is.

Hold the idle at 2000 RPM for 2 minutes then drop it to 1500 for 2 minutes and then 1000 for 2 minutes, then let it return to idle for 5 minutes. Full relearn requires some driving as well, mixed low and highway speed and idle time.

check for emmision problems like the catalic converter clogged, is a good idea to have the fuel injectors cleaned and the fuel pump checked for proper preassure.

It could be a spark plug, plug wire, or a coil. Sometimes thay break down intermittently. If it needs a tuneup, that would be the first thing to do.

Only 1000-2000 RPM? Does it do that in any gear?

If you need further help, I’m available over the phone at https://www.6ya.com/expert/oscar_403cc49e0f00cda7

have your fuel system vacuumed out our excursion did it as well the injectors are not at fault it is the fuel lines