Engine will start at very low idle, sometimes killing, then may fast idle, then low idle, back and forth. What may cause this?

well gee, there are 50 reasons.
did you scan the PCM yet, if not ,why not?
when the idle surges, that means.
NO RPM ranges stated, if super low rpm check EGR.

1: engine lean and ISC dead, or cant handle the huge engine power drop..The PCM can even make this worse!, buy hunting the ISC to find spec, idle and failing... to...
2: or any of 50 other things causing huge engine power drop and PCM can't maintain idle controls.
super lean burn.(bogging)
super rich burn, misfire for 50 reasons.

i cant hear your engine,nor feel it, or see it shake from gross misfiring.
no can i drive it and see/feel power loss,
can I?
asking net bunny's this (blind as bats)
is "OVER THE TOP" no?
so here , ill toss the dice for you
and guess gross misfiring.
ever get a tuneup this decade, if not bingo , no?

and the others...

http://jeepdied.com/Misfire101.html

link even has the 50 cause list, to show the futility of guessing!
we do 1 to 10 tests. and find it.
for sure.

as you can see EFI systems are complex.
gue$$ is not only expensive but can net you 2 problems easy.

Vehicle info engine size model year ? So the engine idles high and low, back and forth and sometimes will not crank and run ?

start by seeing if the check engine lite stays on it could be anything from a fast idle sensor to a fuel issue look for codes

remove and clean or replace IAC (idle air control)

Hi! You might have a Vacuum leak somewhere on your engine or you may have a MAFS Mass Air Flow System failure. Most likely this will cuase the Idle to go high or low, and in the long run this will damage your ignition system and when this happen the car will stall or shut off by it self without any giving warning signal. To find out if you have this problem. Start the engine and check under hood from the top and bottom engine, then listen for any unusual noise or hassing noise, you may want to check the exshust for a leak too. You can also check by starting the ignition and see if the RPM it goest high and low without pressing the gas. This indicate that you may have a Throttle or carburator problem, this will cause the gas line intake when it gets clog. The burned fuel is not circulating properly out in to the Air Intake Filter and sometimes this will cause when
the car has a bad sensor. hope this will solve the problem. good luck.

Glad to help.
Ginskie.

code p 1193 refers to inlet air temp sensor problem --circuit high
a faulty ambient air temperature sensor or barometric pressure sensor will cause this effect as the idle control in the ECM is not getting correct temperature readings to set idle speed

try un-plugging the IAC motor,then start it,if it idles low and stays there then replace the IAC motor----if no change then you may have a knock in the engine and the knock sensor is telling the computer to change the timing to compensate for spark knock due to low grade fuel,so it advances the timing back and forth due to too high of rpm then too low and the computer doesnt know how to smoothen it out that fast,but eventually does.The check engine light would usually come on in this case

Hello,
The car may be hard to start because it may need a normal tune-up. If the spark plugs are pretty used, they will slowly stop burning as much fuel because the spark slowly gets lesser and lesser. On a cold day the fuel is harder to ignite with a small amount of spark.
Normal idle when its cold means that your cars idle is probably too high. If you lower the idle, the choke on a cold day will keep the idle a little higher until the motor warms and then it will kick back off. If the idle is too high, it will run very rich, smothering itself with fuel, and causing the idle to stay low. When the car warms, its creating better spark when its warm, and with the idle being too high its running at a fast idle.
The car wont start sometimes can be the cause of a few things. If the ground is bad on the fuel pump, it wont make a good connection sometimes for when you turn the key on, the fuel pump dont pump the fuel to the engine. Another one can be that your accessory switch can be loose going to the fuel pump. I think its your ground though. Good luck! Hope this helps.
Have a nice day. -Steve

Check the Idle Air Ctrl (IAC) motor. It should allow air to bypass the throttle plate when starting and at idle. if it is not working it could cause veh. to stall.

You need to replace your IAC (idle air control valve). The IAC, like it sounds, control the idle of your car. When they go bad they will cause the low idle and stalling. The IAC is located on the throttle body and is usually held on with a couple of screws. It is an easy relatively cheap fix that you can take care of by yourself. Let me know if you have any more questions. Thanks for rating my response and for using FixYa!