What causes fast idle, and then no engine start?

well 50 causes, to make any engine fail.
bad idle (no year told why?)
egr stuck open
IAC stuck closed gunked up
bad spark plugs or wires.
bad fuel
bad injection
weak fuel pump.
if forced to idle with fast right foot does engine have full power now?
not starter means it cranks fast and engine does not start?
will it start with 5% foot throttle added,? this add air to hide bad IAC..

Not sure what you mean here. Is the engine idling at too low of a speed or do you wish to adjust the idle so it idles at a lower speed because it's running too fast? If it's the latter, the engine control computer has complete control over the base idle speed and it is not adjustable at all. However, if it IS idling too fast then this means that the engine control computer has reached the limit of what it can do and the still too high of idle speed is being caused by a mechanical cause, usually a vacuum leak. I don't believe I've come across a 4200 that idles too slow but I've seen multiple vacuum hose leaks that have caused too high of idle speeds. Hope this helps.

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.

It likely has EFI (electronic fuel injection) and an idle air control valve that regulate idle speed. They can get gummed and fail to have a fast idle when cold, or get stuck the other way, and have a too fast idle all the or regularly take its time slowing down after you take your foot off the gas. An engine temp sensor can also cause a lingering high idle if it reports the engine as still being cold after warm up.

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

It's the choke doing tis job. It reduces the amount of air to the engine to help it start/warm up. As the engine reaches a good operating temp it will drop the RPM's. Some times they do get stuck, so some cleaner wouldn't hurt. Spray it into the throttle body while the engine is running. Also the O2 sensor could be bad.

check and make sure air filter is ok, and there are no restrictions in hose to throttle body, check iac (idle air control.

Could be vacuum leak or bad ignition wire(s).

I had a similar problem on my "93 Ford probe. The engine idled pretty rough with shakes, Turned out to be a head gasket leaking engine coolant into the cylinder during low rpm's. At fast rpm's it was fine becuase the internal cylinder pressure was greater than the hot coolant pressure & would prevent the coolant from leaking in.

A friend has the same problem with his "94 Blazer 4.3 L, throttle body injection, manual tranny. He hasn't fixed it yet, It started a few weeks ago & is getting worse. His gets a fast idle with hiccups which causes the engine shake. He already checked the EGR valve. He also has a severe exhaust problem, Not oilly but very gray & terrible smelly, but only when the fast idle problem happens. THis fast idle hiccup problem starts when the engine is warmed up.

You need to change the thermostat, it must be stuck open and the computer thinks the engine is always cold, hence the fast idle and smoke.

PVC valve is not metering air correctly, it will appear ok shaking it but looses it ability to maintain proper position causing a medium to gross vacuum leak.