wont start unless gas pedal is floored

so this problem comes and goes..its a 93 jeep cherokee 4.0 2wd...my jeep wont start unless the gas pedal is pushed to the floor.. i have replaced spark plugs distributer head and wires also fuel filter.. know i have read on other jeep forums the cps and tps might be bad? has anybody had this problem and have fixed it?.. :doh:

mine would do that when the fuel pump was going bad, just an idea

Flooring it tells the computer to turn the fuel injectors off, so it can dry out after being flooded. My guess would be stuck injectors dumping fuel in it. Or it is getting fuel but not enough spark early on, then later gets spark?

A stuck IAC valve that is not allowing enough air during start up, bad air filter, or both might be starving it for air too?????

assasineee88 said:

so this problem comes and goes..its a 93 jeep cherokee 4.0 2wd...my jeep wont start unless the gas pedal is pushed to the floor.. i have replaced spark plugs distributer head and wires also fuel filter.. know i have read on other jeep forums the cps and tps might be bad? has anybody had this problem and have fixed it?.. :doh:

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Follow along with me here. Carburetors flow air and gas, but with fuel injection the computer looks at the amount of air flowing through the engine and adds the appropriate amount of fuel to match via the injectors (hopefully!). The skinny pedal is an air pedal, not a gas pedal, and all it does is open and close the throttle blades to allow more or less airflow. Meanwhile the IAC controls air flow when you are not pressing the air pedal. If you have to open the throttle manually, then most likely you are having problems with a lack of air flow through the IAC valve.

I had this problem a few weeks ago and while I know it wasnt good for the tranny I drove it home by shifting into neutral and giving it a little gas at stop lights to keep form stalling. By the time I got home the problem never happened again. I read somewhere on CF that it could be the CPS, so I'm awaiting that part to come in the mail.

ehall said:

Follow along with me here. Carburetors flow air and gas, but with fuel injection the computer looks at the amount of air flowing through the engine and adds the appropriate amount of fuel to match via the injectors (hopefully!). The skinny pedal is an air pedal, not a gas pedal, and all it does is open and close the throttle blades to allow more or less airflow. Meanwhile the IAC controls air flow when you are not pressing the air pedal. If you have to open the throttle manually, then most likely you are having problems with a lack of air flow through the IAC valve.

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Very well worded. Worthy of being in an FAQ!!! Nice job!!!!:cheers:

+1 for stuck fuel injector. Put a fuel pressure gauge on it and see how quickly the pressure drops after you shut it off.

I would think if it was an IAC issue, it would start w/ part throttle as well.

I've seen this symptom be related to a bad throttle position sensor.

They can be tested with a manual and a meter. Why not test it.....

The primary purpose of the AIC is to increase air flow at idle, to compensate for the rich mixture used at start-up, when cold. Carbureted engines had a mechanical cam on the choke to increase air flow for the same reason.
The AIC is also used to stop the engine from possibly stalling when you take your foot off of the gas--it allows the throttle to move back to idle slowly.
The fact that you must hold the pedal to the "floor" to get it to start implies too much fuel is being injected: stuck injector, faulty Coolant temp sensor, TPS,---??, whatever sensor/component that affects fuel flow at idle.
Remember the TPS tells the computer how much and how quickly the throttle is being applied. The computer goes into "flood" mode because the TPS tells the computer that the throttle has exceeded ~80% of its range. Also, when accelerating, the ECM interprets a fast change in TPS signal as a need to instantly enrichen the fuel mixture; it serves the same purpose as the [acceleration piston?] ("brain fart" alert) in a carburetor.

xjbubba said:

[acceleration piston?] ("brain fart" alert) in a carburetor.

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Accelerator pump?

fazio8118 said:

+1 for stuck fuel injector. Put a fuel pressure gauge on it and see how quickly the pressure drops after you shut it off.

I would think if it was an IAC issue, it would start w/ part throttle as well.

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That is one I never think of, it should be obvious, LOL. Mental note to remember to check that once year, preventive maintenance!!!

Also, something that just dawned on me, is leaking injectors might cause a high idle, rough idle.

If its a cold start only problem Ill guess a coolant temp sensor. If hot a leaking injector.

Is it throwing any codes? If so look up the codes and go from there. I had a dirty air sensor in the throttle body doing this. I cleaned the throtlle body and sensor and problem solved. Before throwing expensive parts at it, buy a 5 dollar can of throttle body cleaner and clean the throttle body.