Motor Feels Like it Will Die at First Start Up?

Starting a few months back my jeep has some hesitation when it is first started up and is placed under load. It will idle fine but when i give it some throttle it drops down in RPM's and almost dies, I don't let it die by letting off the gas. After feathering the throttle a few seconds it works fine and does not have any other issues. And It only happens when sitting for a while (like over night). If working or driving around it will start up fine and I can punch the peddle to the floor and it does not skip a beat.

There was one isolated incident recently where driving, it just shut down at a light. It started right back up with out any issue the rest of the trip and subsequent trips. (this was a rather hot day 102 as the high and had the AC on Max)

I have had the electrical system tested at AutoZone with the AC off and then with the AC on MAX and both test results show I have good charging and a good battery.

Does this sound like an electrical problem or a fuel problem?

Thanks for any help!

Bump? Any one? Has this occurred to any one else's jeep?

I think I would try cleaning the IAC. I also suggest more info like engine and year.

I will try cleaning the IAC... My Sig shows my information about my jeep..... its a 95 with the 4.0L and Auto AW-4 transmission.

I have recently replaced my whole exhaust system with a new APN manifold, Walker down pipe etc... But spent about 3 hours cleaning the whole intake and TB (DIY bored last fall). I am afrade that if its the IAC then it has just gone bad and needs to be replaced.

Does the FSM show how to test the IAC to see if it is good or not?

*Clean idle air control. The FSM does have a basic test procedure for the IAC
*Check fuel pressure and compare to factory spec
*Test your throttle position sensor (analog meter works best)

Last week, I pulled the brake booster hose off my intake manifold while it was running. The idle shot up, but then quickly recovered and settled back to normal. That convinced me that the IAC was working good.

^^ what result would have shown you that it was not working good?

constant high idle would indicate that the IAC wasn't closing the air bypass.

I'm having a similar issue; I've replaced plugs, cap, rotor, AIC, TPS, CPS, fuel pump & filter, water temp sensor, etc. No codes thrown now, but I can't fix this. If you come to a fix post it up for me to follow!

Magus2727 said:

^^ what result would have shown you that it was not working good?

Click to expand...

Yup. As Shorty said. Idle would have stayed high and not corrected itself. IAC shut down even past it's normal point to correct idle speed.

Well the FSM has you using some special tool, electronics so I think thats out of the question. I will test the IAC using the above method and check the TPS at the same time. It may take a few days to test, I work all day tomorrow and my Wife has the Jeep all Tuesday so it wont be until Wednesday.

Edit: I have fast data loggers at work that I might be able to use to measure and capture the TPS results. When doing the tests should I be measuring resistance and seeing a linear change? or should I have the motor in the ACC mode and measure the voltage (I am guessing also linear). Also does any one know what pins to go off of?

Most of these soleniod valves, including the IAC and the injectors, are operated by a DC constant frequency signal from the ECM. The amount of time the valve is open is determined by the "dwell", i.e., the amount of time the current is "on" vs the time it is "off." Think of the old-time ignition points...