Developed a miss at idle and stumbles when half throttle seems sluggish at full throttle 98 ls1

check out your throttle position sensor voltages w a scan tool , engine off key on, compare to specs given in auto repair manual for your car. Voltages should change smoothly in the specified range, if voltage takes a sudden jump or experiences a zero reading anywhere then the problem is the tps. When is the last time your fuel filter was changed ? Some of these are not replaceable without removing the fuel pump in the tank. When you put in the plugs did you also replace the wires? P.S an intermittent tps won't always leave a trouble code in the computer. Also clean out the throttle body and idle air control unit with an approved spray cleaner and a rag with engine off and key off. Have someone hold the throttle pedal to the floor to make this job easier. After cleaning hold gas pedal to the floor and start up then when engine starts, rev engine half way a couple of times to remove any excess carbon from the system then let engine idle. This should be done once every 6 months whether having idling problems or not, just to avoid having them. If the throttle body is dirty it can and will cause a stumble comparable to a miss.

It can be issue with IAC( idle air control valve) causing the problem.

There are many other possibilities causing these idle issues. Engine idles rough.

There are few help links with solutions to troubleshoot such idle issues.

I suggest you refer the link below and follow its troubleshooting details:------

Click the link below:-----

Ford Expedition IAC valve replacing

http://technoanswers.blogspot.com/2012/12/ford-expedition-iac-valve-replacing.html

Bad miss at idle?

http://technoanswers.blogspot.com/2012/01/bad-miss-at-idle.html

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Stumbling and stalling at idle after engine rebuild?

http://schematicsdiagram.blogspot.in/2012/03/stumbling-and-stalling-at-idle-after.html

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1999 Ford F-150 Idles rough and quits when gas pedal is pressed less?

http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html

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Engine stumbles coming off idle when warm?

http://repairhelpcenter.blogspot.com/2012/01/engine-stumbles-coming-off-idle-when.html

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Erratic idle on car?

http://schematicsdiagram.blogspot.com/2011/12/erratic-idle-on-car.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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Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

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GMC Sonoma Stalls and idles low

http://technoanswers.blogspot.com/2012/10/gmc-sonoma-stalls-and-idles-low.html

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GMC Idle Air Control (IAC) Valve Replacing?

http://schematicsdiagram.blogspot.com/2011/12/gmc-idle-air-control-iac-valve.html

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1996 Camry intermittent engine failure while idle?

http://repairhelpcenter.blogspot.com/2011/10/1996-camry-intermittent-engine-failure.html

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Engine quits at idle?

http://repairhelpcenter.blogspot.com/2011/10/engine-quits-at-idle.html

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Car overheats and Air blows hot at idle?

http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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High Idle Problem

http://repairhelpcenter.blogspot.com/2012/10/high-idle-problem.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed?

http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html

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High rpm in neutral? Inconsistant high RPM in neutral?

http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html

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Vehicle stumbling at idle and shows code 54?

http://schematicsdiagram.blogspot.in/2012/03/vehicle-stumbling-at-idle-and-shows.html

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Rough idle, staling and Erratic shifting?

http://technoanswers.blogspot.in/2012/03/rough-idlestaling-and-erratic-shifting.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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These details will help.

Thanks.

Some makers used Vacuum releases for their emergency brakes which had a Vacuum switch connected to the shift assembly. This would account for a stumble if Vacuum vented out through this switch.

More common for shifting stumble to occur from Park to Drive, engaging a load on the engine, not to stumble to a freewheeling Park position. Sounds like idle speed control is sluggish to stabilize the motor.

You also have idle speed control problem with A/C, kind of shows that A/C was what kept the engine running because when you turned the A/C off, the car died.

I am going to give you some Websites for wiring diagrams. Alldatadiy.com amd Autozone.com with free wiring info upon registering your vehicle.

It seems that the base Idle speed control is not working and the A/C idle speed control is.

Also check for Vacuum leaks on the intake manifold by spraying water from a pumpbottle on the seams of the gaskets.

Check ignition Relay or ignition switch for random cutouts. The fusebox layout is on the Autozone site.

I hope my solution helps.

most common ly it sounds like a crankshaft position sensor, or crankshaft position sensor.... is it throwing a check engine light? if so scan the codes...

This sounds like a slipping transmission , check the fluid level first and make sure its full, if its full then you have ****** problems and may need to have ****** rebuild done,

Another thing is to make sure the trans passing gear cable is connected at the throttle where the gas cable is.

is the air filter full of s...

Wondering if you tested the throttle position sensor and checked that all the fuses in the vehicle are good. You need to have the computer codes cleared to eliminate false codes you may have set while working on it, then go through a couple of drive cycles (about forty miles) of use, then have the codes read. I'm not an expert on your exact system but common to all is that all sensors work with each other and the computer needs a good signal from each of them in its "decision making" Mass air flow sensor and temp sensor included. MAF can be cleaned with carb cleaner and a Q tip, carefully wiping off the two wires that stretch across the inside.I often recommend either haynes or chilton as a reference but in electrical diagnosis chilton is far better...If you are doing the work yourself get one and follow the test procedures in there.
In the end, the manual is more valuable and cheaper than replacing parts 'till the problem goes away. When working with electrical, I often make notes on what I found during testing.. sometimes something that seems meaningless ties in with another item later on.
No reliable shop can make a diagnosis without the benefit of hands on testing. You shouldn't either.

check the battery then if low in electricity, then charge it if missing in Idle throttle then check and repair the fuel pump or carburetor assembly.when check engine lights on it means there is problem that is happening to the engine.like low fuel is going through the engine.and the power produced is less.

Have the vehicle scanned. Autozone will do this for free.

I have run across this problem and discovered that it was the mass air flow sensor.