Rough Idle/Acceleration after warm-up...
 

Have a 98 mustang GT 4.6 with this issue:

1- When the car is cold the idle in park and drive is fine and acceleration is fine as well, no noticeable issue. Now, once the engine reaches normal operating temperature the issues starts

A- Idle is rough both in park and drive. needle oscillates between 500/1100 RPM and engine feels like it's going to die.

B- Car accelerates very rough, feels like an engine misfiring (it's not 'cause it's fine when cold), and I have to floor the pedal for it to pick up speed and stop vibrating.

I'm afraid to take it to a shop 'cause I will pay the price again, just took it out of one a month ago after 5 day diagnostics/$1289.00 and all it needed was a fuel pump in the end. Hope not to have to go that route again....

Already cleaned the throttle body, cleaned MAF, replaced AIC and plugs.

Any real good ideas?

Thanks

Off the top of my head two things happen when the engine reaches operating temperature. #1 is coil packs can fail when they get hot and work properly when they're cold.
#2 The engine starts paying attention to things like the O2 sensors (which actually happens after like 20 seconds).

I suppose one could rule out O2 sensors then, engine takes about 3-5 minutes to reach normal operating temp way longer than 20secs (during this time car is smooth as a baby's A****) -wink?

Shop that replaced the gas pump, check coils and I took it back, they are OK! Car started doing this after fuel pimp was replaced....

apparently there's no EDIT function I can find!

I meant to say: "The Shop that replaced the gas pump, checked coils when I took it back, they are OK."

Coil packs. I don't give two craps what that shop says, it's extremely difficult to test a coil pack due to the changes in heat and load. Get on ebay and you can find a set for $41, then get a 7mm socket and change them out yourself.

I have replaced a lot of cops on ford vehicles, they just go bad. The hesitation on take off is why I suspect the cops, they hesitate under load when they start to go bad. I'm with jc on this one, I have cheap eBay cops on my 99gt for about 2 years now and it still runs strong.

Yeah. When one of my coils went bad the engine ran perfectly until it was under load.

If it is a COP it should throw a code, the first thing i would do is go to your local auto parts store and ask them to scan it, that should be free.

Coil packs are extremely hard to diagnose. When changing spark plugs always do the whole tune up (coil pack, wires, and plugs) because you'll end up having to buy them in the long run. Def sounds like a coil pack problem or a wire isn't giving enough spark to the cylinder. Replace those and if it still doesn't fix the problem, let us know.

Car is running good now! Thanks for the help.
I inspected the wire boots and guess what I found? Someone stuck tissue paper on No5.
Might have something to do with me refusing installation of a $1,280.00 fuel pump and a $2150.00 Brake BOOSTER/Power Steering pump ... The car did start doing this after I removed it from the dealer shop and the service Rep was very rude.

I always believed auto shops are like lawyers: sleazy con-artists, and now I have the proof!!!!!