I have a 1989 Mercury Grand Marquis 5.0L SEFI, and about two months ago it started to have an issue holding idle after start up. Please don't look away just because it is a Mercury, and not the F 150. They had the same motor like most of us know. I could go to start the car with no issues. It's just after it starts, the idle seems high for about 1-2 seconds, but after that it gets very poor and it can't recover. About 60% of the time I can put my foot on the gas to save it, but other times it doesn't matter and will die. Funny thing is, on the third start, it will hold on its own, every time.Once it warms up in about 15 minutes, it won't do it. Mechanics looked over it when it went in for regular servicing, and said it was the Coolant Temp Sensor, and replaced it. Still does it every time when you go to start it unless if it ran previously that day( within a few hours).

The parts that have been replaced, not all are within past few months: Coolant Temp Sensor, IAC Valve, EGR Valve and Valve Position Sensor, Throttle Position Sensor, Plenum cleaned, Tune up w/ plugs, wires, coil, distrubitor cap, PCV Valve, and probably alot more.

I'm open to suggestions. I've went over all of the vaccum hose connections, all sensors with possible relation to issue, and didn't find anything. Not ruling them out.

 

Sounds to me like you need a new IAC motor...

 

What you are describing is the EVR filter getting dirty. Here's a linky to the Lincoln website that shows how to clean it. Yours should be nearly identical.
http://www.lincolnsonline.com/tech/00023.html

FWIW, my 89 LSC had the same issue and cleaning this seemingly unimportant little filter cured the issue. Now my garage queen idles like a champ with all 32,000 miles on it.

 

I also have a 1989 GM with 265000 kilometers on it and it idles perfectly. I would guess the IAC is bad. Try cleaning it with some throttle body cleaner and see if that helps. If it does you may want to spring for a new one. These cars are also bad for vacuum leaks due to the fact that there are about 50 zillion miles of vaccum lines and they are now 21 years old.

Go to grandmarq.net and ask there too. I belong to the site and there are some guys over there that know grand marqs and crown vics inside out!!

Update. I just went over there and searched the EVR filter and it looks like it can cause a magnitude of problems as per Labnerd. Looks like it is located on the passenger side fender on a Grand Marq.

 

Thanks guys, I'll definetly look over what you suggested. You got my attention with the EVR filter. As for the hoses, they would be a good bet too, considering everything behind the motor is just the vaccum hoses!

Thanks again, and I'll let you know what I come up with. It might be a few days because we're expected to get alot of snow out here, but I'll keep you posted.