1990 f150 5.8l e4od won't idle smooth
 

hey guys my truck will sometimes run great and then sometimes sounds like a vacuum leak and will rev up and down and almost die. The smog pump was removed, not sure if that is causing this. thanks

if its throwing a code run by a shop or parts store and have them scan it.. sounds like a possible IAC valve going out.. i think they're around $30 - $40.. quick and easy fix..

it has a new iac

Have you changed or replaced anything else on it?

Did you replace the IAC trying to resolve this issue, or was it for another reason?

Have you had the codes pulled. 1990 should be able to count the blinking lights and do a hand test, and not need the code reader. If you search the forum, I know the procedure has been posted before.

Or you can purchase a code reader, this is the one I have. 30.00$ If you plan to keep the truck, it wouldn't be a bad investment.
http://smile.amazon.com/dp/B000EW0KHW

I replaced it because it ran good but not perfect but it has only gotten worse. The check engine light is not on and I have that code reader but no codes show up. Now I try to use the code reader and it doesn't even work. not sure if it's me or the code reader. thanks for the help

Will it keep running if you give it gas?

yes and it will idle without giving it gas. it's just sometimes it revs up and down and sounds like it might be sucking air when it revs down but maybe that's just because the rpms go so low, I'm not sure.

Neighbor put a new autozone IAC valve on and it was worse than the old one. We put the old one back on today.

i kinda have the same issue.. i replaced the iac and it helped dramatically.. but it still does it a little bit.. and the other day it went all haywire on me while driving it.. unplugged the iac and it quit.. drove it a little bit then plugged it back in and it was running normal.. thinking about buying a new one again.. just cause its new doesnt always mean it works..
im having the problem with the no codes also.. cel is on but every time i hook it up to read it there are no codes or the scanner says no vehicle registered.. my luck the dam hook up doesnt even work.. monday im dropping it off at a mechanic to diagnosis it for me.. ill reply with what he tells me.. it may or may not be the same thing but every little bit of info helps..

ok thanks guys

Both of you should Pull the PCM.

Look for leaking, bulging, blown capacitors.

It takes 30 minutes maybe to pull and inspect it.

Money saved if you catch it failing now or just throwing parts to it.

^^^^ X2
Yes it does sound like a bad PCM.

where exactly is the Pcm? under the dash on the drivers side? and if it is bad where do u recomend getting one from?

I'm having the same issue with my 4.9l il-6. Could it be the PCM also?

On your 1990 it is in the drivers kick panel and it removable from inside the cab. But you have to unplug its plug from under the hood first. 10mm socket.

I get my spare PCM computers from a salvage yard (pick-en-pull). You need open it up before you buy it and check the PC board to see if it has let any smoke out the stuff on it and it has not been ate by a cap. When I get it home I then replace the caps and test it out.

The plug under the hood:
http://i54.photobucket.com/albums/g9...mputerPlug.jpg

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