stangrookie302

Hello again, today I had an interesting encounter with my idling. 88 5.0 just hit 100xxx miles. stock. I was having an issue before were on a cold start my engine would rev to 1400 rpm and then choke and then die. After a second start it would stay started but it would idle like a roller coaster and then settle out to normal RPM. Today, when I got off work, I started my car and I had an idle that would rev to 1400 back down to about 500, not die, but then go back up to 1400 and then back down to 500 RPM. I just replaced my vacuum tree near the firewall since it cracked one day. Do ya'll think I just moved my problem to another vacuum leak somewhere? I tried to spray the lines with wd-40 while the engine was on to see if I could find a difference in idle but nothing. CEL not on. The idle was rough for about 15 secs and then I tapped the pedal and it leveled out. Sounds like a carburetor. Thanks for the help.

dawson1112

Look at the lines under the upper intake, the rubber ones tend to crack.

That hunting idle is typical of a vacuum leak, but a sticky IAC is not out of the question.

stangrookie302

Ya know, this morning I was going to take a video of this "happening" but nothing bogged down on a cold start, and it was chilly, and nothing happened when I started it up after work. Do ya think since I fixed one of the vacuum leaks I may have, and this is a stretch, changed or improved some of the codes in the computer for the proper fuel delivery? Like I said just trying to think outside the box and of course when I get a chance I'll double check my lines and probably just replace all of them for s's and g's.

88 orangepeel notch

It's entirely possible as the computer is now operating within its set of parameters. Clear the codes and drive it, the computer will relearn and adjust accordingly. Pull codes again and see what you get. And I'd stick with your plan on the vacuum hose replacement, it sure won't hurt anything.

stangrookie302

Also, Dawson, I have all ready replaced my IAC. Not that it could be defective but it has helped out.

F&Js90Fox

hey guys.
bringing this back after a search. i'm experiencing the same exact thing.
started happening about 2weeks ago. at cold start up in morning idle doing just like above then die. next start up give it some gas then idle bounces as stated above but runs fine.
for the rest of the day every start up and idle is fine. i was actually going to change the iac but after reading this thread i'll check vacuum lines first.
wondering if OP solved problem and what it finally was.