alnen

It's a 302 motor. The reason I decided to clean the IACV is because when I would cold start my truck, it would rev up to 1500 RPMs, then drop striaght down to 500 RPMs, then bounce back and forth between the two until it wound up around 900-1100 RPMS.

I removed the IAC to clean it, trying to smooth out my idle some. The plunger moves freely. I left the throttle body on the truck, and scrubbed it with a toothbrush and throttle body cleaner. I put everything back together, and now I have a new issue. When I start the truck, it seems to linger around 1500 RPMS, then slowly drops down to 1200 RPMS. Then it will dip down to 1000, then back up to 1200, you can actually hear it revving up and down. I played with the throttle body cables a little and when I did it dropped down to (what I think is) a better RPM, around 900. I made sure all of the residual throttle body cleaner was burned off, so I know that's not it.

What RPM should I be at cold and hot? Since I moved the throttle cable and it dropped the RPMs to a lower #, could it be a TPS, or maybe just a sticky cable? I'd really appreciate the help!

wde3477

The typical suspect with a surging idle problem is the IAC valve being gunked up. Cleaning it usually solves the problem. However, these trucks aren't spring chickens any more, so failure becomes the next likely candidate.

Try starting the truck, and when the surging starts - disconnect the IAC electrical plug to see if things change - the engine should typically slow to a very low idle and possibly die. If so, might try just replacing the IAC since cleaning has already been tried. If the surging continues, the problem is likely somewhere else.

The TPS usually comes into play with throttle changes, not with idle speeds - and a fault will most always be flagged by the computer.

Don't know what the base idle is - thinking likely in the 650-850 range.

Moving the cables as having an effect causes some concern - suggests to me that the cable is binding/hanging, or is not adjusted properly.

A sustained high idle could be a sign of a vacuum leak - those are always fun to chase.

Do not adjust the set screw on the throttle butterfly - this is intended only to keep the butterfly from wedging in the throat, and has nothing to do with idle control.

As always, the first recommendation when troubleshooting an engine problem is to pull the computer codes to see if any faults are active or if any faults have been captured at sometime in the past. The computer is your friend - just have to learn to understand what it's trying to tell you. Most parts stores will read the OBD-II codes for free. Many of these stores no longer have the older OBD-I readers, requiring one to purchase the unit for themselves (about $35), or use the D-I-Y method described in the sticky note in this forum. :-)

Good luck, and keep us posted.

alnen

I disconnected it when it was surging, and the RPMS dropped to 800 and stayed there. So should I replace the IACV? I was thinking RPMS would be a little lower than that with it disabled.

Also, I didn't realize that I don't have a check engine light. I don't know if it was disabled, didn't come one this model, or what, but there's not one that lights up on the dash. I guess I need to buy a scanner to make it easier. I have a digital multimeter and don't feel like buying the mechanical type, plus a scanner would just be easier.

fastpakr

What year is your truck? I don't remember the year that CEL's were added to the cluster, 87 or 88 IIRC. Anything after that had one from the factory. If it doesn't come on when you turn the key to the run position, I'd check the bulb.

alnen

My manufacture date is 2/87. The bulb doesn't come on at any time. What all is involved with replacing the bulb (if it has one)?

fastpakr

I'd guess the '87 doesn't have one, but someone else can answer that for sure. If there's one to replace, you'd just need to get the instrument cluster out. I'm sure someone's posted a walkthrough for that on an 8th gen either here or at one of the other F series or Bronco forums on the web.

Travis_CO

mine is an 89 does the exact same thing in park and I have replaced the IAC with a new motorcraft one and it still does it.

alnen

That's not good... In the mean time of me figuring this out, what's worse for my truck... Leaving the iacv disabled and idling at 800, or leaving it on and idling at 1200 and surging up and down? I messed around with vacuum lines, and found one on the drivers side that caused the revs to go up when pulled, and another on the passenger side that almost stalled the truck. My point being, if there was already a vacuum leak somewhere, would it still rev up 300 rpm if I pulled another vacuum hose?

Sorry, just trying to figure this out.

klricks

I have an '86 and it has the CEL spot on the dash but I found that the socket was pulled out and tucked down behind the dash. Not sure if the PO did that or the factory? I have an 86 Ford shop manual and there is no mention of the CEL anywhere... not in any text or wiring diagrams that I can find.
I put a bulb in it and put the socket back where it should go but it has never lit up for me not even during start-up like the other lights. Does not blink when pulling codes either.
Next time I have my dash apart I will try and trace the wiring.

FYI - I did writeup on reading CEL codes.
http://64.13.91.137:86/F150/86F150.html

mavdad

alnen,
Did you replace the IAC, just wondering if that worked? My son has a 1987 that we just purchased, and we are having similar issues, While at Idle, it will rev up and down on the rpms, Hitting the gas pedal tends to bring it down but it takes a few seconds, also while changing gears it will stay reving up.