89 302 ATX 2WD

when i start it and let it idle in park or neutral the motor is idling pretty high. but when it's in drive or reverse the idle drops.

what would be the problem?

IAC stuck open?

thanks!

 

It could be the IAC but another common issue is a broken vacuum line. You might want to take a look at the rubber vacuum lines under the hood for cracks/breaks.

 

which lines are most common for breaking in the older trucks?

 

Did they have IACs on '89s?

How high is it idling when its in park/neutral?

 

yuup. my truck is EFI. i popped off the IAC and sprayed it with carb cleaner to clean it up. it still idles kinda high.

plus i don't have a tach so i can't really tell you how high it really goes. i'll have to go find one and see.

and in all the service manuals i read it says i cannot adjust the throttle adjustment screw on the TB.

 

You not getting fully informed.
Spray clean the throttle body plate and all around the inside while holding the throttle wide open and use the whole can, to be sure.
The Idle Air controller is bolted to the side. Take it off and clean with the same cleaner until all carbon and oil is gone.
Remove the PCV valve and clean the same way.
Do not adjust the idle stop screw unless you know what you are doing to reset it.
A vacuum leak will raise the idle so do check for hose faults.
This should clear the problem as you had discribed it.
The reason the idle went high in park or neutral is one or more of the conditions above.
The reason the idle dropped in gear is that the computer could not use the IAC to control the bypass air for one or more of the above conditions.

 

ok, so before today i had sprayed the TB and the plates with the carb cleaner. i also popped off the IAC on the side of the TB and sprayed that as well. i grabbed some oil and lubed up the plunger in the IAC.

last night i was checking for vacuum leaks. i've tried looking everywhere i could find spraying brake cleaner at whatever connections i could find and nothing. my brakes also have a hard time stopping and the pedal would sometimes get really soft or really hard. i suspect that a vacuum leak might contribute to the higher idle and the brake booster.

i popped off the brake booster tube at the intake manifold. the idle went up. i plugged the hole with my thumb and the idle went back to 'normal' and then after a little bit the truck died. popped the line back on and everything seemed fine.

ook. now the truck dies occasionally. today i started it up and drove it around. it acted 'normally' as it does before. but then i got stranded on the side of the road when the motor started idling well below it used to and the motor died. tried starting it a couple times and it would kinda start but would stumble.

finally got it started where the idle rose to where it usually is and threw it in gear and limped home. on the way back the motor was stumbling off and on but luckly it didn't die before i got it back home. i also heard some noises on the way back.

i'm getting sick and tired of this thing. i have allready set base timing at 10* i have allready rearranged the plug wires to their correct locations.....

if there is anyone around the detroit area that is willing to come by and help/diagnose wtf my problem is then please do. i'm gonna goo insane with this truck.

 

Sounds like you have more than one problem.
Engine and brakes.
Start with the engine. At hot normal idle, remove the IAC connector and see if the idle drops low. If yes it is working , if no it is not, replace it and don't put any oil in it, doing this only collects dirt.
If the idle is high and removing the brake booster line brings it down, there is a problem with the booster.
The brake booster may have a hole in the diaphram or the check valve is not holding. The booster should hold vacuum at least a short time after the engine is shut off.
Put organization into this and step thru the process same as a mechanic would do. You dealing with a nearly 17 year old truck. Things go wrong and wear out.

 

Check the throttle position sensor on the bottom of the throttle body. It may be giving a false reading causing the PCM to think you have your foot on the throttle. Anytime you open the throttle the IAC opens up too. It is very common for the sensor to go bad, especially with a manual trans as you open and close the throttle more often than with an automatic. If you find the two wire connector under the throttle body, unplug it and tap into the sensor side with an Ohm meter. Then slowly open the throttle and watch the meter for any up or down spikes in the reading. It should move even with the throttle plate. Hope this helps. Good luck.