Vacuum leak

[jtrooney]

Hey guys, have a quick question. I did a PI intake swap about 3 years ago on my 96 GT without a problem. About a month ago my check engine light went on, so I took it to auto zone to pull the codes found out its throwing p0171 and p0174. Yesterday I did some spot checks for vacuum leaks around the throttle bottle and the plenum. With a glimmer of hope I found a loose bold on the IAC ( i think its the IAC, its the part that bolts to the front of the plenum and has a hose connected to the intake). After tightening it up I'm still getting a little bit of a hunt at idle, as well as every once and a while the idle drops to about 500rpms but doesnt die.

My real question is, what is the easiest way to find a vacuum leak? I'm thinking its a leak on the intake manifold, but would hate to pull it off just to find out that it wasnt the problem.

Any help would be greatly appreciated? Thanks

 

[R.J.]

The best way to find a vac leak is to spray starter fluid (ether) on the suspect area when the car is running. If you have a leak the starter fluid will get sucked in and the engine will rev up.
If you do not find a vac leak I would replace your IAC (NOT clean it) and reset your idle like this:


1. Disconnect the negative terminal to your battery and turn on your headlights for about 10 minutes(there's a lot of speculation as to how long. some people will even say to disconnect the positive terminal and some people will say to leave it disconnected over night.)

2. Disconnect the wiring harness connector to the IAC

3. Turn off headlights and reconnect battery. DO NOT TOUCH THE GAS PEDAL!!

4. DO NOT TOUCH THE GAS PEDAL!! Try to start the car. If the car won't start adjust the set screw on the throttle body a 1/4 - 1/2 turn and try again. Repeat until the car will stay running. It will run rough but that is what you want.

5. Let the car sit and idle (DO NOT TOUCH THE GAS PEDAL!!!) until the car reaches operating temperature.

6. DO NOT TOUCH THE GAS PEDAL!!! Step 5 allows the computer to relearn idle settings. It's pretty cool when you follow my procedure because you can actually hear and feel the car figuring out the proper idle settings. It will go from idling rough to idling smooth as it warms up. Once the car is warmed up verify that the car is idling at around 700rpm. If the car is not idling where you want it, adjust the screw until it hits ~700rpm and then let is sit maybe another 10 minutes DO NOT TOUCH THE GAS PEDAL!!!

DO NOT TOUCH THE GAS PEDAL!!!

Turn the car off. Reattach IAC wiring harness. Now remember we adjusted the set screw on the throttle body. That means that the voltage reading from the TPS sensor has changed. It should read between .96 and .99 volts. Anything outside of that range will cause all sorts of issues including misfires and rough idle.

7. You will have to back probe the TPS connector. With the connector attached to the TPS stick a paper clip into the rubber boot on the connector where the top and middle wires go into the connector. The rubber is very flexible the paper clip will slide in between the rubber and the wire.

8. Set your multimeter to volts. Turn ignition to ON. Then place your multimeter leads on the metal pins. If it comes up as a negative that is okay as long as you are -.96 to -.99. If you get this reading then great you are done and hopefully your issues are gone. If not proceed to step 9

9. This is where and extra pair of hands come in handy. Using a large screw driver you will need to loosen the bolts that hold the TPS. DO NOT USE A SCREWDRIVER THAT IS TOO SMALL BECAUSE YOU WILL STRIP THE BOLTS. They probably will be really tight so you have to really lean on the screwdriver and use some finesse.

10. Once the bolts are loose you will need to move the TPS up or down and continue to read the voltage. Once you get it to the desired setting you can retighten the bolts. What I do is I find .97 and then as carefully as possible I tighten the TPS down. what will happen is the voltage will change usually to .96 or .98 but that it okay. Once you are done with this manually open the throttle body a few times and close it then reread the voltage and make sure it is still within our desired range.

11. Depending upon how loose your set screw was you might want to cover it in RTV to hold your setting. At this point you have CORRECTLY reset you idle.

 

[lvmustanggt]

WOW , thanks for taking time to write. I am sure lot's of us will benefit from this.

 

[jtrooney]

Yeah thanks RJ, I'm sure that I'm not the only one that appreciates your help.

However I think I found the general area where the leak is, it seems to be right below the plenum as well as where the throttle body meets the plenum. I replaced the gasket for the throttle body and made sure everything was nice and tight but still seem to be leaking at the plenum.

Thanks again for your help.