nrgetic99

Had some fun this weekend.

I found that the hose on the vacuum side of the idle up valve for the AC was torn. I replaced the hose and the truck will not idle. It surges really badly and even with the idle adjust screw backed almost all the way out, the best idle I can get is 500 rpm and surging.

After 2 hrs of messing, it seem that the truck needs the extra air its getting into the vacuum side of the idle up valve from the atmosphere in order to idle at all.

So...I removed throttle body and gave it a good clean. Checked as best as I could what I assumed was the idle by pass circuit.

Now, put it all back and it seems there are two circuits that come from the air cleaner, go through valving and introduce more air at idle to "idle up" under :

1. AC
2. Power steering.

Here is a little threory in laymans terms :

At regular idle, the idle mixture is adjusted by adding more or less air around the throttle plate (which is closed at idle) to get the desired 800 +/- 50 rpm.

The AC and PS both put a load on the engine (belt driven accesory load) and tend to bring the idle down. Without the driver increasing the throttle position to increase the idle, the two 'idle up' circuits introduce more air downstream of the throttle body direct into the manifold that creates a greater vacuum, this demands a richer mixture and the ECU givs the system a little more fuel to compensate and hence the idle speed increases WITHOUT driver input.

Right now, truck will only idle if one of the two hoses that should be connected via a valve to the air intake (hose downstream of VAF) is disconnected allowing the manifold to suck air downstream of the throttle body.

In theory, the idle circuit allows enough air to idle without the additional air from either of these two "idle up" assist devices but what is foxing me is if the hose ruptured many moons ago , did whoever worked on the vehicle somehow richen the idle mixture enough that it idled with all this extra air coming in from the ruptured hose on the vaccum side of the AC idle up valve.

Also, the AC "idle up" valve has a an adjust screw which meters how much air gets from the intake hose to the manifold so I am guessing you can adjust the idle to a spec when the AC idle up valve is operating (e.g. when the AC compressor is running). I have tested the AC idle up valve itself and the VSV that contolls it an all seems well.

Anyone have ANY experience of this ????

Thanks

David

Toysrme

Set it up like it should be & reset the ecu. Pull the EFI fuse a second & put it back.

nrgetic99

Gonna try that lunchtime....

Ran out of time last night.

David

leiniesred

I'm going to guess you have another vac leak. This one is upstream of the throttle body. My guess is going to be:

Intake trunking up at the throttle body has a split on the bottom of the hose at the seam.

-LeiniesRed

89macrunner

you have a vac leak or maybe somethings not hooked up right...

i can get you a pic of the vaccum line diagram if needed...i found the sticker under my hood the other day