Can the PCM make it difficult to determine if you have a vacuum leak?
 

In trying to determine if I have a vacuum leak, I started the car, let it warm up to temp and reved it up to 3500 RPMs or so.

Then I pulled the IAT sensor out of the intake elbow and the idle did not change. I put the IAT sensor back in and then pulled the PCV hose off of the TB (on the passenger's side of the TB). The idle still did not change. I then put the PCV hose back in and pulled, what looks to me like a PCV valve, out of the driver's side intake manifold .... the idle still did not change.

Did the idle not change because the PCM corrected for it due to still being in closed loop? The car is up on stands, so I did not drive it during thise test.

Or, did the idle not change because I have a vacuum leak somewhere?

As always, any info is appreciated,
Thomas.

Why did you rev it up to 3,500rpm to check for a vacuum leak? Or did you let it return to "normal" idle speed before pulling hoses off? Not clear from your post.

Maybe you could explain a little more clearly exactly what you did.

Sorry, I suppose a little background is in order. I'm trying to determine what's causing a stumble/misfire issue. When I first start the car up and just let it idle, the only way I can tell something is wrong is by listening to the tail pipe pulses. Even after sitting at idle long enough to come up to temp, it seems to be running fine, less the exhaust pulses. If I rev it up and let it settle down to idle, the stumble then becomes bad enough that the motor starts to shake in the engine bay.

When I drive the car, it is very underpowered and has an intermittent hesitation problem.

So to clarify my original post, I did let the idle settle back down before pulling the hoses off. I reved it up to 3500 to make the problem pronounced enough that I would be able to tell if I started closing in on the issue.

Thomas.

I just took it off the stands, drove around the block, then pulled the hoses off again. Same results, the idle did not change at all.

Is this normal? Shouldn't the idle change if you create a manifold leak while it's running?

Thomas.

Pulling the IAT sensor out of the inlet elbow doesn't create a vacuum leak to the manifold. It simply allows a small amount of air to bypass the MAF sensor, which the short term fuel corrections immediately start to correct for. It isn't going to affect idle when you remove the IAT.

On the PCV valve..... If you pulled the PCV valve out of the manifold, you also did not create a vacuum leak. The PCV valve does not go into the plenum portion of the intake manifold. It pulls crankcase vapors from below the intake manifold. Those vapors then flow through the PCV valve, and into a hose.... that hose goes to the front of the intake manifold, under the TB on the 95-97's, or u-bends back into the upper/plenum portion of the intake on 93/94's.

In effect, you haven't noticed any change in idle because you haven't created any vacuum leaks.

The lines you can pull off include:

-EGR solenoid vacuum supply (drivers side of intake)
-Opti vent vacuum connection (drivers)
-fuel pressure regulator vacuum line (passenger side)
-Heat/AC vacuum line (passenger)
-EEC solenoid vacuum line (passenger side of TB)
-power brake booster vacuum line (drivers)

Did you try unplugging the PCV valve (driver side) from the manifold and the Hose its attached too?

Marvin