Track down vacuum leak

[backporch]

I have a high idle when I put the truck in neutral coming to a stop. It may hang at 1500 or so when coasting and usually drops to 8-900. I cleaned up the IAC valve, not sure it is really working but i wanted to track down any issues with vacuum lines. There are a lot.

I noticed a few things on the truck....

The large tube from the oil fillter neck to the rubber intake just inbound from the MAF sensor doesn't have a hose clamp on the end by the oil filler neck... going to fix that.

All the connections seem to be tight and I don't see any broken lines.

I was considering capping off the small hard lines that come from the upper intake manifold and wrap around the back of the motor and feed the fuel rail after it leaves the head and crosses to the passenger side wheel well. Woudl that cause any problems other than temporarily removing power from the HVAC?

 

[Dirtman]

Its suppose to stay at 1,500 rpm while coasting until you come to a stop. It's part of the emissions system.

 

[backporch]

Even when fully warmed up?

 

[Dirtman]

Yes. Engines pollute less at higher rpms so the computer holds the rpms up to 1,500 until you come to a stop. 1,500 at a coast and 850 at a stop is exactly where it should be.

 

[backporch]

I would never have guessed that. Very interesting. And the truck has California emissions, so whatever pollutes less goes.

Another question. How can i test that the vaccuum system is working correctly because now that I have started to think about it I have questions. Should there be some HVAC responses that are expected, like when the fresh air door/recirculate opens and when the lever on the heater hose bypass valve would normally operate. These are probably not easy to give a straight answer to...

Is there an expected vacuum reading for idle? and a best place to hook up a guage?

Thanks!

 

[Dirtman]

There's no specific set number to see on a vacuum gauge, there's simply an "acceptable range". All engines are different based on age and wear. 17-24hg is considered OK.

The engine won't react to the vacuum operation of the hvac controls because A. The amount of vacuum used is minimal and B. The vacuum reservoir holds vacuum so the controls aren't directly operated by the engine.

The best way to determine a vacuum leak is to start the engine and unplug the IAC valve, the engine should drop to around 500 rpm or stall out. If so you're good to go. If it keeps running fine you have a vacuum leak.

 

[backporch]

Really appreciate your answer. I did see the vacuum reservoir by the front passenger tire. Makes sense. Thanks again.