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Erratic idling

I'm searching right now for some instructions. So I can splice the vacuume gauge anywhere in the vacuum system and it should be 15-17" Hg?
 
bigalpha said:
I'm searching right now for some instructions. So I can splice the vacuume gauge anywhere in the vacuum system and it should be 15-17" Hg?
Click to expand...

find somewhere there is a junction (like a check valve) and then insert a t-fitting with a little piece of hose for the bridge, and a longer piece of hose for the gauge.

the engine should be in the 12 hg range at cold start and build up as the combustion chambers tighten up. optimal vacuum is 15-20 hg inches after warm idle.

one fun thing to do is clip it under the windshield wiper arm and drive around town watching it
 
Well, I'd only be using the vacuum tester at this point. Seems easy enough to splice a T into a vacuum line.

I was thinking of doing it where the fuel pressure regulator connects to the manifold.
Here:
 
Hum.

As the engine rev's, it creates more vacuum, right? So as the engine is rev'ing up and down, the vacuum should go up and down?
 
all vacuum originates at the manifold and all hoses have the same relative vacuum pressure (although hose diameter will affect it some). if you notice a big change in pressure on one line then chances are good that you have a leak nearby. that's not how you fix vacuum problems though.

the guage is there to tell you if you have optimal or satisfactory or shitty vacuum levels. if you are down in the 5-10 hg basement you have leaks that need fixing. get 10 feet of vacuum hose from a parts store that is just a little bit small, get a box of vacuum splice pieces (autozone sells a Dorman assortment that is nice), a tube of silicon dielectric grease, and replace every soft hose and fitting you can find. use the grease to make assembly easy, it will dry to a paste after a few heat cycles and make a good seal. if anything is loose after the engine bay is warm you got hose that was too big
 
bigalpha said:
Hum.

As the engine rev's, it creates more vacuum, right? So as the engine is rev'ing up and down, the vacuum should go up and down?
Click to expand...

RPM is one factor but throttle blades are another

vacuum is created when the pistons suck air into the combustion chamber through the intake valves. the intake manifold sucks air through the throttle body to satisfy the vacuum (leaks will allow air to bypass the throttle body which causes numerous problems)

if the RPMs are high then there is more demand for air, however if the throttle blades are open then air will flow more easily so vacuum will actually drop in a working system. on the other hand if you engine brake down a hill the RPMs will be up and the throttle will be closed so more vacuum.
 
I've already replaced a bunch of vacuum lines and made sure they fit tight. The rest of the lines are those hard lines with rubber elbows, and they also fit tightly; they are in good condition, too.

Unless I missed some hose/connection that is large enough to really increase my RPMs to something like 2500RPM. I'm going to try and swing by HF and grab the gauge because the crazy idling is starting to really piss me off.

I'll hook it up and report my findings. Hopefully it's something other than something difficult.
 
Don't use "silicone" unless it is O2 sensor safe.

The vacuum drops as you accelerate, and goes up as you decelerate.

Look for steady versus vibrating needles for diagnosis of things like a head gasket, or valve leak........

Ceck this thread for links to the how to vacuum diagnositcs!

http://www.naxja.org/forum/showthread.php?t=1002311
 
Thanks for the guidance. Now I want to get off work early and hook it all up and see what it's doing. I sure hope it's not a HG problem.

Not sure how I missed that thread, but it's basically amazing.
 
bigalpha said:
Thanks for the guidance. Now I want to get off work early and hook it all up and see what it's doing. I sure hope it's not a HG problem.

Not sure how I missed that thread, but it's basically amazing.
Click to expand...

The key search word was diagnostic. I had trouble finding it until I picked just the right search words. Even with Google, searching can still be a bit of an art.
 
Alright, so I hooked up the vacuum hose and took a video with my phone. It's absolutely terrible, but it shows the variation of the needle.

I can redo this test tomorrow with a better video/audio quality!!

This first video is from a cold start (hadn't been driven all day). This is in PARK the whole time. Idles half-way normal then it gets crazy. You can kind of hear the engine revving and see the needle bouncing.
http://www.youtube.com/watch?v=zuhYGqlMYcE

This second video is after a few minutes of running. At the beginning of the video, the vehicle is in DRIVE (foot on brake). You can see the needle is pretty steady, but much lower than when in PARK. When the needle starts going lower, I had her hold the brake and apply a little gas to see what would happen. At the end, the truck is in PARK, and the idle is going wild, along with the vacuum levels.
http://www.youtube.com/watch?v=j3RzxHQeB6w

Again, if the video/audio quality blows too much, I can redo it.
 
I can drop in the other gauge cluster that I have and extend the hose for the vacuum gauge and get them side by side ... if that'll help more?
 
ehall said:
really hard to correlate that stuff without a tach

blind shot in the dark is timing chain
Click to expand...

Why the timing chain?

Sounds like a vacuum leak that is coming and going, more likely to be valve-head problem IMHO.

You have pulled the IAC and inspected and cleaned it and the throttle body right? A piece of trash could be flopping around the IAC intake area, or the Cat could be busted and rattling around, but I suspect you have eliminated all those by now.
 
Last edited:
He has too much vacuum pressure to have a leak. Actually has too much vacuum pressure period (over 20 is almost too much) which leads me to suspect his RPMs are either really high or his timing is really off. The periodic high blip reinforces the latter theory. As I said though this a completely blind guess since there is no additional data at hand.
 
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