Vacuum and bouncing gauge

[trickc]

I remember reading of someone having a problem with low vacuum readings and a bouncing needle on their gauge. I have a friend with recently rebuilt 6 cyl who's having the same trouble. Can't seem to get a decent vacuum reading, about 12-15 at idle if I recall correctly. Also CO is high on emmission test. He's leaned it out to the point of stumbling and missing, still no better readings on emm. test. Any suggestions?

[mandoneer]

bouncing usually means that you have an air leak at the intake or carb gasket. but you have more than one issue here - the vaccum is to low , it should be between 17 and 22 in./HG -being recently rebuilt it prolly isnt compression but check it ,the low reading could be valve timing or ignition timing or both -make sure that he dialed the cam in right them check the ignition. the fluctuation could be caused by a few things aside from leaky intake/carb gaskets . A sticking valve or ignition misfire would usually show fluctuation between 15 and 20 in./HG If the gauge steadies as the motor speed increases than prolly valve guides (even if they are new)- if it vibrates at all motor speeds it could be a leaky cyl head gasket . hope this helps - first ya got to get the vacuum reading higher though . I will help more if I can

[gsmikie]

could be a bad intake valve

[NomadJack]

I was having the same problem after replacing my timing set, it turned out I simply got the driven gear placed one link off (too far advanced if I remember correctly). It was a stupid mistake but one that can be easy to do if you're not careful.

[letank]

you may be too lean, incomplete combustion.... dial up and see what the gauge reads...

also with high CO, make sure the power valve is connected... and being a 6 cylinder... i am not too sure about the carb....

Michel

[badaboom]

What would be good vacuum reading for the 360?
Just curious.

[NomadJack]

Somewhere around 17in at idle for an engine in reasonable condition seems to be the conventional wisdom. It seems to me that it would be slightly higher for newer or recently rebuilt engines, but a little lower for engines with more aggressive cams.

[Wesdog]

If you have a 2 plane intake manifold and you connect the vacuum gauge to a vacuum port on one of the intake runners your vacuum reading will 'flutter' due to the alternating use of the 2 intake planes by the various cylinders in the firing order. I assume the 6 cylinder engines use a single plane intake and if so then this wouldn't apply. However, even if a single plane intake manifold is used if the vacuum gauge is connected to a vacuum source in one of the runners close to the intake valve you may see some flutter on the gauge due to the pulses in the runner that occur when that particular intake valve opens/closes.

The bottom line is to connect your vacuum gauge to a manifold vacuum source common to both sides of a 2 plane intake above the plenum or your reading will flutter. For a single plane intake you still should be monitoring manifold vacuum at the plenum.

This has nothing to do with a low vacuum indication. Ignition timing/problems, idle mixture and vacuum leaks are typical causes low intake vacuum at idle.

Wesdog

[ August 16, 2003, 05:36 PM: Message edited by: Wesdog ]

[mandoneer]

It could also be a clogged CAT/EXHAUST -I forgot to mention that --it could cause both of the symptoms simultaniously

[trickc]

Thanks for the suggestions, it will be passed on.

[BRAIN]

if it has been recently rebuilt someone probably
missed a vacuum line.
the 1st time i got my 4.2l rebuilt the shop
forgot to plug a vacuum line and a month later i
was in some deep water and the loose vacuum line
sucked the engine full of water, very bad!!

[Wesdog]

When you say you have a low vacuum reading at idle, what rpm are you at and is it an automatic trans and if so is it in gear?

I have my idle adjusted to around 600 rpms with the trans in drive. Under this condition my vacuum is down just below 15 when the engine is at operating temp. Different story if I put the trans in park because there is no load on the engine, the idle and vacuum both increase substantially. I have the idle set low to prevent any dieseling when turning off the ignition.