odd vacuum readings?

[bigchief79]

What would cause intermitent readings on a vac guage? The motor (360) has about 3000 miles on it. It runns rough sometimes and good others. I checked vac a few months ago and was getting 20in/hg. Now it's around 12-15in/hg. What gives? Also, the needle is steady sometimes, but bounces all over the place other times. Hope you guys can point me in the right direction. I need to get this problem solved before it drives me krazy :(

[PhilSine]

Usually if all else has been good then chances are you have a new vacuum leak that needt to be traced. With such a significant drop, it's probably a major one like the brake booster. Have the brakes been acting up any?

Other things that can cause it is a bad valve, misfire, or just valves worn in general. The general valve wear is unlikely because that is usually a gradual decline, not a sudden one.

Good Luck

[Bob Barry]

Worn timing-chain? Ignition missing at times? Sticking valve?

Those might be some of the top culprits.

How's the engine's compression?

[Stuka]

Check ALL your vacuum lines and AL your vacuum caps. You have a intermentent leakk somewhere. Which is exactly what I had a week ago. Redid everything and it runs great now.

EDIT: BOB: it has 3000 miles on it...i doubt poarts are worn out ;p

[ September 08, 2002, 12:11 AM: Message edited by: Stuka ]

[Bob Barry]

Ah; I missed the mileage there. Vacuum leaks are a good suggestion already given. Also look for anything else that would drop your idle-speed randomly; vacuum drops when idle-speed drops. It could be something as simple as a frayed coil wire connection.

[bigchief79]

I checked the vac to the brake booster today and it was fine. Also, i replaced all the vac lines and checked best i could for any leaks. Didn't find any. Seems like it would have to be a major leak to cause this problem. The way the vac guage acts seems like a sticky valve. Hope this isn't the case since the motor is fresh! Is there any other way to verify a bad valve? it doesn't tick, just runns real rough through all rpm's when it's acting up. I haven't checked comp on all cylinders, but the ones I did look at were ok.

[Stuka]

Do you have any thinsg inside run by vacuum? Transfer case, heater flappeer, a/c...anything at all? when I was testing i disconnected the line going to my vacuum canister ont he firwall, so I would leave all that stuff. Problem didnt go away, so I then knew it was a cap or line on the engine.

[Crazy_Jeepman]

If equipped with an EGR Valve, and it is failing. Could cause it to run rough sometimes and good others. ;)

[bigchief79]

I checked egr too. took it off and made a plate to cover the hole. No dice. Could it have anything to do with the carb?

[PhilSine]

It could but if it's running rough at all speeds then check some ignition components. I'd look at the cap/rotor, plug wires, and individual plugs.

Just because stuff is new....

[bigchief79]

Thanks guys! I'll go through and recheck these things and see what happens.

[Rogue]

low vacuum reading can also be caused by a clogged exhaust, retarded timing, ive seen fluctuations like you are referring to caused by broken valve springs - or in the case of a large cam - springs too weak for the application; since i don't know the history of your ride with 3000 miles was this engine just rebuilt? if a valve job was done did the machinist check for installed height and coil bind after the machining? hey it happens to the best of us - i once had a misfiring car come from another shop that had just replaced the head gasket and did a valve job - turns out the machine shop (which is the same we used regularly) forgot to cut the valve stem (installed height) and thus the valves would not fully seat - anything is possible -hopefully w/3000 miles its not an internal problem