vacuum gauge reading problem
 

Today I hook up my vacuum gauge and reading is not steady. It's fluctuating between 20"-22" at the idle and even more during acceleration. I can't read when needle is jumping like that. What could be wrong? :confused: Any help would be appreciated.

hloypaulie,
Have a look at this site which explains vacuum readings and what they mean.

https://www.secondchancegarage.com/public/186.cfm

Scroll down to "The Readings" and go through the various scenarios to most closely match what your car is showing.

Cheers , Pete.

I think it could be something to do with valve clearance, but I'm not sure. The valves runs preety quiet though. Car has only 58,000 miles now.

might be a matter of where the vacuum port you hooked up to is located. if the vacuum port is on or very near a runner, this could be normal. but if the vacuum port you used is located on the plenum i wouldn't expect your result.

I hooked up between intake mainfold and MAP sensor(by throttle body), just like my manual says. I could also put between intake manifold and brake booster but I don't want to cut hose.

The vacuum gauge on my '88 Escort fluctuates slowly between 20-21 inches of vacuum at idle. If you've never used a vacuum gauge it takes some getting use to. It is a very sensitive instrument. Very small changes in the the pressure on the accelerator makes large changes in engine vacuum. Try to be lighter on the gas pedal and hold it steady. Get on a flat stretch of road and hold the gas pedal steady and if there's nothing wrong with the engine the reading should steady out. You'll have to practice in order to learn how to keep the engine vacuum high. I think what you're describing may just be from not understanding how much engine vacuum changes with different accelerator pressures. If you've got 20-22 inches of vacuum at idle it sounds like you have tapped into a good vacuum source.

The difference is my needle fluctuates very fast. When I accelerate is fluctuates even more(about 3"-4"). I have tried to be very gentle and steady on gas pedal as much as I could, but it doesn't change anything. The only time my needle doesn't jump is when I decellerate.

Did you take a look at the link in the second post? It may be a valve that's not seating properly, sticking valve, or a burnt valve. I'm not diagnosing the problem just guessing at what it might be. An experienced mechanic could probably give you a good idea what the problem is without having to tear into it.

NO it is reading the suction pulses in the intake manifold as the cylinder draws in air. Motorcycles really show this because of the very short intakes and free air flow. What you need to do is restrict the air flow going into the vacuum line at the end that connects to the intake manifold with a small opening inside the hose or just pinch it off with a clamp so that the air pulses have a chance to smooth out before they reach the gauge.

I have tried rectriction trick today and I'm disappointed, because is no difference. I'm giving up, there is something wrong with my engine and vacuum gauge is useless for me at this point. If anyone is interneting in buying that vacuum gauge from me, let me know please. Asking $15 shipped to your door for vacuum gauge, 3/16" clear hose with restriction at the end, 3/8" T-connector and velcro tape for mounting. Thanks guys for your good ideas.