How to Use a Vacuum Gauge to Diagnose Engine Problems

Video Description

In this video I show you how to use a vacuum gauge to diagnose engine problems. TRANSCRIPT: Today I am going to show you how to use a vacuum gauge to tell what is going on inside of your engine. Now there are a huge amount of things you can learn about what's going on inside of your engine, with one of these. And in this video I am going to go through pretty much all of them. But first I am going to show you how to connect this and how to actually use this. A normal vacuum condition is....and there is a little region here for it. ..occurs between 17 and 21-22 inches of Mercury (inHg), and inHg is a unit of pressure. So, we want it to be in this green region right here. And another thing we can do to confirm the good condition, is we can "blip" the throttle. If you have an electronic throttle you cant do this from outside, you're going to have to have someone do it from inside. But you just blip the throttle real quick, just open it up and let it go back and it should jump down close to zero, then go above to around 25 and then settle back down in this region. So we are going to do that. And then after that, I am going to go through all of the different scenarios you can come accross when you are using one of these and what you can actually diagnose. So when you read vacuum on an engine, you want to pick a line that comes right from the intake manifold, and is not some routed off line. So we are going to take this line right here, in the middle. I already pulled back the little clamp holding it on, so we are going to take that off and that's the line we are going to tap to read vacuum. Just take our angle pliers here, and take that off... That is the line we are going to tap. Now, if you get a good kit, your vacuum gauge will come with a bunch of fittings that you can put on the end of it. They have a T-fitting, this is really easy to tap into a line. You put the gauge right here and then the line just goes on these two fittings. But what I am going to use... I am going to use this fitting right here. So this can just go into my hose here.. like that. There, so now I am just going to stick this in the line. And since it's wedged it is going to create a seal, and I can get vacuum from that. Okay so when we take a vacuum reading.. we are just going to take this and just... stick it into this line... like that. There. Now, we are going to start the engine and look at the gauge. Okay I am going to start the engine. The scale you are looking at is this inside scale, 15, 20, 25... That is the scale you are looking at, so it is going to come this way. (Engine starts) Alright so we are reading 20 inHg. Now remember what I said, that is a healthy motor, but a way to verify that it is healthy is to blip the throttle real quick and it should drop towards zero, then go above towards 25 and then settle back down. (Engine revs) There you go. (Engine revs again) Now notice that it is staying steady. That is a good sign. If you have any sort of restriction in your intake system, then you will read a higher than normal reading. So this is a normal condition, if it is a steady reading above this, it is likely you have a restriction in your intake system. An exhaust obstruction or damaged exhaust pipe will show itself by.. When you start the engine up, it will be at normal vacuum. But if it steadilly starts to decrease and decrease until about.. between 0 and 5, espectially while you are revving the engine.. If you rev a steady engine, and it starts to go down, that is a sign that there is some sort of exhaust obstruction. Sometimes the pipes can damage on the inside. If you have an intake manifold leak, or any type of large vacuum leak, it will show itself by a steady, very low, steady reading. A steady reading between 8 and 14 inHg indicates incorrect valve timing. There is something wrong with the valve timing, maybe the timing chain slipped or something like that. If you have a periodic drop in vacuum as the motor is running, that indicates leaking valves. Sticking valves also show the same periodic drop in vacuum except it will be less regular. If it is regular periodic drop then that's leaking valves. If it is periodic dtop that is irregular then that's sticking valves. A fast vibrating needle indicates worn valve guides. So the number of guides worn is proportional to how fast it is vibrating. On accelerating the engine, you can diagnose weak valve springs. Because the gauge will swing back and forth and the extent of the swing is dependent on high you are revving the engine. So if the needle is steady, and you start to rev the engine and it starts to swing back and forth, then that is a sign you have weak valve springs. Ahead gasket leak can be seen by a regular swing or float between 5 and 19 inHg. Worn piston rings show themselves by a steady low reading, you can confirm this by accelerating the engine to about 4000 RPM.