Date: Thu, 5 Dec 1996 21:49:41 -0800 From: Mike West Subject: Vacuum Gauge Diagnostics What a can of worms this subject turned out to be. diagnosing a sick pig using a vacuum gauge. Someone, named Dave asked if I'd do one, he'd heard it was the Ultimundo of techniques. Not! I suddenly remembered telling you to get a vacuum gauge for the fuel injection stuff and then I got to wiring in radio parts in the FI and was having so much fun I forgot. The reason I said you needed one is logical enough, there's a vacuum "influenced" pressure regulator in the FI. And then there's the vacuum operated or "influenced" distributor, and every once in a while you run across this note that says "check the vacuum". You check the voltage before the components, right? Ought to be that way with vacuum. I've got this old "Clymers" that we're going to peel some data out of and it says a "fresh" engine is pulling 18-22 in hg.(inches of mercury) At idle rpm. I wanted to double check and went looking. VW will not say. They will tell you anything but. Since they are working around it, we'll just work around the other way. There are disclaimers in that book. "Vacuum gauge data is the easiest to read but the hardest to interpret". What they are telling us there is that there is more than one way on an engine to affect the vacuum reading. So you will have to use this data as a secondary clue. Well let's see what it turns out like. You plug this into the line with a tee on some hose from the intake manifold, as in the hose to your vacuum dizzy, or one of the lower ports on the carburetor. Ok, 18-22"hg on a stock engine at idle. I seem to recall a reading on my bug of 18"hg, just a stock 1600 SP. The book says as low as 15 with a high lift cam and large overlap. These will be steady readings, not big flutters. The third and still a steady reading is late timing. 14"-17"hg that's with normal cam. At idle on these. Here's where it gets hard. You have to put yourself in the engine. "Shades of John Muir" !! "You must be one with the engine and it will repair itself, Grasshopper". Visualize, the piston comes over the top and has already started down on the power stroke before the spark goes off. just milli- seconds off the mark. Incomplete burn, probably increased CO and HC. Wow ! I must have read my own stuff! advance the timing just a hair and see if it affects it. "Ok, give me another". The next one is still a steady reading at idle, 8"-15"hg The cause here is late valve timing. Which mean the piston is there but the valves weren't opening yet. Two answers on this one, 1, your valves need set. 2, your cam is slightly off and there's nothing you can do till re-build. If I was getting the 8"hg reading, I might consider pulling it out soon. By the way, I do know there are some vac distributors that need up to 13"hg but that's not at idle. How else could I check that? Damned if I know. It would be a low power and I think hot engine condition on a new re-build maybe. After everything else was checked out. So there are four ways you could have the same vacuum reading. If this was to continue, I'd just throw up my hands and quit. Still with little thought, you can use that data. The next one is a steady but very low reading, shows a gauge with 4"hg and is an intake manifold leak. That's an easy as that's where the vacuum is made. It could be a small leak and on an engine that used to pull 18"hg is now pulling 15". So now I have 5 ways to get the same reading at idle. Still, with back-firing or high CO, you have another indication. Here's another NORMAL reading: The vacuum drops to 2"hg, then rises to 25"hg when the accelerator is rapidly depressed and then released. For worn rings or diluted oil, the above will drop to 0"hg and rise to 22"hg. The next condition is Sticking valve: it will have a normally steady reading but will flick downward about 4"hg intermittently. A regular drop of about 2" back and forth is a leaking valve. A burned or warped valve is a regular down flicking of about 4"hg. Worn valve guides is an oscillation from the steady reading of 2" above to 2"hg below. Weak valve springs: violent oscillation of about 10"hg as rpm increases. Often steady at idle. So I think we're getting some good out of this. :-) Now for vacuum "float" which is a slow swing of the reading. Improper Idle mixture: floats between 13"-17"hg. Small spark gap or defective points: slight float between 14-16"hg. That would be a tough call but it would give you a place to look. A head leak: it says it floats between 5"-19"hg. Restricted exhaust system: Normal when started but drops to 0" as the rpm increases. may eventually rise to about 16"hg. About all I can say on this subject is it gives you some new clues and a way to use that new vacuum gauge. I would go get some readings while my engine was new and well and record them. Then it would be of some value. On a strange engine of unknown starting point it would have less value except in cases of valves or springs. Hope it's of some value to us all. west didn't get much light out of this. anyone has additional data, let me know and we'll take another look at this.