A little while back, a member posted looking for assistance diagnosing some engine issues.
It struck me that inlet manifold vacuum diagnosis could have been of some assistance, and that it would probably be useful to make a diagnostic chart available here. The best I know of comes from a book called Automotive Service Technology by Clifford Tempest, this is comprehensive, and pretty much self explanatory, with great drawings. Its an Australian publication too.
I decided to request permission from the publisher before posting, and I couldn't scan it properly so I had to rebuild some of the images in a graphics suite, so it took a little while to post this, but it is faithfully reproduced below, with permission for presentation on this site.
Now, to make this useful, the user needs to know what it’s on about! For those unfamiliar with the technique, it’s a pretty standard test, however, was probably used more before engines became computerised. Prior to computers, a vacuum gauge test was a common workshop diagnostic method. Ideally you want a vacuum gauge designed for automotive or similar use. It could however be any sturdy working vacuum gauge that measures in Hg inches to about 30.
Below is a pic of a quite nice one I picked up on the www fairly recently for around $55 brand new. It's worth noting this pic, as it shows some gauges read backwards from the one on the diagrams (zero on the gauge pictured is at the opposite end of the scale than the diagrams) this is because it is reading vacuum as a minus pressure rather than measuring the vacuum as difference from a standard atmosphere expressed as a positive. This doesn't matter, its the numbers that count, and where the needle sits within them, just remember to look at the numbers (and read it the opposite way if you end up with a vacuum gauge that has numbers going in the opposite of the diagrams.)
Now you just need to connect it up, with a good solid airtight fit directly to the inlet manifold.... How? Well, if you are lucky you may have an unused fitting with plug (often brass) already on the manifold somewhere, that would be from where a previous service person has tapped the manifold for exactly this purpose somewhere back in the day, most often though, you wont, so, using the brake vacuum hose is a pretty obvious way to attach the gauge, and that will work fine providing your hose fits the thread/connector on your gauge neatly, otherwise, you'll need to devise an adaptor out of threaded plumbing/air compressor fittings etc, make it tight, and use hose clamps unless you can get a threaded fitting. This is why an automotive vac gauge is recommended, its often easier to connect than a vac gauge that was designed for a different purpose, the size is usually a pretty easy fit.
Technically however, for an absolutely clinical test it is better to have a purpose built tap and plug straight out of the manifold, this way you can test with the brake vac still in the system, but its not usually necessary, the brake vac hose is fine unless the brake vac is leaking, (which is unusual on H series, but if you suspect it, and are prepared to lose the vac hose for the test, you could put a T piece in it to take the vac gauge). But I would think it’s generally easier to go straight off the end of the existing brake vac hose and work it out from there. Everything else that attaches to the manifold needs to be in place, and sealed up normally; pollution control, pcv, vac advance tubes, etc (sometimes you'll fix the prob just doing that).
Then start her up and follow the chart.
(A cable tie or two is sometimes useful to stop your nice new vac gauge shaking off the block and hitting the tarmac/fan blades/whatever if you haven’t got someone on hand to start the motor while your holding the gauge ... ya know, !)
Further info on this is easy to google and there’s a fair few sites out there that give similar information, some perhaps not as comprehensively as the chart above. Interestingly, when I had a look, I found that Autospeed magazine has an article using diagrams from the same chart as above as a reference and have laid it out neatly but differently from the original. I would have just posted a link, but they didn't reproduce the entire chart in full, which I thought was worth doing. They do however add some extra diagnosis points, which may be worth having a look at.
http://www.autospeed.com.au/cms/A_111456/article.html
The chart presented at the top of this thread however is an accurate and complete reproduction of the original work.
(Kind acknowledgements to McGraw Hill Australia).
Anyway, maybe this can help out with engine diagnosis where other techniques have drawn a blank.
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