Old83@pincher

So I had reason to connect a vacuum gauge to the manifold and found this. At idle (700 RPM ish) there was about 19 to 21"Hg on the gauge, that is the needle rapidly moved in that range. When I sped up the engine to about 1500 RPM the vacuum settled and rose to about 22"Hg. Now old school auto (domestic V8 stuff) says to me this is a sign of worn valve guides but the reading is too high (would be under 16"Hg) and there would be other signs such as high oil consumption, fouled plugs, which there isnt. Several months ago I had the intake manifold off and the intake ports and valves didn't have any signs of carbon or oil washing...there are (were) no manifold leaks at that time. Engine has about 215000KM (135000miles)

Now is it reasonable to assume that the intake pulses in a 4cyl are far enough apart (compared to a V8) that it is normal to see a fluctuating vacuum reading at idle?

Old83@pincher

No takers? Not even Wikipedia mechanics want a try?

So...after warming it up a bit a couple trips down the lane and back (its 1/2 mile long or so) the needle still oscillates but in the range of 1 to 1.5 "Hg. I tried the same thing on a Tacoma with 525000KM's on it and it was in the same range of vacuum but oscillates about .5 to .75"Hg.

L5wolvesf

I wish I could offer some info - wiki or other wise - but I got nuthin. I've been waiting to see if someone would offer up something too.
I understand what you said about intake pulses as compared to a V8 but I don't know the answer.
If I had to guess I would say your assumption is reasonable.

Old83@pincher

Well I can add this...I checked a V6 with about 180000KM's on it and the needle at idle was about 21"Hg about was steady within .25 to .5"Hg. Also digging thru some old books I have from my Dad (Fix your Ford, which he had a bunch of clippings in from the Toronto Star from 40-50 years ago and sit down for this...a Rolls-Royce Griffon engine manual; not much info there but it was neat to flip thru!) plus some books from Edelbrock of mine this many be an ignition problem. Since most of mine is either new or rebuilt with the exception of the coil, I'll try a spare that I have and see.

All this is making believe that a vacuum gauge can be a super diagnostic tool...once you know what the readings mean!

L5wolvesf

I just did a quick bit of poking around the inter/web/nets and found a couple things that may help, or just refresh our memories.

https://actron.com/content/engine-pe...g-vacuum-gauge

https://www.motor.com/magazine-summa...vacuum-gauge/#