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Hey,
I checked Chilton's and Bentley, no love there.
I am wondering how much vacuum the car should have on the engine side of the intake manifold when at idle. It seems like knowing this would be a good way to figure out if there's a vacuum problem. My car seems to pull ~5 psi, and is running well. But I have no idea if that's optimal.
Does anybody know?
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Lot said not a whole lot of relavant information, but enough to go on.
Things that can affect engine idle vacuum is , aside from leaks, engine wear, oil type and age, cam type, valve overlap, ignition timing, and to fair degree, the valve lash and adjustment tolerances.
With fresh oil, this is my experience:
M cam -18-19 in Hg
A/B/D cam 13-15 in Hg
VX/VX3 15-16 in Hg
T cam 15-17 in Hg
The valve adjustment if set at +/- 0.002 thou, (0.004" total TIR) at acceptable spec clearance can give 1-2 in Hg less idle vacuum than a total of 0.002" difference across the board.
On the older K-Jet engines it is a good idea to keep close track of engine vacuum, as it's a good measure of the health and performance of the motor and FI. To a lesser degree for the later LH motors.
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All vacuum gauge faces are graduated in inches of mercury. If you have a PSI scale on your gauge it is probably to determine fuel pump pressure.
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My 83 (B23F) and daughter's 940 (B230F) both pull about 18-19 in Hg at hot idle in neutral. That's a bit more than negative 9psi (ie: 9psi below atmospheric).
Cheap vacuum gauges are often quite inaccurate. You might try to borrow a good one from an autoparts shop for a few minutes and check readings right in their parking lot?
An engine in good shape and good tune should show better vacuum than you report.
--
Bob: son's 81-GL, dtr's '94-940, my 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.
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I have a good gauge. What is the correct figure an engine in good shape and good tune should exhibit? 9 psi at idle is higher than I'm getting, but might it need to be 12? 15?
Anecdotal numbers aren't going to suffice...
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I'm wondering about your gauge. I have never seen a vacuum gauge calibrated in psi.
I've seen compound gauges calibrated in psi on the positive side but most are calibrated
in inches or millimeters of mercury on the vacuum side.
Normal atmospheric pressure is about 29.9" or 760mm of mercury but vacuum starts
at zero for atmospheric pressure and most gauges go from zero to 30" or 760mm.
Usually about 20" of vacuum is normal for idling. That would be about 5psi absolute.
(absolute pressure considers atmospheric pressure to be 14.7 psi or so)
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I have a good gauge.
Serious questions arise when you report vacuum in pounds per square inch. Bob is trying to save you from a wild goose chase started with a faulty gauge.
Anecdotal numbers aren't going to suffice...
Well, that is all you're gonna get on the 200 forum, and you should be quite happy at that. Vacuum gauges as test equipment have been moldering in bottom drawers along with dwell/tach meters and ignition files.
Some of us found this Second Chance Garage article interesting enough to dredge up the old gauge, but I've yet to see a post making good use of it in a repair situation. Maybe you can do one.
--
Art Benstein near Baltimore
Give a man a fish and he will eat for a day. Teach him how to fish, and he will sit in a boat and drink beer all day.
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Art,
My 245 would not be on the road if not for a mechanic who took my comment about funny vac gauge behavior seriously. He took one look at it and knew the valve adjustment was off (thanks to the guy who refurbed the head; at the time I had not yet learned how to adjust the valves, so had not checked behind him). Neither fellow brickboarders nor my former mechanic took that observation seriously at all.
John
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John,
I remember that! The engine with the orgy inside! I missed the solution, probably because you put it in a new thread?
The toughest problems to solve over the internet (your fellow brickboarders comment) are those caused by the owner (or his machine shop) because we would have had to make (or almost make) the very same mistakes in our experience to know they could be made.
--
Art Benstein near Baltimore
If I have any beliefs about immortality, it is that certain dogs I have known will go to heaven, and very, very few persons. -James Thurber
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Yes, I totally missed that one. There's a good example of using a vacuum gauge to your advantage, and a reminder that few of us have retained the skill to read one if we ever really had it. And for those of us with turbos, I know I should be watching the gauge when it isn't in boost, but I rarely give it a glance.
Anyhow, for another piece of "soft" data, here's the wife's 245 undergoing suction diagnostics for reference. The needle is fairly steady at 18, dropping about 1/2" whenever the oxygen sensor reports rich. The variations correspond to the sound I'm used to hearing from all these 4-bangers as the ECU makes mixture correction.
--
Art Benstein near Baltimore
My dog is worried about the economy because Alpo is up to $3.00 a can.
That's almost $21.00 in dog money. -Joe Weinstein
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This is very useful. Thank you.
I think my car must have a leak somewhere although it's running fine.
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yeah, that's the dark side of adaptive injection systems. They'll do a beautiful job of compensating for all sorts of problems that would be obvious on a more primitive system.
john
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posted by
someone claiming to be austi012
on
Tue Sep 28 04:29 CST 2010 [ RELATED]
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Thanks for the input. I am using a MityVac to measure the pressure.
Not really the same thing as a Dwell meter at all, since dwell meters are for calibrating obsoleted technology.
I'm not repairing anything. I'm trying to see if there is a hidden error condition. The car runs great, there are no error codes, I'm just being proactive.
If nobody knows the true answer, that's fine. I'm not being insulting by expressing a preference for hard data. I'm just expressing a preference for hard data.
Thanks.
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"True answers" and "hard data" will be especially hard to come by with a low-resolution tool like a vacuum gauge, I think.
It has, what?, a sub-2" face? What accuracy can you derive from that? These little automotive gauges are intrinsically analog, rough, relative. Do you really want to go shopping for a more accurate gauge, say with a 4" dial? And if you did, what would its precision MEAN?
A vacuum gauge is not, for instance, a micrometer, or even a less accurate dial caliper.
This airy-fairy tool's strength seems to be RELATIVE (Is this against your grain?) across TIME:
1. In same vehicle, over its life to note long-term trends.
2. An any vehicle, over minutes or seconds of operation, to note fluctuations.
Not like I am a doctor or anything, but the vacuum gauge seems more like a stethoscope than an MRI: innovative in its time, and still meaningful and useful, but relying strongly on interpretation, rather than precision.
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Do you ever get your blood pressure taken?
Is it useful to know what range that pressure should be?
Is a blood pressure cuff a useful diagnostic instrument or does the fact it's an analog device make it no good?
I think, based on the tone of your reply, you should maybe get yours checked.
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You are right, I am overdue for a physical.
Let's both get BP checked, eh? ;-)
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posted by
someone claiming to be austi012
on
Fri Oct 1 07:01 CST 2010 [ RELATED]
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I'll get my eyes and cranium checked while I'm at it. All this while I've been babbling about PSI and those aren't the untis the gauges are callibrated in at all.
FAIL
LOL
I'm as bad as NASA. At least I caught it before I bounced my 245 off the surface of Mars....
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I don't know what Mity Vac model you're using, but both my old and new one state units at [in./Hg]
--
'92 244 w/ M47 (Hydra, ipd bars and springs, bilstein, urethane bushings)
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"If nobody knows the true answer, that's fine" You probably received as accurate an answer as you're likely to get - from the first person to provide a response--Volvodad reported 18-19 inches with his car. Hooking up a vacuum gauge to any healthy 4 cylinder you are going to see something in the 16-19 inch range at idle and 25-28 under closed throttle deceleration. While dialing in/tuning an offbeat carburetor combo on a B18 I found having two vacuum gauges (one ported, one manifold) mounted on the dash to be invaluable tools. -- Dave
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That seems low. I'm usually about 15-18 at idle. Sure you don't have a vacuum leak?
John
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No, I'm not sure. The car's running great but that doesn't mean there isn't some degradation from a hidden problem. That's why this would be a cool way of diagnosing an issue. :-)
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oops, we're using different units. You're taking readings in psi, I'm remembering readings from a gauge that's in/Hg. My readings translate to about 7-9 psi.
john
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Just a small point regarding the mityvac, mine reads psi from "0" up for pressure and In.Hg for vacuum. So if your "5 psi" is really "5 In.Hg" you are a bit low though for that little gauge it would be hard to deduce much.
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