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Using a vacuum gauge to check an engine.; engine vacuum diagnostics
Topic Started: Apr 27 2010, 04:42 PM (2,432 Views)
crankcase




Well I finally thought the time was right to incorporate vacuum diagnostics into my bag of tricks. :D

I have been avoiding vacuum gauges for some reason I can’t remember. :ermm:

Maybe it was too much subtlety in the differences of readings in relation to the many different things they can tell you? :news

Revisiting the idea, it looks like this type of diagnostics will be a HUGE help, I bought my gauge at the Freight for less than $15.00. :)




There is a really cool website that helps interpret the readings by showing different scenarios (bottom of webpage). ^o)
http://www.secondchancegarage.com/public/186.cfm

My question is how closely does this type of generic representation correspond to what happens in a Metro? :'(

To put it another way, are vacuum readings between 3, 4, 6 and 8 cylinders, straight and V etc., similar enough for interpretations to be generic (given each has a normal idle vacuum)? :hmm

Can anyone recommended any books or websites? :news :type
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Johnny Mullet
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Fear the Mullet

The Haynes manual actual has a pretty decent vacuum gauge diagnostics chart in it.
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smokstac
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The vacuum gauge is the first tool I use when looking at an engine. My Passport with 172k miles on it still pulls a good 20-21in of vacuum at idle and is rock solid at that number. I'm hoping the Swift will read that way once I'm done with it.
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JellyBeanDriver


Please do post #'s. The last time I remember taking vacuum readings on mine was when I was diagnosing a brake booster problem (ended up being a collapsed hose). I was surprised as to how 'not low' the vacuum readings were at the time.
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smokstac
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A severe leak going into the intake will result in a steady but very low reading of around 5in Hg.
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crankcase


Time for an update here:

My Metros pull a vacuum of around 21 (the 1992) or 22 (the 1994), which is good. Also, every one of these cars has some rapid
fluter (hot motor, at idle). From what I've read, this means some valve problems.

My 92 and 94 have both had high idle issues. These have been fixed to a large extent, but my step-daughter's 92 is a problem child.
I've checked all the usual suspects: coolant sensor flow is ok, IAC is blocked off for now, and the TPS has been aligned. Someone
suggested the throttle bushing may be worn, but a vacuum gauge reading of about 21 does not really indicate much of a leak. I suppose
I can see what a shot of propane in that area does. I had to rig the idle RPMs on the 1992 to pass inspection, and it passed with flying
colors, but it's not as it should be.

My 94 does show about 22 on the vacuum gauge (and the idle RPMs are good) - a little better than the 1992 - but it's kind of hard to tell
exactly since it flutters pretty good.

I have aligned the 92's TPS by my 93 factory manual. It says to adjust the TPS so you just get continuity between pins 1 and 2 with the
thinest feeler gauge in the throttle stop. To test the sensor, the 93 book says a resistance of 0 to 5k between pins 1 and 2 is a
good reading.

I just got a 1992 factory book, in the TPS alignment it says to adjust for 0 ohms, not just continuity as in the 93 book. It only goes
down to 33 ohms, which is good continuity, but is not 0 ohms (readings with the thin feeler gauge). To test the sensor, the 92 book
says that a good reading between pins 1 and 2 is 0 ohms.

I would try to redo the tests with voltage readings, but I don't have anything to compare to.

I tried spraying it down with cleaner, but no luck. The 92 and 93 cars seem so close I can't help but wonder if this may be an oversight
in the book? I think the TPS part numbers are also the same for the 92 and 93? The vacuum gauge does not show any signs of a major
vacuum leak, and the only thing that seems funky are the TPS readings. I'm not real sure where to turn next.

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mwebb
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FOG

base ignition timing is .....
should be 8 degrees BTDC no more ...

closed throttle switch is zero volts closed and 5 or 12 volts
not closed
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