Can someone please help me interpret a vacuum gauge reading? I am a newbie to this gauge.

(And thanks, Brickboarders, in helping me try to fix my "great body, but moody personality" 1992 245. If it weren't for the Brickboard I would have had it to a shop by now. ;-) But I am still looking for a solution.
Earlier thread was http://www.brickboard.com/RWD/volvo/1396184/220/240/260/280/even_third_amm_better.html)

Problem, with a 1992 245, LH 2.4:
(1) stumbling idle and lack of power for 1/2 minute after warm restart.
(2) poor mpg, some soot in tailpipe (it IS winter...); and general performance on the lackluster side.

Long story short:

1. my newly-created fuel pressure gauge showed
--> falling (and then rapidly rising) pressure on restart.

(I scratched my head. Especially since the fuel pump is new, and fuel filter, and FPR, etc. etc.)

2. I added a vacuum gauge (since vacuum determines fuel pressure, being mediated by the fuel pressure regulator)


AND NOW: Can anyone please explain what I may be seeing, when, on a typical warm restart (10 minutes to a couple of hours):

1. The car starts and idles OK for several seconds
Vacuum = about 15 inches.
Fuel pres. = about 40 psi

2. Vacuum slowly (over 10-20 seconds) declines to about 10 inches.
Fuel pressure rises to about 44 psi.
Idles for crap (lumpy), and very poor power.

3. Then, fairly quickly (over a few seconds), it rebounds,
vacuum goes to about 16 inches,
and fuel pressure to about 36 psi

4. And immediately settles into steady, typical rough, but decent idle,
with readings as in no. 2 above.


This does not seem to be related to idle speed, I mean, speed fluctuations causing vac/fuel fluctuations.

(I keep thinking some gasket is somehow making up over time... or some electrical connector is overcoming resistance... or ECU is... (dunno; I swapped the ECU anyway)

Thank you!

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Parenthetically: for me it was an insight to explore these relationships...

Throttle: WOT......................closed (idle)
Vacuum: near-0 vacuum............high-teens?
Fuel p.: higher...................lower

Insight: Fuel is injected into an area with vacuum. In a high-vacuum condition (e.g. idle), the fuel requires less "oomph" behind it, since it's being sucked strongly into the intake manifold. Conversely, in a low-vacuum condition (near-ambient; e..g. wide open throttle), the fuel requires more oomph/more pressure.
All is mediated by the fuel pressure regulator, and its vacuum line. (I KNEW there was a reason for the line!)

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(Bases covered section:)

YES, I have continued to test, kneeling in reverence in the snow, before the ECU shrine...

No OBD codes set.

ECT sensor: reads within range (when cold, and warm) at ECU connector
(Sensor was replaced recently)

Fuel injectors:
1. deliver gas consistently between them (sampled into glass jars, though not a cute as Art Benstein's)
2. do NOT leak after shut down (no dribbles into gas jars)
(Aged fuel injectors were replaced recently, FWIW, at CruzinPerformance)

Fuel check valve: fuel pressure holds well after shut down (ca. 40 lbs., for hour+)
(Check valve was replaced recently, FWIW)

Can't ID intake leaks, using (1) Tube to ear, and also (2) Spray carb cleaner... around intake gasket area. (Gasket was replaced recently, FWIW.)

Idle control valve:
1. OBD cycle using mode 2? 3?: IAC cycles OK
2. Proper values read at ECU connector
(I tidied it up, again. I do not yet have a spare to swap.)

Throttle posn checks good using OBD diagnostics.

Oxygen sensor voltage flits in appropriate range across digital meter. (I do not have a scope.)

Spark plugs look OK, and are consistent between them.

Exhaust SEEMS ok... no change when I open the cat's wee port plug and drive the car.
But it could be constipated, I suppose.

etc.? ;-)