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.? ;-)

Just a data point: fully warmed-up idle on my 83 (B23F, LH-2) shows a fairly steady 19 inches vacuum. Daughter's 940 was about the same. There's a little bit of needle jitter - not uncommon on 4-cyls.

Try putting a timing light on it when it's showing the bad idling condition, to check that wonkiness in the ign system is not a factor.

Thanks very much for your advice!

Yep, I tried a timing light yesterday, but with some combination of aging eyes and lack of white paint added where I need it on the harmonic balancer (to-do list!) --won't jump to conclusion that the balancer has spun a bit, but will remember the possibility -- the job is not done. Another taking longer than it "should." ;-)

You are to be commended for the battery of tests you have done.
What seems out of whack is the fuel pressure at 44, isn't that too high?, and the vacuum at 10, that is too low.
It sort of sounds like you have a vacuum leak, but I'd think the 44psi would cause a rich condition, maybe they balance each other out?

44psi just sounds high to me, it shouldn't be there except at WOT, near 0 vacuum.
--
84 242Ti IPD bars&springs, 89 745 16v M46 IPD bars, 89 744 16v M46 IPD bars, 93 945 Turbo AW71, 91 245SE AW70 IPD bars, 93 245 CLassic M47

Thanks, I hear you.

Yes, time to look for something letting in air: less intake vacuum, more fuel pressure.

1. I think I will hook up a good ShopVac to the intake, and pressurize it, and take suds to the whole area... valve cover, dipstick... blech!

2. And I keep thinking the idle air control might be acting up, despite it sounds ok, and delivers numbers to the OBD diagnostics OK. And delivering lotsa air? Just conjecture.

I have lots of older LH 2.2 stuff to swap, but not so much LH 2.4.

I DO seem to have something that those in the know call a dwell meter, and it's TIME to learn to use it! :-) To determine what the IAC is doing in practice.

This might help answer your questions.

http://findarticles.com/p/articles/mi_qa3828/is_200108/ai_n8978956/

Dan

Thanks for the link, Dan.

I had seen another half dozen web pages, which were helpful, but I think the one you provided was a bit more thoughtful, and less "cookbook."

Most such aids describe and interpret:
1. steady-state conditions
2. steadily oscillating conditions, such as a valve dinking up the reading periodically
3. response to user input, such as blipping the throttle

I would also like to see more discussion of transient events, or excursions... trending reading... whatch-callem. Things that take tens of seconds to evolve... or minutes. That's asking a lot. ;-)

Hi Gregg,

While I certainly don't have the answer, I'd say you win the award, hands down, for the very finest in questions.

Your assertions about the FPR's affect on fuel pressure are right on, and when you think about it, absolutely necessary to keep constant the pressure difference between the fuel in the rail and the intake port, so the volume of fuel per unit time is always a known quantity.

I wish I'd kept using a vacuum gauge after leaving carburetors behind (and my gauge) so long ago. My impression is your closed throttle vacuum is low.

That may be for a reason other than a leak, or I should say, it may be low because the idle valve is open wider than it ought to be. Or it might be normal on a 240. I really need to start using a gauge.

Do you have a dwell meter? If you could get it connected across the idle valve, you might see the idle compensation tracking with the vacuum reading, and then at least know it is the ECU that's calling for more air, and not a leak or valve train trouble.

Speaking of valve train, do you have a compression tester? Pardon me if I missed this first go-around.

How different is the vacuum gauge behavior on cold start from warm start? Do you ever see 20+?
--
Art Benstein near Baltimore

Most would rather be ruined by praise than saved by criticism.

Art, you are SO far ahead of me, I am catching up... but never will. ;-)

I FOUND Probst's "Bosch Fuel Injection..." last night and started reading.

Dwell meter! Yes, I suspect the IAC valve, and don't know how to test better. (Cannot swap, since I don't have a spare.) And a dwell meter lets me do a non-invasive EKG on the dealie! Says Probst. Says Art Benstein near Baltimore, earlier!

Yes, I did a compression test. But it was months ago. So may be time for another? It was, if I remember right, a fairly consistent 130 or so.

"How different is the vacuum gauge behavior on cold start from warm start?" Good question. There is similar unimpressive vacuum, BUT NOT the excursion down to e.g. 10 inches vacuum, and the inevitable rebound, to 15 or so.

"Do you ever see 20+?" Um, well, no... not even on closed-throttle deceleration down a hill. 18.

I feel apologetic for all the conjecture. Er, "hypothesis formation."
Just looking for pragmatic, easy, effective ways to test *appropriate* areas.

I think I will:

1. ...having gone to fetch all appropriate clamps I have, and having a small supply of corks: plug and play! Anything in sight gets plugged or clamped.

2. Push a potato up the tailpipe, a ShopVac on the intake, blow, and spritz soapy solution everywhere on head (and above) and to the driver's side. whoopee.

3. Sorry, it's so late for me, and brain a mush. Something about the sequence
(1) Start, and start enrichment
(2) Post-start period (up to 15-20 seconds), its enrichment
(3) Warm-up period, its tapering enrichment.
Ponder the replicability of this problem, and its match to the post-start period: what's the ECU doing that matches so poorly to an underlying mechanical problem? And fix the problem. :-)

Thanks again for your help and encouragement!

Hi Gregg,

Probst is on my shelf, and I am pretty certain I've read the whole thing in bits and pieces, but I honestly didn't recall Charles O. suggesting the use of a dwell meter on the idle valve, but why not? A dwell meter is a tool old(er) guys like me sometimes have around from the 60's, and the new(er) cars control an increasing amount of mechanical muscle by switching their electricity on and off at varying proportion. The idle valve, the injectors, the frequency valve in k-jets -- all good candidates for a device measuring the ratio of on-to-off time, where a modern voltmeter just flashes meaningless digits.

The EKG analogy breaks down if we are trying to determine the health of the idle valve with the dwell meter, because the dwell won't be (meaningfully) affected by a malfunctioning muscle (idle valve). With the dwell meter, we're just trying to learn if the brain is responsible for making your motor take an extra deep breath just as it falters. If the idle valve can screw up that way, it won't show up on the dwell meter, and I can't conjecture why it would only do that on warm re-starts.

Something is suddenly admitting air, and you know it isn't your foot on the throttle, you'd think it isn't the idle valve because the AMM would feel it and match it with fuel (just making the motor race). I tend to discount (a) sticky valve because you would have seen the increased jitter on the vacuum gauge. I think...

So putting the dwell meter on the idle valve is akin to putting the timing light on the spark -- you want to see if one or the other computer is involved in the crazy re-warm-up curve. Then you have to figure out if its involvement is a cause or reaction.

I admire your perseverance. If Mrs. B's 245 was acting like that today, we would likely have a six-week period where it was just worked around, and not worked on. In Mpls six weeks would be optimistic.
--
Art Benstein near Baltimore

People who live on that "New Car Smell" and have their Credit Score memorized better than their own birthday will ALWAYS say you're Crazy. -punk240 Tim

Ah, Art, I am so far behind on this car. I never saw the movie with Robert Redford, well, I don't think it did, "The Candidate." Looks good but acts otherwise?

I think I would have been more severe on this car (more systematic in looking for errors) if it had looked bad or neglected.

Finally, today, for instance, I checked the timing belt position.
Off by one.
Don't know if that's always been so, or part of the recent episode.

We'll take care of THAT... soon, I hope.

Thanks again for your help... and I will get back to the dwell concept!

Timing belt off one? At the cam? Check the i-shaft and distributor too. I've had it happen to me twice now, that someone has adjusted the distributor to make up for a one-tooth error on the i-shaft, so that when I replaced the belt correctly, the distributor needed adjusting. Once, that someone was yours truly.

In your car, the ignition timing isn't directly affected by the distributor position, and its bolt/slot adjustment is locked in place by a plastic insert. But if the PO had the distributor up and out, it might be set back in one tooth off, and the result, combined with the error on the i-shaft cog, could cause the spark to need a hurdle through a lot of thin air at the extremes of spark advance (or not advance) being set by the EZK spark computer. This might fool the knock sensor, which must hear all the true bangs in addition to listening for the early (preignition) explosions.

Once you get your white-out line on the crank pulley (what a great use for an obsolete product!) you and your timing light can watch the action. When I used my light on the coil wire, a second mark, exactly opposite the notch on the pulley, helped me see the advance more quickly and for all cylinders. I painted it on the opposite sheave, so the alternating pattern would highlight an interruption or missing spark, but I was troubleshooting a crank position sensor problem.



--
Art Benstein near Baltimore

"Mantle can hit just as good right-handed as he can left-handed. He's just naturally amphibious."