How much vacuum is too little?

[Remedy]

I have an 87 cali model with the black boxes and vacuum speghetti.
its an auto tranny and in gear at operating temp... idling about 750 rpm my manifold vac is about 16.5"
If I take it out of gear it idles up aroun 1100 rpm and I have 18.5-19" vac.

I am considering taking the car where they have a smoke machine (I have already gone through the carb cleaner spray routine and fixed what I found) but I was wondering if I have a leak based on those readings or if I should just let that go until I (probably) fail the smog check next year.

My engine has ~116k miles on it and the compression is not so good anymore.... its not out of spec, my lowest is 150 and the highest is 165. I dunno if that plays a part in the manifold vac or not.

what do you all think?

[cygnus x-1]

That's really low, you definitely have a leak somewhere. With a stock engine you should be getting at least 20in. at idle.

C|

[bobafett]

yeah it is low for stock. god what i would give for more than 12 in/hg though lol

[shepherd79]

I have an 87 cali model with the black boxes and vacuum speghetti.
its an auto tranny and in gear at operating temp... idling about 750 rpm my manifold vac is about 16.5"
If I take it out of gear it idles up aroun 1100 rpm and I have 18.5-19" vac.
I am considering taking the car where they have a smoke machine (I have already gone through the carb cleaner spray routine and fixed what I found) but I was wondering if I have a leak based on those readings or if I should just let that go until I (probably) fail the smog check next year.
My engine has ~116k miles on it and the compression is not so good anymore.... its not out of spec, my lowest is 150 and the highest is 165. I dunno if that plays a part in the manifold vac or not.
what do you all think?
low compression may have affect on it, but even with those number you should pull at least 20". start searching for vacuum.


yeah it is low for stock. god what i would give for more than 12 in/hg though lol
two words for you, VACUUM CANISTER.

[2oodoor]

I would make sure you have good oil, maybe change the oil, I prefer Castrol GTX 10w30 or if you do not live in freezing temps everyday, 20w50w

Another thing to check would be the brake booster, it could be leaking vacuum. Major vacuum trees off the manifold including the pvc valve, should be inspected for leaks.

The last resort ,if you have done everything to insure you are not losing vacuum, is to look at valve timing, belt, and valve seats.

[Remedy]

OK thank you,
Like I said I have done all I can with my hand vac tester and a can of carb cleaner. The car didn't run when I got it (for free) because the water pump sheared off inside and of course the HG blew. I was meticulous when aligning the cam to the crank and I have checked the valve lash twice. That doesn't rule it out but it puts the possibility lower on the list, also a leakdown test showed good on all cylinders and the majority of leakage was past the rings. The brake booster is a suspect in my mind too, so I capped the line and its not the problem. I replaced the PCV when I had the IM off for the HG job.
Oh I prefer Castrol GTX as well but for my climate I use 10-40w since it gets below freezing in Winter and above 90 every day of the Summer. I was sold on GTX when I saw this
http://www.xs11.com/stories/mcnoil94.htm
about halfway down the page he mentions Using the GTX in an Accord

Thanks again, I will be looking for a place with a smoke machine

[2oodoor]

Dont know what your specific problem was for the reason you started measuring vacuum.
If its idleing too slow, it could be gunk under the carb where the idle air mix is.
I assume you have carb, if not, the same applys for throttle body cleaning.
You may try advancing the timing a little, and check the egr valve to make sure it is not stuck partially open.
Last but not least, your guage may not be calibrated perfectly, does the needle go precisely to zero? Can you compare with another vac guage?

[Remedy]

Dont know what your specific problem was for the reason you started measuring vacuum.
If its idleing too slow, it could be gunk under the carb where the idle air mix is.
I assume you have carb, if not, the same applys for throttle body cleaning.
You may try advancing the timing a little, and check the egr valve to make sure it is not stuck partially open.
Last but not least, your guage may not be calibrated perfectly, does the needle go precisely to zero? Can you compare with another vac guage?

I have a power problem which is a combination of factors. The result is that I can barely come up a hill at 55. The reason I was checking vacuum was for diagnosis of the trouble. I had a lean idle miss that drove me to rebuild my carb and ended with me adding about 2 additional turns on the idle mix. If I should have 20" and have 16.5" there is probably unmetered air. So the way I understand it I will be lean even on the main jets, maybe on the boosters too.

I actually did advance my timing 2 degrees and that didn't give me any extra. The EGR was replaced with a good used one since the original had a diaphragm leak. I cleaned it real good before installing it. Maybe I will blank it off and see if there is any improvement.

The other problem I found was brakes dragging. The rear are good now, but the fronts seem to want to hug the rotor too much for my taste.

[2oodoor]

strange
Just throwing this out there too, if you have a timing light, check to see if your centrifigal timing advance is working, the weights in the bottom of the dizzy, tend to get froze up and do not allow the pick up plate to move.
Lose some weight out of the car if your carrying around a toolbox, and floorjack..lol.. that will slow you on those hills. About the only things you said anything about checking yet.. :)
The vacuum reading could be poorley seating valves like I said before, but if it aint broke dont fix it!

[Remedy]

Put a rebuilt dizzy in last week, there was oil in there and at one point some red dust. The last straw for that one was the vac advance diaphragm was leaking too.

I forgot to add that when I did the compression test I also did a leakdown test and the noticeable leakage was past the rings, but all the cylinders were in the green on the tester.

I am probably going to take it to a shop next week and hand it off to someone who has the right tools.

[A20A1]

yeah it is low for stock. god what i would give for more than 12 in/hg though lol

You can run on 10" like me :)

[Remedy]

Well Im running on 16.5 for now. Any load like headlights, blower, rear defogger, or putting it in gear drops the idle and robs the vacuum.


If I take it out of gear the vac jumps right up to 19.5. Even if I drop the idle down to 700 rpm (in neutral). I found a bad thermovalve and replaced it, but no change in vac or mpg.



Still getting 23 mpg :(

Thinking my oxygen sensor may be bad.

[smufguy]

okay, lemme get this straight. when you say 16.5" you mean 16.5 inches of Mercury right? well that translates to 8.10 psi. When i was trying to trouble shoot my crazy idle (which i later found out to be the TPS), i hooked up a vac gauge and it pulled a -20psi at idle (full warm up). which is 40.72 inHg @ 0deg C.

Am i missing something here or are we talking two different units here? :confused:

[Remedy]

I guess I messed up my last reply, oh well.

So yes I am talking inHg. I have never measure vacuum in PSI personally since only my compund gauge has both units and they each stop at 0.

Anyway my car is effed in a way that could be ignored except for the piss poor MPG. It doesn't run great but it runs. I think my emission system is meant to drive men insane. Everytime I read the operation and test proceedures I get dizzy, my ears ring, and my nose starts to bleed.

All I am looking for with the vacuum is unmetered air screwing up my mixture and making my car run like poo.

The signs I also cannot ignore are the lousy MPG and the black soot in the tailpipe which sugest rich burning (unless I am mistaken)
I am gonna replace the O2 sensor and see if there is any effect. I don't trust it anymore. The sensor is OEM and likely original (116k mi and 20 years) and has had at least a little coolant burned on it.

[cygnus x-1]

Soot in the tailpipe means it's running rich. And a vacuum leak shouldn't cause it to run rich. A bad O2 sensor is a good possibility. It would also explain the bad fuel mileage. It would also cause a loss of power and low vacuum.
Earlier you said you turned out the idle screws to richen the mixture at idle. What happens if you turn them back in? What happens if you disconnect the O2 sensor?

C|

[Remedy]

If I lean the idle mix (turn in) the vac gauage needle starts to bounce about 1 inHg back and forth and if I continue further I develop an idle miss. Making it richer corrects the miss and mostly takes care of the "bouncing" vac needle (when the cooling fan starts the needle bounces and the idle drops until the fan cycles off). I never tried runnig with the O2 sensor disconnected. What should I expect to see that would tell me if its bad or good?

[Remedy]

Bought a universal O2 sensor for like $25 and put it in. I will have to drive it a bit to see if that has any impact. It did seem like my frequency valves were more active after the install... trying to be positive is hard for a pessimist like me :\

[cygnus x-1]

If the O2 sensor is disconnected the computer goes into open loop mode and it will run rich. If there's no change when it's disconnected then it could be bad.

Anyway it sounds like any load on the engine makes the vacuum drop, and bounce as well. Vacuum bounce would indicate a problem affecting one particular cylinder. You're sure the ignition system is ok? Plugs are all ok and gapped right?

Oh, can you cap off the left and right manifold vacuum ports (the big ones) to see if that helps? If one side was leaking it would create an uneven fuel mixture, making the vacuum bounce.

C|

[Remedy]

OK so I took apart my black boxes and tested each solenoid for continuity. Found 2 bad ones that are vacuum bleeds. Both are normally open so 2 constant vacuum drains. One of them is a bleed from a vac canister too. I ran into a brake issue so I cannot confirm that I solved the problem just yet. I will post back when I can get this back on the road and test it out. In the mean time I am sucking air into my brake lines from my right rear wheel, but not leaking fluid. I need to find a sealant to pin down the leak, its either the bleed screw or the fitting to the wheel cylinder.