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What should a vaccum pump run for vac?

GM Guy

GM Guy

Manual Trans. 2WD Enthusiast
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NW Kansas and SC Idaho
Hey all,

I used the search function, and all I found was someones personal reading, no posts telling what the vac level SHOULD be. the one member was getting like 25 inches.

So, I ask you folks, what should the vac level be? This is measured in inches, as in inches on the water column, correct? I got the proper Kent Moore guage, so I am sure that is what it reads. will an idle reading be good enough, or do I need to take it to a certain RPM?

also, is this reading supposed to be the same from 94-00, 1/2 ton to 1.5 ton, or is there differences with EGR equipped trucks? The truck in question that i am working on is the 1995 GMC C2500HD.


also, where should the reading be taken? at the pump I assume? If the pump checks out, I assume the reading should be the same at the solenoid and the turbo?

I will likely have to buy a new set of vac lines, but otherwise I think the rest of the system checks out, just gotta make sure.

It will hold boost if you get into it before the hill, but once on the hill it generally wont clamp shut and make boost, that is why I am checking it out in the first place.

thanks!
 
Good protocol. Last one I checked was on an M1009 6.2 no o.e. EGR. 25 inches, idle, WOT, didn't matter. Measured right off the pump. Had max vacuum in 4-5 pulses.

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V__9208.jpg
 
Yeah, at the pump it should nail 25 in H2O and hold steady. At the turbo or anywhere after the vac solenoid will be less can't remember value but not 25 in and vary according to program. iirc at idle its low then will increase with a bit of throttle and then at around cruise rpm 1700-2000 rpm (along with lite throttle) it again lowers vac some at the turbo.
 
thanks for the info guys, will make sure it will pull 25 inches directly at the pump at idle.

also, if it doesnt pull that much, and doesnt gain much with RPM, its wore out then? will it ever get much more than 25 inches, even healthy?
 
The disc shaped object is just a rubber-ish diaphragm inside. The drive shaft eccentric just pushes it in out in out in out in out... and in out in out. :)

'Spose one could rupture and limit it's capacity.. I think 25 is the top of the mark.
 
yep, failed with flying colors. :)

it turns over with resistance, but wont pull even an inch of vaccum.

luckily the 100 dollar parts engine from nebraska with a few holes punched in the pan (It will be an exciting teardown!) had a good one that seemed to have plenty of pull, wont know for sure how much till I get it put on though.

does anyone know the part number splits on the vaccum harness? (the two plastic "spaghetti" lies in the black split loom) I just got a discontinued one ordered for 18 bucks via the local dealer, and the GM computer claims it fits 1992-1996.

all my 94 and 95 units seem to run a big diameter long leader line on the vac pump inlet, then a reducer, then it connects to the plastic lines. the late parts engine mentioned above is a 97 or 98 (IDK when the switched to the newest oil cap, defintely has the newest acessory bracket, but still 4 bolt water pump) and it has a long spaghetti line that goes straight to a reducer on the pump.

so, with the change in the wastegate location with GM8, and the slightly relocated pump on 96 plus acessories, I would say the harness I just ordered actually only properly (like factory) fits 94 and 95 6.5L?
 
just a small technical correction...this vacuum is measured in inches hg, not water. A "perfect" vacuum is 29.92 "hg which would equate to over 30 feet of water column. You use "h20 for smaller measurements of vacuum.
 
good to know. so what exactly does hg stand for?

the pump off the newe motor didnt have threaded holes, so one from the project burb was used. it had been changed once, casting code was 1997 on it. It pulled over 20 rolling over by hand, so it should be good. I re-used the vac hose, and it was crackly, but no holes or tears, so it hopefully should be fine.

got the new plastic lines today, cost 13 bucks, genuine GM. :)
 
Hg is the chemical suffix for mercury.most vacuum measurements are figured in inches of mercury, or Hg,whereas pressure is figured in pounds per square inch,foot,or liter,ETC...





Mercury, usually as mmHg, millimeters of mercury used in a barometer to measure atmospheric pressure.
 
actually, Hg is not a chemical suffix... it's an Elemental Symbol... Au is Gold, O is oxygen etc... everything else looks good on the above explanation.
 
I used a basic Actron vacuum gauge lika deez: (the 25 figure I arrived at was the outer 'in. Hg.' dial)

bicijyge.jpg
 
ok, got it back together (that part anyways, may pull radiator) I used the big feeder line from a 1994 project pickup, and its a little crackly, but no splits or holes. can old vac hoses leak without holes, but rather through the body of the hose through microscopic holes from aging?

I used the new plastic lines though.

it pulls 22 inches at idle at the leader hose, and 21 inches at the wastegate connector.

Do I need to tear it back off and get a good leader hose?
 
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