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VAC. questions..who knows what would cause this

01maroonz71

01maroonz71

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well, i recently installed some vac. gauges in my truck, and after it's warmed up....at idle i only pull about 13inches of vac. well, my truck has a cam in it, but not sure on how mild/wild it actually is....my good friend(diesel mechanic) said it sounded pretty wild to him, but this truck idles fine and don'est jump around or anything....

what would cause such a low vac????

also, and probably more importantly, when i slowly increase throttle, the vac goes up to about 18inches or so, but when i punch it, it drops off fast, then goes up and finally settles at around 13inches or so....it doesn't fluctuate at all, so i don't think i have a vac. leak, not to mention all vac lines are brand new.

here is how i have everything hooked up...anybody got any ideas????
the timing is set so that it doesn't knock/diesel/runbad at all....

but if i advance or retard the timing at all, the vacc. drops to like 8 or 9inches...even with just 2degrees advance/retard

here's my drawing(not to scale) and no, im not an art major. :laugh:

eaa58b88.jpg
 
I think 13-15 is normal at idle thats what my 350 pulls at idle with a stock cam. It's also normal for the vacuum to drop to 0 when you punch the throttle .
 
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Looks like you may have used the wrong port. Try a take off from the manifold itself or at the base of the carb below the throttle plates. You may have it on a timed vacuum.
Just a thought.
 
by timed , do you mean ported??? so it only draws vac. at accell???? all the ports that i used are manifold vacc. but my buddy, diesel mechanic friend said to hook it up like chevy had it hooked up stock ,and go from there...but i don't see a diff...
basically he said to swap the brake booster and trans lines, and tap into the brake booster line for the vac. gauge. i dunno though.

i thought i read somwhere that at idle i should be pulling like 23inches or something...19-24 aroudn there???? i could be mistaken though.
 
I wouldn't mess with the brake vac line. Your trans line should be on full vac so you could try that. I think I'd get/borrow a good vac gauge (your mech friend should have one) and check true vac incase that gauge reads wrong. My Summit fuel pressure gauge is terrible!
 
awesome man..thanks...yeah, we hooked up to his gauge and it read exactly as mine does...

thanks for the link billy.
 
01maroonz71 said:
awesome man..thanks...yeah, we hooked up to his gauge and it read exactly as mine does...

thanks for the link billy.
Click to expand...

Your welcome hope it helps.
 
could my air/fuel ratio be killing my vac???? maybe if i adjust the screws on the front of the carb (edelbrock 1406) while somebody watches the vac. gauge it will help. hmmmm.....
 
Couldn't hurt to try it out but if that still doesn't increase your vacuum then i would guess you just have a big cam.
 
yeah, that's what i figured....the cam makes alot of low end power i can definately tell you that....not so much on the top end. :) good for what i got i recon'.
 
If the cam is good for low end power it's probably not that hurting your vacuum. Low end cams have a pretty good vacuum. It's the high end stuff that has little idle vacuum.
 
if everything is working fine and you are happy with the way the truck runs then i would say DON'T ADJUST ANYTHING!!!! Just accept that your motor pulls 13" of vacuum
 
low and STEADY??

If the vacuum is steady but low,the timing might be retarted ,try advancing it,see if it increases..if so,test drive it and listen for spark knock..if it does,you'll have to retard it little by little to eliminate the pinging,yet keep the vacuum as high as possible..

Carb adjustment can cause low or eratic vacuum readings also..if the needle wavers around a lot, it could be the cam causing the problem,or poor valve adjustment,stickly lifters if its hydraulic,loose valve guides can cause unsteadly readings too...

Most likely you have a vacuum leak somewhere,like in the PCV valve hose,the PCV valve itself often sticks, or its plastic "T" fitting leaks,and any vacuum operated device like the power booster for the brakes,A/C vacuum cans,etc,can all rob vacuum if they have leaky diaphrams..

The carb base gasket is a common leak source,especially if an adapter was used to put an Edelbrock square bore on a Q-jet intake manifold,or a Holley intake..the adapter get installed with the wrong side facing the manifold often,or just plain doesn't meet up with the mounting surface properly(vacuum can also leak internally ,if the intake manifold is cracked on its underside)..

Jegs and Summit sell a flat plate to correct the problem of the carb not lining up with the intake when using aftermarket intakes....spraying carb cleaner around suspected leaky areas with the engine running finds the leaks in most cases,or use a mechanics stethascope and listen for vaccum suction..If you are running an EGR valve,if it doesn't close fully,you'll have low vacuum too..:crazy:
 
fantastic diesel...thanks so much....im gonig to check my pcv and all the vac t's tomm...all the lines are new, but not the t's....the carb bolted right up to the manifold with no spacers, so that's not it.
i will try to spray a bit of carb cleaner around tomm and see what i can come up with.


what would the low vac. be doing to my truck, it seems to run fine, starts a bit hard sometimes...but other than that ok. would it effect my MPG???? thanks alot ya'll!!!
 
Your low vacuum reading is because of the non stock cam. You can have all the vacuum leaks you want and the vacuum reading won't change. Your engine is nothing more than a big vacuum pump and the cam directly affects how much vacuum the engine makes.
 
Quite possible..

A "lumpy" cam CAN reduce the amount of vacuum in a motor,or make it erratic instead of steady..but 13 inches sounds low to me,I've had motors with pretty hot cams,that still had 16 inches of vacuum..A stock motor usually has between 17 and 21 inches at idle..adding a cam can cause vacuum to drop or become unsteady..

But it won't hurt to check all the possible leak sources,he might have something awry ,like a leaky power brake booster or something..any leaks can reduce performance and mileage..if he's got a carb with a power valve,it might be dumping fuel too soon,that will kill MPG,might need to change it to one that opens at really low vacuum instead..:crazy:
 
Any came change that is made other than stock will start to have an effect on vacuum readings. The hotter the cam the lower the vacuum reading. I ran a cam years ago that would only allow 4 inches of vacuum but it was a nasty idling cam with tons of duration and overlap.
 
As what desiel4me said, where is your PVC valve on your little diagram?
 
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