Lost some vacuum!

BoostedSUV

BoostedSUV

Active member
help me find 5lbs of manifold vacuum!! My truck always idled about 18lbs of vacuum and held it steady... I was driving it home from work the other day and i noticed it was running funnier than normal. Felt kind of laggy just under normal driving, I started checking gauges and noticed I was cruising at 50mph in OD and only had 10lbs of vacuum. If I barely moved the throttle it would zero out. At idle it was was only pulling 12-15lbs of vac. I'm having other issues with my truck (whole other story) but I wasn't even boosting it at all so I don't see what I could have hurt??

I did my own version of the "bike tube" test and did not find any leaks, I pulled the intake manifold off changed the lower intake gaskets even though they looked perfect. I checked the lower and TB gaskets, both were perfect... I used my vacuum tester and checked every line that I disconnected right from the upper plenum. vac ball/hvac/cruise line pumped up to 25lbs and held it. Map/AFPR pumped up to 20lbs and very slowly bled it off, Boost gauge, boost controller, alky controller line pumped up to 20lbs and slowly bled off as well (normal?). The charcoal canister line off the TB would not pump up any vac which i think is normal, but I capped and started the truck with no change.

Truck doesn't idle high and IAC numbers seem to be normal. The truck will randomly idle up and when it does the vac goes to 10lbs... I'm thinking its something mechanical. Any ideas??
 
dgoodhue

dgoodhue

BuSTeD 4.3
Re: Lost some vacuum!

BoostedSUV said:
I did my own version of the "bike tube" test and did not find any leaks, I pulled the intake manifold off changed the lower intake gaskets even though they looked perfect.

Truck doesn't idle high and IAC numbers seem to be normal. The truck will randomly idle up and when it does the vac goes to 10lbs... I'm thinking its something mechanical. Any ideas??
Click to expand...

Did you try the bike tube test again, after you did intake manifold? What you are describing is exactly how my Corvette was with an intake manifold leak.
 
BoostedSUV

BoostedSUV

Active member
Re: Lost some vacuum!

Sy#2661 said:
Did you try the bike tube test again, after you did intake manifold? What you are describing is exactly how my Corvette was with an intake manifold leak.
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I didn't try it again last night after I put everything back together but it would be easy enough to do. I did it one day prior to taking the intake off and everything looked perfect gasket wise. I pumped about 20psi into the motor and I didn't hear any leaks from anywhere. :dunno:
 
dgoodhue

dgoodhue

BuSTeD 4.3
Re: Lost some vacuum!

BoostedSUV said:
I didn't try it again last night after I put everything back together but it would be easy enough to do. I did it one day prior to taking the intake off and everything looked perfect gasket wise. I pumped about 20psi into the motor and I didn't hear any leaks from anywhere. :dunno:
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My Corvette had an improperly machined aftermarket intake and was leaking to the lifter valley. My gaskets look fine on initial inspection but two of the gaskets ports were wet around the bottom of the port. I am just wonder since you have a Vortec Conversion is if perhaps they weren't machined flat to the heads (slightly wrong angle) after they were welded. Perhaps if you have the valve covers off try the bicycle tube test and listen toward the take valley (or if you have an engine stethoscope). I am not sure if you will hear it leak though. The vacuum numbers you are seeing really sound like a vacuum leak to me.
 
BoostedSUV

BoostedSUV

Active member
Re: Lost some vacuum!

Sy#2661 said:
My Corvette had an improperly machined aftermarket intake and was leaking to the lifter valley. My gaskets look fine on initial inspection but two of the gaskets ports were wet around the bottom of the port. I am just wonder since you have a Vortec Conversion is if perhaps they weren't machined flat to the heads (slightly wrong angle) after they were welded. Perhaps if you have the valve covers off try the bicycle tube test and listen toward the take valley (or if you have an engine stethoscope). I am not sure if you will hear it leak though. The vacuum numbers you are seeing really sound like a vacuum leak to me.
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I agree, intake leak was my first suspicion. I've had my racecars suck an intake gasket and you can hear the pitch of the motor change and loose power. That what it feel/sounds like to me but I can't find it. I had JWaller do the heads and intake for me and I don't think the block was decked much to through off angles (have seen that before). I will pull the covers off and bike tube test it again. That's a good idea and will make it easier to hear a leak in the lifter valley if there is one...
 
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BoostedSUV

BoostedSUV

Active member
Re: Lost some vacuum!

Well I pulled the valve covers off to recheck the valve lash and decided to leak test the motor again with the covers off. When I initially pressurized the intake (up to about 15lbs) I had air leaking passed the PCV valve. Shouldn't they close under pressure and seal?

I pulled the PCV valve off and plugged the TB port and tried again. I pumped the intake up to 15psi again and listened. I could hear a very small leak out the TB shaft by the TPS which is normal. I could also hear the intake depressurizing if I put my ears down by the valve coverless heads. I don't know if that's normal, how fast the pressure should bleed off, or where its coming from. I would imagine the pressure would leak down eventually, but how quickly and where would it come from? I would imagine it would leak passed the piston rings... Any tips?
 
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SacDoug

SacDoug

Member
Re: Lost some vacuum!

You may have a loss of power instead of a loss of vacuum. I would not use your vacuum readings under load as an indication of any leak.

If your engine lost power and required the throttle to be open more to produce the same amount of hp as before, then the vacuum would be lower.

I would datalog it and pay attention to your O2 readings. Also check your fuel pressure and ignition timing.
 
BoostedSUV

BoostedSUV

Active member
Re: Lost some vacuum!

I verfied timing and fuel pressure, both are perfect. I am going to pull the plugs and leak down test the motor. I find it hard to believe I hurt something. I barely laid into it, made about 5psi of boost, I pulled my foot and it stalled. I refired it and it hasn't been the same since...
 
Ry368

Ry368

one mile = 5280 feet
Re: Lost some vacuum!

I had a low vac issue when I first did the vortec swap on my old Ty.......turns out that the distributor gear was totally worn out. The teeth on the gear were paper thin almost razor sharp because the distributor sat up just a bit higher then normal.
 
SacDoug

SacDoug

Member
Re: Lost some vacuum!

BoostedSUV said:
They do rattle but I thought they were supposed to act like a one way check valve.
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Not exactly. They are supposed to allow air to flow in both directions depending on the situation. If there is too much vacuum, they are supposed to close. If there is too much crank pressure, they are supposed to close. If there is positive boost, they are supposed to close. Everything in between, they are supposed to be open.
 
BoostedSUV

BoostedSUV

Active member
Re: Lost some vacuum!

SacDoug said:
Not exactly. They are supposed to allow air to flow in both directions depending on the situation. If there is too much vacuum, they are supposed to close. If there is too much crank pressure, they are supposed to close. If there is positive boost, they are supposed to close. Everything in between, they are supposed to be open.
Click to expand...

They do close under "boost" but the air leaks out passed the closed valve. I thought that while under boost they would close and seal boost pressure from going into the motor.
 
Brassmonkey

Brassmonkey

Cruzin' Mel Man
Re: Lost some vacuum!

Have you tried spraying carb cleaner around the intake, TB, and vacuum lines with the motor running to see if your vacuum readings or the engine rpm changes? That is one sure way to tell if you have a vacuum leak. The liquid will temporarily fill the leak.
 
sly dvl

sly dvl

Ya, it's got a Turbo
Re: Lost some vacuum!

There are two PCV's in our trucks. One vent the crankcase under normal conditions (like any N/A engine) and the other works under boost conditions.
It only acts as a check valve in case of a backfire (prevents the pressure from reaching the crankcase).
The crank must always be vented. Too much crank pressure will not close the valve, rather open it to prevent excessive pressure buildup.

If you suspect the PCV's just plug them and run the truck. I doubt this is your problem though.
 
BoostedSUV

BoostedSUV

Active member
Re: Lost some vacuum!

I pulled all of the plugs out and they all looked rich as hell. Most likely because the loss of vacuum = increased FP. 2 plugs were completely fuel soaked. I leaked down the motor and not one cylinder leaked down more than 10 percent stone cold...


On a side note half of the lifters came "un pumped" from sitting. Guess I didn't need to change them after all
 
sly dvl

sly dvl

Ya, it's got a Turbo
Re: Lost some vacuum!

Actually, the rich condition is created not from the increase in f/pressure, but the loss of vacuum. Less vac means more engine load (as far as the MAP & ECM are concerned) so the inj. pulse is increased.

Are you sure your timing is right on? Is the vacuum reading a steady low reading, or is it bouncing?
 
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