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Vacuum Increases With Throttle?

lobo411

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I bought a vacuum gauge today and hooked it up. I thought that the vacuum was supposed to drop when the throttle increases, but instead it increased. Did I get it wrong, or do I have an issue?

Here's what I did:
Test 1
Hooked up the vacuum gauge to the heater control valve vacuum line. Pulled about 20" at idle, and then I had my assistant give the engine a little gas. Vacuum went up to 21" or so.

Test 2
Hooked up the vacuum gauge to the brake booster line. Pulled about 21" at idle, then 23-24" with a little gas.

Thanks!
 



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Dono

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How's your air filter?
 






corkey

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as rpms increase it speeds up the process, it needs more air, that is brought into the engine by vacuum,, thus increase in rpm means an increase in vacuum, but only to a point,
it will drop than stabilize as you drive,, as long as the engine gets what it needs it will maintain rpm, and vacuum,,
think of the engine as a big air pump,, drawing it in, than pushing it back in,,

If the engine is operating under light or no load and low throttle, there is high manifold vacuum. As the throttle is opened, the engine speed increases rapidly. The engine speed is limited only by the amount of fuel/air mixture that is available in the manifold. Under full throttle and light load, other effects (such as valve float, turbulence in the cylinders, or ignition timing) limit engine speed so that the manifold pressure can increase—but in practice, parasitic drag on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.
If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or pulling the car up a hill) then engine speed is limited by the load and minimal vacuum will be created. Engine speed is low but the butterfly valve is fully open. Since the pistons are descending more slowly than under no load, the pressure differences are less marked and parasitic drag in the induction system is negligible. The engine pulls air into the cylinders at the full ambient pressure.
More vacuum is created in some situations. On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast because the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as compression braking, is often used in engine braking to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). Note that although "compression braking" and "engine braking" are sometimes used to describe the same thing, "compression braking" here refers to the phenomenon itself while "engine braking" refers to the driver's usage of the phenomenon. Compression braking can be greatly increased by opening the cylinders to the exhaust with a valve on downstroke while in overrun, which is often done on large trucks (see Jake brake).
from wiki,,
 






lobo411

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How's your air filter?

It should be good. I have a cold air intake and I cleaned/reoiled the element last summer.
 






lobo411

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as rpms increase it speeds up the process, it needs more air, that is brought into the engine by vacuum,, thus increase in rpm means an increase in vacuum, but only to a point,
it will drop than stabilize as you drive,, as long as the engine gets what it needs it will maintain rpm, and vacuum,,
think of the engine as a big air pump,, drawing it in, than pushing it back in,,

If the engine is operating under light or no load and low throttle, there is high manifold vacuum.

Thanks for the info! Ya, the engine was in park so there wasn't any load. I guess as long as I'm getting 20" at idle I'm doing OK then.
 






Dono

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What are we troubleshooting here? Is there any drive-ability issues?
As Corkey said, it is possible that the throttle body is still causing a low load higher restriction, working exactly like its supposed to. It is interesting though, as I have never seen that effect with the last couple of motors I have used a vacuum gauge on.
 






lobo411

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What are we troubleshooting here? Is there any drive-ability issues?
As Corkey said, it is possible that the throttle body is still causing a low load higher restriction, working exactly like its supposed to. It is interesting though, as I have never seen that effect with the last couple of motors I have used a vacuum gauge on.

1996 Explorer XLT 4.0 OHV w/ 158000 mi

Two problems. One long-running, and one recent:

Long Running:
It seems to be pinging under load (8% grade on the interstate is usually where it happens).

Recent:
Engine is running cool. I haven't gotten above 1/3 on the dash temp gauge even on that 8% grade doing 80 MPH. My scan tool says that I'm not getting > 190 deg at any time, and cruising between 180-190. It used to ride right in the middle between C and H, but the problem started when I changed the thermostat 6 mo ago (198 deg). Just installed a new T-stat just now (195 deg).

No driveability issues except for The Big One: passing smog. Last time I was 2 ppm from failing due to high NOx, and I'm worried I might not be so lucky this time. I get great mileage (20 MPG highway, 12-15 city).

My thinking was if one of the intake manifold gaskets was leaking and causing the ping, maybe I could see that in the vacuum gauge.

Tonight:
I noticed what to me was a large accumulation of what I think is oil on the radiator cap and visible passages in the radiator. I'll change the oil tomorrow. Was going to do it anyway, but this is a good chance to see if there's any coolant in the oil.

I changed the head gasket and had the heads pressure tested/valve job done at 125000 mi due to a visible external leak. That was 10 years ago, since I'm a slug at accumulating miles!
 






Dono

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I think Ford had some tsb's on pinging on the ohv's. I'll have to look.

On the vacuum, I just checked my v8. In my case, I was in neutral (Neutral, or park shouldn't matter). As I step on the gas, vacuum does in-fact increase. I just did light throttle, and stopped at 30. Interesting, I never noticed this before.

Now, are you getting any engine codes?
How's your coolant reservoir? Is it empty?

Do you have the ability to pressurize your rad and see if it holds the pressure?

Pinging would usually mean either a buildup of carbon in cylinders, or running lean on a couple of cylinders (Intake gaskets?). Hmm, what else?? Lets think about this.
 






Dono

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type 'pinging' in to the search bar.
Lots of info comes up on the 4.0 ohv.
 






lobo411

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I think Ford had some tsb's on pinging on the ohv's. I'll have to look.

On the vacuum, I just checked my v8. In my case, I was in neutral (Neutral, or park shouldn't matter). As I step on the gas, vacuum does in-fact increase. I just did light throttle, and stopped at 30. Interesting, I never noticed this before.

Now, are you getting any engine codes?
How's your coolant reservoir? Is it empty?

Do you have the ability to pressurize your rad and see if it holds the pressure?

Pinging would usually mean either a buildup of carbon in cylinders, or running lean on a couple of cylinders (Intake gaskets?). Hmm, what else?? Lets think about this.

Nope, no engine codes, and my coolant reservoir is at where it normally is (cold full). I don't have a pressure tester, but I think I can rent one at Autozone so I'll look into that.

I tried injector cleaning on the "can't hurt" theory. I did a bottle of Lucas upper cylinder/injector cleaner through a tank, then a bottle of Lucas injector cleaner (not the lubricant) through a tank, and then a bottle of techron concentrate. Didn't change anything.

I saw the other posts about lower intake manifold gaskets, so that's where I got the idea of vacuum testing. Maybe I'll just try retorquing the bolts tomorrow to see if that helps anything. It's been a long time since I did the intake manifold gaskets (10 years) so I'm just hoping I don't have to do much tear down for what is essentially a gamble.

Also planning to replace my single plat plugs with double plat, also based on one of the other threads.

Replacing the t-stat didn't pan out--I put the one I removed in a pot of water with a candy thermometer and it opened up right at 198. I just don't see how the scanner can be saying that the engine doesn't get above 190 degrees F but everything's running fine.
 






Dono

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I wouldn't worry about the temp showing 190. One of the sensors might be out a few degrees. Maybe even the scan gauge is reading the temp off the intake manifold and it might be 190. At the thermostat, it might be 198. Im betting your temp is just fine. Your pcm is definitely going in to closed loop, and thats giving you the better gas mileage.

As far as the pinging, you could maybe try a can of seafoam thru the intake to loosen up any carbon in the combustion chambers? Im not really sure what to say.

I sure hope you pass smog ok.
 






drbenz

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I bought a vacuum gauge today and hooked it up. I thought that the vacuum was supposed to drop when the throttle increases, but instead it increased. Did I get it wrong, or do I have an issue?

Here's what I did:
Test 1
Hooked up the vacuum gauge to the heater control valve vacuum line. Pulled about 20" at idle, and then I had my assistant give the engine a little gas. Vacuum went up to 21" or so.

Test 2
Hooked up the vacuum gauge to the brake booster line. Pulled about 21" at idle, then 23-24" with a little gas.

Thanks!

This is actually a good quick test for a restricted exhaust, if you measure vacuum at idle (this reading should not be excessively low, 20" is great) and then quickly open the throttle to 3-4 K revs the vacuum reading will go down and then back up as the engine gets up to speed. The reading at higher speed should be at least as high if not higher than the idle reading. A lower reading would indicate a restricted exhaust (likely the catalyst).
 






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