Silly question

[Digital Oxygen]

Ok, I was under the impression that diesels did not develop vacuum.

However, mine is pulling about 140 mmHg at fast idle. That's about 5.5 inches of vacuum. Does this sound about right? Or do diesels actually pull some vacuum, but not enough for the vacuum operated stuff like the heater, etc?

Edit: In case some of you will ask, yes, the airfilter is clean. cleaned it last week. It's an airraid unit.

 

[DaveBen]

No diesels do not create a vacuum. There is no throttle plate for them to use to create it. There is a vacuum pump to run things like the AC and the heater.

Dave

 

[budtoh3zo]

What year is it? there was a throttle plate for one year but later disabled.

 

[Digital Oxygen]

It's an 03. And yes, it is holding vacuum. Please Note picture.

The sensor is installed in the intake manifold. I swapped out sensors with the seonsor out of my mark VIII. same result.

 

[NASCAR Mike]

I would think that there is some vacuum created when the piston is on the downstroke and is sucking in air/fuel.

I know that's what the turbo does (forces it in) but there are non-turbo diesels out there.

Heck, when my turbo broke, the engine still ran, although like crap. There must have been vacuum in the engine or else how would the air/fuel get into the cylinder?

 

[bushpilot]

03's had a "throttle plate" its under the intercooler "elbow" just
in front of the oil filter etc.

 

[Kleetus]

I'm going out on a limb to say it only does this when your foot is off the pedal or idling, right? But as soon as you get into it, and the boost comes on, you start seeing positive pressure, right?

You should. My buddy's 7.3 PS did the same thing. You're seeing the restriction of the air filter and associated piping. It's not much, might be higher if you were in TH mode screaming 3600 rpm coming down a hill. It's a function of how much draw the motor is making just by moving the air through it. Not going to hurt anything.

Now a gasser at idle is what, 20-22 inches? 30 when you tach it up and let it rev down. That's high enough that you can consistantly use it for something.

 

[Digital Oxygen]

Kleetus:
It will still show vacuum (albeit a bit less) when revving the motor, in park.
It will show boost as soon as the turbocharger starts building boost. I know the turbocharger is building boost because I can see it on the autometer gauge.

In TH mode, coming off the freeway, I see... hell, I don't remember now... But it's not higher than idle, IIRC. I'll have to pay attention to it.

On a gasser, the more vacuum you have, the better... up to 30 inches, ofcourse.

I'm not worried about anything being hurt, I just found it curious that there was 4-5.5 inches of vacuum on a diesel.