How many inHg of intake vacuum are considered normal? (1 Viewer)

[lelandEOD]

I'm trying to track down a lean-miss issue with a freshly rebuilt 3FE and I'm consistently getting 15 inHg of vacuum at idle (when cold) and 14 inHg (when warm). I don't see any mention of manifold vacuum in the FSM but I could have sworn I read somewhere on 'mud that someone else reported 20 in/Hg...


TIA!
Lee

 

[mattressking]

First off vacuum is usually measured on this side of the pond by inches of mercury (inHg).

You are looking for roughly 18-22inHg with all accessories turned off at idle. Your elevation also plays a part in what you will see on a gauge. Usually lose an 1inHg per 1000ft in elevation.

 

[lelandEOD]

Sorry, inHg; not pounds... I am using a MityVac and it's giving me 15 inHg at idle. I'm also at about 4500ft/ele so perhaps this is an appropriate reading?

Thanks again!

 

[gregnash]

Dude you got a vacuum leak somewhere that is causing that. Even with my slight intake leak I still pull about 18inHg, which for us should be about right. Normal healthy reading at sea level should be about 21inHg.

 

[lelandEOD]

I’m going to have to get this engine smoke tested. No amount of my squirting propane or carb cleaner everywhere has discovered the source of this lean-running problem I have.

 

[Pighead]

freshly rebuilt? Rings seated yet? Also, adjust your valves again. Also, advance your timing. Some of us use a vacuum gauge to time our engine, advance to best vacuum then back off an inch or two...

 

[lelandEOD]

Yes

freshly rebuilt? Rings seated yet? Also, adjust your valves again. Also, advance your timing. Some of us use a vacuum gauge to time our engine, advance to best vacuum then back off an inch or two...

Yes, freshly rebuilt. I don’t know how long it takes for rings to seat but I plan on doing a compression test at some point this weekend.

Also, I noticed that my air/fuel ratio goes pretty wild when I mash the brake pedal...

 

[Pighead]

Leaky brake booster?

 

[OSS]

The brake booster uses engine vacuum to assist. And that always happens at idle. So if the engine vacuum is low and the idle kinda low, when you mash the brake pedal, there's a momentary big vacuum leak feeding the booster. I wouldn't pay attention to the A/F ratio while braking. It's meaningless.

Here's an altitude adjusted vacuum reading chart I found HERE

Inches of Altitude Vacuum
Sea level-1000 ft. 18-22
1000-2000 ft. 17-21
2000-3000 ft. 16-20
3000-4000 ft. 15-19
4000-5000 ft. 14-18
5000-6000 ft. 13-17

My 32 year old original 2F with an overhauled head at sea level pulls about 21+" on the Autometer gauge but shows about 20+" on the MityVac. Don't know which one is correct. The Autometer gauge sure cost a lot more than the MityVac but who knows.

 

[Vintij74]

Vacuum gauge

• Vintij74
• Feb 16, 2018

february '18
got 2 more inches after rebuild

 

[surfpig]

I'm trying to track down a lean-miss issue with a freshly rebuilt 3FE and I'm consistently getting 15 inHg of vacuum at idle (when cold) and 14 inHg (when warm). I don't see any mention of manifold vacuum in the FSM but I could have sworn I read somewhere on 'mud that someone else reported 20 in/Hg...


TIA!
Lee

freshly rebuilt? Rings seated yet? Also, adjust your valves again. Also, advance your timing. Some of us use a vacuum gauge to time our engine, advance to best vacuum then back off an inch or two...

Maybe a vacuum leak somewhere, as others have said. Lots of spaghetti to sort through there...

I also liked tuning/timing with the vacuum gauge. My 2F liked 19inHg at 10* advance all day long (except for smog check day).

 

[HemiAlex]

Sealevel and about 22 in at 10* initial timing.

 

[Tancruiser]

I’m at 8000 ft and the best can get is 15 lbs .

 

[lelandEOD]

Thanks for the feedback guys. I'm also wondering if there's a chance the air temp from the AFM to the ECU could be erroneous. I don't know how sensitive the connection between the AFM and ECU is as far as the conductor resistance. I have a couple splices in there as I moved the AFM to the driver's fender and the lengthened harness now runs along the fuel rail. My air/fuel ratio at idle is right at 15.5:1 (according to my Innovative wideband O2 sensor) when warm.

So many things to track down... I'm hoping it's something simple like a vacuum leak.

 

[LAMBCRUSHER]

Any air intake tract compromise after the afm and you’re scrawd make sure the dipstick is tight, PCV isn’t backwards, oil cap is tight and vc gasket doesn’t suck air. You should have more tha 16.5” at sea level, the higher up you are, the lower you will see... have you adjusted the valves after break in?

 

[lelandEOD]

Any air intake tract compromise after the afm and you’re scrawd make sure the dipstick is tight, PCV isn’t backwards, oil cap is tight and vc gasket doesn’t suck air. You should have more tha 16.5” at sea level, the higher up you are, the lower you will see... have you adjusted the valves after break in?

I haven't adjusted the valves post-rebuild yet as I was going to give it a little while longer to break-in first. I may just go ahead and do it this weekend though.

Another gearhead buddy of mine brought up a good point about exhaust back-pressure today. I did have a new exhaust setup installed and while I don't know the ID off the top of my head, I know the primary pipes to the collector are larger than the original FJ62 pipes. I wonder if this could be an aggravating factor?

 

[Pighead]

I wonder if this could be an aggravating factor?

Yeah, sure. Probably not. Heck of a lot easier to adjust your valves and check your timing than replace your exhaust...

 

[LAMBCRUSHER]

adjust valves and re evaluate. Do not skip retorquing head bolts. Remember the one under the rocker shaft...