Vacuum reading at idle?

[exgmguy]

What is a normal vacuum reading at idle for a typical Heads/cam/intake 5.0l?
I have to give my car the "anti-idle surge vacuum leak" and it reads 14 inches at idle, and 16 inches with the leak plugged, surging away.

I have a mild E-303, ported TFS heads, Holley intake and a 70 mm throttle body.

[PKRWUD]

Healthy is 19" - 21", mild cam is 15" - 16", built is sub 12"

The trick is to watch the gauge and see what it does. It can tell you alot.

Take care,
-Chris

[Stang951]

So the vacuum will not stay constant at idle, it will bounce a few lbs. Is this correct?

[PKRWUD]

Originally posted by Stang951
So the vacuum will not stay constant at idle, it will bounce a few lbs. Is this correct?

depends. A healthy, stock engine's vacuum gauge will remain rock-steady at idle. use this info to diagnose vacuum readings:

As far as tests with a vacuum gauge, where do I begin...

The vacuum that is created in a normaly aspirated engine (one that is not blown or turboed) gives you a great tool for diagnosing engine problems. Just about any internal engine problem can be identified with a vacuum gauge !!! I'll teach you some of the basics:
1) A steady reading of between 17" and 22" is considered normal for a stock engine.
2) A low but steady reading between 10" and 15" would be caused by retarded ignition or valve timing, or low compression.
3) A very low reading between 4" and 9" means you have a vacuum leak.
4) If the needle fluctuates as you increase the engine speed, you have either an ignition miss, a blown head gasket, a leaking valve or a weak valve spring.
5) A gradual drop in reading at idle means you have excessive back pressure or partial blockage in the exhaust system.
6) An intermittent fluctuation at idle means you have either an ignition miss or a sticking valve.
7) A reading where the gauge needle drifts anywhere between 12" and 22" indicates that your idle mixture is unstable. This is more common on carbureted motors.
8) A high and steady reading indicates that the ignition timing is too far advanced.
9) If, when you quickly close the throttle after running at 2000 rpm for ten seconds or more, the gauge needle DOES NOT jump two or more inches above what it is at idle, your piston rings are shot.
10) If the reading at idle is lower than normal, but the gauge needle fluctuates three inches on both sides of normal, your valve guides are excessively worn.
These are but only a few of the things you can learn about the condition of your engine with a vacuum gauge. Keep in mind that a blower or a turbo will create a false reading because the boost they create offsets the vacuum reading.

The following is the way to find out if you have an internal vacuum leak from a defective intake gasket:
1) Hook up a vacuum gauge to the intake manifold.
2) Fully close the throttle plate by backing out the idle screw all the way.
3) Plug up the PCV valve opening.
4) Disconnect and plug any vacuum hoses attached directly to the intake manifold.
5) Disconnect the wire to the positive side of the coil.
6) Crank the engine and observe the vacuum gauge. From 3" to 7" of vacuum (at a minimum of 250 cranking rpm) is normal. A defective intake manifold gasket will give, in most cases, a zero reading.

With a vacuum gauge, there are two easy tests you can run to determine if your exhaust is restricted (cats clogged).

First test:
1) Attach vacuum gauge to manifold vacuum source.
2) Observe vacuum at idle.
3) Snap throttle to WOT and release, while watching the gauge.

The vacuum gauge should drop to almost zero when you hit WOT. When the throttle snaps closed immediately after, the vacuum should read 4" to 6" higher than what it did at idle. It should then settle back at the same reading it was at idle before the WOT snap. This whole exchange should go from idle reading to idle reading in 2-3 seconds, tops. If it takes longer for the gauge to return to the same idle reading, your exhaust is restricted. The longer it takes, the worse the restriction.

Second test:
1) Attach vacuum gauge to manifold vacuum source.
2) Warm up engine.
3) Observe vacuum at idle.
4) Leave engine idling for 10 minutes. Do not touch anything on the car during this time. In fact, go inside and fix some coffee.
5) After 10 minutes, observe the vacuum.

If the vacuum is the same or higher, you don't have any resctrictions. If the vacuum gauge has dropped, you do have a restriction. The more it drops, the greater the restriction. If it drops 1" or more, you have a serious restriction. 2" or more and you might as well have the exhaust welded shut. Keep in mind that a rich exhaust will clog up cats in a hurry.

Hope this helps a little.

Take care,
~Chris

[93GTDIN]

Chris,

It seems as though I have one of the problems you described in your vacuum diagnosis thread:

" A low but steady reading between 10" and 15" would be caused by retarded ignition or valve timing, or low compression."
-This vacuum reading is taken off of the brake booster
My timing is set to 18*
Compression is 150 PSI cold fairly consistent all cylinders
not sure on valve timing.

If when I set my cam timing I kind of half assed the whole procedure, and did not perfectly line up the timing gear w/ TDC, could this be causing my problem.

It may also be important to mention I have 18-20 inches mercury vacuum cruising on the highway.

[302 LX Eric]

On my new 331, with 9.2:1 comp, I run right around 12-13 in. hg. It's pretty steady maybe moves +/- 1. I was running around 11 with my old heads/cam/intake 302.

E

[93GTDIN]

And what kind of gas mileage did you get on your old HCI 302?

[Mach 1]

Originally posted by exgmguy
What is a normal vacuum reading at idle for a typical Heads/cam/intake 5.0l?
I have to give my car the "anti-idle surge vacuum leak" and it reads 14 inches at idle, and 16 inches with the leak plugged, surging away.

I have a mild E-303, ported TFS heads, Holley intake and a 70 mm throttle body.

I was only getting 10 -11 inches when I had the e-cam and surging problems. Needless to say, the cam is gone, along with the problem.

[302 LX Eric]

93GTDIN - It's hard to say what kind of MPGs I was getting as I drove the car so infrequently. But, if I had to guess, I would say around 16-17 - and that's with drag racing and then a 75 mile round trip to/from the track.

E

[zepherman]

Chris,

can you use the vacume #'s you supplied above for diagnosing most cars, or are those ranges specifically for the mustang?

[Mach 1]

They are for all cars.

Chris, mine is fluctuating back and forth between 12.5-13.5 hg. My idle is also surging, and I assume this is why.

But my combination is fairly mild, shouldnt I have a higher reading, like at least 15?

[93GTDIN]

Mach 1 - What cam did you switch to?

[Mach 1]

A competition cams grind.

114 lsa, .533/.544 lift, 215/220 @ .050....or something like that. If you want me to be exact or want a part number, let me know and I can get it for you. look through their online catalog. Mine was listed under "computer compatible" cams a few years ago when I bought it. It might be in the "magnum" section as well.

[302 LX Eric]

Mach 1 - I've seen three different 5.0 Mustangs with H/C/I combos all in the 11 - 14 range. Now, 4 with yours. So, I don't think that this range is 'abnormal' considering how well all these car run at the track and there overall driveability.

Maybe a stock setup runs 15+, but I dunno.

Also, have you considered an Autologic chip (w/dyno tune, if available)? I got one for the 331 and it idles like a dream. Here are my cam specs (w/1.72s):

115 lsa, .516/.516 lift, 220/226 @ .050

E

[Mach 1]

I would like to get a chip, but I decided to quit putting money into the car, and its more of a daily driver now.

How much was the autologic chip and where do you get one?

[PKRWUD]

Originally posted by 93GTDIN
Chris,

It seems as though I have one of the problems you described in your vacuum diagnosis thread:

" A low but steady reading between 10" and 15" would be caused by retarded ignition or valve timing, or low compression."
-This vacuum reading is taken off of the brake booster
My timing is set to 18*
Compression is 150 PSI cold fairly consistent all cylinders
not sure on valve timing.

If when I set my cam timing I kind of half assed the whole procedure, and did not perfectly line up the timing gear w/ TDC, could this be causing my problem.

It may also be important to mention I have 18-20 inches mercury vacuum cruising on the highway.

I would be suspicious of your valve timing, but everything else looks good. Perhaps you have a leak in your booster. How are your brakes?

[PKRWUD]

Originally posted by Mach 1
They are for all cars.

Chris, mine is fluctuating back and forth between 12.5-13.5 hg. My idle is also surging, and I assume this is why.

But my combination is fairly mild, shouldnt I have a higher reading, like at least 15?

With the cam you have, I'm not surprised by your numbers at idle, but the fluctuation isn't normal. Does it get worse as rpm's increase, or does it go away?

[302 LX Eric]

Mach 1 - I got my Autologic chip from Johnson Motorsports (JMS). They're located in Irvington, Alabama. Website is www.jmschip.com

I really wanted to get a custom dnyo tune, but didn't have the money for it. Plus, I had heard good things about these guys from people at my local track. I've got 3 programs on my chip that I can select b/t. When I spoke with the guys at JMS they wanted to know everything about my 331. After a lenghty discussion they reprogrammed my chip from my old 302 setup and sent it back to me. Original chip cost was $300, and the re-burn fee a year later cost me $80.

The idle is perfect. I'm very happy with it.

So far, I've left the most conservative (ie. rich fuel, lower total timing, etc) program on just to be safe.

Hope this helps.

E

[Mach 1]

Originally posted by PKRWUD
With the cam you have, I'm not surprised by your numbers at idle, but the fluctuation isn't normal. Does it get worse as rpm's increase, or does it go away?

Its only at idle.

if seems to flucuate with the rpms (surging idle)

[93GTDIN]

Actually now that you mention it my brakes kind of suck. They are fine on the highway but when I am doing 5-10 mph they are really really stiff and I kind of have to stand on them to make the car stop.

How would I check/adjust my valve timing?