No Vacuum At Throttle Body Ports

[rossiz]

my 78 280 reads very low vacuum (9-10 at idle) when tested at a manifold nipple at the back near the brake booster, but it picks way up to the middle of the green zone on my gauge when i rev the engine above 2K. i get almost zero vacuum at the ported nipples on the bottom of the throttle body. while the car seems to run well it bugs me... also i notice a slight idle fluctuation - from 700-900 in a constant, sweeping pattern. 

 

i have done quite a bit of work to this car: new head, good compression, i've re-built the entire manifold, replaced all lines, new injectors, new fuel filters, (new afm & ecu by PO) new pcv - the works.

 

i've checked the fsm a dozen times and i'm pretty sure i have everything hooked up correctly, but two things seem not quite right:

1. there are two ported vacuum nipples at the bottom of the throttle body - one goes to the distributor vacuum advance, the other is capped. neither one gives any real vacuum reading on my gauge, the needle barely moves and only when i give it a burst of throttle.
2. when i pull off the vacuum cap on the back of my manifold (by the brake booster) to plug in my gauge, the idle increases with the introduction of air - i thought a vacuum leak would make it idle worse/lower...

in general the engine is running rich - not horrible, but not ideal. i've messed with the afm gear and its at the spot that produces the best vacuum and highest idle.

 

any thoughts??

[siteunseen]

One of those ported vacuums on the side of the throttle body went to the BPT valve on my '77. I removed that with the EGR and plugged that nipple. Mine also had a 2 hose plastic vacuum valve on top of a warming plate that looked like an "F". I can't remember where those wound up.

Check Blues Tech Tips on reading a vacuum gauge. It has all kinds of scenarios.

[Chickenman]

Edit: The two ports on the bottom of the TB are " Ported " vacuum as mentioned. Normally you will see no vacuum at idle with a Ported connection as the vacuum port is located ahead of the Throttle butterfly valve on the AFM side. Only when the throttle is opened will you see vacuum, and it will rise as more air is drawn pats the port. Perfectly normal.

 

Manifold vacuum ports on the other hand, are located behind the throttle butterfly, so they will see high vacuum at idle.

 

Regarding low vacuum readings at brake nipple. You should be seeing 19 to 21 hg/in with a well tuned STOCK motor in good condition.

 

Check all these areas:

 

1: Does the car have a stock cam? A hi perf cam will reduce vacuum readings at low rpm's due to overlap

 

2: Check your ignition timing, you want about 10 - 12 BTDC with the vacuum advance disconnected. Note: Timing light sometimes go wacky. If you have the slightest doubt about your timing light, try a different one. Late ignition timing will produce low vacuum.

 

3: Make sure that the distributor advance is working properly and that it returns properly to zero advance at idle. Twist the rotor to check mechanical advance mechanism. It should " snap " back to the zero position. Often the mechanical advance gets gummed up and sticks.

 

If the Timing mark is hunting back and forth at idle that usually means that the springs have stretched or that you don't have enough tension on them. Timing should not start to advance until approximately 1,000 to 1,1000 rpm.

 

4: Triple check for vacuum leaks. A mighty vac is a good tool to have. Check the small accessory hoses and one way check valves as well. Often the small hoses to the cruise control or AC FIC valves will crack and leak. Obviously check the brake booster hose and check valve for any laeks. . Makes sure that the PCV hose is not cracked.

 

5: Less obvious areas for leaks are the fuel injector bosses. Check the phenolic spacers for cracks. I use a spray bottle of water with the engine running. Spray any suspect areas and if the engine rpm stumbles, you have found a leak. Often you can see the water getting sucked in. Much better than the old Propane trick, as you can't see where the propane is going.

 

6: A rich idle will lower vacuum reading... but it has to be Pig rich. Have you adjsuted the idle air bypass screw in the AFM. It's sometimes hidden with a plug.

 

Properly tuned with no vacuum leaks you should be seeing. 19 to 21 hg/in at idle.

 

One more thing...try a different vacuum gauge. They do fail occasionally and read low. Post back with results and GL.

[Chickenman]

( Snip )

in general the engine is running rich - not horrible, but not ideal. i've messed with the afm gear and its at the spot that produces the best vacuum and highest idle.

 

any thoughts??

 

Errr...that's not good. You shouldn't be messing with the AFM gear to adjust idle mixture. That;s what the idle air bypass screw in the AFM is for ) It may be hidden by an aluminium plug.. but it's there.

 

It's really easy to screw up the AFM settings by changing spring settings and " messing " with the gear. You've really got to be careful there...... Best to  have a 4 gas analyser or Wide Band AF meter hooked up when you play with the AFM inner guts. And mark the original settings so you have a baseline to go back to.

[rcb280z]

Ditto on what was said about the AFM. I have learned my lesson. Has taken way to long to get it adjusted right after chasing my tail trying to fix a vacuum leak that I didn't realize I had! Had a crack in my pcv hose.

[siteunseen]

What about that JB Weld on the EGR tunnel?  Is it all sealed up still?

[rossiz]

thanks  guys - some great input. i'll try to narrow it down a bit.

• stock cam
• timing is at 10 bdc with vac advance disconnected (although it does nothing at idle because it's connected to the ported vacuum at the throttle body)
• vac advance tests correctly (turns plate when i suck on the tube, no leak back)
• pcv hose & valve are new
• no AC, no cruise control
• no egr, all associated plumbing capped, manifold is sealed and holding up well
• brake booster shows no leak down

i will look into the following (haven't messed with these yet):

• mechanical advance seems to be working - when i use the timing light and rev i see the advance happen smoothly, but i haven't tested it really carefully
• check valve at brake booster - not sure how to test or where it is, will consult FSM
• idle control screw on bottom right corner of AFM - i've screwed this in and out with no discernable effect at idle, will consult FSM further
• still unsure why introducing a vacuum leak (pulling off nipple cap to plug in vac gauge) brings the idle up - seems to point to a rich condition at idle, no?
• will need to see about getting a wideband o2 sensor to check mixture - will need one anyway for new build with carbs!

[S30Driver]

What about the timing mark on the front pulley?  Are you certain it is accurate?

I know you did alot of work and replaced the timing set & chain, but I don't think you ever mentioned if you checked the front pulley to see if it had separated which of course would affect the timing accuracy.

 

Mine had slipped & the original owner had the timing retarded by 8 degrees, which produced a corresponding low vacuum reading.

 

When I sold my turbo Z, I kept the boost / vacuum gauge & replaced the clock with it in my 77.  I read about 19hg at idle when warmed up.

 

In other (but possibly related) news .... did your replacement head solve your high oil consumption problem?

[71Nissan240Z]

What about the timing mark on the front pulley?  Are you certain it is accurate?

I know you did alot of work and replaced the timing set & chain, but I don't think you ever mentioned if you checked the front pulley to see if it had separated which of course would affect the timing accuracy.

 

Mine had slipped & the original owner had the timing retarded by 8 degrees, which produced a corresponding low vacuum reading.

 

When I sold my turbo Z, I kept the boost / vacuum gauge & replaced the clock with it in my 77.  I read about 19hg at idle when warmed up.

 

In other (but possibly related) news .... did your replacement head solve your high oil consumption problem?

Yes

[siteunseen]

my 78 280 reads very low vacuum (9-10 at idle) when tested at a manifold nipple at the back near the brake booster, but it picks way up to the middle of the green zone on my gauge when i rev the engine above 2K. i get almost zero vacuum at the ported nipples on the bottom of the throttle body. while the car seems to run well it bugs me... also i notice a slight idle fluctuation - from 700-900 in a constant, sweeping pattern. 

 

 

Idle (consistent but low)

  When gauge needle is steady at about 8 to 14 it generally indicates incorrect valve timing.

 

 

Idle (with constant large fluctuations)

This action usually indicates a leaky head gasket

 

 

Cruising (big variations)

Wide variations of needle increasing with motor speed indicates weak, or broken valve springs

[rossiz]

@ siteunseen:

 

i know the pulley is accurate because i checked it with the head off - #1 was at tdc when the pulley mark was dead on the 0 indicator.

sorry if i wasn't clear about the "constant sweep" during idle - i was talking about at idle is in rpms, not vacuum. the vacuum reading is pretty constant, just really low. the rpms fluctuate in a regular sweep from 700-900 (as viewed by my tach inside the car)

 

the new head is working great - i should update that thread...

[TomoHawk]

the check valve for the brake booster is held by what looks like a black donut on the firewall above the brake booster.  follow the vacuum hose to the donut.  I dunno how you check the leakdown of the brake booster, but I think mine does leak.