You could pinch the hose to the booster and listen for a change in idle speed.  Or disconnect it at the check valve and block it and see if idle speed is different than when connected.  Might be difficult to pinch shut the old stiff hose.

Higuys-

 

I was checking the vacuum advance to the distributor, because the BCDD was removed a few years ago, and the hole was capped. We had the vacuum advance hooked up to the plate that covered the BCDD hole and I wasn't getting any vacuum advance. I tried moving the vacuum advance hose to a T connector that was connected to the intake manifold plenum and FPR, and I suddenly got 40 degrees advance!  That plus the idle went up to 1600 RPM.  If I disconnect the vac advance and plug the hose, the idle goes way down to about normal.

 

Something isn't connected right, obviously.

 

Is there a diagram that shows the connection to the distributor, the FPR, and where it's supposed to connect to the intake manifold?  I can't find anything in the FSM showing vacuum lines and the distributor.

 

thxZ

The vacuum advance is controlled by "ported" vacuum.  The port in the throttle body is closed when the throttle is closed.  That might be why you didn't get any vacuum on that port.

 

Most ports on the throttle body are "ported" or controlled by the throttle plate.  Some may be direct though.  But you can verify by cracking the throttle and feeling with your finger.  If there's only vacuum when the throttle blade is open, that's one for the vacuum advance.

 

Also, on the early 280Z's there is an electrcial solenoid that also stops vacuum unless you're in top gear.  1978 doesn't appear to have that though.

Internet Information is contradictory;  one source states port vacuum is used, another (datsunzgarage) says L engines use manifold vacuum... I prefer what I learned, to use ported vacuum, but the hose fittings are now missing.

 

Otherwise, I'm correct in using the manifold vacuum for the Fuel Pressure Regulator.

Edited August 24, 20159 yr by TomoHawk

I looked into  all the vacuum ports on the intake manifold, and tested each.  The plenum port all had a constant draw on them, making them unsuitable for the distributor vacuum advance.  That port was connected to the FPR only.  It was (PO had it) connected with a Tee fitting to both the FPR and the distributor! The two ports on the BCDD  were then checked-  one was blocked with gunk and had no vacuum at all, even when you  give the throttle a jolt.  The other was not blocked, and had vacuum when you move the throttle, so I  connected that to the distributor. 

 

I think I will cut off & block off the bad BCDD vac port with JB Weld so it doesn't confuse things again.

 

Using the timing light, I confirmed that everything worked right.  Surprisingly, a small movement of the throttle  had a big affect on the timing advance- from 8 degrees to about 30!   I haven't tried it on the highway, only on some local streets. 

Does any one know the correct/factory location of the vacuum port for the distributor advance for the L28E engine?  It's not detailed in any of the reference material I have.

On the throttle body.  Check page EF-39.  There's only two.

It's the one that points toward the radiator. Vacuum from that front port should "T" to go to two places. 1) the distributor vacuum, and 2) the carbon canister control line.

Thanks Bruce.  That's the one I'm using.

On the throttle body.  Check page EF-39.  There's only two.

I can see it (#15) in the graphic, but it doesn't tell you where to look on the engine. It should be a no-brainer diagram to draw ( vacuum advance port & tube, T-fitting to canister,  FPR vac port & tube, etc.)  It's probably that one picture of all the engine hoses & tubes, but that picture is hard to make out

Yeah, I took a quick look through the service manuals looking for some sort of diagram or sketch that did a good job of showing those vacuum lines and came up with nothing very good. Best I found was in the Engine Tune Up section, and the clearest year I found was 1977. So take a look at ET-2 in the 77 FSM manual. You're interested in tube numbers 5, 12, and 14.

 

14 comes off the throttle body and goes to a metal "T". 5 comes off the "T" and goes to the vacuum can. 12 comes off the "T" and goes to the distributor.

And since we're talking about ported vacuum sources... Here's a pic down the throat of the throttle body and you can see the distributor advance port on the left and the EGR port(s) on the right. Been a while since I studied it, but IIRC, the pair of larger holes in the foreground are BCDD.

 

The distributor port is the rectangular slotted one on the left back where the butterfly would contact the housing if it were closed (instead of being blocked open as it is in the pic):