Vacuum line runs cause low reading?

[NC280z]

Good evening all!

 

I've finally had a chance to get back around to messing with my Z. I repaired the BCDD diaphragm with some material I had lying around, but I've still got a low reading on my vacuum gauge (which has tested out as good). Looking at some of the FSM/Datsun vacuum diagrams, it would appear that the large line running from my valve cover should go to a t-fitting on my manifold, shared by the air regulator, and then the t-fitting goes to the throttle body. The current setup goes from the valve cover straight to the throttle body, and the t-fitting is capped. According to the diagrams I've seen, this is the "from '77" setup, which would be in line with the emissions setup as well. Mine is a '75 non-cali, so the manifold has the EGR mount blocked.

 

Could incorrectly-ran vacuum lines cause a low reading? I'm also going to check the vacuum canister/HVAC lines to see if there's a leak present there, as I've got some shady looking splices at the vacuum canister connections. The smoke test I did only showed a leak at the BCDD, and nowhere else. BUT, I didn't think to smoke out the canister/HVAC system (d'oh!)

 

Any help will be appreciated greatly, and I apologize if a similar issue has been started in another thread, but I couldn't find anything that would directly answer my questions. Thanks again. 

[ggzilla]

Does the engine idle well? At 800 rpm?

 

What is the exact vacuum reading?

[NC280z]

I don't appear to have an idle issue, I don't have the exact reading on hand at the moment, but it was a steady needle somewhere around 8-10in, or the "late valve timing/leaky heat riser" portion of the gauge. I've checked the cam/ignition timing about a million times, and everything is where it should be. I've got another thread started on this, the "low vacuum on new engine" one, but wanted to start a separate topic to see if anyone else has had issues caused by running their lines incorrectly, or if their HVAC system has caused low readings.

[ggzilla]

Yes, low vacuum can indeed be caused by the hoses.

[NC280z]

I know a broken or damaged hose can cause the vacuum leak, I just need to know if the line running from the valve cover will cause a low reading if it's not connected to the correct port on the intake manifold or throttle body. It's connected and appears undamaged, but I don't know if I should revert to the connection style pictured in the FSM. The line's current location is how it came to me when I purchased it. Unfortunately, I never did a baseline vacuum reading before rebuilding the engine, so I have no idea what the car ran at prior to where it's sitting at now.

[ggzilla]

Well it didn't come from the factory with only 10" vacuum...

 

Plug that line to see if it brings the vacuum up.

[datzenmike]

You should have 16-18 at idle.

 

A worn sprocket and chain or a head that has been milled will retard the cam timing allowing the piston to rise higher in the cylinder before the intake closes. Basically pushing some of the intake charge back out into the intake again. Very noticeable at idle but can be beneficial at very high RPMs.

 

When checking the cam timing you must set the crank to TDC by turning in a clockwise direction only. If you over shoot, back it up well before and try again stopping at precisely TDC. This will keep the chain slack all on the slack side and give a much more accurate reading. The V notch in the back of the cam sprocket must be below or just slightly to the right of the horizontal etch mark on the cam thrust plate. If to the left the sprocket must be moved one number higher.

[NC280z]

Sprocket, cam, timing chain (and components) are all new, as is 99% of the engine. Timing mark for the cam gear is just slightly right of the tower notch, and the dizzy is in the correct 1125 position. Engine is still very new, I haven't put road miles on it yet, just the standard break-in procedures. I figured the rings not being totally set might be an issue, but I was also told that it should still draw a higher vacuum reading.

[ggzilla]

Ya it should.

[datzenmike]

What cam do you have? Anything with a lot of overlap will have low vacuum too.

[NC280z]

Nothing crazy in terms of overlap, I had thought about that as well. I've ran the valve-cover line as it's depicted in the FSM and my other shop manuals for the '75 model year. Unfortunately the valve-cover gasket isn't sealing correctly, so I've got to wait on a new one before firing it up to see if that's corrected the issue. Also blocked off the lines for the HVAC controls, just as a precaution.

[ggzilla]

Why wait? Plug the hose. If the vacuum doesn't go up to normal, you'll know that's not the cause and can look elsewhere.

[NC280z]

Started up the Z today, still have a low reading on vacuum at around 9" from the brake booster connector at the manifold, and the needle is a little shaky as well. Going to do the old carb cleaner spray test for vacuum leaks, but I'm still at a loss as to what is causing the low reading. No issues with startup at all, at the recommended 7 BTDC for timing, and has a good steady idle at about 750rpms. Could the fact that I don't have any road miles on the engine cause the low reading? I know I clocked the piston rings correctly when installing them, the valve lash is spot on per the factory specs, and timing is as well according to the FSM. Took the cylinder head and had a valve job performed at a local machine shop while putting the bottom-end together, and put the engine together step-by-step with extreme care per the "How to rebuild your Datsun OHC engine" book.

 

My Dad and several friends insist that I put the exhaust back together, and put some road miles on it before worrying too much more about the readings. Anybody else have ideas/opinions on my issue?

 

Oh, here's the cam specs as well (260 intake 270 exhaust duration, 112' LSA .460 lift)

[datzenmike]

Try the gauge on a different motor. We could be chasing a non existent problem.

[NC280z]

Already verified that the gauge works fine, reads at 20" on my Frontier.

 

I started it again today, and recorded the engine running w/ the vacuum gauge readings. I'll post it on Facebook as soon as I can so you guys can see/hear and maybe be able to give a better diagnoses.

 

Strange thing with it running today, when I squeezed the PVC valve tube the gauge went from 7-8inHq to a solid 10", with almost no needle shake. I tried the old carb cleaner trick on the intake manifold, both at the cylinder head and all the vacuum lines mounted on top, and there was no change in the idle.

[smoke]

This is pretty darn rare, so I hesitate to even bring it up, but just in case you end up really chasing your tail on this, I thought I should mention it. I degree all cams when I install them and 99.9% of the time, they are pretty darn close to right on the money for what the card reads. But, last year, I had one ground so far retarded that it would have not worked at ALL like I would have wanted it to. It's highly unlikely it's your problem, but something worth keeping in your mind I think.

[banzai510(hainz)]

I have a 80z car a long time ago.

My heater didn't work right. Ih ooked up some vacuum hose then it worked. Don't know if could be related to your situation

 

 

shounds like a Schneider spec cam. I got that same grind on my 521 and I hade it dialed it to #1 and #2 on Cam sprocket to no ill affect on idle.

[NC280z]

It's a Schneider spec cam, but ground by Delta Cams whom also refurbished my rocker arms. No cam card was included with it, so I installed it as the FSM states for a stock cam. I've also plugged the connection at the back of the intake manifold that would be used for the heater controls, in order to remove that as a possible cause. I'm going to put the exhaust on it this coming weekend and maybe put some miles on it before going too much further, as I'm hoping the lack of road miles has something to do with the low vacuum. As of right now the engine only has the headers connected, so the loudness adds to the difficulty when troubleshooting.

 

Engine also idles smoothly, which throws me off since I figured that low vacuum would affect it. Also, all cylinders did a leak-down test/compression test at about 140psi across all cylinders, all within a few psi of each other. I think this points in the direction of the rings not being fully set, and that having an effect on my readings. Any thoughts? And again, thank you all for the words of wisdom, I do appreciate it.

[ggzilla]

I never heard of low vacuum until the rings seat ?

[datzenmike]

It's basically an unknown cam with unknown duration and overlap. Big cam = low intake vacuum.

[banzai510(hainz)]

Iam very leary of Delta cams.

I bought some and they were not to spec. But it should still run

[smoke]

I think you should do a quick leak down test to eliminate some possible culprits. The rings can cause low vacuum, but not normally that low from just not being seated. Huge end gaps and such, but I doubt it's your issue. You really should not let it idle too much at this point, to seat the rings good, it should have some load put on it and ran kind of hard. But...I really suspect the cam here.

[Nuclear Water Boy]

Lots of really good info here. Now here's an old trick from a semi old guy. Remove a vacuum hose from the intake manifold, strike up a ciggie or whatever (I used cigars as I don't smoke) drag on a big hit hold it, now blow the smoke into the vacuum hose you removed and look for Smoke. I'm surprised Smoke didn't mention this :thumbup: You can also at idle slowly choke off the carbs with your hands (once you've removed everything to get you're hands in there) and if the idle raises you have a vacuum leak if not no leak it's something else: Go to Smoke trick DOH!!

 

Did you check you valve lash, and made sure your cams geometry is correct? Meaning is your cams wipe pattern cwentered on the rocker arms pad? Very important!!!

 

Lou "Heading to Summit Point tomorrow woohoo!!!

[NC280z]

I've already used a smoke machine to test for leaks, the only one I had was the BCDD diaphragm, and I've since repaired it. The '75 is the first year of fuel-injection, so there aren't any carbs to choke off, although I'm sure I could use the same method by capping off the air filter. I used machinist's blue to check the wipe pattern, and it was centered as necessary, so I wouldn't imagine that it's the issue. One thing I may have done wrong is that I set the timing with the vacuum-advance connected, could that cause any issues?

[smoke]

Well, yes and no on the vac advance. It's not the right way to set it. But since you set it at idle and your getting a low reading at idle, probably not it. BUT, you need to set it correctly.