Captain Obvious

About the smoke test... Since the BCDD is simply a bypass around the throttle plate, if that's where your problem is, it won't show up on a smoke test. Both sides of the throttle plate are in the same "closed system". Does that make sense? And, I can tell you with 100% certainty that if you just slapped a flat plate on there where the BCDD used to be, you've got a problem. You don't need a smoke test for that.

Here's a couple threads that might add some additional explanation: https://www.classiczcars.com/forums/topic/58760-1977-280z-with-81-83-engine-persistent-2000-2500rpm-high-idle/ https://www.classiczcars.com/forums/topic/56940-79-280zx-fast-idle-2200-rpm/

Yeah, you can't just plate off the BCDD like that. Creates a big leak around the throttle plate and your idle will be sky high. There are a couple other threads where people had that problem and installed some workarounds. I'll take a quick look and see if I can find something.

How did you block off the BCDD? You can't just pull it off and slap a flat plate in it's place. That won't work:

Agreed. The orifice in the neck of the carbon can is pretty small. Even if my valve never closes, it's not like it creates a big vacuum leak. I bet that over half the 280's out there have other accidental vacuum leaks bigger than that one, Haha! So even if mine is open, I probably just tuned it out like an offset. But my idle is lower than yours. I'm running 600-700. Maybe I could hear a difference if I pinched off the purge line.

Thanks again Dave. I think I'm going to look at those Pathfinder CARB cans next time I'm at the junkyard. My car runs fine, even at cold idle, but if I'm pulling air through that can at idle when I'm not supposed to, I could fix that. One thing I've clearly noticed in colder PA is that the mixture is definitely richer as ambient drops. I think they ran the gain on the air temp sensor a little too high. No way I'm going that deep into the ECU to change it, but I think that's the case. I was running a little lean in the summer, but as the temps have dropped, I can smell that I'm richer now. Especially when cold.

Thanks Dave. Good timing... I was in here at the same time you were. My read on the spring is the same as yours. Under some circumstances, the vacuum could be the same on both sides of the diaphragm, so in order to get it to open, they need to bias it to the "open" side. And I agree that it would be interesting to see how one of the original carbon cans behaved with a new diaphragm. So about your most recent tests... Makes perfect sense that you would run rough (on a hot engine) with a completely disconnected and hanging open purge line. Big vacuum leak. Also agree 100% that the fact that it actually LIKES the additional air on a cold engine is because it's running richer when cold. It's supposed to be richer when cold, but I think they may have gone a little far with that. And there's also a "after start" enrichment that boosts the mixture a little bit more right after you let go of the key. I've measured it and it tapers off over about thirty seconds. So if you had just started the engine, it's even a little richer. So on the most recent test above, on the old can, you don't know if the valve was open or not, right? Maybe you could apply your "push down on the cap" test. If it's really that borderline, then the additional thumb pressure might snap it closed? I'm thinking that forty years of that spring pushing up against the cap may have bowed the cap up a couple thousandths?

Dave, A while ago I was messing with my carbon canister and noticed the exact same operation you described... Able to pull air through the purge line (using lung vacuum) with no vacuum on the control line. My assumption (just like you) was that when there was more vacuum on the purge line (with the engine running), the diaphragm would be pulled down and seal off the purge hole. You're saying that's not normal? You're saying it SHOULD seal with no vacuum on anything? How can that be with the spring in there pushing the diaphragm away from the sealing seat?

Actually for ten miles, those plugs look good to me. Maybe a tiny bit rich, but not bad. Remember this isn't a closed loop engine burning all nice and clean and stuff. You do need to make sure the engine is getting up to temp though. Figure out if you have an instrumentation issue, or are you really running that cold?

Anyone have a confident source for the details about these two cam grinds? Anybody>

I've got a hardcopy of the 83 manual and .pdf of the EM sections of 81 and 82.They don't do any differentiation between turbo and non-turbo. The 83 manual seems to cover both turbo and non-turbo, but on xenon there's also a "turbo" version for 81, but the EM section is incomplete.

I've not seen one for the 260. What tool would you be using to create the diagram? There's a great one for the 77 done in AutoCAD. Would it save you work to start from that and modify it for the 260, or would it be just as easy to start from scratch?

Glad to hear your making progress. LOL. Don't get busted.

I found some of the numbers in the 82 FSM, but not all of them. I'm also unsure if that 82 "EM" section is supposed to cover both NA and turbo. Did you find all the numbers for both turbo and non-turbo versions in the FSM? Which year?

Actually the whole point of my study was to determine where "on the engine load curve" does the peak vacuum (and hence, ignition timing) occur? The reason behind the project was I was adapting the throttle body from a Sentra onto my Z and I wanted to make sure I wasn't changing the ignition timing much by moving the vacuum advance characteristics. So I drove around a bunch with a vacuum gauge teed into the ported vacuum source with the gauge on the interior so I could see it while I drove. I was trying to measure something "portable" between different throttle bodies so I could compare them. In other words... A measurement "standard". My "standard" went like this: "While driving flat and level, what speed would the vehicle be going if the pedal position was held constant at the point where the ported vacuum was at it's peak?" Even though the absolute number does not matter, it gives me an objective way to compare port locations on different throttle bodies. So, directly to your question... The vacuum peak is not right off idle. It's significantly deeper in the pedal than that. In fact, I found that in my car, I would be cruising flat and level on the highway in 5th gear at 65 mph when the ported vacuum was maxed out.

Man, I'm so sorry you're having continued issues!! What the F? You really think oil in one of the cylinders will go back out into the intake manifold and then into the other cylinders? Seems far fetched to me, but I'm no expert.

Looks like we're gonna need a bigger beer.

You want to see movies, or just the summary?

Stick, Yes... We have clearly veered into a purely academic avenue on your thread, but we're good clean fun. We'll help with the math.

Haha!! Just trying to pay it forward. What's the prospect of all three of us having our new shiny motors powering our cars at ZCON 2020?

So @GGRIII and I are putting motor parts together and he's got two cams... One of them is an "F" grind from an 82 non-turbo car and the other one is a "B" grind from an 82 turbo car. We're trying to figure out which one would be better to use. According to the interwebs, the lift numbers for both of them are the same and the valve timing specs for the two are as follows: Format in FSM - a / b / c / d / e / f - ex duration / intake duration / intake open btc / intake close abc / ex close atc / ex open bbc "F" (na) - 248 / 240 / 16 / 44 / 10 / 58 "B" (turbo) - 248 / 240 / 12 / 48 / 14 / 54 It appears to me that the only difference between the two is the non-turbo (F) has all the valve timings four degrees advanced when compared to the turbo version? Seems to me that the turbo cam on sprocket position #2 would be identical to the non-turbo cam? Do the cam experts here concur?

Of course they did. They always do. To add some info to Zed Head's post... The ported vacuum signal only provides vacuum at a "light cruise" pedal position. There's no vacuum at idle, and the vacuum falls off quickly above light cruise. It's peaky right at one throttle blade position (light cruise), not simply when the throttle is open. And there are two ported vacuum connections off the bottom of the throttle body. Make sure you are using the correct one. You want the one on the right side. The other one is for the EGR system.

Lol. I could do that. But it would be waaaaaay down on my priority list! Those metal can circular integrated circuits were used starting in 75. The pins were identified with letters "A", "B", "C", etc instead of pin numbers 1, 2 , 3. Precursors to the plastic DIP. This one is from 77:

So we're clearing into the academic portion of the thread, but I got some pics... Here's the ECU that I took out of an 83 NA with an auto trans: Here's a pic of the inside: And here's a close-up of the microcontroller. Hitachi HD6801VOP. Also note that you can see the 4MHz crystal in the lower left corner of the pic. Now granted... Just because there's a 4MHz oscillator doesn't guarantee that the microcontroller is actually running at 4MHz. I don't know if they are dividing that down or PLL looping it up. But my assumption is that the microcontroller is running at 4MHz : And since I had the camera out... Here's the ECU out of an 82 NA with a 5-speed trans: And the internals are still analog voodoo: So my (one and only) data point seems to indicate that they went to computer control in 1983.

It's my understanding that if the hydraulic lifters are failing, they will fail "short", not too long. By that, I mean... The failure mode is they leak down and then fail to pump up to the proper length. Valve clearances will be too small. And if the valves never open (or only open a tiny bit) because the lifters are too short, If that's severe enough, I'm assuming that could cause low compression.