Dave WM

no obd this early a car. Really don't understand the high resistance. You are reading from the battery post to the engine block right? also from the battery post to the car frame. You should be able to trace from each connection point to find the resistance. start with the battery post neg to the wire lug that connects to the fire wall right off the lead from the battery. Perhaps you have a poor connection thru a fuse link (IIRC there is a fuse link in series with that wire, I may be wrong about that will check later). Its just a matter of wire tracing with the ohm meter.

don't understand what you are saying. Jumping the cold start vale terminal? the engine is turning over? you mean something your are doing is causing the starter to engage? It maybe possible to energize the starter solenoid by applying power to the positive side of the harness that plugs into the CSV I suppose, IIRC they share a common lug on the ign switch "start". But if that is what you are doing that is the incorrect way to activate the CSV to depressurize, You only want to apply power to the csv, not the harness, and frankly you don't need to do that, just put a rag around the fuel line at the fuel rail and loosen the hose clamp. there is not that much fuel stored, the rag will catch it and keep it from spraying.

just to confirm the above, I checked with my DVM the neg post to the engine block, .1 ohms. So yes if you are getting 31.4 ohms from those locations you have a problem. I would start with the easy stuff, like the cleaning the battery post and cable connection. NO use of dielectric grease please. Just check from the center of the post to the battery cable clamp from there chase the battery clamp to the engine fire wall and starter. You have got to get that down to the sub 1 ohm reading.

drove the brand new 260z to the high school prom. Was on the road yesterday on the radio was disco, yikes... but it did take me back! the new car next to me had some kind of thumping noise, think it was music, but I don't think the car will be around for long, sounded like it was rattling apart at the stop light. My old radio has maybe 5 watts max, that and disco not likely to damage spot welds.

here is a prediction for you, I hear a lot about setting up an electric infrastructure for electric cars, I suspect a much bigger problem will be an infrastructure for the internet.

in the 70's driving was the number one objective for teens, at least males (well other stuff as well but we are talking cars here). I got my learners on my 15th b day, same with reg on my 16th b day. drove motorcycle on my 15th b day, every day of the week. same with car when 16. left in the morning home late at night, if not school, then work (min wage), or social. Home is the last place you wanted to be. So Clearly driving was everything, and we worried about running out of gas back then, why cause then we could not do the one thing that was so central to teen existence. The idea of being driven around would get you laugh at, bullying was the way the social order was maintained. VERY different these days, and I presume every generation has the same experience, therefore trying to project into the future becomes a bit of a folly as far as what will be the driving force with large scale problems. when I was young it was common to see gangs of youths wandering the streets looking for action, now I rarely see young folks or old folks outside, but for a few throw backs like myself out cutting the lawn (that is left to "lawn men" for most folks around me). There was some hope though, I took my dirt bike out Saturday for a ride in the woods, there were a ton of people of all ages out enjoying the outdoors. I drive a classic dirt bike (1978 RM400 Suzuki) and there was a lot of attention by mostly older guys digging on the old bike reliving there youth when they had one.

Or we may find that driving is not even an issue, and oil will pile up. I have not commuted to work in the past 12 years, I suspect with the sudden realization that a LOT of the work force does not need to commute there will be even less driving. With VR substituting for family vacations, with amazon delivering your groceries, with doctors seeing patients remotely, with robots taking over all forms of jobs (hastened by the min wage being deemed a 'living wage", there was a robot doing my 1st job, a porter, at a store the other day) so less people will be needed, next up will be taxi and pilots by machines, eventually we become bags of protoplasm sitting in a matrix. In the early seventies this was all the rage (save the earth, running out of oil, global freezing, opec oil crisis) all the young folks were yelling at the old folks telling them they were messing up the world (remember the crying American Indian in the canoe), so this is nothing new, but I have come to realize (with age) that you cant predict what will be the next real problem. I grew up with constant warnings about dui, well I think distracted driving is a bigger problem, also about kids wasting too much time with TV shows, now its video games. Good luck the figuring out what the next gen issues will be for beleaguered parents to deal with. Good luck deciding where resources taken from tax payers by force should be used. Or just live your life as well as possible without taking from or hurting the other guy. Be conservative with all resources, as you will prob get it wrong trying to guess the future.

took my z out and burned some gas, pumped out some carbon, and loved every minute of it. 75 mile round trip so I could feed some tree rats some pecans, harvested no doubt by some huge combine like machine. Now going to change the engine oil after 2500 miles cause it looks a bit brown.

great movie, "the fountainhead"

can you describe the setup that results in the 45psi on #6? I am not sure I follow what a "live compression test" is. 45psi is for sure a problem. I assume the live test on the others was somewhat higher in psi readings? On compression test its all about being even or at least within 10% of the high number. My standard test is a warm engine, throttle plate open, fresh charged battery, use a remote starter, all plugs out, screw in type tester with a one way valve built in, then crank it till it hits highest reading, generally takes about 4 compression stokes to max out. Do this on all noting pressure, then I go back and squirt in some light oil, maybe a 1/2 teaspoon, take another reading. Look for even numbers wet and dry. IF you have an outlier, then do a leak down test, TDC compression stroke, pump up to about 100 psi with shop air, note leakage (should be no more than about 20% max) if more listen to exhaust pipe (exaust valve leaking) intake manifold (intake valve leaking) valve cover opening (rings leaking). Generally most of the leaking will be from the rings into the crankcase (valve cover opening). Very little if any should be heard from the exhaust or intake manifold (you can use a hose to ear connect hose to brake booster port on intake manifold for listening to intake).

as the plate pivots it activates two micro switches. they are located in a small plastic box on the throttle body in line with the rod that the throttle plate is screwed to. the on/off position is adjustable, one switch should be closed at idle, then off as throttle comes off idle, then another switch comes on again at about 2/3 throttle, so both switches are off at just over idle thru about 1/3 throttle. At least on the NA 280z efi.

oh forgot to ask, does the turbo model have the throttle plate position enrichment sensor (idle mid full switch mounted to the throttle body). On non turbos this adds fuel at idle. Its an adjustable series of micro switches that has a 3 prong plug. Small box on the side of the TB.

I doubt a running compression test will add any light to the issue. If you are worried about a loss of seal in rings or valves a leak down test will isolate. OR just do a wet/dry compression test. However all this seems moot since lifting the cap fixes the issue. a few things you should do a simple power test to see if #6 is causing a drop in rpm when disabled. do this with the cap tight vs loose since that is the title of the thread. It would seem to be the issue based on reading the plug, but the power test confirms. This will confirm the issue is #6 and the cap is the fix. If that works go simple, try switching a plug and wire with one that seems to work (cause a power drop when pulled). That will clear the wire and plug. do a wet/dry comparison of compression from a known working cyl to the suspect cyl, if the same that will clear the compression. Next try a new cap and rotor if the problem persist. If new cap and rotor have no effect, I would HIGHLY recommend you pop for the color tune. that will comfirm the fuel mix. That should show if the issue is a lean vs rich, swapping an injector should prove this out. The only thing that that I can think of (given the stated issue) re the loose cap deal is a poor air fuel mix that is hard to ignite coupled with a ign that is just on the edge of working with a lean idle mix. the color tune will show the mix and if its lean that maybe the issue. adding a spark gap to the cap maybe just enough to compensate for a poor air fuel mix and lets it fire. I would start with some brand new standard (cheap) ngk plugs, not sure about those fancy tip plugs (could be adding to the ign just on the edge of dealing with a lean mix). Pure speculation of course on all the above, but remote diagnosis is very hard since we don't even have a video to see/hear what is going on. Sometime there is a tiny clue missing like seeing a spark in the ign wires a night, or some other odd thing that gets overlooked.

would be an interesting mod if you could adapt the wide band readings into the AFM as a way of making the early systems into a closed loop. A small plus or minus voltage added to the wiper of the afm arm.

since I know you have done the research can you explain a few things? diff in "narrow band and wide band" where to mount O2 sensor. How to mount sensor. Before or after cat (if you have one, like I do on my 75 cali Z).

if its 16 ohms it may just be the high impedance coil on the injector, not designed to use the dropping resistor. The main problem I have is how that is fixed by the propped up cap, that observation points to a ign issue. IF you hear it clacking away that would tend to mean the coil is sufficiently energized, but that does not mean its flowing fuel. If its flowing fuel you should be able to see a wet plug. I really wish the lifting of the cap did not fix it, that really throws a metaphorical wrench in the works.

wow, guess I better up my insurance again. My 75 is as nice as this one (the green 76 that is) and I KNOW I have little rust. that's the thing that gets me, unless you personally inspect it how can you bid this kind of money on a car that is known to be rust prone. IF I ever loose my z, I think I would look into an early Mustang, unless the sheet metal support for Z's go up it just seem too big a gamble if you have to pay large sums. I would be to afraid to drive it. While I don't consider a mustang type car to be anywhere near as nice as a Z you cant deny the support makes them a nice classic to own. Hopefully the after market will take notice and make body parts a bit more available for Z's. Perhaps its just a numbers game, lot more mustangs were built and owned here so guess the demand is higher. My fav is the fast back 66, really like the looks of that car, and is one I would consider if I did not have access to a nice Z.

sometimes I get too deep into analysis. Step back, messing with the cap makes it run great, try a new cap and rotor...if that does not fix it then you can get deep into analysis (compression check/fuel/Ign).

hmm well 6 look suspect for sure, the rest seem rich. this is where the color tune would come in handy, you can see if its sparking and evaluate the burn. I would move that #6 injector to another cylinder, get a new set of plugs, and see if the problem follows that injector. Also I noted your are running a plug I don't recognize. generally speaking NGK standard plugs are run with great success on L motors. If moving the injector has no effect then clearly there is a spark issue going on. did you try lifting the plug cap off a bit so there is a spark gap between the plug and the boot? just get it off a tiny bit and see if there is any change in idle. oh and do a compression test to rule out that as a problem, check all cylinders and look for even pressure. Right now I am kinda ignoring the "runs better with cap lifted". I don't doubt you its just something that does not have a clear connection yet. looking for compression, fuel, ignition even on all for now.

so after all the above it runs fine but for hot idle, which can be fixed by lifting the cap, is that right?

here I demo the color tune. now I am not saying this is the issue you have just something to consider if you have a misfire.

Now you got me fired up to look into my own rougher when warmed up engine at idle, cooler weather makes me more likely to wrench on it and see if I can find my problem.

a small brush like used for cleaning paint guns and some contact cleaner from the auto store, followed up with some deoxit will help.

warming up rough idle, things that come to mind is a leaner air fuel mix, which is harder to ignite. I presume you only have this roughness at idle? A leaner mix is more taxing on the ignition system, IIRC fsm calls for .035, but not sure of that. Any weakness in the ign will show up in the leaner idle mix. I have the same issue, idle cold is great get it fully warmed up and the idle is not as smooth. I don't worry about it since its fine at any thing other than idle, guess I could/should look into it. I did try new plug wires and cap recently, no change. Have some new plugs coming. It maybe something as simple as adjusting the idle bypass on the AFM for a slightly richer mix (for my car that is). IIRC I have noted this on a swap I did with a spare AFM (better idle vacuum). You can also see if you have a poor injector, maybe one is flowing a bit less, again a relative power test can isolate this, unplugging one injector at a time and listen for rpm or drop in vacuum is a easy way to find the loafing cylinder. Check it for compression, put a new plug and wire, if still loafing you can try a color tune plug and do a comparative flame color to a good cylinder. if its lean swap an injector and see If the laziness follows that injector. The color tune plug is expensive but a nice tool to add to your diagnostics tricks.

that cap looks a bit beat up from the rotor, maybe that is normal. may as well put some spacers in there and run it with the cap up since that solves the problem. Looks like some alum dust sprinkled around, maybe enough to provide a ground path same as a carbon track? Have you looked at the new plugs since you put them in? Next time this happens you could try pulling the spark plug wire off a little bit and seen if any one plug is effected. I am sticking with the partial fouled plug until you look at them. New plugs can foul if there is a problem.