RPM Resto & Custom

Hey guys I'm fiddling with my buddy's 78 280Z and having a tach issue. His original tach seems to work fine until around 2,500 RPM's and then gets stuck. Sometimes it won't come back down for a while. I assumed it was a mechanical issue inside the tach but I'm not sure. I borrowed another used tach from a local guy....he said it supposedly worked when it was pulled a while back. When I plug that into place it actually works worse than the original. It takes several seconds for it to come up to the correct RPM reading and also won't seem to go above roughly 2,500 RPM's. I cleaned the wiring connections at the coil and the coil was a stock replacement that was just put in recently. I also pulled the resistor out from the pass kick panel and it check out at around 2.18K Ohms....I believe it should be 2.2K so I'm close enough. Is it possible I have 2 bad tachs? That's kind of what I'm leaning towards....one bad electrically and one bad mechanically maybe? Is there anything else in the system I should be checking?

I'm going to read through the 2 articles from Willoughby today sometime. I felt like 12 was a decent number of teeth but not excessive....it's less than 1/4 turn of the wheel but I'm sure it makes a large difference weight wise.

Well I got my father to come over to my shop today for a while and it looks like we solved or circumvented the problem. I swapped ECU's between the 2 cars and it made no difference so I basically ruled that out. This left me with either the resistor mod or trying to recalibrate the AFM itself. Rather than start trying different resistances I decided to pirate the AFM off of the good running 78 here and sure enough the 76 started running like a champ with it installed. No more backfiring through the intake and tons more power all around. Now the AFM out of the 78 had some non original silicone around the cover but not sure if it has ever been adjusted or not so it's a bit of a mystery there. We opened up the original AFM and everything inside looked brand spanking new...no corrosion, lots of shiny connections, etc. It is the original one from the car as my father has had it since new. I loosened the set screw and we started playing around with the adjustment. Just 3 teeth looser made the backfiring through the AFM stop. After a number of trial and error runs around the block with the car a total of 12 teeth looser seemed to make the car run happiest. We will check the plugs after some driving and make sure nothing looks unusual and go from there. If the condition comes back again over time we will most likely just get a new AFM at that time.

Zed what do you mean by the potentiometer in the cooling temp circuit? Do you mean the resistor mod that some guys use, specifically the adjustable dial type that can be installed? Using a "good" ECU would have the same effect at least until it went bad as well right? Or are you referring to the adjustment that can be made inside the AFM itself?

Yeah that was the one diagnostic tool I forgot to grab from my shop before I headed over today. Not that it's an accurate measurement but there shouldn't be any major vac leaks because the idle is pretty smooth and when I had the fpr disconnected I felt a strong pull from the hose. But I do understand that even a small amount of unmetered air after the AFM can throw off the a/f ratio substantially

After researching the topic some more it seems that 30psi may be an acceptable fuel pressure and that it is most likely related to the signal from the temp sensor and the way the computer has worn over time and receives the signal. Is this the more likely scenario if you rule out vac leaks? I have an ECU out of a 78 with the same trans that I can attempt to swap over just for sake of testing the theory. If the 78 doesn't have the backfiring through the AFM then it should stop in the 76 if that is in fact the issue right? I will doublecheck first but I'm almost certain the ECU is the same P/N for both applications.

Hey guys...I'm over at my parents right now looking at my father's 76 280Z. The car is completely stock with no engine modifications and I believe it still has the original fuel pump in it. He has been having an issue with "backfiring" for a while now and after we looked at it together today I can hear it actually coming from the intake side through the AFM. From what I've read using the search function this can indicate a lean condition. The backfiring only happens when you jab the throttle to accelerate...which I also believe would be consistant with a lean problem. All of the vac hoses seem to be in good shape as does the boot between the AFM and throttle body. I also slightly adjusted the timing and idle...about 12* and 800 RPM's. I have a homemade fuel pressure gauge for Z cars that I use....I put it inline between the outlet of the fuel filter and the inlet of the fuel rail. With the car running at idle I'm seeing right around 30lbs of pressure and if I rev it up and hold it the pressure will increase momentarily and then come back down to around 30 again....which should indicate the pressure regulator is working correctly right? Is 30 psi too low and possibly causing the lean condition and backfire through the AFM?

Yes....that is how my pins line up...same as the diagram in the manual.

I definitely had them correct as far as IDing the connector. 45 showed 58 ohms resistance on the thermotime and 46 none when cold. I then wrote the numbers right on the connector to keep them straight through all this. As stated earlier by someone I tried just swapping my wires over at the csv and it actually didn't make the system operate properly. Only when I swapped them at the bullet connectors did it work right still. I ended up just clipping the bullets and putting new connectors on today and leaving them crossed. It all seems to be working right despite making no real sense and the car starts up like a dream now after being out in the cold for hours. First start today in 45* weather was less than 2 seconds of cranking before it caught. Thanks for all of the help guys....I can't believe we haven't been able to identify the actual issue or understand why it works the way it works in my case. One of those great mysteries I suppose.

That's exactly how I grounded it. And if I momentarily ground 46 instead I get smoke. I have to have them right though as 45 gets the 12V signal on start and 46 doesn't when the harness is unplugged from the sensor right?

Captain Obvious.....when I say loss of ground at 45 and 46.....I have a multi meter set to measure continuity with the audible tone on.....one side to ground and one side to whichever pin on the thermotime harness connector I'm probing. As soon as the key goes to Start the tone goes away showing the loss of continuity. Although maybe once the vehicle's voltage passes through it that throws off the meter reading? To answer your second posts question....yes that is basically where I grounded out the system and got fuel. I didn't short it at the bullet....I did it right at the harness connector for the thermotime...with it disconnected from the thermotime switch.

Hahah but did I really fix it or is it somehow bypassing something that shouldn't be bypassed? I suppose it won't hurt anything if it's only active during Start but rather be certain. I'm a stickler for details and doing things the right way whenever possible.

Only if they somehow pulled the female connectors right out of the csv connector on the harness side and switched them. The 45 at the tt seems right as it's getting 12V at Start and the 46 doesn't. I'll look the harness over better tomorrow but I'm pretty certain it's original. Is there any way to determing which pin on the csv should get which wire? I was hoping I was onto something with the loss of ground at both 45 and 46 of the tt as soon as I hit Start.....that sounds proper to you guys though?

Ok so I spent the last 2 hours looking this issue over some more and have come out even more confused. Here is what I can tell you for certain. The CSV works since I can get it to spray fuel. And the new thermotime switch seems ok as well. When cold I get about 58ohms of resistance at Pin 45 and ground to the body of the sensor at Pin 45 and 46. If I put 12V to pin 45 I lose the ground connection to 46 after about 6 or 7 seconds and after a minute or so it'll come back...so the heating element and switching seem proper. Now is where things start to get funky. Remember when I typed a few posts back that if I ground out 45 of the harness at the tt switch I would get fuel and someone said that would yield smoke and 46 would be the correct one to ground for the csv to fire? Well that made sense and I assumed I just remembered wrong. Turns out I'm right.....if I ground out the #45 wire that's when the injector fires. This same wire gets 12V when the ignition is in the Start position.....#46 shows no voltage while the harness is unplugged from the switch. So this being said....just for the hell of it I disconnected the bullet connectors that the tt switch harness leads to and I used jumper wires to criss cross them......sure enough the system seems to operate properly then. When cold the csv sprays for about 5-10 seconds before cutting out and will operate again after a minute or so of cooling down. This makes no sense however as the wiring looks completely untampered with. These are definitely the original bullet connectors and the harness plugs only fit one way onto both the thermotime and csv. And the last thing I found which I don't think is right is this.....with everything hooked up properly if I probe the back side of the connector at the thermotime I have ground at both wires when cold. As soon as the key hits the Start position I lose ground at both of them....I don't understand why. Maybe this is the key to my entire situation? Shouldn't 45 still always have ground and 46 have ground until the thermotime heats up?

You're correct I did in fact mean the thermotime and not the csv. I have to cut and buff the freshly painted rear hatch on it in the morning tomorrow and as soon as that's done I'll be focusing on this issue the rest of the afternoon if possible. I'll report back with that I find or don't find. Thanks for all of the help.

I'm going to have to try and look at it again tomorrow. I had been fighting off a cold all week and it finally caught up to me yesterday. Spent the entire day in bed and all morning today. Have a family function this afternoon so hopefully I'll feel ok tomorrow and check it out more. I believe you're right and I actually grounded 46 and got the injector to operate in the Start position. Do we really think it's possible for the new CSV to be no good? I remember testing it on friday for continuity to ground (the body of the switch) on both pins. When it was cold I had continuity on both pins and one lost continuity after warming up the switch in hot water. Wouldn't that indicate a good switch? For a system that only has 2 or 3 working parts it sure is confusing. When the thermotime switches to ground it does in fact only ground to the block and not some other wire in the harness right?

Oh and did you ever figure out why the ECU receives that signal from the csv via pin 21 in the connector?

Wade I actually found that post before you threw it up in here and read through it. What was your actual initial problem? Your write up makes it sound like your CSV was always on. Was that the case? I'll have to test the continuity change of my new thermotime when 12V is applied to it and then allow it to cool...I believe I did this today but it was a long day. That only occurs on one pin correct? The other seems to be always ground as far as I can tell. Maybe I have an issue with the yellow/white wire you reference that the ground power runs through? That would explain why having the switch body grounded to the frame via an external wire isn't fixing my problem right? Because it has to have the ground through that yellow/white wire to properly activate the csv?

Zed I'm not sure which test on EF-23 you're referring to. I see reference to trigger signal, dropping resitor, power line circuit, and some sort or continuity test. I'm assuming it's the continuity test that flows over onto EF-24? Like I said before for some reason if I ground out wire 45 the injector will spray in the Start position. My switch does have continuity from 46 to ground when it's cold and it goes away if I warm it up...which makes me believe the new switch is operating properly. Also I did not run teflon tape on the thermotime. I checked for ground continuity from the body of the switch to some random bolts on the engine and body and seemed ok. I even went as far as pulling the thermotime out and putting it in a cup of ice water with a ground wire running off of the threads to a good engine ground and it still didn't work. The whole circuit somehow seems to baffle me. I know the manual says when the switch is closed it operates the csv and when open obviously doesn't. What does this mean for power flowing to ground through pin 46 though? I'm assuming if 46 is flowing to ground then the csv fires? If so why doesn't it work if I just manually ground out wire 46? Wade- I'm going to try and search for some of your info on the circuit now. I was having problems with the search feature this afternoon. Do you recall if they were just replies from you or topics you actually started? Thanks for the help guys.

Hello everyone.....I'm working on a 78 280Z 2+2 that my brother recently picked up. I just had the engine rebuilt as well as doing some patch work and other general upkeep. The car starts up and runs really well except for extended cranking in the cold weather. I started digging into the Cold Start Valve system and I'm still unable to get it working properly. Initially I removed the cold start valve and put it into a container to see if I would get fuel when cranking it out in the cold after sitting all night. No fuel. So I jumpered the csv directly off of the battery and despite having fuel pressure it would not spray. I obtained a replacement and the same test off of the battery now yields fuel. I still won't get any fuel however when the injector is hooked up in the harness properly. After verifying that I am getting 12V to the thermotime switch I attempted to ohm the switch out and it looked to be out of spec. I picked up a new one today from my local Nissan dealer and installed it but it still won't fire the cold start valve. The wiring all seems to be ok as I'm getting 12V to the thermotime switch and I have continuity between both sets of wires that run between the csv and thermotime switch. I also have continuity from both down to the 21 pin at the bulk connector that goes into the computer. My ground for the thermotime body to the engine seems to check out ok as well. Given all that what could be causing this to not work properly still? The most interesting thing I've found is that if I disconnect the electrical connector from the thermotime switch and jump the 45 wire to ground the injector will spray when the key is turned to the Start position. Electricity isn't my strong suit and the diagram from EF-49 in the FSM is a bit confusing as it seems to make a circle between the thermotime switch and cold start valve. Any help would be greatly apprecited. I'd like to get it running properly for him even in the cold as it will be his daily driver. Thanks in advance.

Yes there is a metal structure that runs through the mounting edge and I believe bends into the "flat" portion on top but not throughout the entire piece of rubber. I'll have to mess with it some more today. The original sat perfectly flat at the same level that this new one is mounted at. Fillers came from Motorsport. Here's the link for the rear fillers....they also have the fronts. http://www.thezstore.com/page/TZS/CTGY/SBUC01B08

Hey guys I recently bought all new rubber fillers for my 280 and I was trying to button everything up tonight now that my bumpers are back from chrome. The center filler piece in the rear is sitting quite a bit above the bumper and I can't get it to lay flat with the adjustments in it. Does anyone have any suggestions on how to achieve this? I tried a little heat with a heat gun while holding the rubber down with a board and some heavy steel blocks but it popped right back up. Do they need some sort of adhesive? The originals didn't have any from what I can tell so I'm assuming that's not my answer. Any help is greatly appreciated. Trying to get my personal car ready for show season. Thanks!

Found my broken wire. Looks like someone accidently damaged it installing an aftermarket radio. Everything seems to be in working order now.

Zed....I'm keeping all future posts in the other thread. I tried starting this one to weed out all of the circles I had gone in on that thread. I can't remember the last time I had this much wiring trouble. And the harness doesn't look like it was ever tampered with so I don't know why I'm having so much difficulty with this one.

Zed...you are correct this is a customer car, however I have a lot of hours into this issue...of which I will not be billing my customer for. I do not claim to be an expert on every car out there....especially when it comes to electrical issues. Being in the restoration and not the repair business per say my expertiese lays more in refinishing however I'm fully capable of performing mechanical work. I should have mentioned that the car is a 75 in my first post but I referenced it (not very well) in my #8 post. I understand that the injection harness gets power in both the Start and Run positions, however the csv and thermotime both only receive power in the Start position....on the black/yellow ignition switch wire that I referenced. In the run position the harness receives power on the red/white wire. The thermotime and csv should only have power when in the Start position and the csv operates based on the grounding of the thermotime, which is based on if it's heated up or not from extended cranking...in the Start position. It's basically a way to keep from flooding the engine in an extended cranking situation...although it only shuts down the csv and not the actual injectors I think. So unless I'm wrong this system does not have power available all the time....since it's a Start only function...running off of the black/yellow wire...which loses power once the key returns to the Run position. This brings me back to my question in my previous post....do you know where the black/yellow wire from the ignition switch travels up to before splitting off to the yellow (what the FSM diagram still shows as black/yellow)? I believe I have a break in this run of wire somewhere causing my lack of power to the thermotime/csv circuit....but as I said I can't follow the wires up under the dash. I was hoping someone would know where they may split off and I may be able to access them from.