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.