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Check the BCDD. That exhaust pop on decel is a pretty common thing if you BCDD is not operational. You can do a "field" adjustment of the BCDD, but disconnecting the 12v wire to the solenoid to simulate going faster than 10mph, and reving the motor. Turn the screw on the bottom until it has a hard time idling down, then back it out slowly until it idles down gradually (for me this was approx 1/2 turn). If it doesn't do this, then the BCDD is probably bad. They are NLA, and it's hard to find a working one so you might be stuck with it if it's bad. A lot of people just eliminate it and plug the hole in the TB.

Nice install. Did you put your AFM back to the stock spring setting? What resistance did you end up with on the pot? I did the same today, but more of a temporary test install. I just setup a trim pot on some barrel connectors and put it inline with the water temp sensor under the hood. I tweaked, drove, tweaked some more, looked a vac, listened to idle, drove, repeat, repeat ,tweaked afm, drove, etc.. I found that if I put the AFM back to stock settings, and dialed up about 1100-1200 ohms on the pot, it ran smooth but lacked torque and felt a bit flat. So I richened up the AFM a bit again, not as much as before and dialed the pot back. I'm at 4 teeth rich on the AFM now, and right about 800 ohms on the pot. This is pretty close for me. It's rich enough that the lean miss fire is gone, and not too rich that it feels soggy. I may tweak it a bit more but this is close enough for me to give it some drive time to get more familiar before making any more changes. I've also got to get the suspension freshened up, bushings etc,, I've got so much rattle, bounce, and shimmy going on that it makes it hard to feel the subtle stuff on the motor at speed. I've got new tires on it, and I think they might have one not perfectly balanced but I'm sure they will blame it on my lack of strut dampening and rattling ball joint so I'll fix that stuff first. As far as popping on deceleration, do you still have your BCDD valve on the throttle body, and is it operational?

Sounds like a plan. Let me know how you fair with it. I do not think you will be too lean when cold though, if anything I think it will be too rich. 2k ohms is quite a bit of change on the water temp circuit, the car will think it's 68 degrees. My car seems to run pretty good when cold, and gets leaner when warmed up. Have you given any consideration to megasquirt with a wide band o2 setup ? I'm thinking that may be a better long term solution. I may bandaid the car with the same tuning setup as you for the time being and look further into a fuel only MS setup for the future. Like you, I have no desire to turn this into a race car, but I would prefer it to have a decent tune at all throttle conditions. Having a smooth running car is just so much more enjoyable to drive. I've got quite a bit of experience doing EFI tuning on motorcycles for performance (I have a nasty road racing habit), so I'm familiar with wideband tuning and all the variables that come into play with different speed/throttle postions/ gear selection etc.. Thanks for the tip on the idle screw. I'll pull it out and take a look. Also, FWIW I pulled my check valve for the fuel pump yesterday and inspected it, tested it with air. It holds air pressure, so I'm pretty sure it would hold fuel pressure. My fuel rail is boiling within 5 minutes of shutting down, and I now have an inline pressure gauge permanently mounted. I found out just how quick it boils by pulling the fuel rail inlet line off to put the gauge on after it had been sitting for 5 minutes only to be greeted with boiling steaming fuel shooting out. The fuel rail only holds pressure for about an hour or so, so I think my FPR might be leaking down.

Interesting that you are arriving at the same conclusions I have. I re flowed all the solder joints on the entire ECU last night and it didn't help. I too was wondering if degradation of the analog ecu components were to blame. I had thought of wiring a potentiometer in line with the water temp circuit, but worried that it was too broad of a fix and might make the WOT too rich. I can tell you that I've continued to drive, and tweak on mine. I recently adjusted the idle air mixture, I put it three turns in (mine was 6.5 turns out from full rich stock). This with the 6 teeth advance and the timing at 13 is working pretty good for me. It's not 100% and I'd like to find a better work around, but it's close.

Yeah, but if you are like me on this problem you'd be willing to spend $250 to solve it 100%

There are fuel pump contacts in the 78 AFM. I've got two of them. There are not however any wires from those contacts to pins in the connector or the harness. So effectively, it has no contacts. The AFM should be A31 604 000, at least both of mine are

I honestly don't know if the PO was smart enough to diagnose it. I can tell you that the fuel pump safety switch on the AFM module is not operational or wired in on the 78. There are no pins in the harness for it. That would be the only way I could think of that the fuel pump wouldn't run in the key "on" position since the fuel pump has to run in the on while the car is running.

Might have been modded by PO. My fuel control relay is tied to the ignition circuit, which comes on with the key in the "on" position. When I do a hot start, I can hear all kinds of bubbles in the fuel tank coming through the fuel return. Thanks for the tip on the check valve guys. Fastwoman, Another suggestion that's been made to me that seems plausible is "cam walk". Apparently there have been cases where the timing chain tensioner isn't doing a good job keeping tight due to partially blocked oil gallery. I'm checking in to it. My injectors have been flow tested. Tonight I thought that maybe I was sucking air into the fuel pump, so I installed some clear fuel line just before my fuel rail and looked for bubbles while running. There were not any. I'll be pulling the valve cover off and checking for play by rocking the cam back and forth. I tried looking at it with a timing light while running, and it does appear to move a bit but my timing light is a cheapy and it's strobe doesn't seem consistent during this test (although it does when I'm checking timing on the crank)

my fuel pump comes on as soon as i turn key to "on" and I let it prime before starting, however the FPR doesn't allow the fuel rail to circulate prior to getting a vacuum on the FPR ? Am I incorrect in this assumption ? Z train, check valve ? Sorry about the thread jack, I've just wondered if the hot start and lean conditions were some how related.

Interesting. I also get about 15.5" of vacuum when I have the AFM set to 6 teeth richer. FWIW, when I did the leak down test, The motor cooled off quite a bit while I got my setup right and figured out the procedure, at that time I got 20-22% leak down. I did it again today with the motor up to operating temp and got more like 12-15%. Should be well in line with a 100k mile motor. Anyhow, I thoroughly checked out where the air was going, it's all going past the rings, nothing by the valves or through the intake/exhaust. So thats why I ruled out head, etc.. My fuel pressure has also tested good each time I've tested it. Although I do have the same hot start issues that you've mentioned.

Just curious what kind of vacuum you can pull if you richen it up to the best idle? The most I can get is 17 or 17.5 if I richen it up as much as possible. This motor should pull at least 18 or more, so that's why I'm still not convinced it's fuel only.

fastwoman, any progress? I did leak down testing and ruled out any cylinder head issues. I'm still stumped.

The ECU doesn't control when the fuel pump comes on. There is a 5 blade relay in the relay bracket. When you turn ignition on, it should turn the fuel pump on. Look for a white/black wire with 12v going to that relay. Test the relay. The ECU gets power from the 6 blade FI relay right next to it. So you might as well test that relay while you are there. Test procedure for the relays is in the FSM under the EF section.

idle stumbles around, you hear a slight popping in the exhaust, but it's not to any particular rhythm like a standard miss. When you drive it, it will stumble and hesitate randomly at lower rpm. There is a video of it in the thread I started over in the help me forum.

Just FYI fastwoman. I tried another ECU tonight, another AFM and went over my entire intake with propane again and still couldn't not find anything. At this point I've ruled out everything I can think of externally replaceable. Debating on pulling the head off personally.

Yeah, you have some valid points. I feel like I've been running in circles on what seems to be the same issue for weeks. Tonight I will be coming full circle again, jacking the car up and trying to find an intake leak from the bottom. I've tried countless times to find an intake leak on this car, and like fastwoman I have new intake gasket, etc.. I've plugged all the vacuum lines and ruled out any external vacuum leak source. But the truth of the matter is that it sounds like an intake leak, smells like an intake leak, and feels like an intake leak. It's just very frustrating when you bathe the thing in carb spray, do boost tests, smoke tests, etc.. and cant find a darn leak. Especially when you know you just put a new Nissan gasket on. I'm wondering if the EGR plumbing isn't leaking internally some how. I've got a new EGR valve on mine because the old one did have a minor diaphram leak but the plumbing is still the same old rusty cast iron. Personally I just keep getting distracted by all these other possiblities with the EFI etc.. Last night however, I had my AFM apart "again" and fingered the potentiometer to the best idling position to observe vacuum and listen more carefully. For sure it runs smoother, but I did still notice a slight occasional miss. Also I still couldn't get the vacuum over 17.5 no matter how much fuel I added to the mixture. Considering I've seen threads of guys with 15-20psi less compression than me on the same motor getting 18.5 vacuum, I've got to think that there is still some source of unmetered air getting into the intake system (head or manifold) some how. Still running in circles. Check out this copy and paste from another thread: "This problem was present on a 76 that I just rebuilt. The problem was the valves. A valvejob cleared it up nicely. The N42 head (76 has it) has soft valve seats (unless it's already been rebuilt, but they can still be done wrong...) what happens is the seats sink into the head until they come out of adjustment. the valves can never be adjusted properly in this condition. In the case of the 76 I just finished, the car ran rough until just over 2200 and then ran "fine" up to 5500. I say "fine" because after the rebuild the car runs "awesome". Do what ZuperDave tells you, do a compression test (do it right) do a leakdown test (do it right) then put a vaccum gauge on it and it will tell you a lot about the condition of the car. If it's not eating oil and you've adjusted the timing this is where you are headed. One other thing. One (or more) of the o-rings on the injectors could be covering up the spray patterns if it wasn't installed properly. Or, if you are using an intake manifold gasket without the injector holes in it (has happened before) your car will run like @!#$ too. "

Nice looking engine job! Looks like you insulated your fuel rail, I'd be interested in your method. I moved my afm contacts just a hair up so that they would be on new carbon on the trace. It made no difference. I too have a new intake/exhaust manifold gasket. And I've been questioning the install of that plenty too because it does feel like an intake leak, although I've done smoke tests, boost test, carb spray, etc.. and not found anything. I'll be keeping a close eye on your progress. I'm very frustrated at this point.

Yeah, from my understanding, 19-20mpg is pretty much on target for a 4sp at that rpm. Fastwoman. I forgot to mention RE: your soldering on the ECU. I did inspect mine over real well with a magnifying glass. I reflowed all 35 pins on the ECU connector, a couple of those guys looked a bit cold/suspicious. It didn't help. There was an old thread on one of these forums where I guy had some poor solder joints on his ECU, specifically the connector solder joints and it helped him. It's a possibility and no harm in trying, just don't get your hopes up. Especially since you've tried a 2nd ECU. I have been thinking about this problem non stop for weeks. I am beginning to wonder if it is in fact an AFM problem. I've ruled that out with testing a few times, but I wonder if the test procedure is robust enough to detect these minute problems. Basically you are just looking for a resistance test from the various pins. The static resistance from the reference resistor and air temp sensor are easy. The resistance test from the potentiometer leaves a bit open to interpretation. It jumps around a lot as it moves, and according to the atlantic tech tips pages, this is normal. Furthermore, the 12v output test in the FSM seems to work smoothly.. however.. follow my train of thought for a second. The wiper on the potentiometer gets a lot more wear in the first 30% of travel. It gets a lot of wear around idle, and at cruising because that's where it spends the majority of it's life. I performed the 12v output test while moving the flap slowly and it seemed to lower the output progressively. I wonder though if this is misleading. When we loosen up the spring on the AFM and make the mixture richer, we are effectively reducing the voltage output of the AFM to the ECU. Also the physical position of the wiper is moved significantly, possibly out of the more "worn area" of the carbon trace. I just wonder if the test procedure in the FSM, FI Bible, Tech tips pages, etc.. is the end all be all final answer. Is it possible for there to be enough wear on the potentiometer at the idle and cruising areas to have incorrect or intermittent resistance and affect idle/part throttle operation? low vacuum could be just the symptom. These are some reasonable assumptions that can be made: Adding enough fuel seems to correct the problem Any significant source of vacuum leaks has been ruled out When adding more fuel to the idle mixture, higher throttle performance suffers (presumably because the AFM flap would be open a lot more) We aren't the only one's who have suffered this problem and it could be wear related item. Consistent compression across all cylinders seems to rule out a valve or internal engine issue. Even blues tech tips pages has an old worn out 280z motor pulling 18.5hg. The issue does not seem to occur at all when giving the car more than 30% throttle. This could be due to TPS "full enrichment" or possibly the AFM flap opening past the worn area. I know some speculation and suggestions have been made that internal problems could cause this, like worn rings, leaking valve guides, etc.. but two facts seem to contradict this. One is that the vacuum gauge hand is steady, and in a valve leaking scenario this should not be. Second, worn rings would cause engine smoke of some sort. I have none. Even if you make the argument that the rings are seated and working on compression stroke, but leaking or allowing blow by on intake stroke, oil would go by as well and create that puff of smoke on deceleration. I just cant make sense of any internal engine scenario. I've spent countless hours on the internet, google, forums, etc.. researching "280z low vacuum" "280z hesitation", "280z stumble", etc.. and I've found lots of descriptions that seem similar, quite a few discussions, but never a solution. Every discussion I've found abruptly ends. Some discussions are not exact, they might have had fuel pressure issues etc.. But I have found several cases that appeared to be similar, and never a posted resolution. I have found a couple threads where they thought they had a resolution "like bending the TPS full enrichment contact to engage all the time" etc.. But these are just hacks to work around the problem, not real solutions. I think timing should be ruled out as well. I've done the valve timing, and ignition timing until I'm blue in the face. Sure, advancing ignition timing helps. It doesn't solve the problem. The reason it helps seems to be the fact that advanced spark helps a lean mixture by allowing it a longer burn time. But again, aren't we just working around the problem ? The only thing I have a hard time with on ignition timing is this "ported vs. manifold" vacuum. Obviously manifold vacuum on the vacuum advance will advance timing a bunch at idle and part throttle conditions and would be ideal. However It's not the factory configuration, and I'm quite sure this car didn't run like this from the factory. Also, the vacuum advance mechanism wasn't designed for manifold vacuum souce so I'm not sure how that would affect the overall operation of the vacuum advance with a much higher vacuum source. All of this comes full circle and keeps me thinking that there has to be a metering issue of some sort. It's the only thing that has a solid explanation (if you throw out the FSM test proceedure).

oYou can't take a pic of the correct AFM settings. There isn't any mark on the gear. I can tell you for sure mine wasn't tampered with and I'm experiencing the EXACT same issues as her. I've also done a lot of the same test with the same results. I'm sure she marked the original setting on her AFM so she can put it back to spec. The problem is that the thing pops through the intake like crazy at anything below 2500 rpm so she has to do something to be able to drive it somewhat and feel it out. As far as advancing the timing. I'd prefer to have mine on 10 too. I can tell you that the more advance is supposed to be better for a lean mixture, as it takes longer for the lean mixture to burn so in theory the advance should give it more time. I've tried mine on 15 degrees for a few days, I'm thinking about cracking it back down to 12 or so just to see. I'm getting about 19mpg at mostly 70mph driving with this setting. I have a spare ECU, AFM and throttle body of a low mile part out coming. I could have sworn this was an ECU issue, but you've tried that so I'm not so sure now. I've resistance tested my AFM 10 times it seems, and under a load did the voltage output. It tests fine. The only thing I can figure is that maybe the temp sensor is off some, although it seems to test ok too. The flow rate on the injectors I got before cleaning and testing was about 225cc. After, they are all getting right at 240cc.

FWIW, I've got mine set at 6 teeth richer on the AFM, and timing set at 15 degrees. It's pretty drivable set like this. Only temporary in my case until I find the root cause. I don't think the injectors are the source of your issue though. I still have the nissan injectors on mine and I had them cleaned and flow tested so there aren't any issues there. If you suspect the FPR, you can disconnect it and go for a drive. I did that as well. It helped some, but again, it was just a test. I get 37psi with FPR disconnected, 32psi at idle with it connected

Definitely let me know what you think. I think we are fighting the same problem, and two heads (with two cars!) are better than one.

You might consider having them cleaned off the car professionally. Mine still tick a little, but I cannot hear it in the car now and it did seem to help.

Yeah, I thought about that too, but the lean condition actually exist at idle and at slight throttle openings such as cruising or light acceleration. Thats when you get the intake pops, etc.. The best way to richen it up I found was to jumper the "full enrichment" position on the throttle position switch, but you use a lot of gas doing that ( i got like 13 mpg), and it's a hack. I'd rather figure out the real problem then just put in a hack. Where I've got it now makes it plenty driveable and is a work around for now, I've got the AFM advanced 5 teeth, and the timing on 15 degrees advance. That almost completely gets rid of the stumble, appears to get decent mileage, and doesn't loose power so much. It's a compromise for now. it seems to me that there are a few 78 owners that have this issue. I know of 3 so far.

MSA has the boots, I just replaced mine.

Funny that you arrived at 12 teeth. That's exactly the number I arrived at when richening mine up that way with the AFM . I also got up to about 18 or so vacuum when doing that. However, I think that when you go drive the car you may find that it's too rich at mid to full throttle. This is the reason why I put mine back, trying to find another option. I think richening it up with the AFM might be compensating for some other issue. I could not get over this idea. The cure might definitely be a richer mixture at lower rpm, but the upper rpm gets richer too with the AFM. Let me know what you think when you drive it.