Gavin Posted December 12, 2007 Share Posted December 12, 2007 I have had an ongoing problem with my 280zx turbo which, despite my best efforts over the last 4 months or so, I just cant pinpoint. The car is an 83 zxt with the solid-lifter engine. I am using an 83 distributor for my tach input and spark with the stock coil (via VB921). MegaSquirt is a 3.0 board, MSII chip with v 2.86 firmware. Stock injectors and everything, except for an intercooler. Heres the msq- http://homepage.mac.com/gavinwilliams/ZXT.msq The problem that I am having is that I get sudden and repetitive momentary losses of power (skipping), but only above about 5 psi (~135 kpa). Most of the time this will not happen when the engine is cold, but will start happening when its warm. Other times, it will not happen at all, even after extended driving. And on the rare occasion it has occurred with the engine cold. The MAP signal, and the needle on the boost gauge, jump around rapidly while this is happening. My fist thought was that it was a boost leak, but I have repeatedly tested this by hooking the system up to my compressor and, despite finding several leaks, the condition didn't change. I have added ground wires, replaced the spark plug wires and moved them as far as I could from the tach signal wires, replaced the dis cap, rotor, coil, spark plugs, relay board, injectors & wiring... I know there are more things so Ill add to the list as they come to mind. I have also tried adjusting the Lag Factors in MegaTune. The MegaSquirt I am using is the same one I have used on my 280z, which it runs flawlessly (mind you, it was NA). And your all thinking WHAT ABOUT THE DATALOGS! http://homepage.mac.com/gavinwilliams/Good.xls http://homepage.mac.com/gavinwilliams/Bad.xls Here are datalogs from when the engine is running well and when its having the problem. I cannot see any significant difference between the two, other than that the MAP and to a lesser extent the RPM fluctuate a bit as the problem is happening (>~135kpa) in the Bad datalog. From what I can tell this is the effect and not the cause. What do you guys think? What am I missing? Quote Link to comment Share on other sites More sharing options...
evildky Posted December 17, 2007 Share Posted December 17, 2007 your map and boost gauge are all over the place? are they bouncing over say 10 psi? or just 0-5 psi? I had a weird surge taht woudl happed under boost that took me forever to discover was a failing fuel pump, when the boost pressure would rise the pump would shut off, I'd loose power, pressure would drop and the fuelpump would come back on and repeat Quote Link to comment Share on other sites More sharing options...
Gavin Posted December 17, 2007 Author Share Posted December 17, 2007 A REPLY! The fluctuation isnt even as much as 5 psi, just around 1 up and down maybe. Fuel pump is new, and the problem occurred before and after changing it. Filter is also new and I have a FP gauge that reads good, which is sourced from just before the rail. Im going to make a video of the problem happening soon. Why can I not edit my original post?? Quote Link to comment Share on other sites More sharing options...
mobythevan Posted December 18, 2007 Share Posted December 18, 2007 Why can I not edit my original post?? I don't know??? If the MAP and boost gauge jump around I would still think it is a boost leak, it just happens when the temperature is just right and a coupling opens up? When you did the boost leak test did you pressurize the intake above what boost you run? If you run 14psi, pressure test to say 24psi just to see if something is marginal. I realize it does it at 5psi, but under a certain set of conditions. Just thinking out loud. Quote Link to comment Share on other sites More sharing options...
evildky Posted December 19, 2007 Share Posted December 19, 2007 I've heard some people encounter weird problems with the on board ignition circuit, might be worth converting to external and see if it clears up Quote Link to comment Share on other sites More sharing options...
HB280ZT Posted December 28, 2007 Share Posted December 28, 2007 I had a similar problem like that under boost with my V3 board and it was a problem with the spark plugs. What I mean is that I had to drop the gap from 40 to 28 to completely remove the stumble. Just a thought! HB280ZT Quote Link to comment Share on other sites More sharing options...
WizardBlack Posted January 2, 2008 Share Posted January 2, 2008 Yeah, it could be spark blowout. Too much gap or weak/faulty/poorly grounded coil, etc. I have also seen some maps be too violent in the changes from one field to the next cause an issue or one field isn't way too low on timing or fuel (not tuned yet) that will cause a hysteresis like that. I do notice a very rapid change of fuel pulsewidth. Any way to get the sample rate higher? Quote Link to comment Share on other sites More sharing options...
Gavin Posted February 27, 2008 Author Share Posted February 27, 2008 Hey guys, Got it working by reducing the plug gap to 35 from the NGK recommended 44. My reason for being apprehensive to post this as 'fix' is that I am running stock 8psi boost at the moment, so I don't understand why this would be necessary. I am getting some unusual voltage fluctuations (compared to other cars I've MS'd) even after changing the alternator & going over all connections, and so I'm looking to this as a possible culprit. Just wanted to at least give an update. Anybody else experiencing something similar? Quote Link to comment Share on other sites More sharing options...
mobythevan Posted February 27, 2008 Share Posted February 27, 2008 Just curious, what was the AFR when this was happening around 5psi boost? Did it go extremely rich where it was dropping spark? Quote Link to comment Share on other sites More sharing options...
Gavin Posted February 27, 2008 Author Share Posted February 27, 2008 Nope, it would go from the normal ~12 to fluctuating rapidly at 13~14+. From my understanding, I think that would be expected from spark blowout because of uncombusted fuel(?). Quote Link to comment Share on other sites More sharing options...
mobythevan Posted February 28, 2008 Share Posted February 28, 2008 I didn't ask my question correctly, I was curious if it was going rich right before the sparks started dropping. Looks like it was holding at ~12 up to the spark drop. Quote Link to comment Share on other sites More sharing options...
Gavin Posted February 28, 2008 Author Share Posted February 28, 2008 I was suspicious that the problem was something like that, and unfortunately my WBs output to MS crapped out so i cant get a datalog to clearly verify that that wasn't the issue. With the plugs re-gapped though the AFRs are amazingly smooth (at least with the BAT lag factor modified to counter the gremlin I mentioned above.) Quote Link to comment Share on other sites More sharing options...
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