zpeedracer25

Hey all,
I'm having trouble narrowing down this intermittent problem on my 1977 280Z with a 1982 280ZX L28ET swapped into it. It runs lean occasionally at 5k RPM and above, stops reving like it hit a brick wall. I have verified this by watching my O2 sensor trace on an oscilloscope. My fuel pressure is fine the whole time also(have guage and adjustable regulator w/ Porsche high output pump). The only things that seem to change this is adjusting timing will lower the problem down to 4K RPM or so and I swapped in my spare computer and it stopped for about 3 test drives. I can never seem to get it to mess up with all my diag equipment hooked up either!

I am running the stock computer and ignition system, T3/T4 turbo setup, ZCC fuel rail, SX FPR w/ 40 psi of initial pressure, Cartech intercooler, Porsche fuel pump, Porsche BOV. I run between 10 and 13 psi of boost...running more/less boost doesn't seem to matter either. It does the same thing at stock boost pressure. Think thats about it as far as engine goes.

Any ideas would be appreciated.

lifegrddude

I've been told that two things can cause what you're experiencing. One is the AFM, and the other is the cylinder head temp sensor. For me it was the AFM. I also cleaned the TPS contact points as they were corroded too and I'm not sure if that contributed to the problem at all, but after doing those two things, my car has run fine. Before that my car would hit a brick wall at 5300 revs and not go any higher.
John 82ZXT

zpeedracer25

Life,
What did you do with the AFM? Did you replace it? Mine failed completely a few weeks ago(flapper door stuck). I've got a couple spare AFMs and they don't make any difference. It is a very good possibility that this could be the problem or a major player in it. I'm still curious why altering timing slightly lowers the problem to 4K RPM.
I have checked the cylinder head temp sensor and it is in spec every time I check it. I have also eliminated corroded connections also, since I just went through the complete engine harness a few months ago.
Thanks for the input.

lifegrddude

I changed my AFM out with one from the junkyard. I initially forgot to reset the ECU and it ran ok to 5300 revs again. After disconnecting it and grounding the ECU for 15 minutes, I reconnected it and the car ran fine ever since. Your ECU may still be holding the fault code from when the door got stuck.
If changing the timing causes it to hit the wall sooner, it could be ignition related since spark timing will be slightly different. How are the plug wires, coil and rotor? I've fried 2 sets of plug wires so far since owning the car, I think the demands of forced induction are hard on the system especially when the boost is cranked up. That and the super hot environment isn't great for wire life. My car did run bad from another source come to think of it too. A bad O2 sensor did make the car run poorly. However, the a/f constantly read lean no matter what. I'm no expert on these things though, just going by what I've experienced, hope these can help you out and get the Z running up to par.
John 82ZXT

zpeedracer25

Hmm,
New O2, less then 24 hours of driving time on cap/rotor/wires/plugs. Don't think it is ignition unless there is a crank sensor problem and it thinks the car is over revving and cutting fuel. The O2 is doing what its supposed to and it drops to 200mV or less when this problem shows its ugly face. Swapped in a spare computer and problem seemed to disappear for about 3 stop lights. I will check more into the MAF. Driving it the other night, it seemed to do it more often with foot to the floor driving and less often easing into it. So the MAF door may be slapping open all the way on hard pulls. That or I may have more then one problem causing this.

hellzbellz83t

I'm starting to experience the exact same thing. At first it was only when the headlights were on, but now it is happening with them off. I was leaning toward an alternator problem but I really have no clue. I'll keep an eye this post so if you figure it out let me know, too.
Thanks!
-Kel

zpeedracer25

Think I've got it figured out!
It finally messed up with my diag equipment hooked up. I found a bad waveform(spikes) on the AFM output to computer and the TPS when the problem would arise. This made me suspect that these two sensors were ok and I was seeing the effect of the problem rather then the cause. So, I hooked up the scope to the output of the crank sensor for the #1 cylinder sync pulse and hooked up a current probe to the #1 cylinder spark plug and used it as the trigger. These two waveforms should have been on top of each other, but the #1 pulse is happening 20 msec before the #1 fires. Thus I suspect that the misaligned crank sensor is confusing the computer and it is kicking in the rev limiter at a lower RPM and is cutting down the pulse on the fuel injectors. It'll be a while before I can restab the oil pump and distributor as I just put the car in storage about 600 miles away. That way I stop spending all my money on it until after the birth of my first child.........gonna be spensive one....doh!